Effects of hepatic blood flow on hepatic ethanol kinetics measured in cats and predicted from the parallel tube model

In cats anesthetized with pentobarbital, a long-circuit technique was used to measure hepatic blood flow while portal flow was varied from 0 to 300% of normal in random steps. Arterial, portal, and hepatic venous blood samples were analyzed for ethanol concentrations during continuous infusion of ethanol (20 mumol/(min.kg body weight) into the reservoir. Measured values for logarithmic mean sinusoidal ethanol concentration, hepatic venous ethanol concentration, hepatic ethanol uptake, and ethanol extraction were compared with the values predicted by the parallel tube model for hepatic uptake of substrates using Vmax and Km determined in each cat at the start of the experiment. Measured and predicted values were very similar at all blood flows above 65% control, but statistical regression analysis indicated a small but highly significant deviation of the measured values from the predicted values. At low flows, measured values of logarithmic mean sinusoidal and hepatic venous concentrations markedly exceeded the predicted values in most cats. The results indicate that the parallel tube model, which assumes all sinusoids are identical and equally perfused, provides a useful approximation for the effects of hepatic blood flow on hepatic ethanol kinetics except at low flows. However, there appears to be a significant degree of sinusoidal heterogeneity that results in a better fit to the distributed model. Our previously reported data for hepatic galactose uptake followed a similar pattern when reanalyzed in this more rigorous way.     

Derecruitment in cat liver: extension of undistributed parallel tube model to effects of low hepatic blood flow on ethanol uptake

Previous studies showed two deviations from the predictions of the undistributed parallel tube model for hepatic uptake of substrates: a small deviation at high flows and a large deviation at low flows. We have examined whether these deviations could be described by a single correction factor. In cats anesthetized with pentobarbital, a hepatic venous long-circuit technique with an extracorporeal reservoir was used to vary portal flow and hepatic venous pressure, and allow repeated sampling of arterial, portal, and hepatic venous blood without depletion of the cat's blood volume. Hepatic uptake of ethanol was measured over a wide range of blood flows and when intrahepatic pressure was increased at low flows. This uptake could be described by the parallel tube model with a correction for hepatic blood flow: Uptake = Vmax max.(1 - e-kF).c/(Km + c). In 22 cats, Vmax max = 90 +/- 5 mumols/(min.100 g liver), k = 0.021 +/- 0.0015 when flow (F) was in millilitres per minute per 100 g liver, and Km = 150 +/- 20 microM when c is the log mean sinusoidal concentration. (1 - e-kF) represents the proportion of sinusoids perfused and metabolically active. A dynamic interpretation of this proportion is related to intermittency (derecruitment) of sinusoidal flow. Half the sinusoids were perfused at a flow of 33 mL/(min.100 g liver) and the liver was essentially completely perfused (greater than 95%) at the normal flow of 150 mL/(min.100 g liver). Derecruitment was not changed by raising hepatic venous pressure, and it was not related to hepatic venous resistance.     

Quantitative proton nuclear magnetic resonance of plasma for screening hepatic metabolism during ethanol infusion in cats

The effects of increasing blood ethanol levels on hepatic metabolism were studied in anesthetized cats whose prior fluid intake contained ethanol for 24 days. A hepatic venous long-circuit technique with an extracorporeal reservoir was used to allow hemodynamic measurements and repeated sampling of arterial, portal, and hepatic venous blood without depletion of blood volume. For ethanol, Vmax was 106 +/- 15 mumol.min-1.100 g-1 liver and Km was 164 +/- 31 microM. A previous study showed that there were no changes in O2 uptake by the liver, suggesting other oxidative processes were suppressed during ethanol metabolism. In this study, proton nuclear magnetic resonance spectroscopy was used to simultaneously screen several plasma metabolites to elucidate other metabolic processes that may be perturbed in the liver during ethanol infusion. Hepatic lactate uptake remained unaltered when ethanol metabolism was less than 0.5 Vmax but was suppressed on an equimolar basis with ethanol metabolism when ethanol metabolism rose above 0.5 Vmax. Thus, lactate oxidation is one process that can be suppressed to allow ethanol oxidation without additional O2 uptake by the liver. In addition, no release of acetate from the liver occurred during ethanol metabolism in these experiments. This surprising finding suggests ethanol metabolism may, under some conditions or in some species, result in fatty acid synthesis rather than acetate release. Eight other major metabolites remained unchanged during ethanol infusion.     

