Effects of cationic modification on superficial binding and intracellular⁴⁵Ca uptake by decapsulated ob/ob mouse pancreatic islets

Decapsulated ob/ob mouse islets were used to examine the effects of monovalent and divalent cations on45Ca uptake and binding as discriminated by La3+-wash technique. At 3 mM glucose (1.28 mM Ca2+), intracellular45Ca uptake was increased 1.9-4.5-fold by omission of 125 mM Na+, 1.5-fold by omission of 1.2 mM Mg2+and 2.3-fold by reduction of H+(pH 8.5). Omission of 5.9 mM K+was without effect. K+at 30.9 mM increased45Ca uptake 4.7-6.8-fold, whereas addition of H+(pH 6.3) or 12 mM Mg2+decreased45Ca uptake by 1.8 and 1.8-2.0-fold, respectively. In control incubations, 20 mM glucose increased intracellular45Ca uptake 5-fold. Stimulation (2.5-3.9-fold) persisted in the presence of 1 mM Ni2+, Sr2+, Mn2+or Co2+but was abolished by Zn2+, Ba2+or Cd2+. Of these cations, only Sr2+lacked effects on intracellular45Ca uptake. Basal uptake was decreased 3.8-fold by Mn2+and increased 4.7- and 3.9-fold by Zn2+and Ba2+. Mn2+also decreased45Ca uptake 7.2-fold at 20 mM glucose, as did Ni2+, Co2+, Cd2+(1.4-5.8-fold) but not Zn2+or Ba2+. Superficial45Ca binding was increased 7.4- and 10.3-fold by Zn2+and Cd2+, respectively. However, binding was not affected by other divalent cations, by 20 mM glucose in their absence or presence, or by the major ionic modifications made with and without osmotic adjustments using choline chloride or sucrose.