Glycogen phosphorylase, glucose output and vasoconstriction in the perfused rat liver. Concentration-dependence of actions of adrenaline, vasopressin and angiotensin II

Modulation of glucose metabolism by the renin-angiotensin-aldosterone system

The renin-angiotensin-aldosterone system (RAAS) is an enzymatic cascade functioning in a paracrine and autocrine fashion. In animals and humans, RAAS intrinsic to tissues modulates food intake, metabolic rate, adiposity, insulin sensitivity, and insulin secretion. A large array of observations shows that dysregulation of RAAS in the metabolic syndrome favors type 2 diabetes. Remarkably, angiotensin-converting enzyme inhibitors, suppressing the synthesis of angiotensin II (ANG II), and angiotensin receptor blockers, targeting the ANG II type 1 receptor, prevent diabetes in patients with hypertensive or ischemic cardiopathy. These drugs interrupt the negative feedback loop of ANG II on the RAAS cascade, which results in increased production of angiotensins. In addition, they change the tissue expression of RAAS components. Therefore, the concept of a dual axis of RAAS regarding glucose homeostasis has emerged. The RAAS deleterious axis increases the production of inflammatory cytokines and raises oxidative stress, exacerbating the insulin resistance and decreasing insulin secretion. The beneficial axis promotes adipogenesis, blocks the production of inflammatory cytokines, and lowers oxidative stress, thereby improving insulin sensitivity and secretion. Currently, drugs targeting RAAS are not given for the purpose of preventing diabetes in humans. However, we anticipate that in the near future the discovery of novel means to modulate the RAAS beneficial axis will result in a decisive therapeutic breakthrough.

Genetically altered animals in the study of the metabolic functions of peptide hormone systems

PURPOSE OF REVIEW

Here we review the use of genetically altered animals to address the roles of peptide hormone systems in the modulation of energy homeostasis. Despite the disseminated use of transgenic techniques to establish the functional relevance of several peptide hormone systems, we focus on two multifunctional systems, the renin-angiotensin and the kallikrein-kinin systems. Initially, we explored the background information supporting the functional aspects of these systems, followed by novel knowledge obtained with the phenotypic characterization of genetically altered animals.

RECENT FINDINGS

A role for the renin-angiotensin system in the regulation of adiposity and glucose metabolism has been suggested. Studies using genetically altered animals not only confirmed the physiological relevance of angiotensin II in the control of energy homeostasis, but also revealed that the adipose tissue renin-angiotensin system participates in the endocrine modulation of cardiovascular and renal function. On the other hand, the involvement of the kallikrein-kinin system with metabolic processes was not so obvious. Recent reports using genetically altered animals, however, provided strong evidence to support an important role for kinins in the control of glucose homeostasis and energy balance.

SUMMARY

Here we present examples of how genetically altered animals contribute to a final postulation of the physiological roles of certain hormone systems, bringing new insights into the field.

Mechanism for blockade of angiotensin subtype 1 receptors to lower plasma glucose in streptozotocin-induced diabetic rats

AIMS

We investigated the mechanism(s) by which valsartan, a selective antagonist of angiotensin subtype 1 (AT(1)) receptor, decreased plasma glucose in streptozotocin (STZ)-induced diabetic rats.

METHODS

The plasma glucose concentration was assessed by the glucose oxidase method. The concentration of beta-endorphin in plasma or medium incubating adrenal medulla was measured using an enzyme-linked immunosorbent assay. The mRNA levels of the subtype 4 form of glucose transporter (GLUT4) in soleus muscle and phosphoenolpyruvate carboxykinase (PEPCK) in the liver were detected by Northern blotting analysis, while the protein levels of GLUT4 in isolated soleus muscle and hepatic PEPCK were investigated using Western blotting analysis.

RESULTS

A single intravenous injection of valsartan dose-dependently increased plasma beta-endorphin-like immunoreactivity (BER) in parallel with the lowering of plasma glucose concentration in STZ-induced diabetic rats. Naloxone and naloxonazine inhibited the plasma glucose-lowering action of valsartan at doses sufficient to block opioid micro-receptors. In contrast to its action in wild-type diabetic mice, valsartan failed to modify plasma glucose in opioid micro-receptor knockout diabetic mice. Bilateral adrenalectomy in STZ-induced diabetic rats eliminated both the plasma glucose-lowering action and the plasma BER-elevating action of valsartan. In the isolated adrenal medulla of STZ-induced diabetic rats, angiotensin II (Ang II) or valsartan did not affect spontaneous BER secretion. Activation of cholinergic receptors by 1.0 micromol/l acetylcholine (ACh) enhanced BER secretion from the isolated adrenal medulla of STZ-induced diabetic rats, but not in the presence of 1.0 nmol/l Ang II, while valsartan reversed this inhibition by Ang II in a concentration-dependent manner. Treatment of STZ-induced diabetic rats with valsartan (0.2 mg/kg) three times daily for 3 days resulted in an increase in gene expression of GLUT4 in soleus muscle and impeded the reduction of elevated mRNA or protein level of hepatic PEPCK. Both of these effects were blocked by opioid micro-receptor antagonist.

CONCLUSIONS

The results suggest that blockade of AT(1) receptor by valsartan may enhance the adrenal beta-endorphin secretion induced by ACh, activating the opioid micro-receptors to increase glucose utilization and/or to decrease hepatic gluconeogenesis, resulting in the reduction of plasma glucose in STZ-induced diabetic rats.

Preserved direct hepatic insulin action in rats with diet-induced hepatic steatosis

Recent in vivo studies have demonstrated a strong negative correlation between liver triglyceride content and hepatic insulin sensitivity, but a causal relationship remains to be established. We therefore have examined parameters of direct hepatic insulin action on isolated steatotic livers from high-fat (HF)-fed rats compared with standard chow (SC)-fed controls. Direct hepatic action of insulin was assayed in Wistar rats after 6 wk of HF diet by measuring the insulin-induced suppression of epinephrine-induced hepatic glucose output in an isolated liver perfusion system. Insulin-induced activation of glycogen synthase was measured by quantifying the incorporation of radioactive UDP-glucose into glycogen in HF and SC liver lysates. HF diet induced visceral obesity, mild insulin resistance, and hepatic steatosis. Both suppression of epinephrine-induced glycogenolysis and activation of glycogen synthase by insulin were sustained in HF rats; no significant difference from SC controls could be detected. In conclusion, in our model, triglyceride accumulation into the liver was not sufficient to impair direct hepatic insulin action. The data argue for an important role of systemic factors in the regulation of hepatic glucose output and hepatic insulin sensitivity in vivo.

Impact of the renin-angiotensin system on lipid and carbohydrate metabolism

PURPOSE OF REVIEW

This review is intended to provide an update of the impact of the renin-angiotensin system on lipid and carbohydrate metabolism and of its relationship with adipose-tissue and skeletal muscle activities.

