Release of endogenous NE from the mesenteric vasculature of WKY and SHR in response to PNS

The release of endogenous norepinephrine (NE) from the mesenteric vasculature of the isolated mesentery of Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) was determined in response to periarterial nerve stimulation (PNS). PNS caused a frequency-dependent release of NE that did not differ in WKY and SHR preparations at 4 to 10 Hz but was significantly greater from SHR preparations at 12 and 14 Hz. When expressed as NE release/stimulus, release was significantly greater from SHR preparations only at 14 Hz. Mesenteric vascular perfusion-pressure responses to PNS were significantly greater in SHR than in WKY preparations at 6 to 14 Hz. Mesenteric vascular perfusion-pressure responses to exogenous NE were significantly greater in SHR than in WKY preparations at all doses tested. Treatment of mesenteric vascular preparations from WKY and SHR with cocaine at two concentrations plus corticosterone to inhibit neuronal and extraneuronal NE uptake, respectively, increased PNS-induced overflow of NE significantly. There was no difference in the NE release/stimulus in response to PNS between WKY and SHR preparations after either concentration of cocaine. The enhanced PNS responses in SHR at higher frequencies appear to be due in part to decreased NE uptake.     

Effects of propranolol and yohimbine on periarterial nerve stimulation-induced release of endogenous norepinephrine from the mesenteric vasculature of Wistar Kyoto and spontaneously hypertensive rats

The overflow of endogenous norepinephrine (NE) from the mesenteric vasculature of the isolated mesentery of Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) was determined in response to periarterial nerve stimulation (PNS) before and after pretreatment with propranolol or yohimbine. Propranolol pretreatment did not significantly alter spontaneous NE overflow, total NE overflow, NE overflow/stimulus or fractional NE overflow in either WKY or SHR mesenteric vascular preparations at any of the PNS frequencies used. Yohimbine pretreatment did not significantly alter spontaneous NE overflow but did significantly increase total NE overflow, NE overflow/stimulus and fractional NE overflow at all PNS frequencies used in both WKY and SHR preparations. The magnitude of the effect of yohimbine on NE overflow/stimulus did not differ between WKY and SHR over the range of PNS frequencies used. The lack of effect of propranolol on NE overflow suggests that corelease of epinephrine is not sufficient to activate beta adrenergic receptor-mediated modulation of noradrenergic neurotransmission in either WKY or SHR mesenteric vascular preparations under the in vitro study conditions used. However, the effects of yohimbine indicate that prejunctional alpha-2 adrenergic receptor-mediated inhibition of noradrenergic neurotransmission is operative in both WKY and SHR mesenteric vascular preparations. These effects of yohimbine also suggest that no significant differences exist between the functional level of prejunctional alpha-2 adrenergic receptor-mediated autoinhibitory modulation of noradrenergic neurotransmission in mesenteric vascular preparations from adult WKY and SHR under the in vitro study conditions used.     

Defective modulation of noradrenergic neurotransmission by endogenous prostaglandins in aging spontaneously hypertensive rats

Sympathetic nerve stimulation causes a greater vascular response in spontaneously hypertensive rats (SHR) compared to Wistar-Kyoto normotensive rats (WKY), i.e., noradrenergic neurotransmission is enhanced in SHR. Prejunctional and/or postjunctional defects in the regulation of noradrenergic neurotransmission by endogenous prostaglandins could contribute to the increased responsiveness to sympathetic nerve stimulation in SHR. This hypothesis was tested by comparing the effects in SHR vs. WKY of inhibition of cyclooxygenase on vascular responses to periarterial nerve stimulation (PNS), norepinephrine (NE) and angiotensin II (ang II) in the in situ blood perfused rat mesentery. The cyclooxygenase inhibitor, indomethacin, potentiated vascular responses to PNS and NE similarly in 16-week old SHR vs. age-matched WKY. However, in this age group, indomethacin enhanced responses to ang II more in SHR compared with WKY. To determine whether chronic exposure of the vasculature to high blood pressure might alter the physiological significance of prostaglandin-mediated regulation of noradrenergic neurotransmission in vivo, additional studies were conducted in SHR and WKY that were 25 weeks old. In this age group, neither indomethacin nor ibuprofen, an alternative cyclooxygenase inhibitor, significantly potentiated responses to either PNS or NE in SHR, whereas in WKY both indomethacin and ibuprofen potentiated responses to PNS and NE. Also, in these older animals, indomethacin and ibuprofen enhanced responses to ang II equally in SHR vs. WKY. These findings indicate that in aging SHR prostaglandin-mediated regulation of vascular responses to sympathetic nerve stimulation becomes defective. This defect may contribute to the worsening of high blood pressure with age and may be involved in some of the vascular pathology associated with hypertension.     

