ISOPROTERENOL EVOKES EXTRACELLULAR CA2-GLAND CELLS( SPIKES DUE TO SECRETORY EVENTS IN SALIVARY)

Secretory cells should in principle export substantial amounts of calc ium via exocytosis since Ca2+ is sequestered in secretory granules, Ba sed on a new technique for measurements of the extracellular calcium c oncentration in the vicinity of the cell membrane and on the droplet t echnique, we have monitored the rate of calcium extrusion from salivar y gland acinar cells, Isoproterenol (ISP), a beta-adrenergic agonist a nd powerful secretogogue, evoked no change in the cytosolic free Ca2concentration ([Ca2+](i)) but induced vigorous extracellular Ca2+ conc entration ([Ca2+](i)) spiking. The absence of [Ca2+](i) elevation and the pulsatile nature of the changes in [Ca2+](i) indicate that these s pikes are most likely due to calcium release from secretory granules, The cholinergic agonist acetylcholine (ACh), which induces moderate se cretion, evoked a marked rise in [Ca2+](i) and a smooth rise in [Ca2+] (i), most likely induced by plasma membrane calcium pumps, on which sh ortlasting [Ca2+](i) spikes were superimposed. The rate of ISP-induced calcium efflux was very substantial, The calculated calcium loss duri ng the first 100 s of supramaximal stimulation corresponded to a reduc tion of the total cellular calcium concentration of approximately 0.4 mM. We conclude that in salivary glands, calcium release via exocytosi s is one of the main mechanisms extruding calcium from cells to the ex tracellular milieu.