Calcium uptake by subcellular fractions of pancreatic islets. Effects of nucleotides and theophylline.

Uptake of 45Ca2+ by a microsomal fraction isolated pancreatic islets of non-inbred ob/ob mice was studied. ATP strongly stimulated 45Ca2+ uptake, the maximum effect being obtained with 2mM-ATP. GTP and CTP at this concentration did not increase the uptake. Scatchard analysis revealed at least two types of uptake mechanisms in the presence of 2mM-ATP; the apparent association constants were 1.1 x 10(5)m(-1) and less than 2.5 x 10(2)m(-1). In contradistinction to an unaffected low-affinity uptake, the high-affinity uptake was drastically decreased on ommission of ATP. The ATP-dependent and high-affinity uptake was half-saturated at about 10-20mum-Ca(2+) and was inhibited by 10 or 100mum cyclic AMP, 10mum cyclic GMP, 10 mum cyclic GMP, or 5mm-theophylline. 45ca2+ uptake in the absence of ATP was not affected by 100mum-cyclic AMP. In view of its sensitivity to ATP and cyclic nucleotides, the high-affinity Ca2+-uptake mechaniam may play a role in stimulus-secretion coupling in the beta-cells by regulating the cytosolic concentration of Ca2+.