Insulin resistance induced by sucrose feeding in rats is due to an impairment of the hepatic parasympathetic nerves

AIMS/HYPOTHESIS

A considerable proportion of whole-body insulin-stimulated glucose uptake is dependent upon the hepatic insulin-sensitising substance (HISS) in a pathway mediated by the hepatic parasympathetic nerves (HPNs). We tested the hypothesis that a high-sucrose diet leads to the impairment of the HPN-dependent component of insulin action.

METHODS

We quantified insulin sensitivity using the rapid insulin sensitivity test, a modified euglycaemic clamp. Quantification of the HPN-dependent component was achieved by administration of a muscarinic receptor antagonist (atropine, 3 mg/kg).

RESULTS

Insulin sensitivity was higher in standard-fed than in sucrose-fed Wistar rats (305.6+/-34.1 vs 193.9+/-13.7 mg glucose/kg body weight; p<0.005) and Sprague-Dawley rats (196.4+/-5.9 vs 95.5+/-16.3 mg glucose/kg body weight; p<0.01). The HPN-independent component was similar in the two diet groups. Insulin resistance was entirely due to an impairment of the HPN-dependent component in both Wistar rats (164.3+/-28.1 [standard-fed] vs 26.5+/-7.5 [sucrose-fed] mg glucose/kg body weight; p<0.0001) and Sprague-Dawley rats (111.7+/-9.5 vs 35.3+/-21.4 mg glucose/kg body weight; p<0.01). Furthermore, HPN-dependent insulin resistance in Sprague-Dawley rats was already evident after 2 weeks of a high-sucrose diet (28.5+/-7.6 [2 weeks], 35.3+/-21.4 [6 weeks], 17.9+/-5.4 [9 weeks] mg glucose/kg body weight) and was independent of the nature of sucrose supplementation (12.3+/-4.7 [solid] and 17.9+/-5.4 [liquid] mg glucose/kg body weight).

CONCLUSIONS/INTERPRETATION

Our results support the hypothesis that insulin resistance caused by sucrose feeding is due to an impairment of the HPN-dependent component of insulin action, leading to a dysfunction of the HISS pathway.

Hepatic parasympathetic (HISS) control of insulin sensitivity determined by feeding and fasting

In response to insulin, a hormone [hepatic insulin sensitizing substance (HISS)] is released from the liver to stimulate glucose uptake in skeletal muscle but not liver or gut. The aim was to characterize dynamic control of HISS action in response to insulin and regulation of release by hepatic parasympathetic nerves. Insulin action was assessed by the rapid insulin sensitivity test, where the index is the glucose required (mg/kg) to maintain euglycemia after a bolus of insulin. Blocking HISS release by interruption of the hepatic parasympathetic nerves by surgical denervation, atropine, or blockade of hepatic nitric oxide synthase produced similar degrees of insulin resistance and revealed a similar dynamic pattern of hormone action that began 3--4 min after, and continued for 9--10 min beyond, insulin action (50 mU/kg). HISS action accounted for 56.5 +/- 3.5% of insulin action at insulin doses from 5 to 100 mU/kg (fed). We also tested the hypothesis that HISS release is controlled by the feed/fast status. Feeding resulted in maximal HISS action, which decreased progressively with the duration of fasting.

Rapid insulin sensitivity test (RIST)

A rapid insulin sensitivity test (RIST) was recently introduced to assess insulin action in vivo (H. Xie, L. Zhu, Y.L. Zhang, D.J. Legare, and W.W. Lautt. J. Pharmacol. Toxicol. Methods, 35: 77-82. 1996). This technical report describes the current recommended standard operating procedure for the use of the RIST in rats based upon additional experience with approximately 100 tests. We describe the manufacture and use of an arterial-venous shunt that allows rapid multiple arterial samples and intravenous administration of drugs. The RIST procedure involves determination of a stable arterial glucose baseline to define the ideal euglycemic level to be maintained following a 5-min infusion of insulin, with the RIST index being the amount of glucose required to be infused to maintain euglycemia over the test period. Insulin administration by a 5-min infusion is preferable to a 30-s bolus administration. No significant difference was determined between the use of Toronto pork-beef or human insulin. Four consecutive RISTs were carried out in the same animal over 4-5 h with no tendency for change with time. The RIST index is sufficiently sensitive and reproducible to permit establishment of insulin dose-response curves and interference of insulin action by elimination of hepatic parasympathetic nerves, using atropine. This technical report provides the current recommended standard operating procedure for the RIST.

