Effects of Monensin on Conversion of Proinsulin to Insulin and Secretion of Newly Synthesized Insulin in Isolated Rat Islets

When isolated rat islets were incubated with 10⁻¹⁰–10⁻⁶ M monensin, a sodium and proton ionophore, glucose-stimulated insulin release was inhibited in a concentration- and time-dependent manner. After removal of monensin, inhibition of insulin secretion persisted during stimulation with a variety of secretagogues, including 5 mM glucose plus 15 mM arginine, 20 mM glucose, and 20 mM glucose plus 1 mM 3-isobutyl-1-methylxanthine. Within the same low range of monensin concentrations, proteolytic conversion of newly synthesized proinsulin to insulin was also blocked. At each concentration, prohormone-to-hormone conversion was inhibited to almost the same extent as inhibition of insulin secretion. Therefore, both processes may have equal or common dependency on a subcellular ionic gradient. Although monensin decreased total insulin secretion, the glucose-regulated marking process was unaffected. Regardless of the monensin concentration or the overall rate of insulin secretion, the percentage of secreted newly synthesized versus older insulin remained the same, and the threefold differences in the fractional secretory rates of newly synthesized versus total insulin also remained the same. Thus, rather than specifically blocking protein traffic through the Golgi apparatus of the beta cell, monensin probably first inhibited insulin secretion by disrupting proton gradients in secretory vesicles and, thereby, also inhibited other processes occurring within this organelle.