CALCIUM AND STIMULUS-SECRETION COUPLING IN THE NEUROHYPOPHYSIS

Abstract

ABSTRACT Slices of bovine neurohypophyses (wet weight approximately 50 mg), were used as an in vitro model for studying stimulus-secretion coupling for antidiuretic hormone. Release of hormone could be stimulated repeatedly for a period of at least 140 min by a 56 mm K+ medium or in an electrical field. Varying the current caused release of varied amounts of hormone. Several methods were used in an attempt at demonstrating intra-axonal calcium uptake during stimulated release. Stimulation with a 56 mm potassium medium did not cause any significant increase in the influx of 45Ca2+ measured by conventional technique or in total tissue calcium. Kinetic analyses of 45Ca2+ "desaturation" curves showed that incubation in a 56 mm potassium medium for 30 min did increase the 45Ca2+ in a slowly washed out compartment (presumably intracellular). However, the rate of washout of 45Ca2+ was decreased. Electrical field stimulation caused release of hormone in comparable amounts, but this stimulation did not cause any significant increase in 45Ca2+ in the slowly washed out compartment. Application of the "lanthanum method" of van Breemen and coworkers for studying changes in a calcium compartment that is presumably intracellular, gave the same results as the studies of desaturation curves without lanthanum. It is suggested that increased 45Ca2+ "uptake" found during stimulation in a 56 mm K+ medium is a phenomenon which cannot be used as evidence that a calcium uptake is essential for release. Even using these more detailed methods of study, it could thus not be demonstrated that hormone release is always associated with a measurable uptake to the cell interior of calcium. If calcium uptake is obligatory for secretion, it would appear that it is very small. Uptake of a very small amount would, however, be in agreement with recent findings using very special techniques in squid axons. A very small uptake would also agree with recent experiments done in our laboratory in which Ca2+ conductance inhibitors were applied on neurohypophyseal tissue.