The in vitro effect of corticosterone in insulin and glucagon secretion has been examined in the isolated perfused rat pancreas preparation. COrticosterone at physiological concentrations was found to inhibit acutely and strongly the secretion of insulin induced by both glucose and arginine and to potentiate the output of glucagon in a glucose-free medium or when induced by arginine. Phentolamine, an alpha-adrenergic blocking agent, diminished the strong inhibitory effect of corticosterone on insulin secretion. The present results demonstrate that corticosterone has direct effects on pancreatic islet cells, and they suggest that the inhibition of insulin secretion is to some extent related to the alpha-adrenergic receptors.

The direct effects of cortisol and cortisone on insulin and glucagon secretion by the isolated perfused rat pancreas were examined. Cortisone, but not cortisol, in a final concentration of 10(-4) M acutely and strongly inhibited both phases of glucose- and arginine-induced insulin secretion. In repeated pulse administrations, the second infusion of cortisone elicited an inhibition of insulin release equal to the first response. Glucagon output was slightly suppressed by cortisol and more significantly by cortisone in a glucose-free medium. The blockade of alpha-adrenergic receptors by phentolamine attenuated, but did not prevent, the strong inhibitory action of cortisone on insulin secretion.

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Polyethylene glycol (PEG) and killed Staphylococcus aureus cells (S. aureus) were used as agents to separate free insulin from antibody-bound insulin in diabetic sera. Insulin was determined by conventional double antibody radioimmunoassay. The free insulin values after PEG treatment were almost half of those after S. aureus treatment. The free insulin levels in high-antibody containing sera preincubated at 37 degrees C, 2 h were double the value of fresh sera. PEG treatment caused about 40% loss of total serum protein. The addition of bovine serum albumin (BSA) to the PEG-treated serum greatly increased the immuno-reactive insulin values. This may suggest that protein concentration plays a role in insulin radioimmunoassay. PEG treatment may also enhance the interaction between free insulin and free antibodies resulting in underestimation of free insulin level.