P. Belan et al., ISOPROTERENOL EVOKES EXTRACELLULAR CA2-GLAND CELLS (VOL 273, PG 4106,1998)( SPIKES DUE TO SECRETORY EVENTS IN SALIVARY), The Journal of biological chemistry, 273(17), 1998, pp. 10806
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+](o)) spiking. The absence of [Ca2+](i) elevation and
the pulsatile nature of the changes in [Ca2+](o) 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+]
(o), most likely induced by plasma membrane calcium pumps, on which sh
ortlasting [Ca2+](o) 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.