The effects of exercise and adrenaline infusion upon the blood levels of propranolol and antipyrine in the horse

There are now several examples showing that experimentally induced changes in hepatic blood flow can have a marked effect upon the elimination of certain high clearance drugs. Changes in hepatic blood flow produced by exercise might therefore be expected to influence the clearance of these drugs. There was an increase of up to 100%, compound to control values, in the plasma levels of propranolol in horses given either d- or dl-propranolol, 0.2 mg/kg b.wt., and then subjected to sustained exercise for 30 minutes. There was, however, no similar increase with exercise in the plasma levels of antipyrine in horses given antipyrine, 50 mg/kg b.wt. Intravenous infusions of adrenaline, 1 microgram/kg b.wt./min for 10 minutes also produced an increase of about 70% in the plasma levels of d-propranolol.     

Effects of chronic phenobarbital on verapamil disposition in humans

Very little is known about the effects of hepatic enzyme induction with phenobarbital on the disposition of high clearance drugs in humans. Our study was undertaken to investigate the effect of phenobarbital on both alpha-1 acid glycoprotein concentrations and total and free verapamil and its metabolites. Single oral, single i.v., and multiple oral verapamil administrations were evaluated before and after 21 days of phenobarbital treatment in healthy caucasian male volunteers. Significant changes in the pharmacokinetics of total and free verapamil and its metabolites occurred in a predictable manner. Mean total apparent oral clearance after a single dose of verapamil was increased after phenobarbital treatment (75.1 +/- 49.2 vs. 376.2 +/- 221.8 ml/min/kg, P less than .05). Clearance of free drug increased to a similar magnitude. Mean total verapamil systemic clearance was increased (9.95 +/- 1.3 vs. 18.9 +/- 8.7 ml/min/kg, P less than .05); however free drug clearance was not altered. After multiple oral administration, total verapamil apparent oral clearance was increased after phenobarbital pretreatment (21.2 +/- 9.8 vs. 91.2 +/- 28.5 ml/min/kg, P less than .05). Free drug clearance was increased similarly. Finally, the pharmacokinetic theories derived for hepatic extraction of drugs subject to a high metabolic clearance can be successfully applied to men after liver enzyme induction with phenobarbital.     

Effect of antipyrine and phenobarbital on renal gamma-glutamyltransferase excretion in human urine

Serum gamma-glutamyltransferase is used as a marker of hepatic enzyme induction. The kidney contains high activities of gamma-glutamyltransferase in the brush border membrane of the proximal tubule, from which it is released into urine. This study investigated the effect of phenobarbital and antipyrine, two inducers of hepatic monoxygenases and gamma-glutamyltransferase, on the urinary excretion of renal gamma-glutamyltransferase. Three groups (n = 6) of healthy male volunteers received 100 mg phenobarbital for 7 and 14 days and 1200 mg antipyrine for 7 days, respectively. Antipyrine and phenobarbital increased antipyrine elimination, serum gamma-glutamyltransferase, and the urinary excretion of renal gamma-glutamyltransferase, whereas urinary beta-N-acetylglucosaminidase, beta-glucuronidase, and total protein and glucose excretion were unchanged. No correlation was found between serum and urinary gamma-glutamyltransferase or both enzymes and antipyrine elimination. Increases in antipyrine elimination were positively correlated to increases in serum, but not urinary gamma-glutamyltransferase. The findings suggest that antipyrine and phenobarbital increase urinary gamma-glutamyltransferase excretion. However, the increase in urinary gamma-glutamyltransferase does not reflect the magnitude of hepatic enzyme induction.     

