Evaluation of changes in hepatic disposition of phenolsulfonphthalein, indocyanine green and fluorescein isothiocyanate-dextran at low temperatures using a rat liver perfusion system

Objectives The aim of this study was to determine the factor changing the hepatic disposition of a drug during hypothermia using a rat liver perfusion system. Methods The livers of male Wistar rats were perfused at 37, 32 or 28°C in the single‐pass mode. Venous outflow dilution patterns and biliary excretion rate patterns of phenolsulfonphthalein (PSP), indocyanine green (ICG) and fluorescein isothiocyanate (FITC)‐dextran (FD‐4, MW 4400) after the injection of a bolus into the perfused rat liver were analysed based on statistical moment theory. Key findings The first‐pass extraction ratio (E_h) of PSP was significantly decreased at 32 and 28°C compared with 37°C. The biliary recovery of PSP and its conjugate was decreased and the biliary excretion was kept at a high concentration and was prolonged by low perfusion temperatures. ICG was almost extracted by a single‐pass through the liver even at 32 and 28°C. The biliary recovery of ICG was significantly decreased at low temperature. Although the distribution volume of FD‐4 as a vascular reference was not changed by perfusion temperature, the E_h of FD‐4 was decreased at 28°C although not markedly. Conclusion The change in hepatic disposition of a drug at low perfusion temperatures differed according to disposition processes under hypothermia.     

Epirubicin pharmacokinetics after intrahepatic arterial and intraperitoneal administration

The high hepatic clearance of the new doxorubicin analogue epirubicin (4'-epidoxorubicin, epiDX) suggests a possible use of this drug in local and regional therapy where a first pass through the liver is required before the drug can reach systemic circulation. EpiDX pharmacokinetics was followed in advanced cancer patients with liver metastases or a primary tumour after single bolus administration in the hepatic artery, through a surgically implanted catheter and subcutaneous access port. The first-pass effect through the liver was appreciable and only a relatively low fraction of the drug reached systemic circulation. Mild leucopenia and alopecia were observed only in a patient with a hepatopulmonary shunt; this subject was actually exposed to higher epiDX plasma levels. Low intraperitoneal doses of epiDX were administered in a weekly schedule to advanced cancer patients with peritoneal metastases and ascites. Drug concentrations were monitored in the ascitic effusion and in plasma. A high concentration gradient was present between the peritoneal cavity and peripheral circulation. No relevant local or systemic toxicity was observed.     

Disposition of enalapril and its diacid metabolite, enalaprilat, in a perfused rat liver preparation. Presence of a diffusional barrier for enalaprilat into hepatocytes

Enalaprilat (MK-422), a new and potent angiotensin- converting enzyme inhibitor, and its monoethyl ester precursor, enalapril, were studied in a single pass perfused rat liver preparation under constant perfusate flow (10 ml/min) at concentrations of 0.29-0.41 microM for 14C-enalapril and 0.01-0.015 microM for 3H- enalaprilat . During their simultaneous delivery to the same rat liver preparation, the steady state hepatic extraction ratio of 14C-enalapril was high (0.861 +/- 0.02) and 14C- enalaprilat appeared rapidly in effluent perfusate plasma. Of the enalapril dose, 22.7 +/- 6.9% appeared in bile. 14C- Enalaprilat accounted for 79% of the total radioactivity in bile (18% of dose) whereas 14C-enalapril was present only as 10% of the total (2.3% of dose). By contrast, the steady state hepatic extraction of 3H- enalaprilat was very low (0.053) and the disappearance was virtually identical to the appearance of 3H- enalaprilat in bile. These findings suggest that diffusional barrier exists for enalaprilat as the preformed metabolite, which hinders penetration into hepatocytes, and therefore, elimination. The precursor, enalapril, effectively brings enalaprilat into hepatocytes were more extensive biliary excretion of the generated metabolite takes place. This account adds to our further understanding of metabolite kinetics; in addition to the uneven distribution of enzyme system and the intrinsic clearance for metabolite formation and elimination, the presence of a diffusional barrier is another important determinant which may cause deviations between the kinetics of a generated and performed metabolite.     

