AJP - Gastrointestinal and Liver Physiology, Vol 241, Issue 2 150-G158, Copyright © 1981 by American Physiological Society
Transmembrane and transepithelial movement of calcium during stimulus-secretion coupling
J. O'Doherty and R. J. Stark
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]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 was also
determined. Previous studies provided evidence that serosal stimulation
with 10(-8) M 5HT caused a hyperpolarization of Em and short phasic two- to
threefold increases in [Ca]i. In these studies, higher concentrations of
5HT (10(-7) M) resulted in depolarization of Em by 13 +/- 1.2 mV and of ECa
by 64 +/- 2.1 mV, a dramatic increase in [Ca]i, and a decrease in enzyme
release. In addition, 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 X 10(-5) mM to 2.0 +/- 0.1 X 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+, thereby suggesting that, during
secretion, the neurohormone causes the salivary gland to behave as a "leaky
epithelium" by activating the paracellular shunt pathways.
