Role of cyclic adenosine monophosphate in amylase release from dissociated rat pancreatic acini

Effect of adenosine and adenosine agonists on amylase release from rat pancreatic lobules

The effects of adenosine on the amylase secretion from rat pancreatic lobules have been studied. Adenosine induces a dose-dependent stimulation on amylase release, which is maximal at a concentration of 10(-4) M. It has been observed a clear inhibition of this secretory action when atropine was added whereas no amylase release was seen in isolated acini after adenosine. The effect of adenosine is completely blocked by the adenosine receptors antagonist theophylline (10(-4) M), but not by dipyridamole (10(-3) M), a drug that inhibits the transport of adenosine into the cell. The increase of amylase secretion induced by adenosine is inhibited by either the relatively selective A1 receptor antagonist PD116,948 (10(-6) M) and by the A2 receptor antagonist PD115,199 (10(-6) M). Significant increases of amylase release are observed after the relatively selective A1 receptor agonist R-PIA (10(-5) M) and after the relatively selective A2 receptor agonist NECA (10(-4) M). Finally, the effect of R-PIA is not modified by coincubation with PD115,199 and the effect of NECA is not affected by coincubation with PD116,948. These results suggest that the action of adenosine is mediated through the release of acetylcholine and probably by the simultaneous occupation of both A1 and A2 adenosine receptors, whereas the intracellular action of adenosine could be discarded.

Effects of growth hormone releasing factor on pancreatic secretion in vivo and in vitro

Growth hormone releasing factor (GRF), a 44-residue peptide originally isolated from human pancreatic tumors, shows structural similarities to the members of the secretin-vasoactive intestinal peptide (VIP) peptides. This study was designed to determine the effects of human GRF (hGRF-(1-44] on pancreatic secretion in vivo in conscious dogs and in vitro in dispersed rat pancreatic acini. GRF given i.v. in graded doses in dogs caused a small but significant stimulation of pancreatic HCO3- and protein outputs and potentiated secretin- and cholecystokinin (CCK)-induced pancreatic HCO3- but not protein secretion. When given together with somatostatin, GRF failed to reverse the inhibitory action of this peptide on HCO3- and protein responses to secretin plus CCK in dogs. Studies in vitro dispersed rat pancreatic acini showed that GRF added to the incubation medium of these acini caused an increase in basal amylase release and shifted to the left the amylase dose-response curve to caerulein and urecholine but failed to affect the amylase response to VIP. This study indicates that GRF in vivo stimulates basal and augments secretin- or CCK-induced pancreatic HCO3- secretion and that this is probably due to direct stimulatory action of the peptide on pancreatic secretory cells.

Effect of ethanol on amylase secretion and cellular calcium homeostasis in pancreatic acini from normal and ethanol-fed rats

The effects of ethanol on stimulus-secretion coupling were assessed by studying amylase release, Ca2+-homeostasis, and changes in physical properties of membranes in isolated rat pancreatic acini. In acini from normal rats, ethanol (50 mM and above) in vitro caused a dose-dependent stimulation of amylase release and an increase in cytosolic free Ca2+ concentration. Ethanol did not affect amylase secretion stimulated by cholecystokinin-octapeptide (CCK8), a secretagogue that acts by increasing cytosolic free Ca2+ levels, but did potentiate the secretion of amylase induced by vasoactive intestinal peptide (VIP) which raises intracellular cAMP. Ethanol also increased the rate of 45Ca2+ exchange. In acini labeled with the spin-probe 12-doxyl stearic acid, ethanol disordered the pancreatic plasma membranes. By contrast, in acini from animals that had chronically (6-7 weeks) ingested ethanol, the membranes were resistant to this disordering effect of ethanol. Chronic ethanol feeding lowered the total cellular calcium content and ionophore (A23187)-releasable pools of acinar calcium (11 and 24% respectively), and led to a 15-30% decrease in the rate of 45Ca2+ exchange. Chronic ethanol ingestion also lowered the basal rate of amylase secretion, but ethanol in vitro stimulated amylase secretion more than in control preparations. However, these differences in basal and ethanol-induced amylase secretion were not accompanied by corresponding changes in intracellular free Ca2+. The data suggest that ethanol perturbs cell membranes and also disturbs cellular Ca2+ homeostasis. These effects may explain its actions as a weak Ca2+-mediated secretagogue. However, the membrane alterations induced by chronic ethanol feeding do not prevent the ethanol-induced interference with cellular calcium homeostasis.

