Stimulation by calcium and barium of somatostatin release. Evidence for lower sensitivity of D- vis-à-vis B- and A-cells.

To address the question whether a "second messenger" function of calcium differs between D-cells and other cells of the endocrine pancreas, we compared effects of calcium and barium (a calcium substitute) on somatostatin secretion to effects on insulin and glucagon secretion from the perfused pancreas of the rat. 6.5 mmol/l of calcium, when administered early during perfusion, failed to stimulate somatostatin release. 0.05 mmol/l of barium, when added to calcium-deprived media failed to affect somatostatin secretion while 0.5 induced a slight and 2.0 mmol/l a marked and sustained response. Barium-induced insulin release was left-shifted in relation to the somatostatin response, since 0.05 mmol/l of barium stimulated and 0.5 mmol/l evoked a near-maximal insulin response. All concentrations of barium evoked diphasic glucagon responses, i.e. a small (1 min) stimulation followed by sustained inhibition. Addition of 0.5 mmol/l of EGTA to calcium-deprived media abolished D- as well as B- and A-cell secretion. Reintroduction of 0.5-6.5 mmol/l of calcium stimulated somatostatin release; the secretory response was proportionate to the calcium concentration. In contrast, addition of calcium stimulated insulin and glucagon secretion maximally already at 0.5 mmol/l of calcium. We conclude that the D-cell is less sensitive than B- and A-cells to a regulatory effect on secretion exerted by extracellular calcium or barium.