Adenosine Triphosphate Dependent Calcium Uptake by Subcellular Fractions from Bovine Neurohypophyses

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Abstract
Bovine neurohypophyses were fractionated by differential and density gradient ultracentrifugation and the Ca2+uptake and ATPase activities in the microsomal, mitochondrial and secretory granule fractions were studied. The microsomal and mitochondrial fractions accumulated Ca2+in the presence of ATP. The accumulation by the latter per mg protein was at least twice as large as by the former. This Ca2+accumulation was accompanied by liberation of inorganic phosphate (Pi). In the presence of sodium azide (2 mM) Ca2+uptake and Piliberation were inhibited in the mitochondrial, but not in the microsomal fraction. Further studies of the microsomal fraction revealed that the ATP‐dependent Ca2+uptake and Piliberation activities were temperature and pH‐dependent and required Mg2+. Both activities were stimulated by very low concentrations of Ca2+(1–10 μM) and were inhibited by EGTA (2 mM). N‐ethyl‐maleimide (2 mM) inhibited both the Ca2+uptake and ATPase activities of the microsomal fraction. These results suggest the presence of a membrane ATPase that is stimulated by both Ca2+and Mg2+. It is suggested that the observed Ca2+uptake activities are involved in maintaining a low axoplasmic free Ca2+concentration, thus playing an important role in the release mechanism of vasopressin by the neuro‐secretory terminals.