Analysis of enterohepatic circulation of cefixime in rat by fast inverse Laplace transform (FILT)

The enterohepatic circulation of cefixime in rat was evaluated by a nonlinear least square analysis program, MULTI(FILT), into which the fast inverse Laplace transform (FILT) was incorporated. The plasma time course in the bile duct-cannulated rat exhibited a biexponential curve after the rapid iv administration of cefixime. Several pharmacokinetic models for the enterohepatic circulation were constructed based on the recirculatory concept and the Laplace-transformed equations corresponding to these models were derived by means of the method of transfer function. The transformed equations were simultaneously fitted to the time courses of plasma concentration in rats with laparotomy and with bile duct cannula. The optimum model was selected based on the Akaike's information criterion (AIC). The local moment characteristics for a single pass through enterohepatic circulation were further calculated from the time courses of both the plasma concentration and the amount excreted into the bile. The recovery ratio (Fc) and the mean circulatory time (-tc) through a single pass of enterohepatic circulation were estimated 27.9% and 1.07 hr, respectively. The recovery ratio (Fa) and the mean absorption time (-tc) for the absorption process from the intestinal tract into the systemic circulation were 68.3% and 0.0234 hr, respectively. The recovery ratio (Fb) and the mean transit time (-tb) for the disposition process through the systemic circulation into the bile were 40.8% and 1.05 hr, respectively.     

Studies on the uptake of low molecular weight monomeric tris-galactosyl conjugates by the rat liver.

We have attempted to direct low molecular weight compounds to the liver via the internalizing asialoglycoprotein receptor on parenchymal cells by conjugation to a monomeric triantennary galactosyl cluster. Acetate and a hypolipidaemic ansamycin were derivatized and the biodistribution of the conjugates was determined 250 sec and 30 min after administration to Wistar rats. The ansamycin conjugate (CGH46) was rapidly cleared from the circulation by the liver; after 250 sec, 64% of the radiolabelled dose was found in the liver compared to 18% in the blood. However, the distribution of the conjugate did not differ significantly from that of unconjugated ansamycin (CGH45). Tris-galactosyl acetate showed no capacity to localize in the liver, with only 2% recovered from that organ 250 sec after administration compared to 38% in the blood and 13-18% in the kidneys, skin and muscle. Extraction efficiency of CGH46 by isolated perfused rat livers was almost 20% of the administered dose and this value was not significantly changed by co-administration of specific inhibitors of the uptake process. It is concluded that derivatization of low molecular weight molecules with monomeric triantennary galactosyl residues is unlikely to increase their affinity for the liver.     

Hepatic microvascular effects of terbutaline in experimental cardiogenic shock in rats

1. Experimental myocardial infarction was produced in rats by direct electrical cauterization of the myocardium of left ventricle. This produced cardiogenic shock with the accompanying haemodynamic changes of low cardiac output, low mean arterial pressure, raised central venous pressure and an absence of cardiac arrhythmias. 2. The liver microcirculation was observed using in vivo television microscope method. The diameter and erythrocyte flow velocity in the liver sinusoids were measured quantitatively. 3. During experimental cardiogenic shock 80% of the liver sinusoids were constricted; the remaining 20% showed dilatation. In all these liver sinusoids the erythrocyte flow velocity was only 50% of the pre-shock level. 4. Intravenous injection of the selective beta 2-adrenoceptor agonist terbutaline (0.15 mg/kg) restored the systemic arterial pressure to pre-shcok levels and partially raised the cardiac output. In the liver microcirculation terbutaline restored both constricted and dilated liver sinusoids to pre-shock calibres, but only partially raised erythrocyte flow velocity. 5. It is proposed that during experimental cardiogenic shock, terbutaline produces dilatation in the terminal liver microcirculation by opening sphincters of liver sinusoids and restores sinusoid diameters to pre-shock calibres. Therefore, terbutaline has the capacity to decrease peripheral resistance and unload the circulation during cardiogenic shock.     

