Direct cytoplasmic CA(2+) responses to gastrin-releasing peptide in single beta cells.

The neuropeptide gastrin releasing peptide (GRP) stimulates insulin secretion and induces oscillations of the cytoplasmic Ca(2+) concentration (Ca(2+)) in clonal insulinoma cells. It is not known whether GRP affects Ca(2+) in normal beta cells. We investigated, in single, normal, mouse islet beta cells, the effects of GRP on Ca(2+), by dual wavelength spectrophotofluorometry. Beta cells were identified by their typical response to glucose or tolbutamide. At 15 mM glucose, GRP (100 nM) evoked an immediate Ca(2+) transient to 423 +/- 48 nM compared to 126 +/- 18 nM before GRP (P < 0.001). After the initial transient, Ca(2+) exhibited either a sustained elevation or oscillations. At 3.3 mM glucose, in cells with a non-oscillating Ca(2+), GRP stimulated a prompt increase in Ca(2+) (from 60 +/- 6 to 285 +/- 30 nM, P = 0.024) followed by either a sustained increase in Ca(2+) or Ca(2+) oscillations. We conclude that GRP promptly elevates Ca(2+) by a direct action in normal mouse pancreatic beta cells.