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Pancreatic plasma membranes containing a high adenylate cyclase activity and a low contamination by cytochrome c oxidase were isolated from the rat by sucrose density centrifugation. The preparation contained an (Mg,Ca)‐ATPase of high activity with the following characteristics. The ATPase activity was shown to have two apparent Km values for Mg‐ATP (0.24 ± 0.09 mM and 1.15 ± 0.21 mM) and two apparent Km values for Ca‐ATP (0.14 ± 0.09 mM and 0.68 ± 0.10 mM). Mg‐GTP and Ca‐GTP were also hydrolysed by the preparation. The phase transition temperature was 19.3 ± 1.0°C for the Mg‐ATPase and 22.6 ± 1.1°C for the Ca‐ATPase activities. Three lines of evidence suggest that Mg‐ATP and Ca‐ATP were substrates for the same enzyme: Mg‐dependent and Ca‐dependent activities were not additive; the two activities showed the same pH optimum at 8.0; and the nonionic detergents Triton X‐100, Triton X‐305, Triton N‐101, Lubrol P 12 A, and digitonin, produced a parallel solubilization of the two activities. Enzyme activities were insensitive to potassium, sodium, ouabain, pancreozymin, carbamoylcholine, secretin, concanavalin A, wheat germ agglutinin, and soybean lectin.

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(1) The kinetic parameters of rat pancreatic adenylate cyclase were evaluated, using GTP, p[NH]ppG or GTP gamma S as nucleotide activator, cholecystokinin as peptide hormone, and GDP beta S and dibutyryl cyclic GMP as inhibitors of guanosine triphosphate and CCK-8, respectively. The time courses of activation and the degree of activation at steady state (EA/ETOT) were compatible with a simple two-state model of activation-deactivation based on a pseudo-monomolecular activation process (rate constant kappa+1), and a deactivation process (rate constant kappa off) that included, depending on the activating nucleotide, the hydrolysis of GTP (rate constant kappa 2) and/or the dissociation of the intact nucleotide (rate constant kappa-1), so that EA/ETOT = kappa+1/(kappa+1 + kappa 2 + kappa-1). (2) The hormone CCK-8 increased the value of kappa+1 with GTP dose-dependently, from 0.2 to 10.9 min-1. The value of kappa-1 increased 0.01 to 0.3 min-1 but the value of kappa 2 was unaltered at 7 min-1, so that EA/ETOT increased 15-fold, from 4% to 61%. (3) A cholera toxin pretreatment at 30 micrograms/ml allowed also a large increase in EA/ETOT with GTP (up to 51%) but the underlying mechanism was different. It consisted of a 14-fold decrease in the kappa off value of the GTP-activated enzyme (from 7 min-1 to 0.5 min-1) that corresponded to a reduction in GTPase activity. When testing the system with p[NH]ppG, two added effects of the cholera toxin pretreatment were observed: a 4-fold increase in the value of kappa+1 (from 0.2 to 0.8 min-1) and the occurrence of a significant 0.3 min-1 value for kappa-1.

S U MMARYo Pancreozymin l.lo" M and caerulein IdO M provoked a 30-40 % increase in 32p orthophosphate incorporation into proteins preexisting in rat pancréas fragments incubated for 2060 min. This effect was significant in ail subcellular fractions but was most évident in the membrane proteins of zymogen granules. Thèse results suggest th at h ormonal stimulussecretion coupling in the rat exocrine pancréas involves th e activation of a protein phosphotransferase and th e subséquent ph osph orylation of proteins directly implicated in exocytosis. Acinar cells represent at least 90 % of the pancreaso Their sécrétion of hydrolases is stimulated by pancreozymin and caerulein, a related decapep tideo Th e rôle of cyclic AMP as mediator of ecbolic stimulation is still subject to controversy. Dibutyryl cAMP partially reproduces th e effects of pancreozymin on enzymatic sécrétion but it neither induces a ph osph olipidic effect nor does it inh ibit amino acid incorporation into proteins (1). The hormone concentration provoklng a halfmaximal stimulation of adenyl cyclase in pancréas homogenates (1.5 10~^ M) is 200 times h igh er th an th at producing maximal in vitro sécrétion (2,1). Moreover, contradictory results h ave been publish ed on the effects of pancreozymin on cAMP levels in the pancréas (3,4). Th e rôle of cAMP as mediator belng not yet firmly establish ed, we decided to investigate th e effects of pancreozymin and caerulein on th e in vitro ph os ph orylation of proteins in the rat exocrine pancréas. Cyclic AMP and possibly also cyclic GMP indeed activate protein phosphotransferases in the target organs of a number of hormones, and th e resulting ph osph orylation of some proteins modifies th eir biological activity and finally expresses the ph ysio logical effect of the hormones under considération.

Cholecystokinin (CCK) receptors were investigated in the tumoral acinar cell line AR 4-2 J derived from rat pancreas, after preincubation with 20 nM dexamethasone. At steady state binding at 37 degrees C (i.e., after a 5 min incubation), less than 10% of the radioactivity of [125I]BH-CCK-9 (3-(4-hydroxy-[125I]iodophenyl)propionyl (Thr34, Nle37) CCK(31-39)) could be washed away from intact cells with an ice-cold acidic medium, suggesting high and rapid internalization-sequestration of tracer. By contrast, more than 85% of the tracer dissociated rapidly after a similar acid wash from cell membranes prelabelled at steady state. In intact AR 4-2 J cells, internalization required neither energy nor the cytoskeleton framework. Tracer internalization was reversed partly but rapidly at 37 degrees C but slowly at 4 degrees C. In addition, two degradation pathways of the tracer were demonstrated, one intracellular and one extracellular. Intracellular degradation occurred at 37 degrees C but not at 20 degrees C and resulted in progressive intracellular accumulation of [125I]BH-Arg that corresponded, after 1 h at 37 degrees C, to 35% of the radioactivity specifically bound. This phenomenon was not inhibited by serine proteinase inhibitors and modestly only by monensin and chloroquine. Besides, tracer degradation at the external cell surface was still observable at 20 degrees C and yielded a peptide (probably [125I]BH-Arg-Asp-Tyr(SO3H)-Thr-Gly). This degradation pathway was partly inhibited by bacitracin and phosphoramidon while thiorphan, an inhibitor of endopeptidase EC 3.4.24.11, was without effect.