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

Abstract

To address the question whether a 2nd messenger function of Ca differs between D-cells and other cells of the endocrine pancreas, the effects of Ca and barium (a Ca substitute) on somatostatin secretion was compared to the effects on insulin and glucagon secretion from the perfused rat pancreas. Ca (6.5 mmol/l) when administered early during perfusion, failed to stimulate somatostatin release. Ba (0.05 mmol/l) 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. Ba-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 EGTA (0.05 mmol/l) to Ca-deprived media abolished D- as well as B- and A-cell secretion. Reintroduction of 0.5-6.0 mmol/l of Ca stimulated somatostatin release; the secretory response was proportionate to the Ca concentration. In contrast, addition of Ca stimulated insulin and glucagon secretion maximally already at 0.5 mmol/l of Ca. The D-cell is less sensitive than B- and A-cells to a regulatory effect on secretion exerted by extracellular Ca or barium.