Enhanced slow-pressor response to angiotensin II in spontaneously hypertensive rats

Rapid-pressor and slow-pressor responses to angiotensin (ANG) II and norepinephrine (NE) in spontaneously hypertensive rats (SHR) and Wistar Kyoto control rats (WKY) were examined. All animals were treated from 4 wk of age with captopril (100 mg/kg/day in drinking water) to prevent development of hypertension so that changes in responsiveness could not be attributed to disparate base-line blood pressures or to hypertension-induced injury of the cardiovascular system. In 11-wk, conscious, unrestrained, captopril-treated rats, ANG II and NE induced rapid-pressor responses (i.e., a rapid increase in arterial blood pressure that reached a maximum within 10 min) that were of similar magnitude in SHR and WKY. In an additional group of 9-wk captopril-treated rats, both ANG II and NE caused slow-pressor responses (i.e., a slow increase in arterial blood pressure over 2 wk). Although the slow-pressor response to NE was similar in SHR versus WKY, the slow-pressor response to ANG II was much greater in SHR compared with WKY. Further studies were conducted in captopril-treated (from 4 wk of age) SHR and WKY to investigate whether the increased slow-pressor response to ANG II in SHR was mediated by an enhanced ability of ANG II to potentiate peripheral sympathetic neurotransmission, contract vascular smooth muscle, increase sympathetic tone to nonadrenal sites, release aldosterone, and/or reduce renal function. No evidence was found that supported a role for the aforementioned nonrenal actions of ANG II. However, 11-wk captopril-treated SHR were 10-fold more sensitive to the antidiuretic, antinatriuretic, and renal vascular effects of intrarenal infusions of ANG II compared with captopril-treated WKY. Also, chronic (1 wk) intrarenal infusions of a very low dose of ANG II (1 ng/min) caused a marked slow-pressor response in 11-wk captopril-treated SHR but did not alter arterial blood pressure in WKY. We conclude that 1) the slow-pressor response to ANG II is greatly enhanced in SHR, 2) this enhancement is specific with respect to type of response (slow not rapid) and pressor agent (ANG II not NE), 3) a genetic defect underlies the increased slow-pressor response to ANG II in SHR, and 4) the enhanced slow-pressor response to ANG II contributes significantly to the pathophysiology of hypertension in SHR. Finally, the current studies are consistent with our working hypothesis that the kidneys mediate the enhanced slow-pressor response to ANG II in SHR.     

Phorbol-12,13-dibutyrate-induced vasoconstriction in vivo: characterization of response in genetic hypertension

To assess the role of protein kinase C (PKC) in the control of vessel tone in vivo in genetic hypertension, the vascular effects of phorbol-12,13-dibutyrate (PDBu), a PKC activator, was measured in the autoperfused hindlimb of reserpinized spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). PDBu infusion (1-3000 ng/kg/min) into the hindlimb elevated perfusion pressure in a dose-related manner. Vasoconstriction response characteristics (latency, T1/2 to peak effect, decay of effect) of PDBu were significantly longer (2- to 10-fold) than that produced by membrane receptor agonists; phenylephrine, SKF 89748, a lipophilic alpha-1 agonist, angiotensin II and 5-hydroxytryptamine. The tonic vasoconstriction induced by PDBu was not antagonized by prazosin, rauwolscine, cyproheptadine, [Sar1lle8]-angiotensin II but was inhibited reversibly by microbial PKC-inhibitors, K252a and staurosporine at concentrations (1.56-2.8 micrograms/kg/min) which did not block vasoconstriction by phenylephrine or 5-hydroxytryptamine. The EC50 for PDBu was identical in SHR and WKY. However, the maximal response to PDBu was significantly greater in SHR compared to WKY. Staurosporine lowered mean arterial pressure equally in SHR (20%) and WKY (17%) but reduced perfusion pressure in SHR (13%) to a slightly greater extent than in WKY (5%). Unlike the in vivo response, aortic rings removed from SHR were more sensitive to cumulative doses of PDBu than rings from WKY. It is concluded that PDBu-vasoconstriction in vivo is mediated largely through activation of PKC.(ABSTRACT TRUNCATED AT 250 WORDS)     

Defective modulation of noradrenergic neurotransmission by exogenous prostaglandins in aging spontaneously hypertensive rats