Insulin sensitivity tested with a modified euglycemic technique in cats and rats

A new insulin sensitivity test (IST) is described using a modified euglycemic clamp in cats and rats. The IST uses the amount of glucose required to be infused to maintain euglycemia over a 30-min period in rats and 60 min in cats following a bolus administration of insulin as the index of insulin sensitivity. Glucose levels are determined at short time intervals (2-5 min), and variable glucose infusion is used to hold glucose levels within a few percentage points of the basal pre-test glucose level. A new blood sampling procedure is described that allows each IST to be carried out using a total of only 0.5 mL of blood. The IST is sensitive and allows clear insulin dose effects to be demonstrated with 100 mU/kg requiring 355.0 +/- 14.3 mg/kg over 30 min and 50 mU/kg requiring 198.7 +/- 11.1 mg/kg. Five consecutive tests were reproducibly carried out (%CV = 3.0 +/- 0.5) over a 12-hr period in the cat with insulin, glucagon, and glucose levels remaining stable prior to each IST. Glucagon and norepinephrine plasma concentrations do not change significantly during the IST. The IST is sufficiently sensitive to allow demonstration of dose-response relationships for atropine-induced insulin resistance. The IST is thus sensitive, reproducible, and able to demonstrate acute insulin resistance in anesthetized cats and rats. The test is demonstrated in fed (rats) and fasted (cats) state.

Evaluation of vascular tone in portacaval shunts comparing the index of contractility and resistance in cats

A model of prehepatic chronic portal hypertension in cats was used to determine portacaval shunt responses to infused norepinephrine and to possible transmitter overflow into portal blood from nerves supplying the gut. Responses are compared using a new index of contractility. Four weeks after application of a slowly constricting occluder, the portal vein was completely occluded and acute experiments were carried out under pentobarbital anesthesia. Portal pressure was elevated to 15.0 +/- 0.9 mmHg and all portal flow passed through the shunts. In response to intraportal norepinephrine (0.25, 0.5 and 1.25 micrograms.min-1.kg-1) shunt resistance rose by 6% +/- 3%, 19% +/- 4% and 26% +/- 5%, respectively, whereas the index of contractility rose (by 22% +/- 8%, 46% +/- 10% and 89% +/- 20%, respectively), the distending blood pressure also rose (5% +/- 1%, 7% +/- 1% and 14% +/- 3%, respectively). The difference in percentage increase of resistance and the index of contractility is a result of the passive dilator effect of the elevated distending pressure acting on the distensible shunt vessels. Stimulation of mesenteric nerves caused the mesenteric artery to constrict, but the shunt vessels showed no effect. In conclusion, the shunt vessels respond actively to norepinephrine and passively to altered distending pressure. However, transmitter overflow from nerves supplying the intestines is unlikely to play a role in determining resistance in the shunts. Vascular resistance is affected by both active and passive effects, so that the active contractile responses are best evaluated using the index of contractility, which is not altered passively.

Distensibility of portacaval shunts in portal hypertensive cats: index of contractility model

Complete shunting of portal blood flow through portacaval shunts was obtained using a constrictor around the portal vein to gradually produce a total occlusion. After 4 weeks, acute experiments were conducted in anesthetized cats. Blood from the femoral artery was shunted through a pump to supply and control the entire portal blood flow. As shunted portal blood flow was varied over a wide range, the portal shunt resistance showed distensibility. Decreasing portal venous pressure from 15.0 +/- 0.9 to 11.1 +/- 0.6 mmHg (1 mmHg = 133.3 Pa) resulted in elevations of resistance of 58%. The relation between the resistance (R) and the distending pressure (Pd) was a constant, the index of contractility (IC), where IC = R.Pd3. In steady state, the IC was 485 +/- 55 mmHg4.mL-1.min.kg and did not change passively in response to changes in portal blood flow. In conclusion, portacaval shunts are passively distensible, and resistance is altered as a cubic function of the distending pressure. Because resistance is altered both actively and passively, the IC should prove useful to differentiate these alternatives for evaluation of changes in portal hypertensive therapy.