The liver first pass effects and bile concentrations of (-)-, (+)- and (+/-)-praziquantel in rabbits

Praziquantel (PQT) is a racemic mixture. Intragastric gavage of (-)-, (+)- and (+/-)-PQT 100 mg/kg were used to study the liver first pass effects in rabbits. HPLC was used to determine the concentration of each compound in the portal and systemic circulation, and in the bile within 4h. The results showed that all drugs were absorbed rapidly from the gut into the portal vein. The serum concentration in the portal vein were similar. However, over 90% of PQT and its enantiomers were extracted in the first passage through the liver. Intrinsic metabolic clearances of the liver of (-)-, (+)- and (+/-)-PQT were 50.3, 174.4 and 52.6 L/h, respectively. The serum concentrations of the three drugs markedly decreased in the systemic circulation, especially that of (+)-PQT. The AUC of (+)-PQT was apparently lower than that of (-)- or (+/-)-PQT. From these results, it is assumed that the first pass effects of PQT and its enantiomers in the liver are pronounced and most likely stereoselective. Also, unchanged PQT and its enantiomers were found in the bile of the rabbits.     

Pulmonary elimination and metabolism of 5-fluoro-2'-deoxyuridine in isolated perfused rat lung and lung slices

Elimination kinetics and metabolism of the cytostatic drug 5-fluoro-2'-deoxyuridine (FUDR) were studied in isolated perfused rat lung and in incubated lung slices. The intact organ exhibited a low clearance of 0.2 to 0.8 ml/min and a calculated first-pass extraction of 2 to 7% of the drug inflow. Thus, the pulmonary uptake of the fluorinated nucleoside from the circulation is low. Within 120 min of perfusion, however, 30 to 45% of the initial FUDR dose was metabolized by the isolated rat lung. The nucleobase metabolite 5-fluorouracil (FU) represented almost all FUDR metabolites in the medium, indicating that this metabolic pathway, mediated by thymidine phosphorylase, is active in lung while the enzymic activity for further pyrimidine degradation is low. This was demonstrated in incubated lung slices, which have a high capacity to transform FUDR into FU, comprising 83 to 95% of the metabolites in the medium. The final catabolic metabolite, alpha-fluoro-beta-alanine, was present in trace amounts only. It is concluded that the pulmonary tissue contains a marked "intrinsic" capacity to transform FUDR into FU, while the metabolic activity for catabolism of the nucleobase metabolites FU and 5,6-dihydrouracil is virtually lacking.     

Effect of chlorphentermine on the pulmonary clearance of 5-hydroxytryptamine in rabbits in vivo

Previous studies from this laboratory demonstrated that the pulmonary clearance of 5-hydroxytryptamine (5-HT) is perturbed by pulmonary accumulation of chlorphentermine (CP) by isolated perfused lung preparations. This observation raises the possibility that depressed 5-HT clearance may be one factor contributing to CP-induced pulmonary hypertension. The primary objective of the present studies was to determine if the effect of CP could be demonstrated in vivo during single-pass circulation through the lungs. Pulmonary extraction and metabolism of [14C]-5-HT during single pulmonary passage were examined using the reference indicator radioisotope dilution technique in male New Zealand albino rabbits. In control or saline vehicle injected animals, it was established that 81% of administered [14C]-5-HT was extracted by the lung and 25% of total radioactivity in the blood stream was recovered as 5-hydroxyindoleacetic acid (5-HIAA) during single passage through the lungs. Appropriate control incubations of 5-HT with blood and simulated pulmonary circulation through the extracorporeal circulation system yielded only 2 and 5% metabolism, respectively. These results indicate that significant pulmonary metabolism of 5-HT followed by efflux of 5-HIAA into venous output occurs during single-pass circulation. Preadministration of CP (0.5, 1 and 3 mg/kg single dose, i.v.) caused a dose-related inhibition of pulmonary extraction of 5-HT. This effect was also accompanied by a dose-related suppression of the appearance of the metabolite of 5-HT. These results provide in vivo evidence for impairment of pulmonary extraction and deactivation of 5-HT as a result of prior pulmonary accumulation of CP.     