Methods for studying the hepatic metabolism of drugs in man

Various methods can be used to investigate human hepatic drug metabolism in vivo. a) Indirect methods based on the assessment of atoxic substances metabolism in order to investigate: --hepatic oxidative activity by antipyrine or caffeine clearance and aminopyrine breath test determination. Such an activity can be modified by drug-induced enzyme induction or inhibition and in case of hepatic disease; --genetic polymorphism of hepatic drug metabolism assessed by the determination of debrisoquine or dextrometorphane metabolic ratio and of N-acetylation phénotype. Unusual therapeutic or adverse effects could be explained by such a polymorphism; --hepatic blood flow, which variations could modify hepatic clearance of drugs with a high hepatic extraction ratio, by the assessment of indocyanine green clearance. b) Direct methods based on pharmacokinetics data and their alterations under various circumstances: simultaneous administration of other drugs, liver disease.     

Genetic control of the phenobarbital-induced shortening of plasma antipyrine half-lives in man

The mean half-life of antipyrine in the plasma of four sets of identical and four sets of fraternal twins after a single oral dose of 16 mg/kg of antipyrine was 12.7 +/-(SD) 3.3 hr. After 2 wk on sodium phenobarbital (2 mg/kg daily) the half-life of antipyrine in the plasma of these twins was reduced to 8.0 +/-(SD) 1.5 hr. Shortening of the plasma antipyrine half-life occurred in all but one of these 16 normal, adult volunteers, but there was considerable variation in the extent of reduction which ranged from 0 to 69%. Phenobarbital administration decreased individual variations in antipyrine metabolism as indicated by the smaller standard deviation of the plasma antipyrine half-lives after phenobarbital than observed initially and by the narrowed range of variation in plasma antipyrine half-lives from 2.8-fold initially to 1.8-fold after phenobarbital. These results suggest that some inducing agents may be used to minimize individual variations in drug metabolism where such variations create therapeutic problems by exposing patients who slowly metabolize certain drugs to toxicity and other patients who rapidly metabolize some drugs to undertreatment.During the course of phenobarbital administration blood levels were determined. Phenobarbital blood levels correlated neither with the final values for plasma antipyrine half-lives nor with the per cent reduction in plasma antipyrine half-life produced by phenobarbital treatment. There was a direct relationship between initial antipyrine half-lives and the per cent shortening of antipyrine half-life produced by phenobarbital administration: the shorter the initial antipyrine half-life, the less the reduction caused by phenobarbital treatment. Larger intrapair variances in fraternal than in identical twins indicate genetic, rather than environmental, control of phenobarbital-induced alterations in plasma antipyrine half-life.     

Phenobarbital in the genetically obese Zucker rat. II. In vivo and in vitro assessments of microsomal enzyme induction

In vivo and in vitro alterations in drug metabolism and the extent of enzyme induction of the hepatic microsomal cytochrome P-450 system were evaluated in obese and lean Zucker and lean Sprague-Dawley rats. Phenobarbital enzyme-inducing regimens were administered p.o. to achieve similar steady-state phenobarbital plasma concentrations. Control rats received p.o. placebo solution. No significant intra- or inter-strain differences in antipyrine clearance (milliliters per hour) or apparent volume of distribution (liters) were observed between the placebo-treated lean Sprague-Dawley, lean Zucker and obese Zucker rats. Intra- and inter-strain differences in hepatic microsomal protein and cytochrome P-450 content were observed. Compared to placebo, antipyrine clearance (milliliters per hour) after chronic phenobarbital pretreatment was increased in the Sprague-Dawley (198%) and lean Zucker rats (131%), but not significantly altered in the obese Zucker rats. Similarly, increases in hepatic weight, whole liver microsomal protein and cytochrome P-450 content were also observed in the Sprague-Dawley (34, 124 and 352%, respectively) and the lean Zucker rats (24, 96 and 249%, respectively). However, no significant alterations in these parameters were observed in the obese Zucker rats after phenobarbital treatment. Results from these in vivo and in vitro studies implicate alterations in drug metabolism and genetic differences in cytochrome P-450 content in Zucker rats relative to the Sprague-Dawley strain. Obese Zucker rats failed to exhibit a significant induction response after phenobarbital pretreatment.     