Inhibition of cyclooxygenase enhances mesenteric vascular responses to periarterial (sympathetic) nerve stimulation (PNS) in 16-week-old spontaneously hypertensive rats (SHR), but not in 25-week-old SHR. In contrast, cyclooxygenase inhibition enhances mesenteric vascular responses to PNS similarly in 16- and 25-week-old Wistar-Kyoto normotensive rats (WKY). Thus, the modulation of noradrenergic neurotransmission by endogenous PGs becomes defective as SHR age, whereas in WKY this does not occur. The purpose of this study was to determine to what extent alterations in the concentrations of PGs and/or biological response to PGs contribute to this age/hypertension-related abnormality in SHR. All studies were conducted in the in situ autoperfused rat mesentery, and plasma levels of PGE2 and 6-keto-PGF1 alpha were determined by negative-ion, chemical-ionization, gas chromatography-mass spectrometry after derivatization and clean-up of samples by two thin-layer chromatographic steps. Base-line mesenteric venous plasma levels of PGs were similar in 16-week-old SHR vs. 16-week-old WKY; however, base-line levels of PGE2 were approximately 6-fold greater than base-line levels of 6-keto-PGF1 alpha in both strains. PNS at 7 Hz approximately doubled mesenteric venous plasma levels of PGE2 in both 16-week-old SHR and WKY, but PNS did not increase levels of 6-keto-PGF1 alpha in either strain. Inasmuch as mesenteric venous plasma levels of PGE2 were responsive to PNS, the effect of aging on PGE2 levels was studied. In both strains, the base-line mesenteric venous plasma levels of PGE2 and the PNS-induced increase in PGE2 levels were similar in 16-week vs. 25-week-old animals. In 16-week-old SHR, infusions of PGE2, arachidonic acid and PGI2 directly into the mesenteric artery inhibited vascular responses to PNS. However, in 25-week-old SHR, even high doses of PGE2 or arachidonic acid failed to inhibit vascular responses to PNS, and the inhibitory potency of PGI2 was shifted 10-fold to the right compared to 16-week-old SHR. In contrast, PGE2 and arachidonic acid had similar effects on neurotransmission in 25-week-old WKY compared to 16-week-old WKY, and aging had a lesser effect on the inhibitory potency of PGI2 (i.e., 3-fold rightward shift of the dose-response curve). Adenosine also inhibited vascular responses to PNS; however, the inhibitory potency of adenosine was only slightly and similarly affected by aging in SHR and WKY.(ABSTRACT TRUNCATED AT 400 WORDS)     

Endothelial regulation of cyclic GMP and vascular responses in hypertension

The mechanism whereby endothelial modulation of drug-induced vascular responses might change during hypertension was examined. Acetylcholine (ACh) (1 microM) induced maximal relaxation of aortic ring segments with intact endothelium from both Wistar-Kyoto, normotensive rats (WKY) and spontaneously hypertensive rats (SHR) at 5 to 6 weeks of age. At 15 to 18 weeks of age the relaxation response to ACh was reduced in rings from both SHR and WKY (to a greater extent in SHR) and was attenuated even more in the deoxycorticosterone acetate (DOCA)-salt hypertensive rat. The contractile responses of aortic preparations to norepinephrine (NE) (0.1 microM) were similar between 5-6-week-old and 15-18-week-old WKY, but were increased in 15-18-week-old SHR compared to 5-6-week-old SHR. Endothelial cell removal increased contractile responses to NE to a greater extent in WKY than SHR but this did not affect that seen in DOCA-salt hypertensive rats. Methylene blue treatment increased contractions of aortic rings with intact endothelium from 15-18-week-old WKY and SHR to the level detected in rubbed arteries, but it did not affect the NE-induced constriction of intact aortic rings from DOCA-salt hypertensive rats. Basal cyclic GMP concentrations in intact aortic rings were not different between SHR and WKY at 5 to 6 weeks of age. The basal aortic cyclic GMP was unchanged in WKY at 15 to 18 weeks of age, but decreased in SHR and in DOCA-salt hypertensive rats of the same age.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for enhanced inhibitory modulation by cyclooxygenase products of noradrenergic neurotransmission in the mesenteric vasculature of young spontaneously hypertensive rats.

