Effect of losartan on sodium appetite of hypothyroid rats subjected to water and sodium depletion and water, sodium and food deprivation

The involvement of angiotensin AT1 receptors in sodium appetite was studied in hypothyroid rats treated with the angiotensin II antagonist losartan. Losartan was administered chronically by the oral route or acutely by the subcutaneous route after water and sodium depletion or water, sodium and food deprivation. Three days after addition of losartan to the food at the dose of 1.0 mg x g(-1), the rats significantly reduced (P < 0.02) their spontaneous intake of 1.8% NaCl. Increasing the dose of losartan to 2.0 and 4.0 mg x g(-1) did not reduce NaCl intake; in contrast, the intensity of the sodium appetite gradually returned to previous levels. The simultaneous administration of captopril, an angiotensin converting enzyme inhibitor, and losartan significantly increased (P < 0.05) NaCl intake and after captopril removal NaCl intake returned to the levels observed with losartan treatment alone. The administration of losartan 4 days after the beginning of captopril treatment significantly reduced (P < 0.0001) NaCl intake. Following acute administration of losartan, water- and sodium-depleted rats significantly reduced their NaCl and water intake (P < 0.001). The administration of losartan also induced a significant reduction in NaCl and water intake in water, NaCl and food-deprived rats (P < 0.0001 and P < 0.001, respectively). The present results show that chronic treatment with oral losartan inhibited spontaneous sodium appetite in hypothyroid rats. Continuation of treatment rendered rats resistant to the blockade of AT1 receptors. Water and sodium depletion and water, NaCl and food deprivation induced sodium appetite, which in the short term depends on cerebral angiotensinergic activity mediated by the activation of AT1 receptors.     

Dipsogenic stimulation in ibotenic DRN-lesioned rats induces concomitant sodium appetite

The main purpose of this study was to investigate whether dipsogenic stimuli influences the sodium appetite of rats with ibotenic acid lesion of the dorsal raphe nucleus (IBO-DRN). Compared to control, rats microinjected with phosphate buffer (PB-DRN), the ingestion of 0.3M NaCl was enhanced in IBO-DRN at 21 and 35 days after DRN lesion under a protocol of fluids and food deprivation. Despite of similar dipsogenic response observed both in IBO-DRN and PB-DRN treated with isoproterenol (ISO, 300 microg/kg, sc), the 0.3M NaCl intake was again significantly enhanced in IBO-DRN at 21 and 35 days post-lesion. Finally, treatment with polyethylene glycol (PEG, MW=20,000, 20%, w/v, 16.7 ml/kg, sc) induced higher dipsogenic response in IBO-DRN than PB-DRN at 21 day after lesion. In addition, IBO-DRN also expressed higher sodium appetite than PB-DRN, concomitantly with a drinking response. These results suggest that ibotenic lesion of DRN promote an increase of the brain angiotensinergic response, possibly settled within the subfornical organ, through paradigms which increase circulating ANG II levels. The current paper supports the hypothesis that the ibotenic lesion of DRN suppresses a serotonergic component implicated on the modulation of the sodium appetite and, therefore, furthering homeostatic restoration of extracellular fluid volume.     

Dehydration-induced sodium appetite in rats

The effect of 24 hr water deprivation on Na balance was studied in rats. Under baseline conditions, the animals had free access to food and water. During water deprivation, Na excretion was increased, Na intake (i.e., food intake) was decreased and Na deficits of 0.8-1.0 mmol were incurred. During the 24 hr period immediately following the deprivation period when water was returned, Na excretion was decreased and Na balance was restored to baseline or pre-deprivation level. In a second series of experiments, under baseline conditions, the animals had free access to food, water and 0.5 M NaCl. During water deprivation with NaCl solution withheld, Na excretion was not changed relative to baseline but Na deficits of 0.8-1.0 mmol were incurred due to decreased Na intake. During the 24 hr period immediately following deprivation when both water and NaCl solution were returned, intake of the hypertonic NaCl solution was increased and Na balance was restored. In a third series of experiments, under baseline conditions, the animals had free access to food, water, 0.5 M NaCl, 0.5 M KCl, 0.25 M of MgCl2 and 0.25 M CaCl2. During the 24 hr period following water deprivation and also the withholding of the electrolyte solutions, the appetite induced was predominantly for NaCl. The results suggest that the Na appetite observed subsequent to a period of water deprivation may be due to Na deficiency.     