Hepatic blood flow distribution: consideration of gravity, liver surface, and norepinephrine on regional heterogeneity

Blood flow distribution within the livers of cats and dogs was assessed using 15-microns microspheres injected into the hepatic artery and portal vein. Representative vertical core samples (n = 11-18) were taken from the thickest part of each liver. Heterogeneity was assessed in several ways. The difference in total flow to different lobes was greater in dogs than in cats, and in dogs, those lobes with highest portal venous flow had lowest hepatic arterial flow. Overall flow variance was very high in both species, with adjacent surface samples in a single lobe showing variance of 15-22% for both vessels. The ratio of highest to lowest flow within core samples averaged 2.1-3.4 for both vessels in both species. The hepatic arterial flow was highest to the surface 2 mm of the liver. Portal flow most often (31% of all samples) showed a pattern of highest flow to the top, graduating down to lowest flow to the bottom (dorsal side) of the vertical cores. However, this pattern appeared much more frequently in the most ventral liver lobes and very seldom in the lobes lying beneath the liver mass. Norepinephrine reduced heterogeneity. Hepatic arterial occlusion for 10 min produced minor and inconsistent reduction of heterogeneity. Rotating cats from back to front and again to back disrupted patterns of distribution but not in a way that could be interpreted as due to effects of gravity. Flow patterns changed with time. The heterogeneity of perfusion appears to be under dynamic and multiple interacting forces.

Passive autoregulation of portal venous pressure: distensible hepatic resistance

Hepatic resistance to portal blood flow is extremely low and both the pre- and postsinusoidal resistance sites are distensible. Both isolated in situ and in vivo vascular circuitry were used in cats to demonstrate the principle of distensible resistance as a mechanism for the observation that blood flow was able to be decreased from 50 to 20 ml.min-1 x kg-1 while intrahepatic pressure decreased by only 1.4 +/- 0.2 mmHg and portal pressure by 2.0 +/- 0.4 mmHg. Presinusoidal resistance increased by 226% and hepatic venous resistance by 57%, thus accounting for passive autoregulation of portal pressure. The relation between vascular resistance and the distending blood pressure that acts on the resistance is predictable from the relationship IC = R.Pd3, where IC is the index of contractility (does not change passively, but does change with active vascular tone changes), R is vascular resistance (changes actively and passively), and Pd is distending blood pressure (estimated as the average of pressure on either side of the resistance vessels). The relatively minor effect of portal flow on portal pressure is accounted for by a combination of factors including the low basal resistance, the distensible resistance, the hepatic arterial buffer response, and hepatic blood volume compliance. By calculation of IC, the venous distensibility can be quantified and the passive effect of flow changes on portal and intrahepatic pressure determined.

Evaluation of hepatic venous resistance responses using index of contractility

Hepatic vascular responses to 1.25 micrograms.kg-1.min-1 norepinephrine, infused into the hepatic artery, and 8-Hz nerve stimulation were monitored in anesthetized cats using a recently introduced index of contractility (IC). IC was validated in that it did not change passively in response to passive changes in portal flow or distending blood pressure, whereas the distensible venous resistance sites showed dramatic changes in resistance. Resistance is altered by both active contractile responses and passive distensibility; IC is not altered passively but is affected by changes in vascular tone. Resistance was a less sensitive index of vasoconstriction because, although the constriction increased resistance, the subsequent elevation in portal and intrahepatic pressure counteracted the constriction; the extent of active neurogenic response using resistance as the index was grossly underestimated due to venous distensibility. IC showed that pre- and postsinusoidal constriction occurred to both norepinephrine and nerves; extensive vascular escape from neurogenic constriction occurred for the portal vein so that by 5 min almost all the rise in portal pressure was due to hepatic venous constriction.

