Carbachol induces sustained glucose-dependent oscillations of cytoplasmic Ca2+ in hyperpolarized pancreatic beta cells.

The effect of carbachol on the cytoplasmic Ca2+ concentration [( Ca2+]i) was studied in insulin-releasing mouse pancreatic beta cells hyperpolarized by the K(+)-channel-activating agent diazoxide. By mobilizing intracellular Ca2+, carbachol induced an initial [Ca2+]i transient, which was more than tenfold higher after preexposure to 20 mM glucose than in a medium lacking substrate. The transient was followed by a sustained but less pronounced elevation, probably due to activation of the potential-independent entry of Ca2+. In individual beta cells exposed to 20 mM glucose small oscillations with a frequency of 1-4/min were superimposed on the sustained phase. These oscillations were insensitive to methoxyverapamil, and their frequency increased in a Na(+)-deficient medium. However, the oscillations faded away after lowering glucose to 3 mM and reappeared when increasing the sugar concentration. The results indicate that the glucose concentration is an important permissive determinant for sustained oscillations of [Ca2+]i in response to agents stimulating the formation of inositol 1,4,5-trisphosphate.