Effect of triiodothyronine on renal growth and renal sodium reabsorption in hypothyroid rats

Experiments were carried out to compare temporal changes in the paraaminohippuric acid clearance (C _PAH), renal sodium reabsorption ( \(R_{Na^ + } \) ), ribonucleic acid (RNA), and deoxyribonucleic acid (DNA) content in hypothyroid rats after a single injection of triiodothyronine (T₃) (50 μg/100 g body wt).C _PAH and \(R_{Na^ + } \) showed no changes at 24 and 48 h. At 72 h, however, significant increases of 41% and 42% (per g kidney wet wt) were observed inC _PAH and \(R_{Na^ + } \) , respectively. The cortex in T₃-treated hypothyroid rats showed a significant increase in the protein/DNA and RNA/DNA ratios at 24 h and progressive increases to a level of 24%, and 37%, respectively, at 48 h. No changes in DNA content were observed at either time-points. The results show that the increases in RNA/DNA and protein/DNA ratios upon T₃ treatment preceded the increases inC _PAH and \(R_{Na^ + } \) , suggesting a direct effect of T₃ on renal cortical growth, rather than a secondary response to a primary increase in renal functions.     

Contractile activation properties of ventricular myocardium from hypothyroid,euthyroid and juvenile rats

Contractile activation properties of intact and chemically skinned ventricular myocardium preparations were studied in juvenile (3–4 weeks old), adult euthyroid and adult hypothyroid rats. The rats were made hyperthyroid by treatment with iodine-131 and propylthiouracil. The ventricular muscle of euthyroid rats contains a mixture of isozymes of myosin while the myocardium of juvenile and hypothyroid rats are relatively pure in regard to V₁ and V₃ types of myosin respectively. No significant differences were found in either the maximum Ca²⁺ activated or rigor force developed by chemically skinned preparations in either the juvenile or hypothyroid groups compared with euthyroid adults, suggesting that there is no difference between myocardia with different isozymes of myosin in the intrinsic capacity to generate force. In the hypothyroid (V₃) preparations there was a significant shift in the force/pCa relation to the left compared with the euthyroid adult (mixture of V₁ and V₃ isozymes). The force/pCa relation for the juvenile lay in between that for the hypothyroid and euthyroid adults. The greater apparent Ca²⁺ sensitivity to activation in the hypothyroid group may relate to a slower cross-bridge cycling rate or altered Ca²⁺ kinetics in ventricular myocardium with exclusively V₃ isozyme. In intact papillary muscles differences were found in the dependence efforce on extracellular [Ca²⁺] such that a higher extracellular [Ca²⁺] was required for muscles from hypothyroid animals to attain maximum twitch force than those from juveniles. The force/frequency relations also differed, with the hypothyroid group being better able to sustain force as stimulation frequency increased than the juvenile group. Also, in the hypothyroid group, the contraction following a 3-min period of quiescence was potentiated in relation to the preceding steady-state contractions, whereas in the juvenile group it was not. These results indicate that the thyroid state may influence the pattern of calcium translocation as well as the myosin isozyme type and that both of these factors may influence contractile properties. Furthermore, the pattern of responses seen depends not only on the pattern of isozymes present but may also depend on the age of the animal because there was no simple relationship between the apparent sensitivity of the contractile apparatus for calcium and proportion of isozyme in the myocardium.     

Evaluation of oxidative phosphorylation in hearts from euthyroid, hypothyroid, and hyperthyroid rats.

The energy relationships between cytosolic and mitochondrial metabolism were studied in the hearts from euthyroid, hypothyroid, and hyperthyroid rats. Isolated mitochondria showed high respiratory control ratios and impermeability to exogenous NADH. Hypo- and hyperthyroidism, respectively, resulted in lower and higher contents of both cytochromes per mitochondrion and mitochondrial protein per gram of wet weight of heart without changes in the ratio of cytochrome c to cytochrome aa3. In isolated perfused heart, the hyperthyroid state led to an increase in work rate and thereby an elevation of Vo2, which resulted in an increase oxidation-reduction turnover number for the cytochromes. An agreement was found between [ATP]/[ADP][Pi] of cytosolic free adenine nucleotides and the value calculated from a mathematical model of mitochondrial respiration. This implies that mitochondrial respiration is controlled at the cytochrome oxidase reaction and that oxidative phosphorylation in intact tissue is tightly coupled irrespective of thyroid state. It is concluded that thyroid hormone causes an increase in the mitochondrial mass, mitochondrial cytochrome content, and respiratory rate, and consequently expands the capacity of oxidative metabolism without an uncoupling effect on oxidative phosphorylation.     

