Adenosine triphosphate dependent calcium uptake by subcellular fractions from bovine neurohypophyses.
Bovine neurohypophyses were fractionated by differential and density gradient ultracentrifugation and the Ca-2+ uptake and ATPase activities in the microsomal, mitochondrial and secretory granule fractions were studied. The microsomal and mitochondrial fractions accumulated Ca-2+ 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) Ca-2+ uptake and Pi liberation were inhibited in the mitochondrial, but not in the microsomal fraction. Further studies of the microsomal fractions revealed that the ATP-dependent Ca-2+ uptake and Pi liberation activities were temperature and pH-dependent and required Mg-2+. Both activities were stimulated by very low concentrations of Ca-2+ (1-10 muM) and were inhibited by EGTA (2 mM). N-ethylmaleimide (2 mM) inhibited both the Ca-2+ uptake and ATPase activities of the microsomal fraction. These results suggest the presence of a membrane ATPase that is stimulated by both Ca-2+ and Mg-2+. It is suggested that the observed Ca-2+ uptake activities are involved in maintaining a low axoplasmic free Ca-2+ concentration, thus playing an important role in the release mechanism of vasopressin by the neurosecretory terminals.
D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D001386:Azides / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D005033:Ethylmaleimide / Q000494:pharmacology; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000494:pharmacology; D008861:Microsomes / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism; D010904:Pituitary Gland, Posterior / Q000201:enzymology / Q000378:metabolism*; D013347:Subcellular Fractions / Q000378:metabolism; D013696:Temperature; D013997:Time Factors
D001386:Azides; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D008274:Magnesium; D005033:Ethylmaleimide; D002118:Calcium