Isoproterenol evokes extracellular Ca2+ spikes due to secretory events in salivary gland cells

Abstract

Secretory cells should in principle export substantial mounts of calcium via exocytosis since Ca2+ is sequestered in secretory granules. Based on a new technique for measurements of the extracellular calcium concentration in the vicinity of the cell membrane and on the droplet technique, we have monitored the rate of calcium extrusion from salivary gland acinar cells. Isoproterenol (ISP), a β-adrenergic agonist and powerful secretogogue, evoked no change in the cytosolic free Ca2+ concentration ([Ca2+](i)) but induced vigorous extracellular Ca2+ concentration ([Ca2+](i)) spiking. The absence of [Ca2+](i) elevation and the pulsatile nature of the changes in [Ca2+](i) indicate that these spikes are most likely due to calcium release from secretory granules. The cholinergic agonist acetylcholine (ACh), which induces moderate secretion, evoked a marked rise in [Ca2+](i) and a smooth rise in [Ca2+](i), most likely induced by plasma membrane calcium pumps, on which shortlasting [Ca2+](i) spikes were superimposed. The rate of ISP-induced calcium efflux was very substantial. The calculated calcium loss during the first 100 s of supramaximal stimulation corresponded to a reduction of the total cellular calcium concentration of approximately 0.4 mM. We conclude that in salivary glands, calcium release via exocytosis is one of the main mechanisms extruding calcium from cells to the extracellular milleu.