Effects of cationic modification on superficial binding and intracellular<sup>45</sup>Ca uptake by decapsulated ob/ob mouse pancreatic islets
Flatt PR., Rorsman P., Swanston-Flatt SK.
Decapsulated ob/ob mouse islets were used to examine the effects of monovalent and divalent cations on 45 Ca uptake and binding as discriminated by La 3+ -wash technique. At 3 mM glucose (1.28 mM Ca 2+ ), intracellular 45 Ca uptake was increased 1.9-4.5-fold by omission of 125 mM Na + , 1.5-fold by omission of 1.2 mM Mg 2+ 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 increased 45 Ca uptake 4.7-6.8-fold, whereas addition of H + (pH 6.3) or 12 mM Mg 2+ decreased 45 Ca uptake by 1.8 and 1.8-2.0-fold, respectively. In control incubations, 20 mM glucose increased intracellular 45 Ca uptake 5-fold. Stimulation (2.5-3.9-fold) persisted in the presence of 1 mM Ni 2+ , Sr 2+ , Mn 2+ or Co 2+ but was abolished by Zn 2+ , Ba 2+ or Cd 2+ . Of these cations, only Sr 2+ lacked effects on intracellular 45 Ca uptake. Basal uptake was decreased 3.8-fold by Mn 2+ and increased 4.7- and 3.9-fold by Zn 2+ and Ba 2+ . Mn 2+ also decreased 45 Ca uptake 7.2-fold at 20 mM glucose, as did Ni 2+ , Co 2+ , Cd 2+ (1.4-5.8-fold) but not Zn 2+ or Ba 2+ . Superficial 45 Ca binding was increased 7.4- and 10.3-fold by Zn 2+ and Cd 2+ , 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.