Physical Characteristics of the Ca++Compartments Associated within VitroACTH Release

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Abstract
Two separate Ca++ compartments necessary for in vitro release of ACTH from the rat adenohypophysis are revealed by respective mild or vigorous washing procedures with Ca-free media. Vasopressin (500 mU/ml), like elevated [K+], requires the presence of a loosely bound compartment for ACTH release. A crude acid extract of hypothalamus-stalk-median eminence (HSME) or theophylline (10 mM) require a tightly bound compartment, since their potency as ACTH secretagogues is not diminished when the loosely bound compartment is removed. Estimations of the intracellular concentrations of Ca+ + show that incubation in Krebs-Ringer bicarbonate media increases the Ca+ + level as much as threefold from the normal in vivo value of 2.70 mM/kg wet wt. The mild wash procedure produces only a slight reduction in this elevated value. The vigorous wash procedure reduces the internal concentration to about 65% of the in vivo level. We conclude that Ca+ + need not play a critical role in the release process of adenohypophysial hormones at the level of the plasma membrane. Its critical involvement would appear to be at some specific cytoplasmic site involving translocation of Ca+ + ions within the cell. (Endocrinology94: 435, 1974)

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