RECENT FINDINGS

The components of the renin-angiotensin system are fully represented in the adipose tissue and appear to be upregulated in obesity--a condition associated with enhanced circulating angiotensinogen levels. The local renin-angiotensin system plays a role in adipocyte differentiation and possibly in body-fat accumulation. In humans, angiotensin II produced by mature adipocytes appears to inhibit the differentiation of adipocyte precursors, thus decreasing the percentage of small insulin-sensitive adipocytes. In turn, the lipid-storage capacity of adipose tissue could become reduced and triglycerides might accumulate in liver and skeletal muscle, contributing to insulin resistance. Randomized controlled trials indicating that pharmacological renin-angiotensin system blockade improves insulin sensitivity and reduces the risk of type 2 diabetes are in keeping with this possibility. The local renin-angiotensin system in skeletal muscle may affect exercise performance and the individual response to different types of muscular performance. The concept that the local renin-angiotensin system plays a role in body-fat storage and in lipid and carbohydrate metabolism is further supported by genetic studies showing that susceptibility to weight gain and possibly insulin resistance is greater in individuals carrying certain renin-angiotensin system allelic variants associated with alterations in systemic and local angiotensinogen levels and angiotensin-converting enzyme activity.

SUMMARY

In summary, the aforementioned data imply that the renin-angiotensin system plays a substantial role in obesity, insulin resistance and the associated increase in blood pressure.

Antihyperglycemic action of angiotensin II receptor antagonist, valsartan, in streptozotocin-induced diabetic rats

OBJECTIVES

In the present study, we use valsartan, a highly selective antagonist for angiotensin(1) (AT(1)) receptor subtype, to investigate the effect of AT(1) receptor on the plasma glucose metabolism in streptozotocin-induced diabetic rats (STZ-diabetic rats).

METHODS

The plasma glucose concentration was assessed by glucose oxidase method and plasma insulin was measured using enzyme-linked immunosorbent assay. Systolic blood pressure (SBP) was determined by the tail-cuff method. The intravenous glucose challenge test (IVGCT) was carried out to evaluate the effect of valsartan on the glucose utilization in vivo. The mRNA levels of the subtype 4 form of glucose transporter (GLUT4) in soleus muscle and phosphoenolpyruvate carboxykinase (PEPCK) in the liver were detected by Northern blotting analysis. Moreover, the protein levels of GLUT4 in isolated soleus muscle and hepatic PEPCK were investigated using Western blotting analysis.

RESULTS

A single intravenous injection of valsartan decreased the plasma glucose concentrations in a dose-dependent manner in STZ-diabetic rats. Plasma glucose-lowering action of valsartan also observed in normal rats although in a way not so effective as that in STZ-diabetic rats. Valsartan at the dose of 0.2 mg/kg that produced the maximal plasma glucose-lowering activity in STZ-diabetic rats is also effective to lower the SBP. However, oral treatment with nifedipine or nicorandil in STZ-diabetic rats at the dose sufficient to decrease SBP showed no change of plasma glucose. Otherwise, infusion of saralasin (10 microg/kg per min) into STZ-diabetic rats produced a plasma glucose-lowering activity similar to that by valsartan at 0.2 mg/kg. Moreover, valsartan (0.2 mg/kg) significantly attenuated the raise of plasma glucose induced by IVGCT in normal rats. Repeated intravenous administration of valsartan (0.2 mg/kg) in STZ-diabetic rats resulted in the lowering of plasma glucose after 3 days. The mRNA and protein levels of GLUT4 in the soleus muscle were increased after repeated intravenous administration of valsartan in STZ-diabetic rats for 3 days. Moreover, similar repeated treatment with valsartan reversed the elevated mRNA and protein levels of PEPCK in the liver of STZ-diabetic rats.

CONCLUSIONS

These results suggest that the plasma glucose-lowering activity of AT(1) receptor antagonism was associated with an increase in the glucose utilization in peripheral tissue and/or a reduction in hepatic gluconeogenesis in the absence of insulin.

Possible involvement of the adipose tissue renin-angiotensin system in the pathophysiology of obesity and obesity-related disorders

Angiotensin II (Ang II), acting on the AT1 and AT2 receptors in mammalian cells, is the vasoactive component of the renin-angiotensin system (RAS). Several components of the RAS have been demonstrated in different tissues, including adipose tissue. Although the effects of Ang II on metabolism have not been studied widely, it is intriguing to assume that components of the RAS produced by adipocytes may play an autocrine, a paracrine and/or an endocrine role in the pathophysiology of obesity and provide a potential pathway through which obesity leads to hypertension and type 2 diabetes mellitus. In the first part of this review, we will describe the production of Ang II, the different receptors through which Ang II exerts its effects and summarize the concomitant intracellular signalling cascades. Thereafter, potential Ang II-induced mechanisms, which may be associated with obesity and obesity-related disorders, will be considered. Finally, we will focus on the different pharmaceutical agents that interfere with the RAS and highlight the possible implications of these drugs in the treatment of obesity-related disorders.

Splanchnic blood flow and hepatic glucose production in exercising humans: role of renin-angiotensin system

The study examined the implication of the renin-angiotensin system (RAS) in regulation of splanchnic blood flow and glucose production in exercising humans. Subjects cycled for 40 min at 50% maximal O(2) consumption (VO(2 max)) followed by 30 min at 70% VO(2 max) either with [angiotensin-converting enzyme (ACE) blockade] or without (control) administration of the ACE inhibitor enalapril (10 mg iv). Splanchnic blood flow was estimated by indocyanine green, and splanchnic substrate exchange was determined by the arteriohepatic venous difference. Exercise led to an approximately 20-fold increase (P < 0.001) in ANG II levels in the control group (5.4 +/- 1.0 to 102.0 +/- 25.1 pg/ml), whereas this response was blunted during ACE blockade (8.1 +/- 1.2 to 13.2 +/- 2.4 pg/ml) and in response to an orthostatic challenge performed postexercise. Apart from lactate and cortisol, which were higher in the ACE-blockade group vs. the control group, hormones, metabolites, VO(2), and RER followed the same pattern of changes in ACE-blockade and control groups during exercise. Splanchnic blood flow (at rest: 1.67 +/- 0.12, ACE blockade; 1.59 +/- 0.18 l/min, control) decreased during moderate exercise (0.78 +/- 0.07, ACE blockade; 0.74 +/- 0.14 l/min, control), whereas splanchnic glucose production (at rest: 0.50 +/- 0.06, ACE blockade; 0.68 +/- 0.10 mmol/min, control) increased during moderate exercise (1.97 +/- 0.29, ACE blockade; 1.91 +/- 0.41 mmol/min, control). Refuting a major role of the RAS for these responses, no differences in the pattern of change of splanchnic blood flow and splanchnic glucose production were observed during ACE blockade compared with controls. This study demonstrates that the normal increase in ANG II levels observed during prolonged exercise in humans does not play a major role in the regulation of splanchnic blood flow and glucose production.

