Specific decrease of secretin/VIP-stimulated adenylate cyclase in the heart from the Lyon strain of hypertensive rats

The physiological roles of secretin and its receptor

Secretin is secreted by S cells in the small intestine and affects the function of a number of organ systems. Secretin receptors (SR) are expressed in the basolateral domain of several cell types. In addition to regulating the secretion of a number of epithelia (e.g., in the pancreas and biliary epithelium in the liver), secretin exerts trophic effects in several cell types. In this article, we will provide a comprehensive review on the multiple roles of secretin and SR signaling in the regulation of epithelial functions in various organ systems with particular emphasis in the liver. We will discuss the role of secretin and its receptor in health and biliary disease pathogenesis. Finally, we propose future areas of research for the further evaluation of the secretin/secretin receptor axis in liver pathophysiology.

Secretin and body fluid homeostasis

Body fluid homeostasis is critical for the survival of living organisms and hence is tightly controlled. From initial studies on the effects of secretin (SCT) on renal water reabsorption in the 1940s and recent investigations of its role in cardiovascular and neuroendocrine functions, it has now become increasingly clear that this peptide is an integral component of the homeostatic processes that maintain body fluid balance. This review, containing some of our recent findings of centrally expressed SCT on water intake, focuses on the actions of SCT in influencing the physiological, neuroendocrine, and cardiovascular processes that subserve body fluid homeostasis.

Genesis and regulation of the heart automaticity

The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated. Nevertheless, ion channels and intracellular Ca(2+) signaling are necessary for the proper setting of the pacemaker mechanism. Here, we review the current knowledge on the cellular mechanisms underlying the generation and regulation of cardiac automaticity. We discuss evidence on the functional role of different families of ion channels in cardiac pacemaking and review recent results obtained on genetically engineered mouse strains displaying dysfunction in heart automaticity. Beside ion channels, intracellular Ca(2+) release has been indicated as an important mechanism for promoting automaticity at rest as well as for acceleration of the heart rate under sympathetic nerve input. The potential links between the activity of ion channels and Ca(2+) release will be discussed with the aim to propose an integrated framework of the mechanism of automaticity.

Synergistic hypotensive effect of vasoactive intestinal polypeptide and alpha-blockade with phentolamine. Evidence for vasoactive intestinal peptide alpha-adrenoceptor coupling in the cardiovascular system of newborn dogs

Vasoactive intestinal polypeptide (VIP) is a neuropeptide with potent circulatory effects in the adult animal and human. Little is known about its effects or mechanism of action in the immature animal. These series of experiments evaluated the effects and possible mechanism of action of VIP on the developing canine cardiovascular system. In all three series, measurements of mean heart rate and blood pressure were taken in the control state, after parasympathetic denervation with bilateral cervical vagotomies, and after autonomic blockade with propranolol (1 mg/kg) and phentolamine (0.5 mg i.v.). In series 1, we characterized the role of alpha-adrenergic receptors in early newborn puppies by investigating the hemodynamic effects of phentolamine alone in five early newborn puppies. In series 2, the hemodynamic effects of intravenous VIP infusion (0.2 microgram/kg/min) were recorded and compared in six early newborn puppies and in 10 late newborn puppies. In series 3, the hemodynamic effects of phentolamine in the presence of VIP receptor binding inhibitor were studied. In early newborn puppies, VIP had essentially no effect on heart rate or blood pressure until phentolamine was given; then, blood pressure decreased by 17% (p less than 0.005). In late newborn puppies, VIP resulted in an increase in heart rate in the control state but not after parasympathetic or sympathetic denervation. In early newborn puppies, phentolamine alone resulted in a 24% decrease (p less than 0.005) in blood pressure, compared with a 54% decrease (p less than 0.005) in early newborn puppies preexposed to VIP infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations of rat cardiac adenylate cyclase activity with age