Hepatic blood flow: accuracy of estimation from galactose clearances in cats

Experiments were carried out to determine the accuracy and validity of estimations of hepatic blood flow from clearance data during infusions of galactose in anesthetized cats. Clearance calculations were compared directly with the measured hepatic blood flows using a hepatic venous long-circuit technique. This technique allowed direct measurement and alteration of hepatic blood flow and collection of arterial and mixed hepatic venous blood samples without depletion of the animal's blood volume. It was found that infusions of galactose could not be used to estimate accurately hepatic blood flow. Infusion rate could not be used as an estimate of hepatic or splanchnic uptake owing to substantial and variable extrasplanchnic uptake. As a result, estimated hepatic flows allowing for incomplete extraction overestimated the true flow. On the other hand, extraction was less than 100%. This caused systemic galactose clearance to underestimate hepatic blood flow. These errors could cancel each other giving an apparently good estimate of hepatic flow from systemic galactose clearance. This agreement was fortuitous and occurred only at a specific dose and blood flow. We conclude that in the absence of independent measurements of both extrasplanchnic uptake and splanchnic extraction of galactose, systemic galactose clearance is not a reliable measure of hepatic blood flow in anesthetized cats. Until proved otherwise, it seems likely that this is also true in humans.     

Kinetics of Regional Blood-Brain Barrier Glucose Transport and Cerebral Blood Flow Determined with the Carotid Injection Technique in Conscious Rats

Anesthetics, particularly barbiturates, have depressive effects on cerebral blood flow and metabolism and likely have similar effects on blood-brain barrier (BBB) transport. In previous studies utilizing the carotid injection technique, it was necessary to anesthetize the animals prior to performing the experiment. The carotid injection technique was modified by catheter implantation in the external carotid artery at the bifurcation of the common carotid artery. The technique was used to determine cerebral blood flow, the Km, Vmax, and KD of glucose transport in hippocampus, caudate, cortex, and thalamus-hypothalamus in conscious rats. Blood flow increased two to three times from that seen in the anesthetized rat. The Km in the four regions ranged between 6.5 and 9.2 mM, the Vmax ranged between 1.15 and 2.07 mumol/min/g, and the KD ranged between 0.015 and 0.035 ml/min/g. The Km and KD in the conscious rat did not differ from the values seen in the barbiturate anesthetized rat. The Vmax, on the other hand, increased two- to three-fold from that seen in the anesthetized rat and was nearly proportional to the increase in blood flow seen in the conscious rat. The development of the external carotid catheter technique now allows for determination of BBB substrate transport in conscious animals.     

Effects of hemorrhage and hepatic nerve stimulation on venous compliance and unstressed volume in cat liver

Intrahepatic blood volume-pressure relationships were studied using plethysmography to measure hepatic blood volume and a hepatic venous long-circuit to control intrahepatic pressure. In cats anesthetized with pentobarbital or with ketamine-chloralose, hemorrhage (to reduce hepatic blood flow to 60% of control) caused marked reductions in hepatic blood volume and intrahepatic pressure but did not significantly change hepatic blood volume-pressure relationships. We were unable to demonstrate an active reflex venous response to hemorrhage in these preparations, although a large passive response occurred. The volume-pressure relationships in innervated livers were different from those in denervated livers: apparent venous compliance was much greater and apparent unstressed volume was zero or negative. Hepatic nerve stimulation in denervated livers caused a marked decrease in hepatic blood volume at low intrahepatic pressures but failed to alter hepatic blood volumes at high intrahepatic pressures (15 mmHg) (1 mmHg = 133.3 Pa). This resulted in large apparent compliances and apparently negative unstressed volumes, as seen in the innervated livers. Thus blood volume-pressure relationships in innervated livers may not give valid measurements of compliance and unstressed volume. A remarkable feature in all these experiments was the linearity of the relationship between hepatic blood volume and intrahepatic pressure. Exudation of fluid begins at higher intrahepatic pressures in innervated compared with denervated livers.     