Structural and functional aspects of the actomyosin complex from fast-twitch muscle of euthyroid and hypothyroid rats

The effects of hypothyroidism on structural and functional properties of the actomyosin-ATPase complex of rat fast-twich gastrocnemius muscle were examined and related to energetic and mechanical parameters. Hypothyroidism resulted in the appearance of a small band of the myosin heavy chain subunit of the slow form (MHCs) 8% of total MHC) which was absent in the euthyroid group. This observation corresponded with lower activities of myofibrillar ATPase (–14%) and Ca-activated myosin ATPase (–9%) in the hypothyroid group, although these changes were not significant. No effect of hypothyroidism on the Ca²⁺-sensitivity of the myofibrillar-ATPase activity was observed and tetanic force was not changed. Twitch force, however, was significantly increased by hypothyroidism. The degree of myosin P-light chain phosphorylation (percentage of total amount of P-light chain) determined after 5 and 10 s of tetanic stimulation (130 Hz, 35°C), respectively, proved to be significantly lower in the hypothyroid group (5 s: 57%; 10 s: 61%) vs the euthyroid group (5 s: 79%; 10 s: 82%). There was no difference in P-light chain phosphorylation at rest between eu- and hypothyroids. The results suggest that a decreased actomyosin-ATPase activity can only in part contribute to the 30% lower energy turnover during force development found for fast-twitch skeletal muscle of hypothyroid rats. Moreover the increase in twitch force by hypothyroidism cannot be explained by a change in myosin P-light chain phosphorylation. Isometric twitch tension potentiation after a 2 s tetanus and during lowfrequency repetitive stimulation was reduced (up to –60%) in muscles of hypothyroid rats, which may well be related to the lower extent of P-light chain phosphorylation in hypothyroids.     

Effect of lithium intake on sodium and lithium appetite in sodium deficient cattle

Sodium deficient, unlike normal calves, will push a panel vigorously for Li⁺ or Na⁺ salts. In 5 min (VI 20 sec) operant tests calves (n=5) pressed for 10 ml rewards of 0.3 M NaHCO₃, NaCl, Na₂SO₄, LiCl and Li₂SO₄. When the tests were extended to 20 min, LiCl responses were reduced significantly in a few days and the aversion transferred to NaCl, Na₂SO₄ and Li₂SO₄ but the NaHCO₃ response remained at a high level. No signs of lithium toxicity were detectable. Lithium naive calves (n=2) given a single LiCl IV infusion which raised the plasma lithium to 12.5±1.3 mEq/l did not exhibit any reduction in Li or Na appetite. In 3 of the 5 calves mentioned above which had ingested lithium salts an IV injection of LiCl reduced their appetite for all Na and Li salts tested except for NaHCO₃. This suggests that either there is some retention of lithium within the body to produce avoidance behaviour when plasma lithium is raised or a taste memory of the aversive effects produced by raised plasma Li⁺ exists. The Li⁺ aversion, which varies between calves, appears to be connected directly with intake and the effect fades in weeks or months. The distinct selection made for NaHCO₃ illustrates the importance of this salt for the survival of Na⁺ depleted calves.     

Enhanced sodium appetite in rats with lesions centered on nucleus medianus

Recent experiments have demonstrated that rats with lesions of the ventral portion of nucleus medianus (vNM) frequently exhibit a chronic and robust hyperdipsia, which occurs only at night. This study indicates that the same brain damage may produce a nocturnal appetite for sodium that is similarly pronounced and persistent. Of 68 rats with vNM lesions, 33 were observed to drink at least 15 ml of 0.51 M NaCl solution per day, and 11 of them consumed more than 30 ml daily. The basis for this impressive consumption of saline is uncertain; the brain-damaged rats had normal sodium concentrations, renin activities, and aldosterone levels in plasma during basal maintenance conditions, and they conserved sodium in urine when maintained on a sodium-deficient diet. Nevertheless, the present results indicate that vNM and/or local fibers of passage may play an important role in the control of sodium appetite, as it does in the control of thirst.     