The effects of cyclooxygenase inhibition by indomethacin and meclofenamate on pre- and postjunctional aspects of noradrenergic neurotransmission were determined in mesenteric vascular preparations from 4- to 6-week-old spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto rats (WKYs). Perfusion pressure responses to periarterial nerve stimulation and to exogenous norepinephrine (NE) were significantly greater in SHR than in WKY preparations, whereas fractional NE overflow was equivalent between the two strains. Indomethacin and meclofenamate enhanced perfusion pressure responses to periarterial nerve stimulation in both strains. Fractional NE overflow was significantly enhanced by indomethacin but only at 14 Hz in SHR preparations, whereas it was unaffected in WKY preparations. The combination of indomethacin and cocaine resulted in a significant enhancement of perfusion pressure responses to periarterial nerve stimulation in both strains that was significantly greater than that produced by either drug alone. This effect was significantly greater in SHR than in WKY preparations. This combination also resulted in a significant enhancement of responses to exogenous NE in both strains. Fractional NE overflow was significantly increased in SHR preparations in the presence of the combination of cocaine and indomethacin, whereas it remained unaltered in WKY preparations. These findings suggest that a cyclooxygenase product exerts both pre- and postjunctional inhibitory effects on vascular noradrenergic neurotransmission that differ in these two strains of rats. The prejunctional inhibitory effect of this cyclooxygenase product was observed only in SHR preparations and was especially evident in the presence of cocaine.     

Time course of changes in the norepinephrine content of tissues from spontaneously hypertensive and Wistar Kyoto rats

The change in norepinephrine (NE) content with age (from 2 days to 17 weeks old) was examined in a variety of tissues from spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats. NE content was determined by either a catechol-O-methyltransferase-based radioenzymatic assay or high performance liquid chromatography with electrochemical detection. Regardless of the age of the animal, NE content per gram of tissue was significantly greater in mesenteric arteries and kidneys from SHR compared to WKY tissues, whereas NE content per whole kidney was similar between the two rat strains. The time course of enhanced NE content in caudal arteries and aortas from SHR followed the development of hypertension. In the spleen, NE content per gram of tissue was elevated in young SHR; however, in adult rats NE content was not significantly different between the two rat strains. Because spleens from WKY rats were substantially larger, total NE content per spleen was significantly greater in tissues from WKY rats. Cardiac contents of NE were similar in SHR and WKY rats at all ages examined. Adrenal epinephrine concentrations were similar in SHR and WKY rats, whereas NE content was elevated in the SHR at 46 and 81 days of age. The results of the present study demonstrate that the appearance of increased NE levels in some SHR tissues occurs before the development of hypertension in this model. If NE content is a valid index of sympathetic innervation, enhanced innervation may contribute to the vascular medial hypertrophy observed in young SHR and the elevation of blood pressure in these rats.     

Decreased supply-dependent oxygen consumption in the skeletal muscle of the spontaneously hypertensive rat during acute hypoxia

The purpose of this study was to correlate microvascular oxygen delivery (DO2) and consumption (VO2) in the skeletal muscle of spontaneously hypertensive rats (SHRs) and Wistar Kyoto rats (WKY) with hemodynamics during acute hypoxia. We expected greater abnormalities in central and microvascular hemodynamics during hypoxic induced shock in the SHR compared with the WKY due to microvascular rarefaction. The inspired oxygen fraction (FiO2) was lowered from 0.21 to 0.15, 0.1, 0.08, and 0.05 in anesthetized, mechanically ventilated rats. Lactate and base deficit values were similar for both groups at 0.21 and 0.15 FiO2, but higher in SHR at lower FiO2. Baseline aortic blood flow (SHR, 56.2+/-4.0 mL min; WKY, 61.8+/-5.3 mL min) and systemic DO2 (SHR, 9.02+/-0.82 mL min; WKY, 9.32+/-0.54 mL min) increased similarly when FiO2 was lowered to 0.15. Further reductions in FiO2 caused lower aortic flow and systemic DO2 in the SHR than WKY at 0.08 and 0.05 FiO2. Spinotrapezius blood flow increased from baseline (SHR, 24.8+/-1.8 nL s; WKY, 22.7+/-2.1 nL s) in both groups when FiO2 was reduced to 0.15; further reductions in FiO2 decreased blood flow in both groups, with lower values in the SHR group at 0.1 and 0.08 FiO2. The SHR group demonstrated higher venous oxygen saturation at low values of FiO2 compared with WKY. This reduced oxygen extraction in SHR resulted in a lower supply-dependent VO2 at low values of spinotrapezius DO2, perhaps attributed to arteriolar thickening and rarefaction seen in chronic hypertension.     