Acute effect of leptin on hepatic glycogenolysis and gluconeogenesis in perfused rat liver

Leptin circulates in blood and is involved in body weight control primarily via hypothalamic receptors. To examine its direct metabolic action, effects of short-term portal leptin infusion: 1) on postprandial basal and epinephrine-stimulated glycogenolysis; and 2) on postabsorptive lactate-stimulated gluconeogenesis were studied in isolated perfused rat livers. Incremental epinephrine (150 pmol x min-1 x g-1 liver)-stimulated glucose release (in micromol/g liver within 30 minutes; control: 28.3 +/- 2.8) was suppressed (P <.05) by 44% (15.8 +/- 1.6), by 48% (14.6 +/- 4.1), and by 53% (13.3 +/- 2.1) during insulin (3 pmol x min-1 x g-1 liver), leptin (30 pmol x min-1 x g-1 liver), and simultaneous leptin + insulin infusion. Perfusate cyclic adenosine monophosphate increased approximately twofold during epinephrine stimulation in all groups. Neither leptin nor insulin affected hepatic lactate production, bile flow, or portal pressure in the fed state. In the postabsorptive state (20-hour fasting), rates of lactate (10 mmol/L)-dependent hepatic glucose release (in micromol. min-1 x g-1 liver; control: 0.12 +/- 0.01) were increased (P <.01) to 0.35 +/- 0.02 and to 0.24 +/- 0.01 by glucagon (3 pmol x min-1 x g-1 liver) and by leptin (15 pmol x min-1 x g-1 liver), respectively. In parallel, lactate uptake rates (in micromol x min-1 x g-1 liver) were higher in the presence of both glucagon (0.90 +/- 0. 03) and leptin (0.84 +/- 0.02) compared with control (0.68 +/- 0.04). In conclusion, leptin modulates hepatic glucose fluxes and may contribute to direct humoral regulation of liver glycogen stores in the fasted as well as in the fed state.

13C NMR study of the effects of leptin treatment on kinetics of hepatic intermediary metabolism

The recent discovery of leptin receptors in peripheral tissue raises questions about which of leptin's biological actions arise from direct effects of the hormone on extraneural tissues and what intracellular mechanisms are responsible for leptin's effects on carbohydrate and lipid metabolism. The present study is focused on the action of leptin on hepatic metabolism. Nondestructive 13C NMR methodology was used to follow the kinetics of intermediary metabolism by monitoring flux of 13C-labeled substrate through several multistep pathways. In perfused liver from either ob/ob or lean mice, we found that acute treatment with leptin in vitro modulates pathways controlling carbohydrate flux into 13C-labeled glycogen, thereby rapidly enhancing synthesis by an insulin-independent mechanism. Acute treatment of ob/ob liver also caused a rapid stimulation of long-chain fatty acid synthesis from 13C-labeled acetyl-CoA by the de novo synthesis route. Chronic leptin treatment in vivo induced homeostatic changes that resulted in a tripling of the rate of glycogen synthesis via the gluconeogenic pathway from [2-13C]pyruvate in ob/ob mouse liver perfused in the absence of the hormone. Consistent with the 13C NMR results, leptin treatment of the ob/ob mouse in vivo resulted in significantly increased hepatic glycogen synthase activity. Chronic treatment with leptin in vivo exerted the opposite effect of acute treatment in vitro and markedly decreased hepatic de novo synthesis of fatty acids in ob/ob mouse liver. In agreement with the 13C NMR findings, activities of hepatic acetyl-CoA carboxylase and fatty acid synthase were significantly reduced by chronic treatment of the ob/ob mouse with leptin. Our data represent a demonstration of direct effects of leptin in the regulation of metabolism in the intact functioning liver.

Glucose and the insulin-releasing drug tolbutamide attenuate the effects of morphine and angiotensin on alcohol consumption

Animals studies have shown that insulin injections reduce alcohol intake, implicating glucoregulatory processes in alcohol consumption. Angiotensin (ANG) II reduces alcohol intake and promotes glycogen breakdown in the liver but no studies have assessed the role of glucoregulatory processes in ANG II's effect. Similarly, glucose injections attenuate the analgesic and cognitive effects of opiates, yet no studies have assessed the effect of glucose on the well-documented ability of opiates to enhance alcohol consumption. The present experiments further examine the role of glucoregulatory processes in alcohol intake by assessing the effect of glucose injections on morphine-enhanced alcohol consumption and by evaluating the effect of the insulin-releasing drug, tolbutamide, on ANG II-reduced alcohol consumption. Adult male Wistar rats acquired alcohol drinking using the limited access procedure that offers daily 40-min access to both 6% w/v alcohol and water and ensures reliable alcohol drinking in bouts large enough to produce pharmacologically relevant intakes. Experiment 1: after intake stabilized, four groups of rats were first pretreated with vehicle injections and in the next phase, three of the four groups received either 50, 100, or 200 mg/kg glucose intraperitoneally (i.p.) prior to access to alcohol. Neither the vehicle injections nor any of the glucose doses had an effect on alcohol intake. In the final phase all groups continued to receive their respective glucose doses or vehicle but were now also treated with 5 mg/kg morphine sulphate i.p. prior to alcohol access. Morphine stimulated alcohol intake to a similar degree in all groups except the 200 mg/kg group, which showed a significant attenuation in morphine-enhanced alcohol intake. Experiment 2: after intake stabilized, different groups of rats were pretreated with vehicle injections and in the next phase received either 5, 25, 50, or 100 mg/kg tolbutamide or vehicle subcutaneously (s.c.) prior to alcohol access. The vehicle injections did not alter alcohol intake, and only the 100 mg/kg dose of tolbutamide produced a reduction in alcohol intake. In the final phase the groups continued to receive their respective doses of tolbutamide or vehicle but were also treated with 400 micrograms/kg ANG II s.c. immediately prior to alcohol access. ANG II reduced alcohol intake a similar extent in the groups pretreated with 5-50 mg/kg tolbutamide. However, the 100 mg/kg dose of tolbutamide significantly attenuated ANG II's ability to reduce alcohol intake. These results demonstrate that manipulations that engage glucoregulatory processes can influence the mechanism(s) by which morphine and angiotensin respectively increase and decrease alcohol drinking.