Effect of somatostatin on amylase secretion from in vivo and in vitro rat pancreas

In the present study the effect of somatostatin on amylase secretion was determined using in vivo cannulation and isolated acini from rat pancreas. In vivo somatostatin-14 inhibited amylase secretion in basal state and that stimulated with CCK8 and acetylcholine. Somatostatin-14 and somatostatin-28 failed to inhibit amylase secretion from isolated acini in basal state and that stimulated with CCK8 and bethanechol. Somatostatin-14 did not increase 45Ca uptake or efflux of label from acini preloaded with 45Ca. Cellular cyclic AMP levels were not significantly increased. Somatostatin-14 did not alter the synthesis of proteins in vitro, as judged by incorporation of a mixture of fifteen 14C-labeled amino acids. Somatostatin-14 stimulated phosphoprotein phosphatase in higher doses, whereas no effect was observed at lower doses. Inhibition of secretion in vivo and lack of stimulation of amylase secretion in isolated acini suggest that the somatostatin effect in vivo is mediated by an indirect effect similar to other peptides, for example, opiates and neurotensin. Stimulation of phosphoprotein phosphatase suggests that somatostatin may bind to the acinar cells and affect functions other than secretion and synthesis of enzymes.

Amylase secretion from isolated pure acinar cells

Isolation of pure acinar cells of the rat pancreas was achieved employing counterflow sedimentation filtration technique (CSFT). The preparation of purified acinar cells contained an occasional red blood cell (RBC, 200:1) with total absence of endocrine and duct cells. A significant stimulation of amylase secretion from isolated pure acinar cells was produced by octapeptide of cholecystokinin (CCK8) and insulin produced potentiation of the effect of CCK8. Synthetic glucagon inhibited basal and CCK8 stimulated amylase secretion. Non-synthetic purified glucagon stimulated amylase secretion and potentiated the effect of CCK8. Vasoactive intestinal polypeptide (VIP) did not stimulate amylase secretion but potentiated the effect of CCK8. No leakage of lactic dehydrogenase (LDH) was detected from the cells in any of the secretion studies. Thus a highly purified preparation of isolated pure acinar cells of rat pancreas could be obtained with excellent morphologic and functional integrity.

Effect of endorphins on amylase secretion from rat pancreas in vivo and in vitro

The present study was done both in vivo by cannulating pancreatic duct of rats and in vitro using pancreatic slices and dissociated acini to determine the mode of action of endogenous opiate peptides on pancreatic acinar cell. Pancreatic slices were incubated with beta-endorphin or (Met)5-enkephalin alone and in combination with CCK8. Dissociated acini were incubated with naloxone, substance P, VIP, (Met)5- and (Leu)5-enkephalin and alpha-, beta-, and gamma-endorphin alone or in combination with CCK8. In vivo, both beta-endorphin and (Met)5-enkephalin did not alter basal secretion but inhibited CCK8-stimulated amylase secretion. This effect was not reversed by administration of naloxone. In the slices, neither beta-endorphin nor (Met)5-enkephalin altered basal or CCK8-stimulated secretion. In the dissociated acini, substance P and VIP significantly increased amylase secretion, whereas naloxone, enkephalins, and endorphins failed to alter amylase secretion. CCK8 increased amylase secretion greater than sixfold. In combination with enkephalins and endorphins, there was neither inhibition nor potentiation of CCK8 effect. These data indicate that the effect of opiate peptides on pancreatic acinar cells in the rat are nonspecific and appear not to be mediated by opiate receptors.

