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

Validation of an integrative mathematical model of dehydration and rehydration in virtual humans

Water homeostasis is one of the body's most critical tasks. Physical challenges to the body, including exercise and surgery, almost always coordinate with some change in water handling reflecting the changing needs of the body. Vasopressin is the most important hormone that contributes to short‐term water homeostasis. By manipulating vascular tone and regulating water reabsorption in the collecting duct of the kidneys, vasopressin can mediate the retention or loss of fluids quickly. In this study, we validated HumMod, an integrative mathematical model of human physiology, against six different challenges to water homeostasis with special attention to the secretion of vasopressin and maintenance of electrolyte balance. The studies chosen were performed in normal men and women, and represent a broad spectrum of perturbations. HumMod successfully replicated the experimental results, remaining within 1 standard deviation of the experimental means in 138 of 161 measurements. Only three measurements lay outside of the second standard deviation. Observations were made on serum osmolarity, serum vasopressin concentration, serum sodium concentration, urine osmolarity, serum protein concentration, hematocrit, and cumulative water intake following dehydration. This validation suggests that HumMod can be used to understand water homeostasis under a variety of conditions.

Adenosine restores the hepatic artery buffer response and improves survival in a porcine model of small-for-size syndrome

The aim of the study is to define the role of the HABR in the pathophysiology of the SFS liver graft and to demonstrate that restoration of hepatic artery flow (HAF) has a significant impact on outcome and improves survival. Nine pigs received partial liver allografts of 60% liver volume, Group 1; 8 animals received 20% LV grafts, Group 2; 9 animals received 20% LV grafts with adenosine infusion, Group 3. HAF and portal vein flow (PVF) were recorded at 10 min, 60 min and 90 min post reperfusion, on POD 3 and POD 7 in Group 1, and daily in Group 2 and 3 up to POD 14. Baseline HAF and PVF (ml/100 g/min) were 29 +/- 12 (mean +/- SD) and 74 +/- 8 respectively, with 28% of total liver blood flow (TLBF) from the HA and 72% from the PV. PVF peaked at 10 mins in all groups, increasing by a factor of 3.8 in the 20% group compared to an increase of 1.9 in the 60% group. By POD 7-14 PVF rates approached baseline values in all groups. The HABR was intact immediately following reperfusion in all groups with a reciprocal decrease in HAF corresponding to the peak PVF at 10 min. However in the 20% group HAF decreased to 12 +/- 8 ml/100 g/min at 90 min and remained low out to POD 7-14 despite restoration of normal PVF rates. Histopathology confirmed evidence of HA vasospasm and its consequences, cholestasis, centrilobular necrosis and biliary ischemia in Group 2. HA infusion of adenosine significantly improved HAF (p < .0001), reversed pathological changes and significantly improved survival (p = .05). An impaired HABR is important in the pathophysiology of the SFSS. Reversal of the vasospasm significantly improves outcome.

Purinergic receptors in the splanchnic circulation

There is considerable evidence that purines are vasoactive molecules involved in the regulation of blood flow. Adenosine is a well known vasodilator that also acts as a modulator of the response to other vasoactive substances. Adenosine exerts its effects by interacting with adenosine receptors. These are metabotropic G-protein coupled receptors and include four subtypes, A(1), A(2A), A(2B) and A(3). Adenosine triphosphate (ATP) is a co-transmitter in vascular neuroeffector junctions and is known to activate two distinct types of P2 receptors, P2X (ionotropic) and P2Y (metabotropic). ATP can exert either vasoconstrictive or vasorelaxant effects, depending on the P2 receptor subtype involved. Splanchnic vascular beds are of particular interest, as they receive a large fraction of the cardiac output. This review focus on purinergic receptors role in the splanchnic vasomotor control. Here, we give an overview on the distribution and diversity of effects of purinergic receptors in splanchnic vessels. Pre- and post-junctional receptormediated responses are summarized. Attention is also given to the interactions between purinergic receptors and other receptors in the splanchnic circulation.