Pharmacological characterization of a vasopressin V1 receptor in the isolated human gastric artery

Species-related specific differences in the pharmacological profile of vasopressin V1a receptors have been reported. Thus, the aim of the present study was to identify a vascular preparation of human origin expressing V1a receptors. Vasopressin was found to contract human gastric artery strips without endothelium with high affinity (pEC50 8.9). The maximal effect induced by vasopressin was inversely related to the diameter of the vessel. Oxytocin was found to contract the human gastric artery strips with low potency (pEC50 7.2). Contraction induced by vasopressin was competitively antagonized by the non peptide vasopressin receptor antagonists SR 49059 (pA2 9.2), OPC 21268 (pA2 6.2) and OPC 31260 (pA2 7.1). The order of potency of agonists (vasopressin > > oxytocin) and of antagonists (SR 49059 > > OPC 31260 > OPC 21268) indicate the contraction induced by vasopressin in the isolated human gastric artery is mediated by the V1a receptor type. The present data are similar to those obtained in different preparations expressing the native human V1a receptor as well as to those obtained in cell transfected with this receptor. The human gastric artery is a monoreceptor system of great utility for studying the effects of new drugs interacting with the human V1a receptor.

Molecular pharmacology of V1a vasopressin receptors

1. Vasopressin, a mammalian neurohypophysial peptide hormone, has diverse physiological actions. 2. Pharmacological studies, using a range of mammalian tissues, have identified three subtypes of vasopressin receptor. 3. The V1a subtype of vasopressin receptor is widely distributed and mediates many central and peripheral actions of vasopressin. 4. The development of subtype-selective vasopressin analogues has provided valuable tools for pharmacological and physical studies of the V1a receptor protein. 5. Pharmacological differences indicate species heterogeneity in the characteristics of V1a receptors and in the expression of hepatic V1a receptors. 6. The cloning of neurohypophysial hormone receptor proteins allows structural and functional comparison of the V1a vasopressin receptors with other G-protein-coupled receptors.

Spontaneous periodic hypothermia

Spontaneous periodic hypothermia is a rare syndrome of recurrent, centrally mediated hypothermia without an identifiable systemic cause or brain lesion. Most patients defend a temporarily lowered temperature "set point" during episodes of hypothermia, despite manifesting many well-known systemic consequences of core temperature hypothermia. No case of death directly attributable to an episode of spontaneous periodic hypothermia has been reported, although many of the serious systemic effects of hypothermia have been documented in these cases, so it is not unlikely that death may occur. The syndrome's cause, and that of Shapiro syndrome, remains unknown. Pharmacologic trials to date have been only modestly successful. Anticonvulsant agents, clonidine, and cyproheptadine appear the most likely to succeed, with cyproheptadine being a reasonable first choice. Given that the term "spontaneous periodic hypothermia" describes a syndrome, and not a pathophysiologic mechanism, it is likely to encompass a common eventuality, arrived at via several different pathways. One can postulate mechanisms such as structural abnormalities, trauma, infection, irritation, and degeneration involving strategic locations which create a focus for epileptic or other periodic dysfunction whose scope involves the centers for thermoregulation. The existence of 2 distinct, oppositional thermoregulatory centers would allow for speculation of similar mechanisms accounting for cases of both periodic hypo- and hyperthermia (61). Postmortem data regarding the hypothalamic and surrounding areas from future cases of Shapiro syndrome and spontaneous periodic hypothermia would be of great interest. Further, more sensitive in vivo testing methods are clearly needed. The role of PET or single photon emission computed tomography (SPECT) with technetium 99m-labeled hexamethylpropylene amine oxime (Tc 99m HMPAO) performed acutely during an episode remains to be characterized (64, 103, 105). The term "diencephalic epilepsy" may in fact be accurate, given the periodic episodes of the case presented here and similar cases resulting from non-generalized seizure activity, with or without an underlying predisposing lesion. The label diencephalic epilepsy has been merely speculative so far, however, as definitive evidence of seizure activity has not been documented. Further, it is expected that the descriptive terms "spontaneous periodic hypothermia" and "episodic spontaneous hypothermia with hyperhidrosis" will outlive their usefulness as researchers gain greater understanding of this syndrome, and be replaced with a more pathophysiologically meaningful nomenclature.

Characterization of the human liver vasopressin receptor. Profound differences between human and rat vasopressin-receptor-mediated responses suggest only a minor role for vasopressin in regulating human hepatic function

The [Arg8]vasopressin (AVP) receptor expressed by human hepatocytes was characterized, and compared with the rat hepatic V1a vasopressin receptor subtype. In addition to determining the pharmacological profile of the human receptor, the cellular responses to AVP were measured in human and rat hepatocytes by assaying glycogen phosphorylase alpha activity and DNA synthesis. Marked differences were observed between human and rat hepatocytes regarding vasopressin receptors and the intracellular consequences of stimulation by AVP. Data presented in this paper demonstrate the following, (i) Vasopressin V1a receptors are present in low abundance on human hepatocytes. (ii) Species differences exist between human and rat V1a receptors with respect to the affinity of some selective antagonists. (iii) AVP-stimulated glycogen phosphorylase a activation in human hepatocytes was approx. 5% of that observed in rat cells. (iv) In contrast with rat hepatocytes, DNA synthesis in human cells in culture was not stimulated by AVP. It is concluded that vasopressin plays only a minor role in the regulation of human hepatic function. Furthermore, conclusions drawn from observations made with AVP and its analogues on rat hepatic function cannot be directly extrapolated to the human situation.

A selective biotinylated probe for V1a vasopressin receptors

We have designed and synthesized a biotinylated vasopressin antagonist which is a selective probe for studying the V1a subtype of vasopressin receptor. Initially we synthesized the novel vasopressin analogue d(CH2)5Tyr(Me)2LysNH2(9)AVP (ALVP). Biotinamidocaproate was subsequently coupled to the epsilon-amino group of ALVP to generate the novel biotinylated probe d(CH2)5Tyr(Me)2Lys(N epsilon-biotinamido-caproate)NH2(9)AVP (ALBtnVP). Pharmacological characterization of ALVP and ALBtnVP established that both ligands were high affinity antagonists at V1a receptors, and that both displayed marked V1a/V2 selectivity. The observation that receptor-bound ALBtnVP was bi-functional, and thereby able to bind conjugated derivatives of avidin or streptavidin, allowed ALBtnVP to be utilized as a selective probe for V1a receptors. This strategy allowed the visualization of V1a receptors on the surface of WRK-1 cells and hippocampal neurons, by using streptavidin-gold with electron microscopy and fluorescein-avidin with light microscopy. We conclude that ALBtnVP is a useful probe for V1a receptors.