The effects of mannitol diuresis on digoxin and phenobarbital handling by the kidney: implications for tubular reabsorption and secretion of the cardiac glycoside

The effect of mannitol diuresis on the renal clearance of digoxin and phenobarbital was studied in dogs. Mannitol diuresis significantly increased the clearance of digoxin and the ratio digoxin: inulin clearances (from 0.7 +/- 0.2 to 1.1 +/- 0.25). The increase in phenobarbital: inulin clearance ratio was significantly higher than the increase in the digoxin: inulin clearance ratio (4.9 fold vs 1.66 fold) (p less than 0.005). Mannitol diuresis did not significantly affect inulin clearance, nor digoxin protein binding during the experimental period while there was a significant increase in PAH clearance. Significant correlations were found between urine flow rate and digoxin renal clearance or digoxin: inulin clearance ratio. The increase in the ratio drug: inulin clearance with diuresis correlated inversely with the initial ratio; animals with more predominant net reabsorption had a higher increase in ratio. These studies suggest that the mannitol-induced increase in digoxin clearance stems from a combination of increased renal blood flow enhancing digoxin secretion, and increased urine flow rate inhibiting its reabsorption. We conclude that urine flow rate and renal blood flow are important determinants of the renal clearance of digoxin, independent of GFR. Any study assessing the effect of pathophysiological states or drug interactions on digoxin renal clearance must control for these factors.     

EFFECT OF TURPENTINE-INDUCED INFLAMMATION ON THE DISPOSITION KINETICS OF PROPRANOLOL, METOPROLOL, AND ANTIPYRINE IN THE RAT

Plasma concentrations after oral administration of the high extraction drug propranolol are increased in patients and animals with inflammation. This could be due to increased serum propranolol binding, but also to decreased first-pass metabolism. We studied the pharmacokinetics of 3 drugs in control rats and in rats with turpentine-induced inflammation: propranolol, which is bound extensively to alpha 1-acid glycoprotein (alpha 1-AGP); metoprolol, another high extraction drug, but which is negligibly bound to alpha 1-AGP; and antipyrine, a low extraction drug, not bound to serum proteins. After IV administration of propranolol in rats with inflammation, systemic clearance, volume of distribution, and free fraction decreased, and the area under the curve (AUC) increased, whereas the half-life did not change. As the systemic clearance of a high extraction drug such as propranolol depends on hepatic blood flow only, a fall in hepatic blood flow or transition to a low extraction situation should be postulated. After oral administration of propranolol, the AUC was increased 20-fold in rats with inflammation; as the decrease in free fraction was only 4-fold, it can be concluded that a considerable decrease in hepatic intrinsic clearance was present. For metoprolol, in contrast to propranolol, after IV administration, no changes in pharmacokinetic parameters as a result of inflammation were observed. After oral administration, the AUC was increased about 4 times in rats with inflammation; as metoprolol is only negligibly bound to serum proteins, the increase in AUC can be attributed to a decrease in hepatic intrinsic clearance.(ABSTRACT TRUNCATED AT 250 WORDS)     

Alterations in the disposition of differently cleared drugs in patients with cirrhosis.

We have compared the disposition of antipyrine orally (15 mg/kg) and the new antiarrhythmic drug lorcainide intravenously (1.5 mg/kg) in 8 patients with alcoholic cirrhosis. Antipyrine (AP) serves as a model drug for drugs which are eliminated independently of liver blood flow and lorcainide (L) elimination as a model for drugs which depend on liver blood flow. Since in healthy subjects elimination half-life (t 1/2) of L increased with age (r = 0.68, p less than 0.01) due to parallel change in the volume of distribution (r = 0.52, p less than 0.05), its disposition had to be compared to age-matched controls. Elimination of both AP and L was impaired in cirrhotic patients, expressed either in terms (mean +/- SD) of t 1/2 (AP = 26.8 +/- 15.0 hr and 12.3 +/- 1.8; L = 12.5 +/- 4.5 hr and 7.7 +/- 2.2 hr, p = 0.002) or of clearance (Cl) (AP = 21.9 +/- 7.9 ml/min and 41.7 +/- 5.5 ml/min; L = 814 +/- 144 and 1002 +/- 304 ml/min, p = 0.06). While the alterations in plasma Cl were great for AP, they were smaller for L. This suggests that elimination of drugs in cirrhotic patients is associated with relatively more impairment of enzyme activity than of hepatic blood flow. The slightly decreased Cl of L in patients with alcoholic cirrhosis would suggest that L should be carefully handled in patients with dysfunction of the liver.     