Differential cardiovascular effects of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), flesinoxan, 5-methyl-urapidil and MDL 75,608A in conscious spontaneously hypertensive rats

Summary— The effects of intravenous (iv) administration of four agonists at central 5-HT_1A receptors were investigated and compared. Acute iv injection of 0.1 mg/kg of 8-OH-DPAT induced a decrease in blood pressure both in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The maximal hypotensive effect was observed 15 and 10 min after injection, respectively, but the effect was greater and longer-lasting in the SHR. 8-OH-DPAT significantly decreased heart rate in WKY and, to a lesser extent, in SHR. The iv injection of 1 mg/kg of flesinoxan caused a similar fall in blood pressure and heart rate in SHR and WKY. The iv administration of 1 mg/kg of 5-methyl-urapidil or MDL 75,608A caused a fall in blood pressure which was significantly more pronounced in SHR than in WKY. 5-methyl-urapidil induced a significant tachycardia in WKY, but had little effect on heart rate in SHR. MDL 75,608A caused a short-lasting tachycardia in SHR and WKY. In conscious SHR, the intracerebroventricular (icv) injection of 10 μg of 8-OH-DPAT or 100 μg of either flesinoxan or MDL 75,608A caused a decrease in blood pressure and heart rate. The icv injection of 100 μg of 5-methyl-urapidil caused only a decrease in blood pressure. Chronic pre-treatment with these compounds, by daily iv injection, did not significantly influence the hypotensive or bradycardic effects in an acute experiment. The involvement of α₁-adrenoceptors in the effects of these compounds was studied by administering phenylephrine (1 μg/iv) at 5- min intervals before and after the iv injection of the experimental compounds. The injection of phenylephrine reproducibly increased blood pressure by 35–40 mm Hg after saline pre-treatment, and these responses were not affected by the iv injection of 0.1 mg/kg of either 8-OH-DPAT or 1 mg/kg of flesinoxan. In contrast, the phenylephrine-induced pressor responses were markedly diminished at 5 min after treatment with 1 mg/kg of either 5-methyl-urapidil or MDL 75,608A, but slowly recovered thereafter. These results show that the 5-HT_1A recptor agonists 8-OH-DPAT, flesinoxan, 5-methyl-urapidil and MDL 75,608A show antihypertensive properties in conscious SHR after iv or icv injection. However, the mechanism of action of the compounds differs: 8-OH-DPAT and flesinoxan may act predominantly as 5-HT_1A receptor agonists, where as 5-methyl-urapidil and MDL 75,608A also seem to have an effect on peripheral α₁-adrenoceptors.     

Reduced function of the stimulatory GTP-binding protein in beta adrenoceptor-adenylate cyclase system of femoral arteries isolated from spontaneously hypertensive rats

Arterial relaxant responses to beta adrenoceptor agonists are decreased in spontaneously hypertensive rats (SHR) when compared with normotensive Wistar-Kyoto rats (WKY). To determine which component of the beta adrenoceptor-adenylate cyclase (AC) system is involved in the decreased beta adrenoceptor responses, effects of two activators of AC-forskolin and cholera toxin and of dibutyryl cyclic AMP (DBcAMP) were compared between the strips of femoral arteries isolated from 13-week-old SHR and age-matched WKY. Arterial relaxant responses to either forskolin, an activator of AC or DBcAMP were not significantly different between the SHR and WKY, whereas the relaxant responses to norepinephrine (NE) via beta adrenoceptors were significantly weaker in the SHR than in the WKY. In the absence of timolol, a beta adrenoceptor antagonist, contractile responses to NE were significantly greater in the SHR than in the WKY. Timolol augmented the contractile responses to NE to a greater extent in the WKY than in the SHR. After the blockade by timolol of beta adrenoceptors, contractile responses to alpha adrenoceptor stimulation with NE were not significantly different between the two strains. The pretreatment of the strips with cholera toxin, an activator of stimulatory GTP-binding protein (Gs), antagonized the alpha adrenoceptor-mediated contractions much greater in the WKY than in the SHR. The alpha adrenoceptor-mediated contractions after the pretreatment with cholera toxin were comparable to the contractile responses to NE determined in the absence of timolol in either the SHR or the WKY. Forskolin and DBcAMP also antagonized the alpha adrenoceptor-mediated contractions. However, these antagonisms were not significantly different between the two strains. The cellular cAMP content in arterial strips after the stimulation with NE was significantly less in the SHR than in the WKY, whereas the cAMP contents were similar in arterial strips from both strains which were stimulated with forskolin. These results suggest that the reduced function of Gs is involved in the abnormality of beta adrenoceptor-AC system in the SHR femoral artery.     