Hepatic circulation: potential for therapeutic intervention

In recent years, knowledge of the physiology and pharmacology of hepatic circulation has grown rapidly. Liver microcirculation has a unique design that allows very efficient exchange processes between plasma and liver cells, even when severe constraints are imposed upon the system, i.e. in stressful situations. Furthermore, it has been recognized recently that sinusoids and their associated cells can no longer be considered only as passive structures ensuring the dispersion of molecules in the liver, but represent a very sophisticated network that protects and regulates parenchymal cells through a variety of mediators. Finally, vascular abnormalities are a prominent feature of a number of liver pathological processes, including cirrhosis and liver cell necrosis whether induced by alcohol, ischemia, endotoxins, virus or chemicals. Although it is not clear whether vascular lesions can be the primary events that lead to hepatocyte injury, the main interest of these findings is that liver microcirculation could represent a potential target for drug action in these conditions.

Regulation of vasopressin secretion

With the development of sensitive and specific radio-immunoassays to measure the low circulating concentrations of vasopressin there has been a quantum leap in our understanding of the physiological processes involved in the regulation of its secretion. The results of Verney's pioneering studies in dogs led to the concept of 'osmoreceptors'. It is now appreciated that osmoregulation of vasopressin release is of principal importance in the maintenance of water balance. Functional characteristics of the osmoregulatory system have been defined clearly by independent laboratories, and more recently the physiological influences that can subtly alter this very finely controlled system have been described. Non-osmotic factors that release vasopressin have been recognized for many years. Secretion of vasopressin in response to haemodynamic influences has been characterized, and significant hypotension and/or hypovolaemia are potent stimuli to hormone release. Other non-osmotic factors--nausea/emesis, hypoglycaemia--may play important roles in disturbances of water balance. Vasopressin should not, however, be regarded as a stress hormone, since recent careful studies in a variety of species indicate that secretion is not enhanced following a series of different noxious stimuli.

Vasopressin secretion during insulin-induced hypoglycaemia: exaggerated responses in people with type 1 diabetes

Insulin hypoglycaemia causes a rise in plasma vasopressin concentrations in man and the rat, and vasopressin stimulates glucagon secretion and increases hepatic glucose output in man. Vasopressin has also been suggested to have an important synergistic role with corticotrophin releasing factor in the release of adrenocorticotrophin hormone, and a counter-regulatory role for the hormone has been proposed. As diminished anterior pituitary hormone responses to hypoglycaemia have been reported in diabetes mellitus, we studied the plasma vasopressin responses to insulin-induced hypoglycaemia in 10 patients with established Type 1 diabetes and 10 matched control subjects. Blood glucose fell from 4.5 +/- 0.3 to 1.6 +/- 0.1 mmol l-1 (p less than 0.001) in the diabetic group and from 4.6 +/- 0.2 to 1.5 +/- 0.2 mmol l-1 (p less than 0.001) in control subjects, with delayed blood glucose recovery in the diabetic patients. Plasma vasopressin rose in the diabetic patients from 0.9 +/- 0.2 to 6.9 +/- 2.8 pmol l-1 (p less than 0.001), a significantly greater rise (p less than 0.05) than in the control subjects, 0.8 +/- 0.1 to 2.4 +/- 1.0 pmol l-1 (p less than 0.001). Plasma osmolalities remained unchanged and haemodynamic changes were similar in both groups. There is an exaggerated rise in plasma vasopressin concentrations in diabetic patients in response to insulin-induced hypoglycaemia. The putative mechanisms and potential significance of the exaggerated rise are discussed.

Guanine nucleotide regulation of [3H]vasopressin binding to liver plasma membranes and solubilized receptors. Evidence for the involvement of a guanine nucleotide regulatory protein

A guanine nucleotide regulatory protein may be involved in vasopressin-receptor-mediated polyphosphoinositide breakdown in rat liver. Therefore we examined the effects of the non-hydrolysable guanine nucleotide guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG) on [3H]vasopressin ([3H]AVP) binding to hepatic plasma membranes and detergent extracts. [3H]AVP bound to a single set of high-affinity binding sites in membranes. Addition of p[NH]ppG decreased the affinity of receptor binding without altering the maximal binding capacity. The rate of dissociation of [3H]AVP from membrane-bound receptors was also enhanced by p[NH]ppG. Solubilization of [3H]AVP-prelabelled membranes with dodecyl beta-D-maltoside resulted in a [3H]AVP-receptor complex that was unstable in solution. Incubation of these extracts for 5 min at 30 degrees C resulted in a 40% loss of bound [3H]AVP, whereas in the presence of p[NH]ppG there was a 54% loss. However, when membranes were prelabelled with [3H]AVP and p[NH]ppG and then solubilized, the resulting hormone-receptor complex was still temperature-labile but insensitive to the further addition of p[NH]ppG. The molecular size of soluble vasopressin receptors was estimated by gel filtration. The [3H]AVP-receptor complex was eluted as a single peak with an apparent molecular size of 258 kDa. However, no peak was detected when solubilized extract was made from membranes prelabelled with [3H]AVP and p[NH]ppG, suggesting that this receptor complex had dissociated during chromatography. It is possible therefore that the high-Mr complex contains the hormone, its receptor and a guanine nucleotide binding protein.

Suppression of triglyceride secretion by epinephrine in isolated rat hepatocytes

The effect of epinephrine on triglyceride synthesis and secretion was examined in isolated rat hepatocytes. Epinephrine potently inhibited triglyceride secretion but did not affect cellular triglyceride content or the rate of incorporation of radiolabelled glycerol into cell triglyceride. The inhibitory effect of epinephrine was abolished by inclusion of the alpha-adrenergic antagonist prazosin but not the beta-antagonist propranolol.

Alpha-adrenoceptor involvement in catecholamine-induced hyperglycaemia in conscious fasted rabbits

In conscious fasted rabbits an intravenous infusion of phenylephrine (20 micrograms kg-1 min-1) induced hyperglycaemia. The increase in blood glucose was accompanied by a modest increase in insulin secretion and a reduction of liver glycogen. Muscle glycogen and blood lactate levels were not altered by treatment with phenylephrine. Prazosin, 1 mg kg-1 s.c., partially attenuated phenylephrine-induced hyperglycaemia. Phenoxybenzamine infusion (16.6 micrograms kg-1 min-1) for 15 min suppressed the increase in blood glucose and the reduction in liver glycogen evoked by phenylephrine. This alpha-adrenoceptor blocker also clearly attenuated the blood glucose elevation observed on infusing adrenaline at 0.3 microgram kg-1 min-1. Blockade by phenoxybenzamine of phenylephrine- and adrenaline-induced hyperglycaemia was not accompanied by a significant increase in immunoreactive insulin plasma levels. Yohimbine infused at a rate of 20 micrograms kg-1 min-1, also completely blocked phenylephrine-induced hyperglycaemia. This suppressor effect was accompanied by a marked rebound in insulin secretion. It is concluded that in normal fasted rabbits stimulation of alpha-adrenoceptors induces hyperglycaemia. The increase in blood glucose depends mainly on liver glycogenolysis and inhibition of insulin secretion. Separate blockade of each component suffices to reduce alpha-adrenoceptor-mediated hyperglycaemia.

