Using medium with a low ionic strength, a low concentration of Ca2+ and Mg2+ and 
devoid of K+, we have measured Ca(2+)-ATPase activity in the homogenates of rat 
islets preincubated for 3 min with several hormones in the presence of 3.3 mmol 
glucose/l. Insulin secretion was also measured in islets incubated for 5 min 
under identical experimental conditions. Islets preincubated with glucose (3.3 
mmol/l) and glucagon (1.4 mumol/l) plus theophylline (10 mmol/l), ACTH (0.11 
nmol/l), bovine GH (0.46 mumol/l), prolactin (0.2 mumol/l) or tri-iodothyronine 
(1.0 nmol/l) have significantly lower Ca(2+)-ATPase activity than those 
preincubated with only 3.3 mmol glucose/l. All these hormones increased the 
release of insulin significantly. Dexamethasone (0.1 mumol/l) and somatostatin 
(1.2 mumol/l) enhanced the Ca(2+)-ATPase activity while adrenaline (10 mumol/l) 
did not produce any significant effect on the activity of the enzyme. These 
hormones decreased the release of insulin significantly. These results 
demonstrated that islet Ca(2+)-ATPase activity was modulated by the hormones 
tested. Their inhibitory or enhancing effect seemed to be related to their 
effect on insulin secretion; i.e. those which stimulated the secretion of 
insulin inhibited the activity of the enzyme and vice versa. Hence, their effect 
on insulin secretion may be due, in part, to their effect on enzyme activity and 
consequently on the concentration of cytosolic Ca2+. These results reinforce the 
assumption that Ca(2+)-ATPase activity participates in the physiological 
regulation of insulin secretion, being one of the cellular targets for several 
agents which affect this process.