CA++ FLUXES IN ISOLATED CELLS OF RAT PANCREAS - EFFECT OF SECRETAGOGUES AND DIFFERENT CA++ CONCENTRATIONS

Secretagogues of pancreatic enzyme secretion, the hormones pancreozymin, carbamylcholine, gastrin I, the octapeptide of pancreozymin and caerulein as well as the Ca2+-ionophore A23187 stimulated 45Ca efflux from isolated pancreatic cells. The nonsecretagogic hormones adrenaline, isoproterenol, secretin, and dibutyryl cyclic[c]AMP and dibutyryl cGMP had no effect on 45Ca efflux. Atropine blocked the stimulatory effect of carbamylcholine on 45Ca efflux completely, but not that of pancreozymin. A graphical analysis of the Ca2+ efflux curves revealed at least 3 phases: a 1st phase, probably derived from Ca2+ bound to the plasma membrane; a 2nd phase, possibly representing Ca2+ efflux from cytosol of the cells; and a 3rd phase, probably from mitochondria or other cellular particles. The Ca2+ efflux of all phases was stimulated by pancreozymin and carbamylcholine. Ca2+ efflux was not significantly effected by the presence or absence of Ca2+ in the incubation medium. Metabolic inhibitors of ATP production, Antimycin A and dinitrophenol, which inhibited Ca2+ uptake into mitochondria, stimulated Ca2+ efflux from the isolated cells remarkably but inhibited the slow phase of Ca2+ influx, indicating the role of mitochondria as an intracellular Ca2+ compartment. Measurements of the 45Ca2+ influx at different Ca2+ concentrations in the medium showed saturation type kinetics which were compatible with a carrier or channel model. The hormones mentioned above stimulated the rate of Ca2+ translocation. Secretagogues of pancreatic enzyme secretion acted by increasing the rate of Ca2+ transport most likely at the level of the cell membrane, and Ca2+ exchange diffusion did not contribute to the 45Ca2+ fluxes.