Acute and chronic effects of synthetic atrial natriuretic factor on blood pressure and sodium excretion in spontaneously hypertensive rats

To assess possible roles of atrial natriuretic factor (ANF) in the regulation of blood pressure in spontaneously hypertensive rats (SHR), we performed two series of experiments. First, we studied acute hypotensive, natriuretic and diuretic effects of ANF in pentobarbital-anesthetized SHR and age-matched Wistar-Kyoto rats (WKY). A synthetic ANF of 25 amino acid residues was intravenously administered as a bolus at doses of 0.1, 1.0, 2.5 and 5.0 micrograms/kg. In SHR group, a significant decrease in mean arterial pressure (MAP) was observed at a dose of 1.0 micrograms/kg, and the decrease was dose-dependent. On the other hand, in WKY group, the hypotensive effect of ANF was not observed until a dose of 5.0 micrograms/kg. The diuretic and natriuretic effects of ANF were observed at a dose of 2.5 micrograms/kg in SHR and 5.0 micrograms/kg in WKY, respectively. Second, we also studied chronic effect of ANF on the development of hypertension in 6-week-old SHR. The SHRs, on regular diet or given 1% NaCl solution for drinking, were continuously infused into the jugular vein by osmotic minipumps with ANF (15, 75 and 150 micrograms/kg/day) or vehicle (physiological saline) as controls for up to 14 days. ANF at a dose of 150 micrograms/kg/day attenuated transiently the development of hypertension in the sodium-loaded SHR. However, the blood pressure returned to control levels by day 5. ANF at doses of 15 and 75 micrograms/kg/day did not affect the development of hypertension. In SHR on regular diet, ANF at a dose of 150 micrograms/kg/day did not affect the development of hypertension. In addition, ANF did not induce any significant changes in urine volume, fluid intake, and urinary excretion of sodium and potassium in SHR, whether they were sodium-loaded or not, when compared to those in vehicle-infused SHR. These results indicate that there may be a significant difference in the sensitivity to diuretic, natriuretic and hypotensive actions of ANF between SHR and WKY. Moreover, it is suggested that ANF may play significant roles by its vascular effect at the early stage of development of hypertension in sodium-loaded SHR.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vascular endothelial growth factor-mediated endothelium-dependent relaxation is blunted in spontaneously hypertensive rats

The vasodilatory effect of VEGF has not been characterized in the setting of hypertension. This study investigated the in vitro vasorelaxant effects of VEGF in organ chambers in the aorta of the adult (12-week-old) spontaneously hypertensive rats (SHR), young (4-week-old) SHR without hypertension, and age-matched Wistar-Kyoto (WKY) rats compared with acetylcholine (ACh). Cumulative concentration-relaxation curves were established for VEGF (approximately 10(-12)-10(-8.5) M) and ACh (approximately 10(-10)-10(-5) M) in U46619 (10(-8) M)-induced contraction. VEGF induced endothelium-dependent relaxation that was significantly reduced in the adult SHR compared with the age-matched WKY control (87.8 +/- 2.8 versus 61.4 +/- 8.6%, P = 0.01). These responses were significantly attenuated by pretreatment with N(omega)-nitro-L-arginine (L-NNA, 300 microM) alone (SHR: 25.1 +/- 1.9%; WKY: 21.0 +/- 2.6%; P = 0.01) or indomethacin (7 microM) + L-NNA (SHR: 30.2 +/- 2.1%; WKY: 35.0 +/- 2.9%; P = 0.01). Further addition of oxyhemoglobin (20 microM) abolished the residual relaxation and reduced the relaxation induced by nitroglycerin. ACh induced similar responses to VEGF. In contrast, pretreatment with indomethacin alone enhanced VEGF- or ACh-induced relaxations and the effect was greater in the adult SHR than in WKY rats. In contrast to the adult SHR versus WKY rats, there were no significant differences of VEGF- or ACh-induced relaxations between young SHR and WKY rats. The results demonstrate that VEGF induces endothelium- or nitric oxide-dependent relaxation, which is blunted in the adult SHR. The mechanism of this impairment may be related to decreased release of NO although increased release of contracting factors from the dysfunctional endothelium may also be involved.     

Angiotensin II-induced renal vasoconstriction in genetic hypertension.