Comparative evaluation of the effects of isradipine and diltiazem on antipyrine and indocyanine green clearances in elderly volunteers

Calcium antagonists have been shown to depress hepatic enzymes and accelerate hepatic blood flow. This study was designed to compare the effects of two calcium antagonists, isradipine and diltiazem, on antipyrine and indocyanine green (ICG) clearances in the elderly. Eighteen elderly subjects (aged 65 to 80 years) received either isradipine (5 mg every 12 hours), diltiazem (90 mg every 8 hours), or placebo (every 12 hours) for 4 days. On the third day after the study treatment, a 0.5 mg/kg dose of ICG was administered. Blood samples were obtained over 20 minutes for HPLC determination of ICG plasma concentrations. Ten minutes later, subjects ingested 1.2 gm antipyrine. Blood samples were obtained over 48 hours for HPLC determination of antipyrine plasma concentrations. Mean +/- SD antipyrine clearance after diltiazem (0.0258 +/- 0.0065 L/hr/kg) was significantly lower than that observed after isradipine (0.0334 +/- 0.0098 L/hr/kg) or placebo (0.0329 +/- 0.0082 L/hr/kg). Antipyrine clearance after isradipine was not significantly different from that after placebo. Mean +/- SD ICG clearances after diltiazem (9.17 +/- 1.35 ml/min/kg) or isradipine (9.57 +/- 1.82 ml/min/kg) were significantly higher than that observed after placebo (8.06 +/- 1.45 ml/min/kg). These findings suggest that diltiazem, but not isradipine, affects hepatic enzyme activity in the elderly. Both agents accelerate ICG clearance, a marker of hepatic blood flow.     

Hepatic clearances of antipyrine, indocyanine green, and galactose in normal subjects and in patients with chronic liver diseases

Blood clearance of antipyrine, indocyanine green, and galactose were measured to evaluate the alterations of effective hepatic blood flow and hepatic intrinsic clearances in chronic liver diseases. Galactose blood clearance, which may be taken as effective hepatic blood flow, decreased by approximately 30% in patients with cirrhosis (12.49 +/- 0.76 ml/min/kg; mean +/- SE; n = 17) compared with normal subjects (18.17 +/- 1.03 ml/min/kg; n = 5). In patients with cirrhosis, intrinsic clearances of antipyrine (0.178 +/- 0.014 ml/min/kg; n = 17) and indocyanine green (6.19 +/- 1.38 ml/min/kg; n = 7) showed 61% and 85% reduction, respectively, compared with those of normal subjects (0.462 +/- 0.048 ml/min/kg; n = 5; 41.72 +/- 7.75 ml/min/kg; n = 5). Considering that indocyanine green and antipyrine are eliminated by different hepatic mechanism, these mechanisms may not be equally sensitive to decrements in hepatic function. In addition, fractional reductions of intrinsic clearances for these compounds are thus much greater than that of effective hepatic blood flow.     

The effect of antipyrine, phenobarbitol and rifampicin on thyroid hormone metabolism in man

