Jean Christophe

Characterization of VIP-sensitive adenylate cyclase in guinea pig brain

This paper demonstrates that VIP activates an adenylate cyclase from a synaptosomal fraction of guinea pig brain. This activation was not potentiated by guanyl triphosphate nucleotides, and was unaffected by α- and β-adrenergic blockers and by atropine. Furthermore, peptides related to VIP, like secretin, glucagon and somatostatin, were devoid of significant agonistic or antagonistic activity. EGTA was also without effect on basal and VIP stimulated activities while calcium at concentrations higher than 10-5 M inhibited both activities.

In vitro action of bombesin and bombesin-like peptides on amylase secretion, calcium efflux, and adenylate cyclase activity in the rat pancreas: a comparison with other secretagogues.

Bombesin (a tetradecapeptide), the C-terminal nonapeptide of bombesin (bombesin-NP), and litorin (a parent nonapeptide), each stimulated amylase secretion from rat pancreatic fragments. These responses were not affected by atropine. The concentrations that produced half-maximal stumulation of secretion were 0.25 nM for bombesin, 0.30 nM for bombesin-NP, and 0.07 nM for litorin, as compared to 0.12 nM for caerulein and 0.80 muM for the cholinergic agent carbamylcholine. When used at maximal concentrations, bombesin, bombesin-NP, and litorin showed no action on cyclic AMP levels in the presence of 5 mM theophylline. By contrast, caerulein and secretin increased cyclic AMP levels by 27 and 208%, respectively. Bombesin, bombesin-NP, and litorin did not activate adenylate cyclase in a purified pancreatic plasma membrane preparation, whereas caerulein and secretin increased this activity 20 and 16-times, respectively...

VIP activation of rat anterior pituitary adenylate cyclase

The brain gut octacosapeptide VIP (vasoactive intestinal peptide) is found in discrete areas in the cerebral cortex, the hypothalamus and the posterior pituitary gland. Specific VIP binding sites are coupled to an adenylate cyclase system in synaptic membranes from guinea pig brain. Besides, the concentration of VIP in the hypothalamo-hypophysial portal vessels is much higher than in the systemic blood. The peptide has however no established function in the hypophysis. These data document the presence in the rat pituitary of functional VIP receptors existing in the form of a VIP-stimulated adenylate cyclase system and suggest that VIP might be a major peptidic activator of rat adenopituitary membrane adenylate cyclase.

Short-term adaptation of pancreatic hydrolases to nutritional and physiological stimuli in adult rats

Abstract o 1. Adult rats, previously maintained on a chow containing 48 p. cent carbohydrates, were fasted 24 h, then refed for 5 days on various diets. The nutritional stimuli were made up either of 4 p. cent corn oil, 67 p. cent of a carbohydrate, and 18 p. cent casein, or of 50 p. cent lipid, 0 p. cent carbohydrates and 18 p. cent casein (w:w). Other rats were simply starved for 3 or 6 days, and a group was diabetic after 7 days on chow, following alloxan administration. 2. The opposite regulations of α-amylase and lipase were striking. The specific activities of both hydrolases tripled within 5 days in the presence of their best inducers (starch and corn oil respectively). The specific activity of the second enzyme decreased simultaneously to one third of its initial value. These variations were reversible. 3. The specific activity of amylase on 67 p. cent starch, glucose, fructose or sucrose did not exceed that on the 48 p. cent carbohydrate chow. The induction by galactose and lactose was mediocre. Amylase decreased sharply during fasting and in diabetic rats. The enzyme was always depressed on fat-diets, but somewhat less so on tricaprylin. 4. When compared to control values on chow, several circumstances favored lipase induction. The specific activity of lipase in the pancreas doubled in diabetic rats. The increase on 67 p. cent galactose was probably due to reduced glucose tolerance. Diets rich in starch, fructose and sucrose also increased lipase. All 50 p. cent fat diets stimulated lipase. Triolein, and various oils (olive, corn, sunflower, walnut and lineseed oils) yielded results twice as good as those with more saturated fats (tricaprylin, tristearin and lard). 5. The activities estimated in the small intestine partially reflected inductions.