Neurotensin-mediated inhibition of cyclic AMP formation in neuroblastoma N1E115 cells: involvement of the inhibitory GTP-binding component of adenylate cyclase

The tridecapeptide, neurotensin, inhibited prostaglandin E1-stimulated cyclic AMP production in intact plated neuroblastoma N1E115 cells. The peptide effect was concentration dependent (EC50 = 2 nM) and maximal inhibition reached 55% with 100 nM neurotensin. Acetyl neurotensin (8-13) was as active as neurotensin whereas neurotensins (1-8), (1-12), and (10-13) were barely active in inhibiting cyclic AMP production, thus showing the requirement of the carboxy terminal hexapeptide sequence of neurotensin for biological activity. The inhibitory effect of neurotensin on cyclic AMP production was largely prevented by pretreatment of N1E115 cells with islet-activating protein (pertussis toxin). In contrast, pertussis toxin did not inhibit neurotensin-stimulated cyclic GMP production in neuroblastoma cells. In cell membranes, the toxin promoted the selective ADP-ribosylation of a single protein having the same molecular weight (41,000) as the alpha-subunit of Ni, the inhibitory regulatory protein of adenylate cyclase. In membranes prepared from N1E115 cells, monoiodo[125I-Tyr3]neurotensin bound to a single population of receptors characterized, at 25 degrees and in the absence of monovalent cations and guanyl nucleotides, by a dissociation constant (Kd) of 56 pM and a maximal binding capacity (Bm) of 30 fmol/mg of protein. Na+ (10-100 mM) and GTP (0.1-100 microM) inhibited neurotensin binding in a concentration-dependent manner. At 100 mM Na+ and 100 microM GTP, receptor affinity was decreased by 5- and 2-fold, respectively. Li+ and K+ were less effective than Na+, and the effect of GTP was shared by GDP and guanyl-5'-yl-imidodiphosphate, but not by GMP, ATP, ADP, or adenyl-5'-yl-imidodiphosphate. It is concluded that in N1E115 cells, neurotensin attenuates cyclic AMP production by exerting an inhibitory effect on adenylate cyclase through an interaction of the peptide receptors with the regulatory GTP-binding protein Ni.     

Effect of 3,4-Dihydroxyphenylpyruvic Acid on the Metabolism of L-DOPA in Isolated Perfused Rat Liver

Abstract: The effect of 3,4-dihydroxyphenylpyruvic acid (DHPPA) on the hepatic metabolism of L-DOPA was investigated in isolated perfused rat liver. DHPPA decreased the initial hepatic extraction and prolonged the elimination half-life of L-DOPA when they were added simultaneously at the ratio 1:4 (L-DOPA: DHPPA) to the perfusate. At the ratio 1:1 DHPPA had no effect on the elimination half-life, but decreased the initial hepatic extraction of L-DOPA. When L-DOPA was added alone the initial loss of L-DOPA was 30% at 5 min. At that time the perfusion medium had passed the liver 2.5 times. Between 5 to 60 min. about 5% of the dose was extracted in a single pass through the liver. DHPPA added alone or together with L-DOPA was rapidly converted to L-DOPA, only about half of the DHPPA dose remained unmetabolized in the “plasma” at 2.5 min. The main metabolites in bile of all the compounds tested were conjugates of homovanillinic acid and 3,4-dihydroxyphenylacetic acid.     

The action of neurotensin on single myenteric neurones.

The action of neurotensin was studied on single myenteric neurones within ganglia of the myenteric plexus isolated from the guinea-pig ileum. Drugs were applied by adding them to the perfusing Krebs solution. Extracellular recording with glass suction electrodes indicated that neurotensin (100 pM-300 nM) caused a dose-dependent excitation of about 50% of myenteric neurones; the remaining neurones were unaffected. This effect persisted in calcium-free solutions. Intracellular recording showed that a similar proportion of Type 1 myenteric neurones were depolarized by neurotensin: this was associated with an increase in membrane resistance. Type 2 cells were either depolarized or hyperpolarized by neurotensin. The depolarization persisted in calcium-free solutions. The hyperpolarization disappeared in calcium-free solutions, suggesting either that the potential change itself is calcium-dependent or that it was due to release by neurotensin of a hyperpolarizing substance.     