Previous studies demonstrate that renovascular responses to angiotensin II (Ang II) are enhanced in spontaneously hypertensive rats (SHRs); however, it is possible that this hyperresponsiveness is mediated by Ang II-induced release of substances from the adrenal gland. Previous studies also show that pertussis toxin normalizes renovascular responses to Ang II in SHRs; however, it is possible that this response is mediated by effects of pertussis toxin on endogenous Ang II levels and/or the sympathoadrenal axis. The purpose of this study was 2-fold: 1) to determine whether the renovascular response to Ang II in SHRs is enhanced even in adrenalectomized SHRs and 2) to determine whether pertussis toxin normalizes enhanced renovascular responses to Ang II when pertussis toxin-induced changes in the renin-angiotensin system and the sympathoadrenal axis are prevented. SHRs and Wistar Kyoto (WKY) rats were anesthetized and administered 20 ml/kg 0.9% saline, and an infusion of aldosterone and hydrocortisone was initiated. After bilateral adrenalectomy, left renal denervation, and pretreatment with captopril, animals received an intrarenal artery infusion of Ang II at 10 ng/kg/min for 5 min. Ang II-induced changes in renal vascular resistance were greater in SHRs compared with WKY rats (p =. 010, n = 19/group). Pertussis toxin (10 microgram/kg i.v. 3 days before the experiment) attenuated Ang II-induced changes in renal vascular resistance in SHR (p <.05), but not in WKY rats (strain x treatment interaction: p =.046). These results suggest that the enhanced renovascular response to Ang II in SHRs is mediated by a G(i)-dependent pathway within the renal vasculature.     

Somatostatin infusion increases intestinal ischemia and does not improve vasoconstrictor response to norepinephrine in ovine endotoxemia

Hemodynamic support of patients with septic shock is often complicated by a tachyphylaxis against exogenous catecholamines. Because an increase in somatotropic hormones may play a pivotal role in the regulation of the inflammatory response to endotoxin, intravenous supplementation of the neuroendocrine hormone somatostatin (SOMA) may attenuate hemodynamic dysfunction resulting from endotoxemia. The objective of the present study was to assess the short-term effects of SOMA alone and in combination with norepinephrine (NE) on cardiopulmonary hemodynamics, global oxygen transport, plasma nitrate/nitrite levels, and intestinal integrity compared with single NE therapy in ovine endotoxemia. After a baseline measurement in healthy sheep (n = 16) had been performed, Salmonella typhosa endotoxin was centrally infused (10 ng x kg(-1) x min(-1)) to induce a hypotensive-hyperdynamic circulation using an established protocol. Animals surviving 16 h of endotoxemia were randomly assigned to one of the two groups (each n = 6). Sheep allocated to the SOMA + NE group received SOMA as a loading dose of 10.5 microg x kg(-1) x min(-1) for 1 h, followed by a continuous infusion of 3.5 microg x kg(-1) x min(-1) for the next 2 h. After the SOMA loading dose had been given, NE was concurrently infused (0.3 microg x kg(-1) x min(-1)) for 2 h. In the NE group (control), NE (0.3 microg x kg(-1) x min(-1)) was continuously infused for 3 h. Endotoxemia caused a decrease in MAP and systemic vascular resistance index in both groups, but to a greater extent in the NE group. Arterial hypotension persisted despite administration of the study drugs. Infusion of SOMA alone and in combination with NE did not significantly increase systemic vascular resistance index. Neither SOMA nor NE infusion alone affected pulmonary vasoregulation. Plasma nitrate/nitrite levels did not differ between groups. However, combined infusion of SOMA and NE significantly increased arterial lactate concentrations, oxygen consumption index, and oxygen extraction rate (P < 0.05) and aggravated ileal mucosal injury. In conclusion, short-term treatment with SOMA failed to attenuate cardiocirculatory shock resulting from endotoxemia and did not improve vasopressor response to NE. In addition, combined SOMA and NE therapy resulted in intestinal injury. Therefore, SOMA does not seem to represent a therapeutic option to treat arterial hypotension resulting from sepsis and systemic inflammatory response syndrome.     