In vivo effects of pancreozymin, secretin, vasoactive intestinal polypeptide and pilocarpine on the levels of cyclic AMP and cyclic GMP in the rat pancreas

Pancreozymin (PZ) and muscarinic cholinergic agents powerfully stimulate the secretion of pancreatic hydrolases. Their mode of action is at the present time unclear, though Yransmembranous ionic movements involving primarily Ca2+ [ 1,2] , and cyclic AMP [3-51 have been implicated. However, the fact that calcium is involved [6] in ‘stimulus-secretion coup ling’ does not prove that this ion is a necessary mediator of emiocytosis. A pancreatic adepylatecyclase can be stimulated by pancreozymin [5,7,8] yet the status of cyclic AMP as a second messenger of the secretagogues is ambiguous. The occurrence in the rat pancreas of phosphodiesterase activity capable of hydrolyzing physiological concentrations of cyclic GMP [9] and of cyclic GMP-dependent protein kinase [lo] suggests that cyclic GMP may intervene in pancreatic secretion. It was decided therefore to compare the in vivo time course of cyclic AMP and cyclic GMP levels following administration of pancreozymin, pilocarpine, secretin and vasoactive intestinal polypeptide (VIP).

Effects of somatostatin on pancreatic exocrine function. Interaction with secretin

Abstract The release of hydrolases from rat pancreas was stimulated in vivo and in vitro by somatostatin. In vitro this hypersecretion was accompanied by a moderate but significant rise in intracellular cyclic AMP. In addition, the tetradecapeptide inhibited rises of cyclic AMP provoked by secretin. The existence of the same sequence of four amino acid residues in the two peptides suggests that somatostatins activation of the exocrine pancreas depends on its interaction with secretin receptors.

Vasoactive intestinal peptide: Levels and functional receptors in rat brain before and after weaning

Abstract The average level of VIP was found to be 17 pmol/g wet weight in the brain of the newborn rat. This level ramained constant during the first two weeks after birth then increased progressively to 40 pmol/g wet weight at 20 days, a value comparable to that found in adult animals. VIP was already able to stimulate brain membrane adenylate cyclase activity at birth. The stimulation with 10 −6 M VIP allowed a 2.5-fold increase in basal activity in membranes from 1 to 14-days-old pups as compared to a 1.7-fold stimulation in membranes from adult brain. The apparent activation constant for VIP adenylate cyclase stimulation was 4.10 −7 M at all ages. The efficiency of VIP activation amounted to as much as 70% of that of fluoride at birth and to 35% only of fluoride activation in brain membranes from adult rats.

The influence of detergents and trypsin on the stimulation of amylase secretion by either pancreozymin or sodium fluoride in the perfused rat pancreas

Abstract (1) Rat pancreas fragments were perfused for 2hr with Krebs-Ringer bicarbonate buffer enriched with 10 mM glucose and Trasylol (500 UIK/ml). Amylase output was estimated at 5-min intervals on successive samples of the effluent. (2) Pancreozymin at a concentration of 7.10−9 M doubled amylase output when introduced after 1 hr of preincubation. Administration of 10 mM NaF promoted a biphasic effect. The initial and transient hypersecretory peak was followed by a second and more prolonged period of hypersecretion. It is assumed that the primary component of the biphasic response was due to hyperosmolarity, and it is tentatively suggested that the secondary response to NaF was the result of variations in the phosphorylation of membrane proteins. (3) Paired tissue fragments were pre-exposed for 30 min to digitonin, sodium dodecylsulfate, Triton X-100, or bovine trypsin (the proteolytic enzyme in the absence of Trasylol). The basal output of amylase rose with increasing detergent concentrations (from 100 to 500 μg/ml but not after trypsin pretreatment. The four agents were equally effective in reducing the sensitivity to pancreozymin. They did not impair the initial osmotic response to NaF, but did curtail the prolonged second NaF hypersecretory effect. Digitonin and dodecylsulfate were less effective in this respect than either Triton X-100 or trypsin though being equally detrimental to pancreozymin action. (4) These observations suggest that the regulation in vitro of secretion by pancreozymin and NaF in intact acinar cells of the rat pancreas involves two distinct loci of a membrane-lipoprotein complex.