Inhibition by extracellular ATP of organic anion transport in the perfused rat liver

The action of extracellular ATP on organic anion transport in the bivascularly perfused rat liver was investigated, using bromosulfophthalein as a model substance. Transport was measured by means of the multiple-indicator dilution technique. The action of portal 100 microM ATP presented the following characteristics: (a) inhibition of bromosulfophthalein single pass extraction; the inhibition degree decreased with increasing bromosulfophthalein doses; (b) diminution of the influx rate coefficients; (c) 86.7% decrease of the maximal activity of the saturable component for bromosulfophthalein transport, but 100% increase of the non-saturable component; (d) diminution of the bromosulfophthalein flow-limited distribution space; (e) no significant alteration of the rate coefficients for metabolic sequestration. The action of ATP on organic anion transport in the intact liver occurred at much lower concentrations (10x) than those previously reported for isolated hepatocytes. This reinforces the suggestion that inhibition of organic anion transport could be a physiologically relevant effect of extracellular ATP.     

The Possible Role of Cytochrome P-450 in the Liver “First Pass Elimination” of a P-Receptor Blocking Drug

The highly lipid soluble β-receptor blocking drug alprenolol interacts with high affinity with rat liver microsomal cytochrome P-450, is rapidly metabolized in the liver and exhibits a marked liver “first pass elimination” (FPE) in the rat. It thus has a low oral bioavailability in this species. In order to investigate the possible role of the cytochrome P-450 system in the FPE we studied the influence of the three P-450 inhibitors SKF 525-A, imipramine and metyrapone and of phenobarbital treatment on the disposition kinetics of alprenolol in a series of experimental models. Alprenolol rapidly gave rise to a type I spectral change on addition to intact liver cells, indicating a rapid hepatic uptake. The maximal magnitude of this spectrum increased about twofold after phenobarbital treatment of rats in both microsomes and isolated liver cells. Imipramine, SKF 525-A and metyrapone partly displaced ³H-al-prenolol from non-metabolizing partly purified cytochrome P-450 and liver cell preparations (20°). Imipramine and SKF 525-A were about equally effective in this respect whereas metyrapone was much less potent. At 37° the metabolism of alprenolol was rapid and of about similar activity (per nmoles of cytochrome) in liver microsomes, isolated liver cells and in the perfused liver (at a high dose). The K_m-value was similar in microsomes and in isolated liver cells. A similar metabolic inhibitory pattern was found in microsomes and isolated liver cells. SKF 525-A was the most efficient inhibitor followed by imipramine and then metyrapone. The same inhibitory pattern was found for the hepatic extraction of alprenolol. Moreover, the hepatic extraction of alprenolol was dose dependent. Imipramine in a high dose increased the area under the blood concentration curve by a factor of ten after oral administration of alprenolol in the conscious rat. The above findings suggest that cytochrome P-450 is, at least partly, responsible for the degree of hepatic extraction and metabolism (FPE) of alprenolol. This view was also supported by the findings that the perfused liver showed an increased capacity for the extraction and metabolism of alprenolol after phenobarbital treatment. The cytochrome P-450 system may influence the hepatic extraction of alprenolol in rat liver by providing an intracellular “high affinity binding pool” for the unchanged drug. The subsequent metabolic step seems to be important since it “unloads” P-450 so that it can bind new drug molecules.     

Systemic translocation of particulate matter-associated metals following a single intratracheal instillation in rats.