Effect of hepatic nerve stimulation on hepatic uptake of lidocaine in the cat

Hepatic blood flow and lidocaine uptake were measured using a hepatic venous long-circuit preparation in cats anesthetized with pentobarbital-Na. The processes involved with hepatic elimination of lidocaine were not affected by stimulation of the hepatic nerves. The lack of neural influence on hepatic extraction ratios of lidocaine supports the contention that nerve stimulation does not result in shunting or redistribution of blood to non-nutritive sites. In species which do not show complete vascular escape from neurogenic vasoconstriction, a reduced lidocaine elimination would be anticipated since it was shown that reduced hepatic blood flow results in reduced lidocaine elimination. In the intact rat one third of the lidocaine in the blood was extracted on each passage through the liver. This extraction ratio is not affected by arterial levels of lidocaine, by changes in blood flow or by activation of the hepatic nerves.     

Kinetics of chylomicron triglyceride removal from plasma in rats: a comparison of the anesthetized and the unanesthetized states

The kinetics of chylomicron-TG removal were studied using an experimental method which allows measurements to be made under optimal physiological conditions. Chylomicrons, labeled with palmitic acid-(14)C, were constantly infused at a rate of 0.5 mg total lipid per min into chronically cannulated, unanesthetized, unrestrained rats which had been fasted for 18 hr. Serial blood samples were withdrawn from an arterial cannula during a 20 min infusion period and for 10 min following the infusion. Plasma lipoproteins were separated into two fractions in the ultracentrifuge, and the lipids were extracted. Radioactivity in the low-density fraction (d<1.006) was taken to represent chylomicron-TG radioactivity. Using this method we studied the influence of anesthesia on the kinetics of removal of chylomicron-TG. The following three phases of the radioactivity-time curve were plotted: (a) the increase in (14)C during infusion of chylomicrons, (b) the steady-state phase during the infusion, and (c) the decay of (14)C after chylomicron infusion was stopped. The values for the anesthetized rats failed to reach a steady-state phase during the course of the experiment. From the disappearance of (14)C following the end of the infusion, the apparent half time of removal of chylomicron-TG was estimated to be 2.8 +/- 0.37 min in unanesthetized rats, 4.5 +/- 0.37 min in rats anesthetized with sodium pentobarbital, and 4.4 +/- 0.44 min in rats anesthetized with halothane. Thus, two anesthetics with different physical properties markedly slowed the removal of chylomicron-TG from the circulation. The reduced rate may have resulted from alterations in cardiac output or distribution of blood flow induced by the anesthetic agents.     

The use of 8-phenyltheophylline as a competitive antagonist of adenosine and an inhibitor of the intrinsic regulatory mechanism of the hepatic artery

Reduction of portal blood flow results in compensatory vasodilation of the hepatic artery, the hepatic arterial buffer response. The hypothesis tested is that the regulation of the buffer response is mediated by adenosine, where the local concentration of adenosine in the region of the hepatic arterial resistance vessels is regulated by washout of adenosine into portal venules that are in intimate contact with hepatic arterioles. In anesthetized cats, portal flow was reduced to zero by complete occlusion of all arterial supply to the guts. The resultant dilation of the hepatic artery compensated for 23.9 +/- 4.9% of the decrease in portal flow. Dose-response curves were obtained for the effect of intraportal adenosine infusion on hepatic arterial conductance in doses that did not lead to recirculation and secondary effects on the hepatic artery via altered portal blood flow. The dose to produce one-half maximal response for adenosine is 0.19 mg X kg-1 X min-1 (intraportal) and the estimated maximal dilation is equivalent to an increase in hepatic arterial conductance to 245% of the basal (100%) level. The adenosine antagonist, 8-phenyltheophylline, produced dose-related competitive antagonism of the dilator response to infused adenosine (but not to isoproterenol) and a similar, parallel antagonism of the hepatic arterial buffer response. If supramaximal blocking doses were used, the hepatic artery showed massive and prolonged constriction with blood flow decreasing to zero. The data strongly support the hypothesis that intrinsic hepatic arterial buffer response is mediated entirely by local adenosine concentration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of the temperature factor in determining the nature of the chronotropic response of the heart of cats in the simulation of increased venous inflow

The heart rate changes in response to the imitation of venous input to the heart by infusion of an additional blood volume to the right atria have been studied on anesthetized by chloralose cats. It has been discovered that patterns of the heart rate changes are determined by the infused blood temperature if it is equal to the blood temperature in heart, tachycardia appears, if it is below this temperature, bradycardia develops. These data explain, from our point of view, the contradictory results obtaining by different authors in experiments with the imitation of the increased venous input (Bainbridge reflex).     