Basal, 5'-guanylimidodiphosphate, GTP-, NaF-, forskolin-, D,L-isoproterenol-, glucagon- and secretin-stimulated adenylate cyclase activities were investigated in cardiac membranes from young adult (6 month old), old (20 month old) and senescent (24 month old) Sprague Dawley rats. The only significant difference between old and young adult rats was a 43% decrease of the glucagon-stimulated enzyme activity. In senescent rats compared to young adult rats, we observed a 23% decrease in forskolin-stimulated enzyme activity, a more severe (-73%) decrease in glucagon-stimulated adenylate cyclase activity and a decrease (-38%) of the response to secretin. The response to the beta-adrenoreceptor agonist D,L-isoproterenol was unaffected. These results suggest an alteration with age in the vicinity of the catalytic unit of adenylate cyclase and a selective decrease of functional glucagon and secretin receptors.

Alteration of secretin-stimulated cardiac adenylate cyclase activity in rats with portacaval shunt

Wistar rats were submitted to portacaval anastomosis (PCA). Control rats were sham-operated and pair-fed (SOPF). After 3 weeks, PCA led to the hypertrophy of right atrium (+50%), left atrium (+67%) and both ventricles (+26%). The response of adenylate cyclase activity to secretin was specifically and markedly decreased in membranes from atria (-51 to 59%) and ventricles (-68 to 69%). These data suggest a decrease in the number of functional secretin receptors in heart considering that: the half-maximal stimulatory secretin concentration was unchanged; glucagon stimulations were unaltered and D,L-isoproterenol stimulations were hardly affected; the Gpp(NH)p-, NaF-, and forskolin-stimulated adenylate cyclase activities were moderately decreased (in ventricles, by 14-28%) or unchanged (in atria).

Heart receptors for VIP, PHI and secretin are able to activate adenylate cyclase and to mediate inotropic and chronotropic effects. Species variations and physiopathology

We have assessed the presence of VIP/PHI/secretin receptors in heart by: (1) testing the ability of the corresponding peptides to activate adenylate cyclase in cardiac membranes from rat, dog, Cynomolgus monkey and man, and (2) examining the ability of the same peptides to exert inotropic and chronotropic effects on heart preparations from rat and Cynomolgus monkey in vitro. Based on their affinity for natural peptides and synthetic analogs, two types of VIP/PHI/secretin receptors were characterized: the relatively nonspecific "secretin/VIP receptor" of rat heart (that is "secretin-preferring" only in that secretin was more efficient than VIP in stimulating adenylate cyclase), and the "VIP/PHI-preferring" receptor of man, monkey and dog heart. Four physiopathological situations affecting secretin/VIP receptors in rat heart were explored: In male rats from the Okamoto strain and the Lyon strain, two strains presenting spontaneous hypertension, heart membranes exhibited a markedly decreased response of adenylate cyclase to secretin/VIP, with lesser alterations in the responses to isoproterenol and glucagon. This impairment developed in parallel with the occurrence of hypertension and was reproduced in normotensive rats submitted to chronic isoproterenol treatment (but not in Goldblatt hypertensive rats). These findings are consistent with a hyperactivity of norepinephrine pathways in spontaneously hypertensive rats, leading to a reduced number of cardiac post-junctional secretin/VIP receptors bound to adenylate cyclase. Heart membranes from genetically obese (fa/fa) Zucker rats also exhibited severely decreased responses to secretin/VIP with lesser alterations in the responses to glucagon and isoproterenol. These anomalies were specific for the heart, and developed in concomitance with obesity. The first anomaly could not be corrected by severe food restriction. Secretin stimulation of heart adenylate cyclase was also selectively altered in streptozotocin-diabetic rats. Thus, two types of diabetic cardiomyopathy were characterized by a severe local alteration of secretin/VIP receptors coupled to adenylate cyclase. Hypothyroidism, provoked in rat by thyroidectomy or propylthiouracil treatment, again induced a marked decrease in secretin-stimulated cardiac adenylate cyclase activity. In rat papillary muscle electrically stimulated in vitro, secretin exerted a positive inotropic effect. This effect was reduced in obese (fa/fa) Zucker rats. In rat right atrium, secretin also exerted a positive chronotropic effects.(ABSTRACT TRUNCATED AT 400 WORDS)