Magnetic resonance imaging quantification of regional cerebral blood flow and cerebrovascular reactivity to carbon dioxide in normotensive and hypertensive rats

Hypertension afflicts 25% of the general population and over 50% of the elderly. In the present work, arterial spin labeling MRI was used to non-invasively quantify regional cerebral blood flow (CBF), cerebrovascular resistance and CO(2) reactivity in spontaneously hypertensive rats (SHR) and in normotensive Wistar Kyoto rats (WKY), at two different ages (3 months and 10 months) and under the effects of two anesthetics, α-chloralose and 2% isoflurane (1.5 MAC). Repeated CBF measurements were highly consistent, differing by less than 10% and 18% within and across animals, respectively. Under α-chloralose, whole brain CBF at normocapnia did not differ between groups (young WKY: 61 ± 3ml/100g/min; adult WKY: 62 ± 4ml/100g/min; young SHR: 70 ± 9ml/100g/min; adult SHR: 69 ± 8ml/100g/min), indicating normal cerebral autoregulation in SHR. At hypercapnia, CBF values increased significantly, and a linear relationship between CBF and PaCO(2) levels was observed. In contrast, 2% isoflurane impaired cerebral autoregulation. Whole brain CBF in SHR was significantly higher than in WKY rats at normocapnia (young SHR: 139 ± 25ml/100g/min; adult SHR: 104 ± 23ml/100g/min; young WKY: 55± 9ml/100g/min; adult WKY: 71 ± 19ml/100g/min). CBF values increased significantly with increasing CO(2); however, there was a clear saturation of CBF at PaCO(2) levels greater than 70mmHg in both young and adult rats, regardless of absolute CBF values, suggesting that isoflurane interferes with the vasodilatory mechanisms of CO(2). This behavior was observed for both cortical and subcortical structures. Under either anesthetic, CO(2) reactivity values in adult SHR were decreased, confirming that hypertension, when combined with age, increases cerebrovascular resistance and reduces cerebrovascular compliance.     

Effect of clentiazem on arterial pressure and renal function in normotensive and hypertensive rats.

The present study evaluated the effects of a new benzothiazepine calcium channel antagonist, clentiazem, on arterial pressure and renal function in spontaneously hypertensive rats (SHR), normotensive Wistar-Kyoto (WKY) and Munich-Wistar rats (MWR). Administration of clentiazem in doses from 1 to 20 micrograms/kg/min produced dose-dependent increases in sodium and water excretion in MWR, reaching maximum values of 292 and 376% of control, respectively, at the 20-micrograms/kg/min dose. Clentiazem (10 micrograms/kg/min) lowered arterial pressure by 16% and doubled glomerular filtration rate (GFR) in MWR. The rise in GFR was associated with an increase in glomerular capillary pressure of 16 mm Hg, produced by a combination of preglomerular vasodilation and efferent arteriolar vasoconstriction. In SHR, administration of clentiazem (10 micrograms/kg/min) lowered arterial pressure by 30 mm Hg and increased urine flow and sodium excretion by 137 and 200%, respectively. In WKY rats, the same dose of clentiazem decreased arterial pressure by only 10 mm Hg, whereas urine flow and sodium excretion increased 62 and 38%, respectively. A high dose of clentiazem (1 mg/kg bolus plus 1 mg/kg/hr infusion i.v.) lowered arterial pressure by 63 mm Hg in SHR. Renal vascular resistance fell by 39% and there was a 5-fold increase in sodium excretion. In WKY rats, the same dose of clentiazem reduced arterial pressure by 20 mm Hg, but it had no significant effect on sodium excretion. These results indicate that clentiazem increases sodium excretion and GFR in normotensive rats in part by preferentially dilating the renal preglomerular vasculature. This compound is also an antihypertensive agent that lowers arterial pressure and promotes sodium excretion in SHR.     

Direct and acute cardiotoxic effects of ultrafine air pollutants in spontaneously hypertensive rats and Wistar--Kyoto rats

It is hypothesized that preexisting cardiovascular disease could affect the susceptibility to direct and acute cardiotoxic effects of ultrafine air pollutants. Ultrafine particles (UFP) isolated from 12.5 mg of diesel particulate matter (National Institute of Standards and Technology) were infused into isolated Langendorffperfused hearts obtained from spontaneously hypertensive rats (SHR) and normotensive control Wistar- Kyoto rats (WKY). Perfusion for 30 minutes with UFP reduced cardiac function in both groups-but to a greater extent in WKY. In SHR, developed pressure was reduced by 24.1 +/- 4.4% of baseline and maximal dP/dt was reduced by 19.8 +/- 4.9%; in WKY, developed pressure was reduced by 43.5 +/- 7.3% and maximal dP/dt by 41.8 +/- 8.2% (P < .05 for maximal dP/dt in SHR vs WKY). Coronary flow was decreased by 30.3% versus 53.7% in SHR versus WKY ( P < .05). The results of this study suggest that although UFP depress myocardial contractile response and coronary flow in both SHR and WKY the underlying hypertension does not necessarily worsen the response.