Calcium movement in the neurohypophysis of the rat and its relation to the release of vasopressin

Abstract

We have recently described experiments on the neurohypophysis of the rat designed to reveal something of the events which occur when the neurosecretory terminals are stimulated and which lead to the extrusion of their stores of hormone (Douglas, 1963;Douglas & Poisner, 1963a, 1964. The experiments indicated that this train of events, which we refer to as 'stimulus-secretion coupling', involves depolarization followed by some calcium-dependent link. Thus vasopressin was released when the isolated neurohypophysis was stimulated electrically or was exposed to excess potassium; the stimulant effect of excess potassium varied with the extracellular calcium concentration over a wide range and was inhibited by magnesium; and, finally, the introduction of calcium was itself sufficient to evoke vasopressin release when the terminals were exposed to excess potassium. These findings led us to propose that 'stimulus-secretion coupling' in the neurohypophysis involves the following steps: first, the arrival of impulses propagated down the hypothalamo-hypophysial tract; second, depolarization of the neurosecretory terminals; and, third, an increased uptake of calcium. In this view the appearance of an excess of calcium somewhere in the neurosecretory terminals is the factor which sets in motion the process of extrusion of the posterior pituitary hormones.The present study was undertaken to obtain information on calcium movement in the neurohypophysis under the same experimental conditions. METHODSPreparation. The experiments were carried out on neurohypophyses from male rats weighing close to 140 g. The glands were removed and prepared for incubation as described previously (Douglas, 1963;Douglas & Poisner, 1964) except that the cut made along the sagittal plane to facilitate diffusion was incomplete to allow the individual glands to be recovered at the end of the experiment for measurement of radioactivity.Incubation media. The principal incubation medium was a modified 'Locke's solution' of the following composition (mm): NaCl, 154; KCI, 5-6; NaHCO3, 6-0; CaCl2, 2*2; MgC1,2 1-0; glucose 10-0. Several variations of this solution were used which had, for example,

Fusion of neurohypophyseal membranes in vitro

“…It is well established that the passage of.Ca 2+ across the cell membrane is a critical step in stimulus-secretion coupling in the neurohypophysis [1,2,3]. Although Ca 2+ influx is apparently too small to be reliably quantitated [4], experiments with Ca 2+ transport inhibitors [2] bear out this contention.…”

Exocytotic Secretion of Catecholamines From the Cat Adrenal Medulla by Sodium Deprivation: Involvement of Calcium Influx Mechanism

“…Similar tendencies were also observed in the inhibitory effects of Mg and Co on the two parameters. These ions thus inhibit secretion by blocking inward Ca movement, although the possibility has been raised that in some secretory cells, including the adrenal medulla, that the inhibitory action of Mg is exerted at the intracellular Ca sites (Douglas & Poisner, 1964;Rahman, Borowitz & Miya, 1973;Lastowecka & Trifar6, 1974; Cochrane & Douglas, 1976). We did not investigate the effects of D-600 and tetracaine as both agents were found to increase the spontaneous secretion of CA.…”

Ca2+‐independent regulation of neurosecretion by intracellular Na+

“…extraceUular Ca -' + does not result in a ~ecretory response [21]. Therefore, its induction to application ofNa + following NM treatment is not a direct result of membrane depolarization (see also permeabilized nerve ending results below).…”