Transmembrane and transepithelial movement of calcium during stimulus-secretion coupling

Publisher Website
Google Scholar
Abstract
Electrophysiological studies were undertaken to determine the transmembrane and transepithelial changes in free Ca2+ concentration that occur during serotonin-induced secretion in the salivary glands of the blowfly, Phormia regina. Ca-selective and conventional microelectrodes were used to measure intracellular and luminal Ca2+ concentrations ([Ca]i, [Ca]L), serosal membrane and transepithelial potentials (Em, Etr), and their changes during serotonin (5-hydroxytryptamine, 5HT)-induced salivary secretion. The effect of stimulus concentration on these parameters and enzyme release also was determined. Previous studies provided evidence that serosal stimulation with 10-8 M 5HT caused a hyperpolarization of Em and short phasic 2- to 3-fold increases in [Ca]i. In these studies, higher concentrations of 5HT (10-7 M) resulted in depolarization of Em by 14 .+-. 1.3 mV decrease in enzyme release. Serotonin (10-8 M) reduced the normal spontaneous Etr (+19.6 .+-. 1 mV) to near zero while causing an increase in [Ca]L from 1.3 .+-. 0.3 .times. 10-5 mM to 2.0 .+-. 0.1 .times. 10-3 mM, a concentration isomolar with that of the bathing medium. These results provide direct electrochemical evidence that, during stimulus-secretion coupling of the salivary epithelial cells, the neurohormone serotonin controls the secretory response by the regulation of intracellular Ca2+ and induces transepithelial transport of Ca2+. During secretion, the neurohormone probably causes the salivary gland to behave as a leaky epithelium by activating the paracellular shunt pathways.