Regulatory processes interacting to maintain hepatic blood flow constancy: Vascular compliance, hepatic arterial buffer response, hepatorenal reflex, liver regeneration, escape from vasoconstriction

Constancy of hepatic blood flow (HBF) is crucial for several homeostatic roles. The present conceptual review focuses on interrelated mechanisms that act to maintain a constant HBF per liver mass. The liver cannot directly control portal blood flow (PF); therefore, these mechanisms largely operate to compensate for PF changes. A reduction in PF leads to reduced intrahepatic distending pressure, resulting in the highly compliant hepatic vasculature passively expelling up to 50% of its blood volume, thus adding to venous return, cardiac output and HBF. Also activated immediately upon reduction of PF are the hepatic arterial buffer response and an HBF-dependent hepatorenal reflex. Adenosine is secreted at a constant rate into the small fluid space of Mall which surrounds the terminal branches of the hepatic arterioles, portal venules and sensory nerves. The concentration of adenosine is regulated by washout into the portal venules. Reduced PFreduces the washout and the accumulated adenosine causes dilation of the hepatic artery, thus buffering the PF change. Adenosine also activates hepatic sensory nerves to cause reflex renal fluid retention, thus increasing circulating blood volume and maintaining cardiac output and PF. If these mechanisms are not able to maintain total HBF, the hemodynamic imbalance results in hepatocyte proliferation, or apoptosis, by a shear stress/nitric oxide-dependent mechanism, to adjust total liver mass to match the blood supply. These mechanisms are specific to this unique vascular bed and provide an excellent example of multiple integrative regulation of a major homeostatic organ.

Blockade of nitric oxide synthase potentiates the suppression of vasodilators by norepinephrine in the hepatic artery

We have previously shown that nitric oxide (NO) and adenosine suppress vasoconstriction induced by norepinephrine infusion and sympathetic nerve stimulation in the hepatic artery and superior mesenteric artery. NO is involved in the control of basal vascular tone in the superior mesenteric artery but not the hepatic artery. The vasodilation induced by adenosine is inhibited by NO in the superior mesenteric artery but not in the hepatic artery. Based on these known interactions of catecholamines, adenosine, and NO, the objective of this study was to test the hypothesis that NO modulates the interaction between vasoconstrictors and vasodilators in the hepatic artery. We examined the ability of norepinephrine to suppress adenosine-mediated vasodilation and the role of NO in this interaction. Hepatic arterial blood flow and pressure were monitored in pentobarbital-anesthetized cats. The maximum hepatic arterial vasoconstrictor response to norepinephrine infusion was potentiated by blockade of NO production using Nomega-nitro-L-arginine methyl ester (L-NAME), and the potentiation was reversed by L-arginine. The maximum dilator response to adenosine was only slightly suppressed (14.0+/-5.8%, P < 0.05) by norepinephrine infusion; however, after the NO blockade, the suppression by norepinephrine of the vasodilation induced by adenosine was substantially potentiated (45.2+/-9.1%, P < 0.05). Similar results were obtained for isoproterenol-induced vasodilation. We conclude that the interaction between these vasodilators and norepinephrine was modulated by NO which inhibited the vasoconstriction and the suppression of vasodilators caused by norepinephrine and that in the absence of NO production, norepinephrine-induced constriction and the ability to antagonize dilation is substantially potentiated.

The 1995 Ciba-Geigy Award Lecture. Intrinsic regulation of hepatic blood flow

Intrinsic regulation of hepatic blood flow is mediated only through the hepatic artery because the liver is not able to directly regulate portal vein blood flow. Hepatic metabolic activity does not affect hepatic artery flow. Although the hepatic artery is affected by sympathetic nerves and blood-borne agents, the intrinsic regulation of the hepatic artery can be demonstrated if these factors are controlled. The primary intrinsic regulator of the hepatic artery is the hepatic arterial buffer response, which is the inverse response of the hepatic artery to changes in portal vein flow. The hepatic arterial buffer response is sufficiently powerful that doubling portal vein flow leads to maximal constriction in the hepatic artery, while low portal vein flow can result in maximal dilation. The mechanism of the hepatic arterial buffer response is based on adenosine washout, whereby adenosine is produced at a constant rate, independent of oxygen supply or demand, and secreted into a small fluid compartment that surrounds the hepatic arterial resistance vessels. If portal vein flow decreases, less adenosine is washed away into the portal blood and the accumulated adenosine leads to hepatic arterial dilation. Similarly, hepatic arterial autoregulation operates by the same mechanism, whereby a decrease in arterial pressure leads to a decrease in hepatic arterial flow, thus resulting in less adenosine washout into the hepatic artery blood. The accumulated adenosine leads to hepatic artery dilation. These intrinsic regulatory mechanisms tend to maintain total hepatic blood flow at a constant level, thus stabilizing hepatic clearance of hormones, venous return, and cardiac output.