Oestrogenic influence on brain AT1 receptor signalling on the thirst and sodium appetite in osmotically stimulated and sodium-depleted female rats

The present work was carried out to investigate the role of angiotensin II type 1 (AT(1)) receptors in nocturnal thirst and sodium appetite induced by classical models of osmotic and sodium depletion challenges in ovariectomized rats chronically treated with oil or oestradiol benzoate (EB, 20 microg per animal, s.c. daily). In both conditions, the animals were given saline or losartan (108 nmol per animal, i.c.v.), a selective AT(1) receptor blocker. Oestrogen therapy significantly reduced the water intake induced by water deprivation, sodium depletion produced by frusemide injected 24 h before, and s.c. acute frusemide plus captopril injection (FUROCAP protocol), with no alteration following s.c. hypertonic saline injection. In contrast, EB therapy decreased the salt intake induced by sodium depletion and FUROCAP protocols, with no alteration following water deprivation and s.c. hypertonic saline injection. Central AT(1) blockade inhibited the dipsogenic response induced by water deprivation, osmotic stimulation, chronic sodium depletion and FUROCAP protocols and inhibited the natriorexigenic response induced by sodium depletion in ovariectomized rats. Oestrogen therapy significantly attenuated the losartan-induced antidipsogenic and antinatriorexigenic actions following sodium depletion and FUROCAP protocols. These results indicate that ovariectomized rats express increased AT(1) receptor signalling related to thirst and sodium appetite responses. Oestrogen therapy and brain AT(1) receptor blockade weakened or markedly decreased the behavioural responses during the nocturnal period, a time at which brain angiotensinergic activity is expected to be more prominent. Finally, we demonstrated through different experimental protocols a clear-cut influence of oestrogenic status on the behavioural AT(1)-induced signalling response.     

Effect of stimulation of brain serotonergic system on mouse-killing behavior in rats

Mouse-killing behavior was induced in male Wistar rats due to 6 weeks isolation. The selective stimulation of the dorsal raphe nucleus markedly reduced muricide behavior. On the other hand rats with electrodes situated outside of the dorsal raphe (within the substantia grisea centralis) showed no changes in muricide behavior when compared with control (non-stimulated) animals. Pharmacological stimulation of whole serotonergic brain neurons by quipazine, an agonist of serotonin receptors, and compound CGP 6085-A, a selective inhibitor of serotonin uptake, strongly decreased mouse-killing behavior. Our data indicates that serotonergic neurons inhibit predatory aggression in rats.     

Intracellular sodium and potassium activities of skeletal muscle fibres of hypothyroid rats.

In soleus muscle fibres of hypothyroid rats, the membrane potential (Vm) and the intracellular K+ activity (aKi) were significantly lower than in control muscles. These results are consistent with the well-documented decrease in the number of Na(+)-K+ pumps which occurs in hypothyroid muscles. aNai was unchanged in the hypothyroid muscles but this may reflect a change in passive Na+ fluxes which has been reported to occur in association with changes in the number of pump units.     

Ontogenetic role of angiontensin-converting enzyme in rats: Thirst and sodium appetite evaluation

We investigated the influence of captopril (an angiotensin converting enzyme inhibitor) treatment during pregnancy and lactation period on hydromineral balance of the male adult offspring, particularly, concerning thirst and sodium appetite. We did not observe significant alterations in basal hydromineral (water intake, 0.3 M NaCl intake, volume and sodium urinary concentration) or cardiovascular parameters in adult male rats perinatally treated with captopril compared to controls. However, male offspring rats that perinatally exposed to captopril showed a significant attenuation in water intake induced by osmotic stimulation, extracellular dehydration and beta-adrenergic stimulation. Moreover, captopril treatment during perinatal period decreased the salt appetite induced by sodium depletion. This treatment also attenuated thirst and sodium appetite aroused during inhibition of peripheral angiotensin II generation raised by low concentration of captopril in the adult offspring. Interestingly, perinatal exposure to captopril did not alter water or salt intake induced by i.c.v. administration of angiotensin I or angiotensin II. These results showed that chronic inhibition of angiotensin converting enzyme during pregnancy and lactation modifies the regulation of induced thirst and sodium appetite in adulthood.     

Importance of postingestional factors in the satiation of sodium appetite in rats

Two experiments using the technique of intragastric infusion of isotonic saline were conducted to examine the role of postingestional factors in the regulation of sodium intake by sodium deficient rats. Experiment 1 demonstrated that the sodium appetite of a Formalin injected rat is satiated by intragastric infusion of sufficient NaCl to restore the rat's sodium balance. Experiment 2 demonstrated that a sodium deficient rat's bar pressing for NaCl is influenced by the amount of NaCl previously intragastrically infused. As the amount of saline delivered intragastrically to the rats increased, the amount of saline they worked for decreased. In addition, the infused sodium apparently was as effective as sodium taken orally in satiating the rats' sodium appetites.