Abstract. The effect of three different liver microsomal enzyme inducing drugs on thyroid hormone metabolism was investigated. Seven volunteers were randomly allocated in a crossover design to either antipyrine (1200 mg), phenobarbital (100 mg) or rifampicin (1200 mg) daily for 14 days. Before and after each treatment the following parameters of enzyme induction were measured: antipyrine clearance, y-glutamyltranspepti-dase, d-glucaric acid and 6-β-hydroxycortisol urinary excretion. In addition, thyroxine-binding globulin (TBG), T₃-resin uptake (RT₃U), thyroxine (T₄), free thyroxine (FT₄), triiodothyronine (T₃), reverse T₃ (rT₃), and thyroid stimulating hormone (TSH) were estimated. Following antipyrine and phenobarbital antipyrine clearance increased by about 45%, while with rifampicin an increase of 125% was observed. The indices of thyroid function did not change following phenobarbital and antipyrine, but after rifampicin T₄, FT₄ and rT₃ decreased by about 14%, and T₃ increased by 25%. In addition, the impact of rifampicin on the clearance of injected ¹²⁵I-T₄ was investigated in six additional volunteers by blocking thyroid iodine uptake. The ¹²⁵I-T₄ halflife decreased from 155 to 106 h and its clearance increased from 35 to 50 ml/h, while a fall in T₄, FT₄ and rT₃ by about 40% and no rise but a decrease in T₃by 25% occurred. Therefore an increased clearance of T₄ and rT₃ but not of T₃ seems likely following rifampicin, which might be due to enhanced hepatic metabolism and biliary excretion.     

Effect of aging on the pharmcokinetics of atenolol, metoprolol and propranolol in the rat

The pharmacokinetics of three beta adrenoceptor blocking drugs atenolol, metoprolol, propranolol and of the model drug antipyrine were studied in 3-, 12- and 24-month-old rats. Alpha-1-acid glycoprotein in plasma was slightly increased and the free fraction of propranolol was slightly decreased in the 24-month-old rats. For the four drugs studied, the volume of distribution and the clearance were decreased in the older rats. For propranolol, metoprolol and antipyrine these decreases were already observed in 12-month-old rats, for atenolol only in the 24-month-old rats. There was no change in half-life as a function of age for the three beta blockers; for antipyrine a significant increase in half-life was seen. The decreased volume of distribution suggests a lower tissue binding or a decrease in total body fluid. The change in clearance in function of age can probably be explained for metoprolol and propranolol by a decrease in hepatic blood flow as no change in metabolism or protein binding occurs, for atenolol by a decrease in renal function and for antipyrine by a decrease in metabolism. After p.o. administration of the beta blockers, no significant increase in area under the plasma drug concentration-time curve was observed for propranolol and metoprolol as a function of age, but for atenolol, a significant increase in area under the plasma drug concentration-time curve was seen in the 24-month-old rats, due to a decrease in renal function, as bioavailability of atenolol did not change in function of age.     

Hepatic drug clearance in children with leukemia: changes in clearance of model substrates during remission-induction therapy

We administered a "cocktail" of three model substrates for hepatic processes: antipyrine for oxidation, lorazepam for glucuronidation, and indocyanine green for hepatic blood flow, to children with acute lymphocytic leukemia (ALL). The plasma clearance of these substrates was determined before and after remission-induction therapy in 14 children with ALL. We found an improvement in clearance of antipyrine (0.65 to 0.95 ml/min/kg; P less than 0.01) and lorazepam (0.93 to 1.20 ml/min/kg; P less than 0.05) in 12 of 14 patients, with a mean increase of 67% and 52%, respectively, from before to after remission. There was no significant difference in mean indocyanine green clearance from before to after induction (14.8 vs. 14.4 ml/min/kg). There were no significant differences in liver function test results (SGOT, prothrombin time, or serum bilirubin) from before to after induction. Plasma concentrations of albumin, alpha 1-acid glycoprotein, and apolipoprotein A changed by a mean of +11.1%, -38.2%, and +68.6%, respectively, from before to after remission. However, these changes did not account for the changes in total plasma clearance of lorazepam, because lorazepam free fraction did not change and lorazepam free clearance increased by a mean of 83%. Our hypothesis is that eradication of hepatic leukemic infiltration by ALL remission therapy resulted in an improvement in microsomal metabolism of antipyrine and lorazepam.     