Hormonal regulation of the tricarboxylic acid cycle in the isolated perfused rat liver

The effect of Ca2+-mobilizing hormones, vasopressin, angiotensin II and the alpha-adrenergic agonist phenylephrine, on the metabolic flux through the tricarboxylic acid cycle was investigated in isolated perfused rat livers. All three Ca2+-mobilizing agonists stimulated 14CO2 production and gluconeogenesis in livers of 24-h-fasted rats perfused with [2-14C]pyruvate. Prazosin blocked the phenylephrine-elicited stimulation of 14CO2 and glucose production from [2-14C]pyruvate whereas the alpha 2-adrenergic agonist, BHT-933, did not affect the rates of 14CO2 and glucose production from [2-14C]pyruvate indicating that the phenylephrine-mediated response involved alpha 1-adrenergic receptors. Phenylephrine, vasopressin and angiotensin II stimulated 14CO2 production from [2-14C]acetate in livers derived from fed rats but not in livers of 24-h-fasted rats. In livers of 24-h-fasted rats, perfused with [2-14C]acetate, exogenously added pyruvate was required for an increase in the rate of 14CO2 production during phenylephrine infusion. This last observation suggests increased pyruvate carboxylation as one of the mechanisms involved in stimulation of tricarboxylic acid cycle activity by the Ca2+-mobilizing agonists, vasopressin, angiotensin II and phenylephrine.

Formation of activated oxygen in the hypoxic rat liver

The biliary GSSG efflux rate of normoxic perfused rat liver was 1.5 +/- 0.2 nmol/min/g liver wet weight. The GSSG efflux rate as indicator for the flux through the glutathione peroxidase reaction and, therefore, for an oxidative loading increased with the extent of hypoxia. 2.6 +/- 0.5 nmol/min/g were released from the severely hypoxic liver. The hydroxyl radical scavenger formate as well as the xanthine oxidase inhibitor allopurinol reduced the efflux rate of GSSG. GSH was released from the perfused liver at a rate of 15.5 nmol/min/g which was nearly unchanged in severe hypoxia. The high rate of glucose liberation from the hypoxic liver declined to almost that of the normoxic organ in the presence of formate. There is an 'oxidative stress' during hypoxic liver perfusion which probably originates from increased generation of activated oxygen species in the degradation of purine nucleotides.

The stimulation of glycogenolysis in isolated hepatocytes by opioid peptides

Addition of the opioid peptides, [Leu]enkephalin and [Met]enkephalin, to isolated hepatocytes was shown to produce a stimulation of glycogenolysis comparable with that observed in the presence of maximal concentrations of glucagon, adrenaline or angiotensin. This stimulation was demonstrated to be the result of an activation of phosphorylase by a rapid Ca2+-dependent mechanism and was not decreased by the presence or either alpha- or beta-adrenergic antagonists, although it was dependent on the presence of the N-terminal tyrosine residue in the enkephalin molecule. It is suggested that this may be further evidence for specific opioid receptors in the liver. Addition of [Leu]enkephalin also inhibited lactate formation, indicating that the opioid peptides exert a concerted effect on hepatic carbohydrate metabolism to enhance glucose output. The transient nature of the effect of the enkephalins was shown to be the result of a rapid breakdown of the peptides in the incubation as a result of aminopeptidase activity, the initial product being the inactive des-tyrosine derivative.

The effect of vasopressin infusion on glucose metabolism in man

Studies on intact animals and isolated rat hepatocytes have shown that arginine vasopression (AVP) stimulates glycogen phosphorylase to break down glycogen and raise plasma glucose concentrations. Since no similar work has been performed on healthy human adults, the effect of moderate (25 pmol/min) and high (75 pmol/min) dose AVP infusion on plasma glucose, intermediary metabolites, glucose kinetics, and circulating glucagon and insulin concentrations was investigated. After AVP infusion, plasma glucose rose from 4.9 +/- 0.1 to a peak of 5.7 +/- 0.2 mmol/l (P less than 0.001), but no changes in blood lactate, pyruvate, alanine, glycerol or 3-hydroxybutyrate concentrations were observed. The glucose rise was accounted for entirely by an increase in the rate of appearance of glucose from 11.19 +/- 0.43 to 13.38 +/- 0.63 mu mol/kg/min (P less than 0.001). Infusion of AVP also increased plasma glucagon concentrations from 38 +/- 8 to 79 +/- 20 pg/l (P less than 0.01). The hyperglycaemic effect of AVP may be mediated solely by stimulation of glucagon release, but we cannot exclude direct stimulation of glycogen phosphorylase activity.

Glucose turnover during insulin-induced hypoglycemia in liver-denervated rats

The role of hepatic autonomic nerves in glucose production during hypoglycemia was studied. Selective, surgical denervation of the liver was performed in rats, which reduced hepatic norepinephrine concentrations by 96%. Hypoglycemia was induced by 250 mU of insulin intra-arterially in anesthetized as well as in chronically catheterized, awake rats. Half of the anesthetized denervated or sham-operated rats had previously been adrenodemedullated. Glucose turnover was measured by primed, constant intravenous infusion of [3-3H]glucose. Before as well as during hypoglycemia the arterial glucose concentration and rates of production and utilization of glucose were similar in denervated rats and control rats. Also hepatic glycogen depletion was similar in the groups. The lack of effect of denervation could not be ascribed to compensating changes in hormone or substrate levels. In adrenodemedullated rats lack of glucose recovery from hypoglycemia was accompanied by delayed normalization of glucose clearance. In fed rats, activity in hepatic autonomic nerves is not a primary mechanism increasing glucose production during acute hypoglycemia. Epinephrine enhances glucose recovery by decreasing glucose clearance rather than by increasing glucose production, at least when glucagon is present.

Hormonal control of pyruvate dehydrogenase activity in rat liver

A detailed study of the control of liver pyruvate dehydrogenase activity by various hormones was carried out with perfused liver and isolated hepatocytes. Vasopressin produced a significant increase in the enzyme activity in fed rats, and the time course and sensitivity of the response was similar to that of glycogen phosphorylase a. The enzyme from starved animals was resistant to hormonal activation. The possible factors involved in the above effects are discussed. Angiotensin and phenylephrine also increased pyruvate dehydrogenase activity, and the magnitude of the response was of the same order as that to vasopressin by the liver enzyme. The effects of these hormones on pyruvate dehydrogenase activity were critically dependent on extracellular Ca2+, thus suggesting a role for this ion in the mechanism of action of the hormones. Insulin did not appear to have a role in the control of the enzyme activity, as shown by its lack of effect on the enzyme. Glucagon, in contrast with previous reports, produced a rapid, transient and significant increase in pyruvate dehydrogenase activity. The physiological importance of the above effects is discussed.

