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Atropine; Monophosphate; Guanosine; Secretin; Isoproterenol
Abstract:
Secretagogues of pancreatic enzyme secretion, the hormones pancreozymin, carbamylcholine, gastrin I, the octapeptide of pancreozymin, and caerulein as well as the Ca++ -ionophore A 23187 stimulate 45Ca efflux from isolated pancreatic cells. The non-secretagogic hormones adrenaline, isoproterenol, secretion, as well as dibutyryl cyclic adenosine 3',5'-monophosphate and dibutyryl cyclic guanosine 3',5'-monophosphate have no effect on 45Ca efflux. Atropine blocks the stimulatory effect of carbamylcholine on 45Ca efflux complately, but not that of pancreozymin. A graphical analysis of the Ca++ efflux curves reveals at least three phases: a first phase, probably derived from Ca++ bound to the plasma membrane; a second phase, possibly representing Ca++ efflux from cytosol of the cells; and a third phase, probably from mitochondria or other cellular particles. The Ca++ efflux of all phases is stimulated by pancreozymin and carbamylcholine. Ca++ efflux is not significantly effected by the presence or absence of Ca++ in the incubation medium. Metabolic inhibitors of ATP production. Antimycin A and dinitrophenol, which inhibit Ca++ uptake into mitochondria, stimulate Ca++ efflux from the isolated cells remarkably, but inhibit the slow phase of Ca++ influx, indicating the role of mitochondria as an intracellular Ca++ compartment. Measurements of the 45Ca++ influx at different Ca++ concentrations in the medium reveal saturation type kinetics, which are compatible with a carrier or channel model. The hormones mentioned above stimulate the rate of Ca++ translocation. The data suggest that secretagogues of pancreatic enzyme secretion act by increasing the rate of Ca++ transport most likely at the level of the cell membrane and that Ca++ exchange diffusion does not contribute to the 45Ca++ fluxes.
