Slow interaction of islet-activating protein with pancreatic islets during primary culture to cause reversal of alpha-adrenergic inhibition of insulin secretion

Abstract

The manner in which islet-activating protein (IAP), a protein purified from the culture medium of Bordetella pertussis, interacts with the islet B-cell was studied by following the progressive development of IAP-induced reversal of alpha-adrenergic inhibition of insulin release during maintenance of islets in culture with glucose and epinephrine. This action of IAP developed in an exponential manner dependent on its concentration after a true lag period of about 1 h. The lag period was not grossly dependent on the concentration of IAP added but highly dependent on temperature of culture, and was still seen upon adding a second dose of IAP to partially stimulated cells. After 24-h culture significantly more insulin was secreted with IAP at a concentration as low as 1 pg/ml and the half-maximal effect was observed at 0.1 ng/ml. The development of IAP action occurred even in the islets that had been exposed to IAP for only 30 s, but was significantly prevented by anti-IAP serum added before the end of the lag period. IAP was effective in the presence of cycloheximide, an inhibitor of protein synthesis, or of vinblastine or cytochalasin B, microtubular-microfilamentous modifiers. It is suggested that the IAP molecule is rapidly bound to the receptor area of the islet B-cell and then is gradually inserted into the cell membrane before appearance of its action to activate native calcium ionophores. This slow interaction of IAP with the membrane may be responsible for potentiation of insulin secretory and cAMP responses of the cell to various stimuli as well as for reversal of alpha-adrenergic inhibition.