Vasopressin effects on food-rewarded learning tasks might be due to its action on carbohydrate/lipid metabolism, not memory

Vasopressin (VP) has been implicated in memory processes on the basis of effects observed in aversively motivated learning situations. Therefore researchers have tried to confirm this role by using food-motivated learning tasks. However, the well-established physiological influences of VP on carbohydrate and lipid metabolism were not taken into consideration. At various times following administration, VP might act as a feeding stimulant or as a satiating agent. Experimental designs should allow for these effects when food-rewarded learning paradigms are used to determine whether VP acts on memory.

Vasopressin release by D-aspartic acid, morphine and prolyl-leucyl-glycinamide (PLG) in DI Brattleboro rats

The L-asparaginase activities of the brains of the Wistar, heterozygous and homozygous Brattleboro rats divided into three parts namely the anterior, middle and posterior which respectively contained cerebral cortex, hippocampus + midbrain + thalamus + hypothalamus cerebellum + pons + medulla oblongata were estimated. The L-asparaginase activities of all the three parts in the homozygous Brattleboro rats were significantly higher than in the Wistar rats as well as in the heterozygous Brattleboro rats. Twenty min following the injections of 200 mg/kg D-aspartic acid, 20 mg/kg morphine, 200 mg/kg D-aspartic acid + 20 mg/kg morphine, 6 mg/kg prolyl-leucyl-glycinamide (PLG) and 6 mg/kg PLG + 20 mg/kg morphine the L-asparaginase activities of all three parts of the homozygous Brattleboro rat brains were found to be significantly inhibited. After having seen the suppressive effect of the drugs and their combinations used before the homozygous Brattleboro rats were given D-aspartic acid, morphine, D-aspartic acid + morphine, PLG and PLG + morphine for seven days. Then their plasma vasopressin levels were determined by RIA. The treatments applied to the homozygous Brattleboro rats caused the appearance of a significant amount vasopressin in the plasma. The results were interpreted as evidence for the fact that the inhibition of the brain L-asparaginase provides and/or accelerates the biosynthesis and/or release of vasopressin. As morphine has a vasopressin releasing and a brain L-asparaginase inhibiting effect the antidiuretic action of morphine was considered to be linked to its inhibitory effect on the brain L-asparaginase.

Inhibition of lipogenesis by vasopressin and angiotensin II in glycogen-depleted hepatocytes

Vasopressin and angiotensin II inhibited lipogenesis (measured with 3H2O) in hepatocytes from fed rats. Inhibition was also observed with hepatocytes from fed rats which had been depleted of glycogen in vitro and incubated with lactate + pyruvate (5 mM + 0.5 mM) as substrates. The inhibitory actions of the hormones are therefore independent of hormone-mediated changes in glycogenolytic or glycolytic flux from glycogen, and thus the site(s) of hormone action must be subsequent to the formation of lactate. (-)Hydroxycitrate, a specific inhibitor of ATP-citrate lyase, decreased lipogenesis in hepatocytes from fed rats incubated with lactate + pyruvate by approx. 51% but had little effect on lipogenesis in glycogen-depleted hepatocytes similarly incubated. There was parallel inhibition of incorporation of 14C from [U-14C]lactate into fatty acid and lipogenesis as measured with 3H2O in each case. Thus depletion of glycogen, or conceivably the process of glycogen-depletion (incubation with dibutyryl cyclic AMP) causes a change in the rate-determining step(s) for lipogenesis from lactate. Vasopressin and angiotensin II also decreased lipogenesis and incorporation of 14C into fatty acids in glycogen-depleted hepatocytes provided with [U-14C]proline as opposed to [U-14C]-lactate. However, proline-stimulated lipogenesis was inhibited by (-)hydroxycitrate, and proline-stimulated lipogenesis and incorporation of 14C from [U-14C]-proline were not decreased in parallel by this inhibitor (inhibition of 52% and 85% respectively). It is inferred that lactate and proline stimulate lipogenesis by different mechanisms and incorporation of 14C from [U-14C]proline and [U-14C]lactate into fatty acid occurs via different routes.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycogenolysis in liver of phosphorylase kinase-deficient rats during liver perfusion and ischaemia

Liver glycogen degradation and phosphorylase activity were measured in normal and phosphorylase kinase-deficient (gsd/gsd) rats. During perfusion or ischaemia, gsd/gsd-rat livers showed a brisk glycogenolysis. There was also a small (1.9-fold) but significant transient increase in their phosphorylase alpha activity during ischaemia, despite their phosphorylase b kinase deficiency; it seems unlikely, however, that this was the main determinant of the glycogenolysis.

Metabolic effects of vasopressin infusion in the starved rat. Reversal of ketonaemia

The effects of vasopressin on the metabolism of starved rats were investigated by using a constant-infusion regimen (50 pmol/kg body wt. per min, after an initial loading dose of 150 pmol/kg body wt.). 2. Blood ketone bodies decreased by 50% in 10 min, and this was accompanied by a 60% decrease in the plasma non-esterified fatty acids. 3. Blood glucose increased by 0.9 mM within 5 min and decreased to control values over the 40 min infusion. Small increases in lactate and pyruvate also occurred. 4. Plasma insulin was not increased by vasopressin infusion. 5. The net decrease in blood ketone bodies caused by vasopressin was similar when somatostatin was infused simultaneously (1 nmol/kg body wt. per min). 6. Hepatic ketone bodies were significantly decreased by vasopressin, as was the 3-hydroxybutyrate/acetoacetate ratio. A small increase in the hepatic concentration of several glycolytic intermediates also occurred. 7. Vasopressin did not decrease the ketonaemia produced by infusions of octanoate or long-chain triacylglycerol in rats that had been pre-treated with the anti-lipolytic agent 3,5-dimethylpyrazole. 8. In comparison with vasopressin, the infusion of adrenaline or glucose had much smaller effects in decreasing the ketonaemia of starvation, despite the 4-fold increase in plasma insulin, at 10 min, with the glucose infusion. 9. The primary metabolic effect of vasopressin in the starved rat appears to be that of decreased supply of non-esterified fatty acid to the liver. It is suggested that vasopressin has a direct anti-lipolytic effect in adipose tissue.