Dilazep potentiation of adenosine-mediated superior mesenteric arterial vasodilation

The present study investigated the effects of dilazep, an inhibitor of adenosine uptake, on adenosine-mediated vasodilation in vivo. Intravenous and intraportal venous infusions of exogenous adenosine (0.04-1.0 mg/kg/min) did not recirculate to cause increases in superior mesenteric arterial conductance (SMAC) or arterial plasma adenosine levels except at the higher doses tested (0.4-1.0 mg/kg/min). After administration of dilazep, however, even low doses (0.04-0.1 mg/kg/min) of exogenous adenosine significantly increased SMAC and elevated arterial plasma adenosine concentration. The increased adenosine levels were highly correlated with the increased percentage of change of SMAC and values for Rmax and EC50 were 193.4 +/- 27.3% change of SMAC and 2.8 +/- 1.3 microM, respectively. Administration of bolus doses of 8-phenyltheophylline abolished the ability of dilazep to potentiate vasodilation, but did not affect isoproterenol-induced relaxation. Together, these results suggest that potentiation of the vasodilating effect of exogenous adenosine by dilazep is mediated through inhibition of adenosine uptake in vivo which increases the availability of plasma adenosine to act on adenosine receptors.

Index of contractility: quantitative analysis of hepatic venous distensibility

The low-pressure resistance vessels of the splanchnic circulation are passively distensible, and changes in regional blood pressures can lead to large changes in vascular resistance. The relationship between distending blood pressure (Pd) and vascular resistance (R) is described as a constant, the index of contractility (IC) where IC = R x Pd3. IC was derived in an isolated blood-perfused liver and was confirmed in vivo for both pre- and postsinusoidal resistance sites. IC does not change passively in response to wide changes in blood flow or hepatic outflow pressure. IC is dramatically altered in response to active vasoconstriction. In vivo, the presinusoidal IC rose from a control level of 12.2 +/- 4.2 to 92.7 +/- 20.6 IC units (mmHg4.ml-1.min.kg body wt) in response to 1.25 micrograms.kg-1.min-1 norepinephrine intraportal; the postsinusoidal IC rose from 20.4 +/- 2.3 to 59.6 +/- 14.2 IC units. IC reflects resistance changes secondary to active contractile responses independent of the passive consequences of the distensible nature of the resistance sites. We suggest that these concepts can be applied to any vascular bed with distensible resistance vessels.

Effect of adenosine and glucagon on hepatic blood volume responses to sympathetic nerves

Hepatic blood volume responses were studied in cats using in vivo plethysmography. The maximal response (Rmax) to sympathetic nerve stimulation and to infusions of norepinephrine into the hepatic artery or portal vein was similar (12-14 mL expelled per liver in 2.9-kg cats; average liver weight, 76.8 +/- 6.8 g). The ED50 for norepinephrine intraportal (0.44 +/- 0.13) and intrahepatic arterial infusions (0.33 +/- 0.08 micrograms.kg-1.min-1) were similar indicating equal access of both blood supplies to the capacitance vessels. Adenosine (2.0 mg.kg-1.min-1) did not cause significant volume changes but produced a mild (27%) suppression of Rmax due to nerve stimulation with no change in the frequency (3.4 Hz) needed to produce 50% of Rmax. Rmax tended (not statistically significant) to decrease during glucagon (1.0 micrograms.kg-1.min-1) infusion but the nerve frequency needed to produce 50% of Rmax rose to 5.6 Hz. Thus both adenosine and glucagon produced modulation of sympathetic nerve-induced capacitance responses without having significant effects on basal blood volume. Adenosine, by virtue of its marked effects on arterial resistance vessels (at substantially lower doses than those used here) and the relative lack of effect on venous capacitance vessels, may be useful for producing clinical afterload reduction without venous pooling.