Ca2+ and cyclic nucleotide dependence of amylase release from isolated rat pancreatic acinar cells rendered permeable by intense electric fields

Enzyme digestion of rat pancreatic tissue yielded a preparation of isolated acinar cells, over 90% of which excluded trypan blue. These isolated cells responded to a variety of secretagogues, the responses being sensitive to the removal of extracellular calcium, increasing extracellular magnesium, and by trifluoperazine, an antagonist of Ca-dependent processes. When exposed to intense electric fields, isolated acinar cells became permeable to CaEGTA and MgATP, these markers gaining access to over 60% of the intracellular milieu within minutes. The accessibility to these markers seemed independent of the ionised Ca2+ level. Less than 0.5% of the cellular amylase was released when cells were rendered leaky in a medium containing about 10(-9) M Ca2+, but typically 4% was released when the Ca2+ level was subsequently raised to 10(-5)M levels, the EC50 for Ca2+ being 2 microM. This amount of amylase released was comparable to the amounts secreted from intact cells in response to a variety of agonists. The cytosolic marker lactate dehydrogenase was also released from leaky cells, but the extent was independent of Ca2+ concentration. No amylase was released at 10(-7)M Ca2+ when permeable cells were exposed to cyclic 3',5'-AMP or cyclic 3',5'-GMP. The calcium activation curve for amylase release seemed to be independent of cyclic nucleotides, but was markedly increased in both the extent of release and apparent affinity for Ca2+ in the presence of the phorbol ester 12-0-tetradecanoyl phorbol 13 acetate. These results suggest that when "functionally normal" isolated acinar cells are rendered permeable, Ca2+-but not cyclic nucleotides-acts as a second messenger for amylase secretion, and furthermore that protein kinase C may be involved in the secretory process.

Interaction of cholecystokinin and vasoactive intestinal polypeptide on function of mouse pancreatic acini in vitro

In isolated mouse pancreatic acini, vasoactive intestinal polypeptide (VIP) and secretin potentiated amylase release stimulated by cholecystokinin (CCK). VIP (1-100 nM) or secretin (100-1000 nM) alone elicited a negligible secretory response, whereas in combination with CCK, these agents induced a significantly larger response. VIP increased maximal amylase release elicited by CCK without affecting the potency with which CCK stimulated secretion. The phosphodiesterase inhibitor, 3-isobutyl-1-methyl xanthine (IBMX), from 0.03-1.0 mM had effects on secretion similar to those of VIP. VIP, IBMX and 8-Br-cyclic AMP, all of which act through or mimic the action of cyclic AMP, potentiated the secretory response to maximal concentrations of CCK, carbamylcholine and the ionophore A23187, all of which act via intracellular calcium. In contrast to amylase release, stimulation of acinar glucose transport by CCK or carbamylcholine was not augmented by VIP, secretin, IBMX or 8-Br-cyclic AMP. The results indicate that for amylase release from mouse pancreas, secretagogues acting via cyclic AMP potentiate those acting via calcium. However, potentiation does not apply to all biological responses of the pancreatic acinus and each response must be studied individually.

Stimulatory effects of Gila monster venom on rat pancreatic acini

The effects of Gila monster venom on dispersed rat pancreatic acini were compared with those of secretin and VIP. The efficacy of the venom in terms of amylase release was much higher (a 24-fold increase over basal secretion) than that of secretin (a 4-fold increase) and VIP (+ 40% only). On the other hand, cyclic AMP levels increased 12-fold with the venom, as compared to 18-fold with secretin and 16-fold with VIP. The venom, VIP and secretin all displaced 125I-VIP and the competition curve with the venom was steeper, suggesting that all VIP-recognizing receptors bound the venom with the same affinity. VIP receptors were, however, not responsible for the release of amylase provoked by the venom since VIP (and secretin) did not inhibit the secretory action of the venom. The venom exerted no effect on 45Ca efflux and its secretory effect did not depend on the presence of external calcium. Besides, the effect of CCK-8 on amylase release was additive with the effect of the venom. A first exposure to the venom induced a refractoriness to itself with respect to amylase release but not in terms of cyclic AMP increase. In conclusion, Gila monster venom may contain one component binding to VIP/secretin receptors with resulting cyclic AMP elevation. A second venom component may be responsible for the high secretory efficacy, without involving cyclic AMP or calcium efflux.