Respirable ambient particulate matter (PM) exposure has been associated with an increased risk of cardiovascular disease. Direct translocation of PM-associated metals from the lungs into systemic circulation may be partly responsible. We measured elemental content of lungs, plasma, heart, and liver of healthy male WKY rats (12-15 weeks old) 4 or 24 h following a single intratracheal (IT) instillation of saline or 8.33 mg/kg of oil combustion PM (HP-12) containing a variety of transition metals with differing water and acid solubility. Tissues were digested with a combination of quaternary acid, amine, and nitric acid and analyzed using inductively coupled plasma-atomic emission spectroscopy. Lung levels of metals were lower at 24 h than at 4 h. Metals with high water solubility and relatively high concentration in HP-12 were increased in extrapulmonary organs. Water-soluble nonessential metals, like vanadium and nickel, were increased in plasma, hearts, and livers of exposed animals at both time points. Exposure-related small increases in essential metals, like zinc and manganese, were also noted in extrapulmonary tissues at both time points. Lead, with low water solubility but high acid solubility, was detected in liver only at 24-h postinstillation. Elements with low water or acid solubility, like silicon and aluminum, were not detected in extrapulmonary tissues despite decreased levels in the lung suggesting mucociliary clearance. We have shown that HP-12-associated metals translocate to systemic circulation and extrapulmonary organs following IT exposure. This translocation is dependent upon their relative levels and water solubility. Thus, following inhalation, PM-associated metals deposited in the lung may be released into systemic circulation at different rates depending on their water/acid solubility, thereby providing a means by which metals may elicit direct extrapulmonary effects.     

Stoichiometric Interaction of a β-Receptor Blocking Drug with Fraction of Liver Cytochrome P-450 as a Possible Cause for its Dose-dependent Availability

Abstract The liver's ability to bind and metabolize “first-pass” drugs is a major factor responsible for their blood concentrations on oral administration. Therefore we have characterized the binding of such a drug, the adrenergic β-receptor antagonist alprenolol, to rat liver. The microsomal fraction accounted for almost all the liver binding capacity. On titration of microsomes with alprenolol, the induced type I spectral change, reflecting alprenolol-cytochrome P-450 interaction, showed a high and a low affinity binding phase with identical “optical capacity”. The spectral dissociation constant for the high affinity binding was 0.17 μM. By use of spectral methods we found that 13–14 per cent of the total microsomal cytochrome P-450 participated in the alprenolol binding at the end of the first phase and 27 per cent at the end of the second phase. Spectral titrations showed that SKF 525-A and imipramine interact with the same sites of cytochrome P-450 as alprenolol. Binding studies with radioactive alprenolol showed the presence of a minimum of three classes of microsomal binding sites. The last had a very high capacity. The first phase caused a marked non-linearity in the overall microsomal binding of the drug, and it had almost identical capacity (0.12 mol alprenolol/mol P-450) and affinity (K_s = 0.21 μM) as the first type I spectral binding phase. This strongly indicates that this first “overall” binding phase reflects selective binding to P-450, which is further strengthened by the fact that SKF 525-A and imipramine markedly influenced this phase. Moreover, this phase was not present in plasma membranes, which do not contain cytochrome P-450. Hence we conclude that a stoichiometric (1:1) interaction of alprenolol with a 12–14 per cent fraction of rat liver microsomal cytochrome P-450 causes a non-linear overall microsomal binding. Such a phenomenon probably explains this drug's dose-dependent availability. We suggest that when this high affinity site is saturated the low affinity site of cytochrome P-450 oxidizes a large fraction of the alprenolol escaping ‘first pass’ liver extraction.     

Neurotensin receptors on the ileum of the guinea-pig: evidence for the coexistence of inhibitory and excitatory receptors