Evaluation of hepatic venous balloon occluder to estimate portal pressure

The hepatic venous balloon occluder method of estimating portal venous pressure (PVP) was evaluated in cats and dogs, during basal state and active vasoconstriction, and during passive presinusoidal resistance elevation in cats. In the dog, the balloon catheter measured a pressure not different from PVP when the balloon was inflated both in basal state and during active vasoconstriction induced by hepatic nerve stimulation, intraportal infusion of histamine or norepinephrine, regardless of whether the balloon was distal or proximal to hepatic venous sphincters. In the cat, the inflated balloon measures a pressure not different from PVP in basal state but slightly overestimated PVP during nerve stimulation or norepinephrine infusion in some protocols. Blood clots were injected intraportally in cats to produce a pure, passive presinusoidal resistance as shown by unchanged intrahepatic pressure but elevated PVP. The balloon method accurately measured PVP in this condition and clearly cannot differentiate pre- from postsinusoidal resistance sites in cats or dogs. The balloon method and the classical wedged pressure method will represent PVP when resistance is primarily postsinusoidal; they provide different measurements when resistance is presinusoidal, the wedged method representing intrahepatic pressure but the balloon method reflecting portal pressure. These differences require confirmation in human presinusoidal cirrhosis.

Quantitation of the hepatic arterial buffer response to graded changes in portal blood flow

Hepatic arterial blood flow changes inversely in response to altered portal blood flow. The hepatic arterial capacity to buffer portal flow changes was studied over a wide range of portal flow with arterial pressure held steady (the active buffer response) or uncontrolled. The active component of the buffer response led to nearly full dilation of the hepatic artery at low portal flows as shown by inability to dilate further in response to adenosine infusion; at high portal flows the hepatic artery was nearly fully constricted as shown by lack of further constriction to norepinephrine. With pressure uncontrolled, active and passive effects combined to produce an increased compensation with similar efficiency (44% +/- 4%) over the full range of portal blood flows. Thus, although the active component of the hepatic arterial buffer response becomes less efficient at very high and low portal flows, the combination of active and passive effects leads to a larger buffer capacity which is equally efficient over a wide range of portal blood flow changes.

Vascular escape from vasoconstriction and post-stimulatory hyperemia in the superior mesenteric artery of the cat

Vascular escape is seen as a partial recovery from initial vasoconstriction despite continued constrictor stimuli. Escape in the feline intestine (superior mesenteric artery) occurred for i.a. norepinephrine (NE) infusions (56% escape for low dose, 40% for high dose NE) and for sympathetic nerve stimulation (SNS) (65% for 1 Hz, 49% for 3 Hz, 44% for 9 Hz). Adenosine infusion or blockade of adenosine receptors (8-phenyltheophylline) did not alter the escape, showing that endogenous adenosine levels are unlikely to play any role in the mechanism of escape. Other aspects of escape were studied: equiconstrictor doses of NE given i.a. or i.v. lead to similar degrees of escape; propranolol and ouabain did not alter escape; the degree of escape was significantly greater for the low dose NE and the 1-Hz SNS than for higher intensities of stimulation, however, escape did not inversely correlate significantly with the initial degree of vasoconstriction when all data were pooled. Post-stimulatory hyperemia occurs upon cessation of vasoconstrictor stimuli, reaches a peak conductance within 1 min, and returns to baseline within about 3 min. Hyperemia was quantitated from the peak vasodilation and from the area under the flow-hyperemia curve. The hyperemias were not related to NE dose or SNS frequency nor did they correlate with initial vasoconstriction or extent of vascular escape. Contrary to the hypothesis that adenosine may mediate hyperemia, adenosine infusions reduced the response and adenosine receptor antagonism tended to elevate the response. Propranolol and ouabain did not produce significant effects on post-stimulatory hyperemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine modulation of vasoconstrictor responses to stimulation of sympathetic nerves and norepinephrine infusion in the superior mesenteric artery of the cat