Modulation of the kinetics and the steady-state level of intermediates of mitochondrial coupled reactions by inhibitors and uncouplers

In oxidative phosphorylation and ATP-driven uphill electron transfer from succinate to NAD, double-reciprocal plots of rates vs. substrate concentrations of the energy-driven reactions are a family of parallel lines at several fixed subsaturating concentrations of the substrates or at several moderate concentrations of the inhibitors of the energy-yielding reactions. Thus, as shown elsewhere [Hatefi, Y., Yagi, T., Phelps, D. C., Wong, S.-Y., Vik, S. B., & Galante, Y. M. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 1756-1760], partial uncoupling decreases the Vappmax and increases the Kappm of the substrates of the energy-driven reactions, resulting in a decrease of Vmax/Km as a function of increased uncoupling. However, partial limitation of the flow rates of the energy-yielding reactions decreases both the Vappmax and the Kappm of the substrates of the energy-driven reactions, resulting in no change in Vmax/Km. This is true as long as the rate limitation is moderate (e.g., less than 60%), under which conditions the steady-state membrane potential (delta psi) remains essentially unchanged. At high inhibition of the energy-yielding reactions, or at moderate inhibition in the presence of low levels of an uncoupler to cause partial uncoupling, then the family of double-reciprocal plots is no longer parallel and tends to converge toward the left. Under these conditions, steady-state delta psi and Vmax/Km also decrease as inhibition is increased. The relationship between the magnitude of steady-state delta psi and the rate of the energy-driven reaction was studied in oxidative phosphorylation, ATP-driven electron transfer from succinate to NAD, and respiration-driven uniport calcium transport by intact mitochondria.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of cardiac preload reduction and dobutamine on hepatosplanchnic blood flow regulation in porcine endotoxemia

Insufficient cardiac preload and impaired contractility are frequent in early sepsis. We explored the effects of acute cardiac preload reduction and dobutamine on hepatic arterial (Qha) and portal venous (Qpv) blood flows during endotoxin infusion. We hypothesized that the hepatic arterial buffer response (HABR) is absent during preload reduction and reduced by dobutamine. In anesthetized pigs, endotoxin or vehicle (n = 12, each) was randomly infused for 18 h. HABR was tested sequentially by constricting superior mesenteric artery (SMA) or inferior vena cava (IVC). Afterward, dobutamine at 2.5, 5.0, and 10.0 μg/kg per minute or another vehicle (n = 6, each) was randomly administered in endotoxemic and control animals, and SMA was constricted during each dose. Systemic (cardiac output, thermodilution) and carotid, splanchnic, and renal blood flows (ultrasound Doppler) and blood pressures were measured before and during administration of each dobutamine dose. HABR was expressed as hepatic arterial pressure/flow ratio. Compared with controls, 18 h of endotoxin infusion was associated with decreased mean arterial blood pressure [49 ± 11 mmHg vs. 58 ± 8 mmHg (mean ± SD); P = 0.034], decreased renal blood flow, metabolic acidosis, and impaired HABR during SMA constriction [0.32 (0.18-1.32) mmHg/ml vs. 0.22 (0.08-0.60) mmHg/ml; P = 0.043]. IVC constriction resulted in decreased Qpv in both groups; whereas Qha remained unchanged in controls, it decreased after 18 h of endotoxemia (P = 0.031; constriction-time-group interaction). One control and four endotoxemic animals died during the subsequent 6 h. The maximal increase of cardiac output during dobutamine infusion was 47% (22-134%) in controls vs. 53% (37-85%) in endotoxemic animals. The maximal Qpv increase was significant only in controls [24% (12-47%) of baseline (P = 0.043) vs. 17% (-7-32%) in endotoxemia (P = 0.109)]. Dobutamine influenced neither Qha nor HABR. Our data suggest that acute cardiac preload reduction is associated with preferential hepatic arterial perfusion initially but not after established endotoxemia. Dobutamine had no effect on the HABR.     