Effects of adrenaline on ketogenesis from long- and medium-chain fatty acids in starved rats

1. Injection of adrenaline into 24 h-starved rats caused a 69% decrease in blood [ketone-body] (3-hydroxybutyrate plus acetoacetate), accompanied by a decreased [3-hydroxybutyrate]/[acetoacetate] ratio. Blood [glucose] and [lactate] increased, but [alanine] was unchanged. 2. Adrenaline also decreased [ketone-body] after intragastric feeding of both long- and medium-chain triacylglycerol. The latter decrease was observed after suppression of lipolysis with 5-methylpyrazole-3-carboxylic acid, indicating that the antiketogenic action of adrenaline was not dependent on the chain length of the precursor fatty acid. 3. The actions of adrenaline to decrease blood [ketone-body] and to increase blood [glucose] were not observed after administration of 3-mercaptopicolinate, an inhibitor of gluconeogenesis. This suggests that these effects of the hormone are related. 4. The possible clinical significance of the results is discussed with reference to the restricted ketosis often observed after surgical or orthopaedic injury.

Effect of vasoactive intestinal polypeptide on glycogen metabolism in rat hepatocytes

The effects of vasoactive intestinal polypeptide (VIP) on several enzymes of glycogen metabolism in rat hepatocytes were compared with those of glucagon and of vasopressin (ADH). VIP caused phosphorylase activation and glycogenolysis in hepatocytes from fed rats. In hepatocytes from fasted rats incubated with glucose, lactate, and pyruvate, VIP inhibited net glycogen deposition, inactivated glycogen synthase, and activated phosphorylase. VIP was about 100-fold less potent than glucagon and 1,000-fold less potent than ADH in causing activation of phosphorylase. The ability of VIP to activate phosphorylase was not altered by chelation of the calcium in the medium. The half maximal effective doses of VIP for both phosphorylase activation and stimulation of glycogenolysis were 10-30 nM. Treatment with VIP, ADH, or glucagon did not decrease phosphorylase phosphatase activity. Each of these hormones, however, lengthened the lag time before synthase phosphatase activity was expressed in vitro. Other gut hormones tested did not affect hepatocyte glycogen metabolism. These results do not support the concept of physiologic control of hepatic glycogen metabolism by VIP or by other gut hormones.

Stimulation of inorganic-phosphate incorporation into phosphatidylinositol in rat thoracic aorta mediated through V1-vasopressin receptors

Vasopressin stimulates the incorporation of [32P]Pi into phosphatidylinositol but not into other phospholipids in rat thoracic aorta strips. The relative abilities of three vasopressin analogues to stimulate phosphatidylinositol labelling in rat aorta are similar to their relative pressor potencies in vivo and to their relative potencies in stimulating the metabolism of rat hepatocytes, but very different from their relative antidiuretic potencies. The vasopressor antagonist [1-(beta-mercapto-beta, beta-cyclopentamethylenepropionic acid),8-arginine]vasopressin competitively inhibits [Arg8]vasopressin-stimulated phosphatidylinositol labelling in rat aorta with a pA2 of 8.1. It is concluded that the Ca2+-mobilizing vasopressin receptors (V1-receptors) of the rat aorta stimulate phosphatidylinositol metabolism, probably by enhancing phosphatidylinositol breakdown.

The control by vasopressin of carbohydrate and lipid metabolism in the perfused rat liver

Vasopressin-induced glucose release from the perfused livers of fed rats is diminished in the presence of insulin or following adrenal ablation. The reduced rate of glucose release following vasopressin treatment in the perfused livers of adrenalectomized rats was restored towards the control value by cortisol treatment in vivo. Vasopressin did not influence the total rate of fatty acid synthesis in the livers of fed rats perfused with medium containing glucose and two concentrations of lactate. The contribution of these precursors to hepatic fatty acid synthesis and CO2 production was similarly uninfluenced by vasopressin. Vasopressin casued a transient increase in the release of K+ by the perfused liver which was observed within 2 min of hormone administration. These results are discussed in relation to the possible mode of action of vasopressin in the liver.

The influence of vasopressin and related peptides on glycogen phosphorylase activity and phosphatidylinositol metabolism in hepatocytes

The relative abilities of seven vasopressin-like peptides to activate hepatic glycogen phosphorylase and stimulate phosphate incorporation into phosphatidylinositol were compared. Although the individual peptides differed in their potencies, the concentrations required to stimulate phosphatidylinositol metabolism were always greater (about 10 times) than those needed to activate phosphorylase. The molecular specificity of the hepatic vasopressin receptor and the role of vasopressin-stimulated phosphatidylinositol turnover are discussed.

Stimulation by vasopressin, angiotensin and oxytocin of gluconeogenesis in hepatocyte suspensions

1. In hepatocytes from starved rats, vasopressin, angiotensin (angiotensin II) and oxytocin stimulated gluconeogenesis from lactate by 25--50%; minimal effective concentrations were about 0.02pM, 1 nM and 0.2 nM respectively. 2. Vasopressin and angiotensin also stimulated gluconeogenesis from alanine, pyruvate, serine and glycerol. EGTA decreased gluconeogenesis from these substrates. 3. Hormonal stimulation of gluconeogenesis from lactate was abolished in the absence of extracellular Ca2+. 4. Insulin did not prevent stimulation of gluconeogenesis by vasopressin or angiotensin. 5. The potency of the stimulatory effects of vasopressin and angiotensin on hepatic gluconeogenesis suggests they are operative in vivo. Also, the data suggest that Ca2+ plays a role in the stimulation by these hormones.

The metabolic and endocrine effects of circulating catecholamines in fetal sheep

1. Adrenaline and noradrenaline have been infused into the fetal sheep to produce plasma concentrations comparable to those seen during hypoxia. The effects have been compared with those of isoprenaline and methoxamine and the sensitivity to beta- and alpha-adrenergic antagonists has been followed. 2. Adrenaline caused an alpha-mediated increase in blood glucose that is associated with a fall in plasma insulin concentration. It also caused a beta-mediated increase in plasma lactate, free fatty acid and amino acid concentrations. 3. Noradrenaline was much less effective than adrenaline at eliciting metabolic responses. It had not cause a significant change in plasma glucose concentration, although this was associated with a small increase in plasma insulin concentration. It caused a small rise in the concentration of lactate and free fatty acids in fetal plasma but had no effect on plasma amino acids. 4. The beta- and alpha-adrenergic antagonists propranolol and phentolamine alone were without effect on any of the plasma metabolites or hormones assayed. Isoprenaline increased plasma glucose, lactate, free fatty acid, alanine and insulin concentrations, while methoxamine only increased plasma glucose and lactate, and this was associated with a fall in insulin concentration. 5. The concentration of ACTH in fetal plasma was increased by adrenaline and to a lesser extent by noradrenaline and methoxamine; these were blocked by phentolamine. Isoprenaline also caused a small increase in ACTH. There were no corticosteroid changes associated with the increase in ACTH. 6. The results have been discussed in relation to the adrenergic and pancreatic control of metabolism in the fetal sheep.

Activation of pyruvate dehydrogenase in the perfused rat liver by vasopressin

The proportion of pyruvate dehydrogenase in its active form is doubled in rat liver within 5 min of addition of vasopressin to the perfusing medium.