Vasoconstriction induced by sympathetic nerve stimulation and by norepinephrine infusion in the superior mesenteric artery of cats anesthetized with pentobarbital was inhibited by adenosine infusions in a dose-related way. The responses to nerve stimulation were not inhibited to a greater extent than the responses to norepinephrine, thus suggesting no presynaptic modulation of sympathetic nerves supplying the resistance vessels of the feline intestinal vascular bed. Blockade of adenosine receptors using 8-phenyltheophylline did not alter the degree of constriction induced by nerve stimulation or norepinephrine infusion, indicating that in the fasted cat, endogenous adenosine co-released or released subsequent to constriction does not affect the peak vasoconstriction reached. Isoproterenol caused similar degrees of vasodilation as adenosine but did not show significant antagonism of the pooled responses to nerve stimulation or norepinephrine infusion; there was no tendency for the degree of dilation induced by isoproterenol to correlate with the inhibition of constrictor responses. Thus, the effect of adenosine on nerve- and norepinephrine-induced constriction is not secondary to nonspecific vasodilation.

Effect of hepatic venous sphincter contraction on transmission of central venous pressure to lobar and portal pressure

In dogs anesthetized with pentobarbital, central vena caval pressure (CVP), portal venous pressure (PVP), and intrahepatic lobar venous pressure (proximal to the hepatic venous sphincters) were measured. The objective was to determine some characteristics of the intrahepatic vascular resistance sites (proximal and distal to the hepatic venous sphincters) including testing predictions made using a recent mathematical model of distensible hepatic venous resistance. The stimulus used was a brief rise in CVP produced by transient occlusion of the thoracic vena cava in control state and when vascular resistance was elevated by infusions of norepinephrine or histamine, or by nerve stimulation. The percent transmission of the downstream pressure rise to upstream sites past areas of vascular resistance was elevated. Even small increments in CVP are partially transmitted upstream. The data are incompatible with the vascular waterfall phenomenon which predicts that venous pressure increments are not transmitted upstream until a critical pressure is overcome and then further increments would be 100% transmitted. The hepatic sphincters show the following characteristics. First, small rises in CVP are transmitted less than large elevations; as the CVP rises, the sphincters passively distend and allow a greater percent transmission upstream, thus a large rise in CVP is more fully transmitted than a small rise in CVP. Second, the amount of pressure transmission upstream is determined by the vascular resistance across which the pressure is transmitted. As nerves, norepinephrine, or histamine cause the hepatic sphincters to contract, the percent transmission becomes less and the distensibility of the sphincters is reduced. Similar characteristics are shown for the "presinusoidal" vascular resistance and the hepatic venous sphincter resistance.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of histamine, norepinephrine, and nerves on vascular pressures in dog liver

In the control state, lobar venous pressure (LVP) measured proximal to a hepatic venous sphincter in dog liver (9.9 +/- 0.3 mmHg) is insignificantly different from portal venous pressure (PVP = 9.9 +/- 0.3 mmHg). Essentially all of the pressure drop occurs across the hepatic veins. Intraportal infusion of histamine constricts the hepatic venous sphincter and leads to similar elevations of LVP and PVP, indicating that all of the rise in PVP (except at small doses = 1 microgram X kg-1. min-1) can be accounted for by hepatic venous sphincter constriction. Norepinephrine at doses from 0.25 to 1.25 micrograms X kg-1. min-1 (intraportal) caused both hepatic venous sphincter constriction and constriction proximal to hepatic venous sphincters to roughly equal proportions, with approximately 44% of the rise in PVP due to hepatic sphincter constriction. Hepatic nerves activated both resistance sites, with 90% of the rise in PVP due to hepatic venous constriction at 2 Hz stimulation. By 4 Hz stimulation, the postsinusoidal sphincters were nearly maximally activated, but the "presinusoidal" resistance continued to increase until, at 10 Hz, the hepatic venous sphincter component accounted for only 59% of the rise in PVP. The proportion of PVP rise accounted for by hepatic venous sphincter resistance was not significantly altered by prior occlusion of the hepatic artery.