Hepatic venoconstrictor effects of isoproterenol and nifedipine in anesthetized cats

In cats anesthetized with pentobarbital, isoproterenol infused into a peripheral vein causes a reduction in hepatic blood volume measured by plethysmography. As this response is accompanied by increases in portal and hepatic lobar venous pressures, the decrease in hepatic volume cannot be a passive emptying secondary to reduced intrahepatic pressure. We conclude that intravenous isoproterenol causes an active hepatic venoconstriction. Nifedipine produced similar responses. From this and our previous data, we conclude that in anesthetized cats, arteriolar vasodilators which increase cardiac output cause hepatic venoconstriction (hydralazine, adrenaline, dopamine, isoproterenol, and nifedipine), while those which do not increase cardiac output have no effect on the hepatic venous bed (nitroprusside and diazoxide) or cause venodilatation (nitroglycerine). The mechanism of the hepatic venoconstrictor effect of isoproterenol was investigated further. Because previous work has shown that this response does not occur when isoproterenol is infused locally into the hepatic artery or portal vein, the venoconstrictor effect of peripheral intravenous infusions must be indirectly mediated. The response was still present after hepatic denervation, adrenalectomy, nephrectomy, and after indomethacin administration indicating it is not mediated by the hepatic nerves, adrenal catecholamines, the renal renin-angiotensin system, or prostaglandins. The mechanism remains unknown.     

Changes in Regional Blood–Brain Transfer of l-Leucine Elicited by Arginine-Vasopressin

Arginine-vasopressin (AVP), injected into the carotid artery in physiological concentration together with L-leucine, changed kinetic constants of the blood-brain barrier (BBB) transport of this neutral amino acid without changing the cerebral blood flow (CBF). The maximum velocity of transport (Vmax), the half-saturation constant (Km), the nonsaturable transport constant (KD), and CBF were estimated in nine brain regions of male Wistar rats anesthetized with ether. In cerebral hemisphere, Vmax decreased from 21 nmol . min-1 . g-1 (control) to 7.6 nmol . min-1 . g-1 (AVP). Km decreased from 0.11 to 0.029 mM. Regional differences of the kinetic constants were found in controls as well as in AVP-treated animals. In all regions, the calculated constants Vmax and Km of animals coinjected with AVP were significantly decreased when compared to controls. A direct or indirect interaction of AVP with the transport system of large neutral amino acids is suggested.     

Epinephrine-induced hepatic potassium movements before and after adrenergic blockade.

The epinephrine-induced loss and subsequent uptake of K+ by the liver was studied by measuring hepatic arterio-venous K+ differences and splanchnic blood flows in anesthetized dogs with chronically implanted portal vein catheters and celiac and superior mesenteric artery flow probes. When epinephrine was administered intraportally, neither alpha- nor beta-adrenergic blockade, singly or in combination, had significant effects upon the hyperkalemic or the hypokalemic phases in either hepatic venous or systemic arterial blood. It was concluded that the movements of K+ into and out of the liver caused by epinephrine are not mediated by the classical adrenergic receptors as defined by inhibition by specific blocking agents.     

Effects of bromocryptine on hepatic blood volume responses to hepatic nerve stimulation in cats

Arterial pressures, portal pressures, and hepatic blood volumes were recorded after hepatic denervation in cats anesthetized with pentobarbital. Bromocryptine (50 micrograms/kg) lowered arterial pressure but did not significantly change portal pressure or hepatic blood volume. However, both portal pressure and hepatic blood volume responses to hepatic nerve stimulation were significantly depressed after bromocryptine especially at low frequencies of stimulation. Responses to intraportal infusions of norepinephrine were significantly impaired only at the highest dose. The inhibitory effect of bromocryptine on the neural responses may, therefore, involve a presynaptic inhibition of norepinephrine release, but the mechanism requires further study. These data provide further support for the hypothesis that drugs which impair hepatic venous responses to sympathetic stimuli cause significant impairment of postural reflexes and orthostatic hypotension during clinical use.     