Effect of exercise on pharmacokinetics

The increasing popularity of sporting events, even for people on drug treatment, has raised the question of the interaction of exercise and pharmacokinetics. Exercise reduces splanchnic blood flow, but possible changes in the absorption of orally given drugs seem to be of minor clinical significance. Absorption from intramuscular, subcutaneou and transdermal application sites may be accelerated by exercise, possibly causing harmful consequences, e.g. in diabetics treated with insulin. Exercise or physical work increases the rate and depth of respiration thus increasing alveolar exchange of gases and vapours. Physical activity increases muscular blood flow and the binding of digoxin to muscular structures, with a simultaneous fall in the concentration of serum digoxin. Reduction in blood flow to adipose and other inactive tissues may delay the distribution of some drugs that are stored or removed by these tissues. The change from supine to upright position can affect the distribution of a drug. Exercise reduces the blood flow in the liver and deactivation of drugs with flow-limited (high clearance) hepatic metabolism such as nitrates and lidocaine. Metabolism of capacity-limited (low clearance) drugs, e.g. antipyrine, diazepam and amobarbital, is not influenced by exercise. Renal plasma flow, urine excretion rate and urine pH are also reduced by exercise. This is an important reason why the serum levels of drugs eliminated through the kidneys increase during physical stress. The changes in parenteral absorption and distribution volume of some drugs caused by exercise, as well as the short half-life of drugs, are properties resulting in altered therapeutic/toxic response in those drugs with a narrow therapeutic range.     

Drug pharmacokinetics in the obese

In the obese, modifications in body constitution (higher percentage of fat and lower percentage of lean tissue and water) can affect drug distribution in the tissues. For slightly liposoluble molecules (e.g., digoxin, antipyrine), the equilibrium distribution volume (V), total and per kilogram weight, is significantly less than that of control subjects. With lipophilic drugs (e.g., barbiturates, benzodiazepines), this parameter is significantly increased, explaining the prolongation of the plasma elimination half-life. For drugs that are almost equally soluble in water and oil (methyl xanthines, aminoglycosides), the V is slightly increased in the obese. The other main factors involved in drug diffusion in the tissues are binding to plasma and tissue proteins, and regional blood flow. In the obese the binding of drugs to albumin does not seem to be altered. A marked increase in plasma alpha-glycoprotein acid and in propranolol binding has been reported in some studies; this has not been corroborated by other authors. Although the cardiac output and total blood volume are increased in the obese, the blood flow per gram of fat is less than in nonobese subjects. This could limit diffusion in the tissues of some lipophilic drugs. Studies on hepatic clearance of drugs are not available in the obese, but hepatic histological alterations have been described. In most publications concerning drugs with biotransformation as the principal elimination route, the total plasma clearance is not reduced. Up to the present, there are no reports of any impairment involving renal elimination of drugs in the obese. Dose-adjustment of hydrophilic drugs is assessed according to the ideal weight of the individual obese subject; with lipophilic drugs the loading dose can be fixed according to the total weight; calculation of the maintenance dose depends on possible changes in the total clearance.     

Disposition of drugs in cystic fibrosis. II. Hepatic blood flow

To determine whether the increased clearance of high extraction-ratio drugs in cystic fibrosis is caused by an increase in hepatic blood flow, the blood flow in main branches of the hepatic vein and portal vein was measured by use of noninvasive duplex ultrasound scanning in 10 adult subjects with cystic fibrosis and in 10 healthy age-, gender-, and height-matched control subjects. No statistically significant differences between subjects with cystic fibrosis and control subjects were detected in either the hepatic vein (217 +/- 103 ml/min for subjects with cystic fibrosis versus 211 +/- 135 ml/min for control subjects) or the portal vein (205 +/- 114 ml/min for subjects with cystic fibrosis versus 190 +/- 101 ml/min for control subjects) blood flows. These data indicate that a large (greater than or equal to 100%) increase in the clearance of high extraction-ratio drugs in patients with cystic fibrosis is unlikely to be primarily caused by an increase in hepatic blood flow. It is probable that alternative mechanisms such as enhanced secretory or metabolic pathways account in large part for increases in clearance of high extraction-ratio drugs.     