Hepatic venous resistance site in the dog: localization and validation of intrahepatic pressure measurements

Intrahepatic pressure (9.4 +/- 0.3 mmHg; 1 mmHg = 133.32 Pa), measured proximal to a hepatic venous resistance site, was insignificantly different from portal venous pressure (9.6 +/- 0.4 mmHg). This lobar venous pressure is not wedged hepatic venous pressure as it is measured from side holes in a catheter with a sealed tip. Validation of the lobar venous pressure measurement was done in a variety of ways and using different sizes and configurations of catheters. The site of hepatic venous resistance in the dog is localized to a narrow sphincterlike region about 0.5 cm in length and within 1-2 cm (usually within 1 cm) of the junction of the vena cava and hepatic veins. Sinusoidal and portal venous resistance appears insignificant in the basal state and large increases in liver blood volume (histamine infusion or passive vena caval occlusion) or large decreases in liver blood volume (passive vascular occlusion) do not alter the insignificant pressure gradient between portal and lobar venous pressures. Norepinephrine infusion (1.25 microgram X kg-1 X min-1 intraportal) and hepatic sympathetic nerve stimulation (10 Hz) led to a significantly greater rise in portal venous pressure than in lobar venous pressure, indicating some presinusoidal (and (or) sinusoidal) constriction and this indicates that lobar venous pressure cannot be assumed under all conditions to accurately reflect portal pressure. However, most of the rise in portal venous pressure induced by intraportal infusion of norepinephrine or nerve stimulation and virtually all of the pressure rise induced by histamine could be attributed to the postsinusoidal resistance site.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hepatic nerves, norepinephrine, angiotensin, and elevated central venous pressure on postsinusoidal resistance sites and intrahepatic pressures in cats

Portal venous pressure was controlled by resistance localized to specific sites in hepatic lobar veins in cats. All of the pressure drop from the portal vein to the vena cava occurred across postsinusoidal vessels; portal pressure, lobar venous pressure, and, therefore, sinusoidal pressure were not significantly different. Norepinephrine and angiotensin infusions (intraportal) caused elevation in portal pressure due to constriction of hepatic venous resistance sites as well as some constriction of presinusoidal (portal or sinusoidal) resistance sites. At low doses of norepinephrine presinusoidal constriction dominated whereas at higher doses the postsinusoidal constriction increased proportionately more. Hepatic nerve stimulation produced a similar response measured at an early time (1 min), but by 3 min the presinusoidal constriction showed complete escape so that elevated portal pressure was entirely due to hepatic venous constriction. The same site that provided basal vascular resistance also provided the increased hepatic venous resistance with nerve stimulation and infusion of angiotensin and norepinephrine. Rapid elevation of central venous pressure (CVP) caused elevated sinusoidal pressure. At high CVP (16 mm Hg), 75% of a rise in CVP was transmitted whereas at normal CVP (less than 4.5 mm Hg) less than 20% transmission occurred. The presence of a high resistance in the hepatic veins protected intrahepatic pressure from the effects of normal fluctuation of CVP.

Localization of intrahepatic portal vascular resistance

The pressure drop from the portal vein to the vena cava occurs primarily across a postsinusoidal site localized to a narrow segment (less than 0.5 cm) of hepatic veins (roughly 1.5 mm diam) in the anesthetized cat. Portal venous pressure (PVP = 8.9 +/- 0.3 mmHg) and lobar hepatic venous pressure (LVP = 8.7 +/- 0.4 mmHg) are insignificantly different, and pressure changes imposed from the presinusoidal or postsinusoidal side are equally transmitted to both pressure sites. Several types of experiments were done to validate the LVP measurement. The portal vein, hepatic sinusoids, and hepatic veins proximal to the resistance site are all under a similar pressure. Previously reported calculations of hepatic vascular resistance are in error because of incorrect assumptions of sinusoidal pressure and localization of the portal resistance site as presinusoidal. Stimulation of hepatic sympathetic nerves for 3 min caused LVP and PVP to increase equally, showing that the increased "portal" resistance is postsinusoidal across the same region of the hepatic veins that was previously localized as the site of resistance in the basal state.