Neurotensin caused a complex muscular response of the longitudinal muscle-myenteric plexus preparation of guinea-pig ileum: picomoles of neurotensin produced inhibition while larger concentrations caused an inhibitory effect followed by a delayed dose-dependent contraction. The inhibitory phase of the neurotensin-induced muscular activity was not modified by tetrodotoxin but was potently antagonized in a non-competitive manner by apamin, a bee venom toxin. The contractile component was blocked by tetrodotoxin but not by apamin. These toxins were used to dissect the neurotensin muscular response into an inhibitory phase and an excitatory component. It was possible to further characterize the two neurotensin muscular components by their kinetics of desensitization. The inhibitory neurotensin response showed a fast rate of desensitization and presented a relatively low spare receptor capacity. In contrast, desensitization to the excitatory action of neurotensin was much slower, the excitatory receptors apparently having a larger spare receptor capacity. Desensitization to the action of neurotensin was selective for the neuropeptide not altering the contractile activity of substance P, angiotensin II, bradykinin, histamine or acetylcholine. These results strongly suggest the presence of two subsets of neurotensin receptors in the ileum: the inhibitory set probably localized at the postsynaptic effector level and excitatory neurotensin receptors probably of neuronal origin whose function is probably to modulate the release of neurotransmitters. The physiological implications of these two subtypes of neurotensin receptors in the control of gastrointestinal motility are discussed.     

Evaluation of hepatic uptake clearance of tetrodotoxin in the puffer fish Takifugu rubripes

In this study, we investigated the hepatic uptake clearance (CL(uptake)) of tetrodotoxin (TTX) in the marine puffer fish Takifugu rubripes by integration plot analysis after a single bolus injection of 0.25mg TTX/kg body weight into the hepatic vein at 20 degrees C. The blood concentration of TTX decreased over time after the injection, from 1451+/-45ng/mL at 10min to 364+/-59ng/mL at 60min. TTX concentrations in the spleen and kidney decreased in parallel with the blood concentrations, whereas those in the muscle and skin remained almost the same throughout the experiment. In contrast, the TTX concentration in the liver gradually increased, reaching 1240+/-90ng/g liver at 60min after injection. The amount of TTX that had accumulated in the liver 60min after injection accounted for 63+/-5% of the administered dose. Integration plot analysis indicated a CL(uptake) of 3.1mL/min/kg body weight in the liver for TTX, a rate far below that of the hepatic portal vein blood flow rate (at most, 9%). This finding is consistent with negligible extraction of TTX by the liver. The results demonstrated conclusively that the liver-specific distribution of TTX in T. rubripes is achieved by removal from the systemic circulation, but not by the hepatic first-pass effect.     

Dose-dependent disposition of oral propranolol in normal subjects

The pharmacokinetics and relative systemic availability or oral propranolol were studied in three healthy volunteers following administration of 10, 40, and 80 mg of propranolol hydrochloride. Plasma concentrations of propranolol were determined using a sensitive and specific fluorometric high pressure liquid chromatographic technique. In the dosage range studied, the amount of propranolol reaching the systemic circulation increased with dose, while half-lives remained unchanged. The apparent ‘threshold dose’ for propranolol was much smaller than previously reported, and its contribution to the observed dose-dependent availability is doubtful. Apparent intrinsic clearance values were shown to decrease with increase in dose, with a true maximal intrinsic clearance of 5·4 1kg^?1 h^?1. These data suggest the saturation of a low capacity enzyme system in the liver and are consistent with theoretical characteristics of a drug that is extensively metabolized during its first pass through the liver.     

Effect of neurotensin and its fragments neurotensin-(1-6) and neurotensin-(8-13) on dopamine release from cat striatum

At low concentrations, neurotensin (10(-9) M) enhanced electrically evoked release of dopamine. At higher concentrations, neurotensin (10(-7) M) also enhanced basal release of dopamine. The carboxy-terminal sequence of neurotensin-(8-13) was fully active and enhanced both electrically evoked and basal release. In contrast, the amino-terminal fragment neurotensin-(1-6) did not enhance electrically evoked release of dopamine even at high concentrations (10(-6) M). However, it retained the ability to enhance basal dopamine release. Combination of different doses of apomorphine with a subthreshold (10(-10) M) and submaximal concentrations (10(-9) M) of neurotensin gave clear additive effects. It is concluded that in the cat striatum neurotensin stimulates dopamine release by a direct effect on its own neurotensin receptor, and does not modulate the sensitivity of presynaptic dopamine autoreceptors. Furthermore, it is suggested that there exist at least two types of neurotensin receptors in the cat striatum. One type stimulates evoked dopamine release and another influences basal release of dopamine.