Modulatory effect of hormones on insulin secretion in vitro in the toad

The aim of this work was to demonstrate the possible direct effect of several hormones upon glucose-induced insulin secretion in amphibians. Hence, pancreas pieces of Bufo arenarum were incubated for 60 min at 25 degrees with 2 and 8 mM glucose plus the addition of hormones known to affect insulin secretion in mammals, measuring the release of insulin by radioimmunoassay. Glucagon (1 microM), ACTH (2.5 microM), human and bovine growth hormone (4.6 and 2.1 microM), prolactin (0.27 microM), corticosterone (0.4 microM), androstanolone (10(-2) microM), estradiol and estrone (10 microM), triiodothyronine and thyroxine (1 microM) enhanced significantly the glucose-induced insulin secretion. Androstanolone, human and bovine growth hormone, triiodothyronine and thyroxine only exerted such effect in the presence of 8 mM glucose. Conversely, somatostatin (1 microM), adrenalin (1 microM), clonidine (2 microM), dexamethasone (0.4 microM), and 2-hydroxyestradiol (5 microM) decreased significantly the glucose-induced insulin release. However, the effect of somatostatin was only apparent in the presence of high glucose. The direct effect of all these hormones--tested for the first time in the amphibian pancreas--was similar to that described in the mammalian pancreas, thus suggesting that such hormones might participate, at least in vitro, in the fine-tuning of insulin secretion in amphibians.