Adenosine modulation of hepatic arterial but not portal venous constriction induced by sympathetic nerves, norepinephrine, angiotensin, and vasopressin in the cat

Intrinsic regulation of hepatic arterial blood flow depends upon local concentrations of adenosine. The present data show that i.a. infusions of adenosine cause dilation of the hepatic artery and inhibition of arterial vasoconstriction induced by norepinephrine, vasopressin, angiotensin, and hepatic nerve stimulation. Vasoconstriction induced by submaximal nerve stimulation (2 Hz) and norepinephrine infusions (0.25 and 0.5 micrograms X kg-1 X min-1, i.p.v.) were equally inhibited by adenosine. Supramaximal nerve stimulation (8 Hz) was inhibited to a lesser extent. The data are consistent with the hypotheses that (a) adenosine causes nonselective inhibition of vasoconstrictor influences on the hepatic artery; and (b) adenosine antagonizes neurally induced vasoconstriction by a purely postsynaptic effect and does not decrease norepinephrine release. In contrast with the hepatic artery, the intrahepatic portal resistance vessels are not affected by even large doses of adenosine; neither responses in basal tone nor antagonism of vasoconstrictor effects of nerve stimulation, norepinephrine, or angiotensin could be demonstrated. The data are consistent with the hypothesis that the smooth muscle of the portal resistance vessels does not contain adenosine receptors, whereas adenosine receptors on the smooth muscle of the hepatic arterial resistance vessels are of major regulatory importance. Whether endogenous levels of adenosine can reach sufficient concentration to modulate endogenous constrictors remains to be determined.

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)

Adenosine as putative regulator of hepatic arterial flow (the buffer response)

In anesthetized cats, reduction of portal flow by occlusion of the superior mesenteric artery results in rapid increase in hepatic arterial (HA) flow that compensates for (buffers) 25.5 +/- 2.7% of the decreased portal flow. The hypothesis tested is that adenosine concentration produced near the HA resistance vessels is regulated by washout into portal vessels in intimate contact with the HA. Reduced portal flow leads to accumulation of adenosine and HA dilation. Several criteria for this hypothesis are met. First, adenosine is a potent dilator of the HA. Second, portal blood has access to HA resistance vessels as shown by a marked dilator effect of adenosine infused into the portal vein; it is therefore possible for adenosine produced locally to diffuse into portal blood. Third, dipyridamole potentiated the dilator response to adenosine as well as potentiating the buffer response from a 23% compensation for reduced portal flow to 34%. Fourth, 1-methyl-3-isobutylxanthine (MIX) antagonized exogenous adenosine and reduced the buffer response from 19% down to 5%. These data strongly support the hypothesis that the hepatic arterial buffer response is mediated by local concentrations of adenosine that are controlled by the rate of washout into portal blood.

The comparative effect of administration of substances via the hepatic artery or portal vein on hepatic arterial resistance, liver blood volume and hepatic extraction in cats

Compounds reaching the liver do so via either the hepatic artery or the portal vein. This paper reports on the effectiveness of administration of compounds into these alternate routes for their effects on the hepatic parenchymal cells, the hepatic arterial resistance vessels (blood flow) and hepatic capacitance (blood volume responses). All tests were done on cats under pentobarbital anesthesia. Perfusion of the parenchymal cell mass was assessed by comparing the hepatic elimination of indocyanine green (ICG) administered via the two vascular routes. The ICG uptake was assessed by measuring relative areas under the hepatic venous outflow curve obtained following bolus injections of ICG into the artery and portal vein. In a separate series, using different methods, the hepatic venous levels reached early (2 min) and later (5 min) during a constant infusion were compared during administration via the two routes and found to be equal. Parenchymal cell functions (ICG extraction, bile salt stimulation of bile flow) indicate that blood from the artery and portal vein supplies the hepatic parenchymal cells equally well. This suggests a well-mixed blood supply prior to exposure of either blood stream to parenchymal cells. Substances being processed by the liver are thus equally well handled if reaching the liver via either the arterial or portal blood stream. This has significance in validating the use of some isolated liver perfusion methods that perfuse only via the portal vein. Access of vasoactive compounds in the two blood streams to hepatic arterial resistance vessels was assessed using electromagnetic flow probes.(ABSTRACT TRUNCATED AT 250 WORDS)