Lack of effect of hepatic enzyme induction on metabolic control in patients with type 2 (non-insulin-dependent) diabetes

A placebo-controlled, double-blind crossover study was carried out in 11 non-insulin-dependent (type 2) diabetic patients to find out the effects of a hepatic enzyme inducer (phenobarbital, 100 mg/day for 2 months) on the metabolic control, plasma C-peptide, insulin, serum, and lipoprotein lipid levels. Phenobarbital induced a significant increase in hepatic antipyrine metabolizing activity, but no significant changes were found in fasting or postload blood glucose, plasma C-peptide, or insulin levels during the study. There was a significant increase in serum total cholesterol, high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol, as well as in serum total and very low-density lipoprotein triglycerides, during phenobarbital treatment as compared with placebo.     

Treatment of noninsulin-dependent diabetes mellitus with enzyme inducers

Although treated adequately with antidiabetic drugs, diet, exercise, and education, patients with noninsulin-dependent diabetes mellitus (NIDDM) may develop resistance to treatment. In NIDDM hepatic microsomal enzyme activity is reduced and since postreceptional glucose metabolism is influenced by these enzymes, we treated the subjects with enzyme-inducing drugs. These inducers (phenobarbital and medroxyprogesterone acetate) when added as adjuvant therapy to sulfonyl urea regimen, reduced blood glucose and plasma insulin, and increased microsomal enzyme activity (as indicated by increased antipyrine metabolism). A trial with placebo did not alter serum glucose levels. Body weight fell and serum aminotransferase levels were normalized. These changes were reflected by reduction of liver fat content (determined by light microscopy), by increased surface density of smooth endoplasmic reticulum, and by repairation of the plasma cell membrane of hepatocytes, as seen in electron micrographs. Activation of postreceptional events in hepatocytes may thus be a new approach in the treatment of therapy-resistant type II diabetes.     

Stimulation of hepatic sodium and potassium-activated adenosine triphosphatase activity by phenobarbital. Its possible role in regulation of bile flow.

Since phenobarbital administration produces a profound increase in bile flow without changing bile acid secretion, we examined whether this drug increases the activity of hepatic sodium-potassium-activated ATPase [Na+-K+)-ATPase], the postulated regulating enzyme in the secretion of bile salt independent bile flow. After freeze-thawing to increase substrate accessibility, (Na+-K+) ATPase activity was determined by ouabain inhibition of total ATPase activity. Its activity was highest in isolated liver surface membrane fractions enriched in bile canalicult. Phenobarbital administration significatly increased (Na+-K+)-ATPase activity in both liver surface membrane fractions as well as liver homogenates. This enhanced activity is apparently selective for other membrane phosphatases and the enzyme activity in other tissues is either unaltered or decreased. Kinetic analysis of (Ka+-K+)-ATPase indicates that phenobarbital treatment increased maximum velocity and half-maximum activation constant was unchanged, consistent with activation of latent molecules or an increased number of enzyme molecules. The latter process seems more likely because cycloheximide prevented phenobarbital induction and activators were not demonstrated in vitro. Examination of the full time course of phenobarbital induction to determine whether phenobarbital increased synthesis or decreased degradation was consistent with increased synthesis since the apparent degradation rates were similar with or without phenobarbital treatment. The apparent half-life for (Na+-K+)-ATPase was estimated to be approximately 2.5 days, consistent with liver surface membrane protein turnover. The correlation of changes in bile flow with (Na+-K+)-ATPase was examined under several experimental situations. Phenobarbital caused a parallel increase in each during the 1st 2 days of greatment: thereafter other factors become rate limiting for flow, since enzyme activity doesn't reach a new steady state until 4-days. Consistent with increased sodium-potassium exchange, bile sodium was unchanged while potasium concentrations were significantly reduced. Changes in both bile flow and (Na+-K+)-ATPase induced by phenobarbital are independent of thyroid hormone. These studies support the postulate that (Na+-K+)-ATPase is an important factor in regulation of bile flow. In addition, phenobarbital enhancement of both bile flow and (Na+-K+)-ATPase is dependent upon de novo protein synthesis.