Effects of propylthiouracil and methylthiouracil on cyclic AMP and ion movements in rat pancreatic islets.
Propylthiouracil and methylthiouracil have been shown to potentiate glucose-induced insulin secretion from rat pancreatic islets: the effect of methylthiouracil being less pronounced than that of propylthiouracil. In this study the effects of these substances on cAMP levels, 86Rb+ efflux, 45Ca2+ net uptake, and 45Ca2+ efflux were tested in isolated rat islets in order to obtain information on their possible mechanism of action. Propylthiouracil and to a lesser extent methylthiouracil increased islet cyclic AMP in a concentration-related manner. Maximum increases at the highest concentrations tested were 261% and 190% respectively. In the presence of 3 mM glucose propylthiouracil and methylthiouracil led to a decrease in the 86Rb efflux rate. With 5.6 mM glucose, both thiourea derivatives produced an increase in the 86Rb+ efflux rate which was independent of the presence or absence of calcium in the medium. Propylthiouracil and methylthiouracil augmented the 45Ca2+ efflux rate in the presence as well as in the absence of external calcium at various glucose concentrations. Propylthiouracil did not change, and methylthiouracil only slightly augmented, 45Ca2+ net uptake into the isolated islets. It is suggested that the synergistic effect of propylthiouracil and methylthiouracil on glucose-induced insulin release is at least in part due to an increase in islet cAMP levels. Whether the two substances have additional direct effects on ionic fluxes which contribute to their insulinotropic action or whether the observed changes in ion movements are secondary to the elevation of cAMP levels remains to be unclear and needs further investigation.