Dopamine and hepatic oxygen supply-demand relationship

The present study examined the effect of small, vasodilating doses of dopamine on the hepatic oxygen supply--uptake ratio. Thirteen miniature pigs weighing 18-27 kg were studied under sodium pentobarbital anesthesia. Hepatic arterial and portal blood flows were measured. Oxygen content in arterial, portal, and hepatic venous blood was determined. Dopamine was infused in doses of 5, 10, and 15 micrograms.kg-1.min-1. Dopamine infusion was associated with a dose-related increase in hepatic oxygen uptake and a dose-independent increase in hepatic oxygen delivery with a maximal increase (30%) in the hepatic oxygen delivery at 10 micrograms.kg-1.min-1. The hepatic oxygen delivery--uptake ratio remained unchanged during dopamine infusion in doses of 5 and 10 micrograms.kg-1.min-1 and significantly decreased during the dose of 15 micrograms.kg-1.min-1. The study demonstrated that an increase in cardiac output and hepatic oxygen delivery during dopamine administration was not associated with an improvement in hepatic oxygen supply--demand relationship since hepatic oxygen uptake also increased.     

Kupffer Cell Function in Thyroid Hormone-Induced Liver Oxidative Stress in the Rat

The influence of thyroid hormone (L-3, 3', 5-triiodothyronine, T₃) on Kupffer cell function was studied in the isolated perfused rat liver by colloidal carbon infusion. Rates of carbon uptake were determined from the influent minus effluent concentration difference and the flow rate, and the respective carbon-induced respiratory activity was calculated by integration of the area under the O₂ curves during carbon infusion. In the concentration range of 0.2 to 2.0 mg of carbon/ml, livers from euthyroid rats exhibited a sigmoidal-type kinetics of carbon uptake, with a Vmax of 4.8 mg/g liver/min and a concentration of 0.82 mg/ml for half-maximal rate; carbon-induced O₂ uptake presented a hyperbolic-type kinetics, with a Vmax of 4.57 μmol of O₂/g liver and a Km of 0.74 mg of carbon/ml, which significantly correlates with the carbon uptake rates. Light-microscopy showed that carbon was taken up exclusively by non-parenchymal cells, predominantly by Kupffer cells. Thyroid calorigenesis was found in parallel with increased rates of hepatic O₂ consumption and thiobarbituric acid reactive substances (TBARS) formation, glutathione (GSH) depletion, and higher sinusoidal lactate dehydrogenase (LDH) efflux compared to control values. In the concentration range of 0.25 to 0.75 mg/ml, carbon infusion did not modify liver LDH efflux in control rats, while it was significantly enhanced in T₃-treated animals. In this latter group, higher carbon concentrations (1 and 1.3 mg/ml) led to loss of viability of the liver. At 0.25 to 0.75 mg of carbon/ml, both the rates of carbon uptake and the associated carbon-induced respiratory activities were significantly increased by T₃ treatment, effects that were abolished by pretreatment of the rats with gadolinium chloride (GdCl₃). In addition, GdCl₃ decreased by 50% the changes induced by T₃ in hepatic GSH content and TBARS formation. It is concluded that hyperthyroidism enhances Kupffer cell function, correlated with the increased number of liver macrophages observed histologically, which may represent an alternate source of reactive O₂ species to that induced in parenchymal cells, thus contributing to the enhanced oxidative stress status developed.     

Effect of human epidermal growth factor (hEGF) on splanchnic circulation in dogs

The effect of intravenous administration of human epidermal growth factor on the splanchnic blood flows was examined in anesthetized dogs, using an ultrasonic transit-time volume flow meter. Human epidermal growth factor (0.1, 0.5 and 1 microgram/kg) significantly increased blood flows in the portal vein (36.9 +/- 7.4% at 1 microgram/kg) and the superior mesenteric artery (49.0 +/- 16.8% at 1 microgram/kg). Systemic blood pressure monitored simultaneously was significantly decreased (8.4 +/- 1.2% at 1 microgram/kg). This study is the first to demonstrate that intravenous administration of epidermal growth factor increases the portal venous blood flow.