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Application of the K-pyroantimonate technique combined with glutaraldehydeosmium fixation results in a reproducible intracellular distribution of mineral precipitates in the mouse hypophysis. Control experiments--with chelators and electron probe microanalysis--reveal that these precipitates consist mainly of calcium. Regularly present in the mitochondria, Ca also seems to be stored in the Golgi apparatus of the glandular cells and in the axoplasmic reticulum and the "synaptic" vesicles of the neurosecretory fibres. These structures thus appear able to control intracytoplasmic calcium movements. These observations agree with physiological data showing the existence of an intracellular Ca pool that can be mobilized by specific stimulation. The presence of diffuse precipitates in the pituicytes, together with the existence of gap junctions between them, suggest that these cells regulate the ionic environment of the neurosecretory nerve fibres; in this way, they too might participate in neurohypophysial hormonal release.

The effects of K⁺-enhanced and Ca⁺⁺-free media on α-MSH and adrenocorticotropic hormone (ACTH) release from superfused neurointermediate lobe (NIL) were studied in the same experiments. High K⁺ caused reversible and repeatable inhibition of α-MSH release and stimulation of ACTH release. Removal of Ca⁺⁺ impaired the effect of K⁺, irreversibly for α-MSH and reversibly for ACTH. The existence of both melanotrophic and corticotrophic cells within the intermediate lobe (IL) of rat hypophysis could account for the different effects of ionic modifications on in vitro α-MSH and ACTH release.

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