Calmodulin-Binding Proteins in a Cloned Rat Insulinoma Cell Line

Calmodulin concentrations were measured in isolated hamster isiets or in a cloned rat insulin-secreting cell line RIN-m-5F treated with high glucose. There was no change in the cellular calmodulin content of either islets or RIN-m-5F cells despite increases of insulin concentrations in the media. Treatment of cells with the anti-calmodulin drug W13 inhibited insulin-stimulated glucose release, whereas a small effect on insulin accumulation in the media was observed with W12, the dechlorinated, less active analogue of W13. At the lowest dose tested (30 μM) the effect of W13 on insulin accumulation in the media was completely reversible. To further investigate the possible role of calmodulin in insulin secretion, calmodulin-binding proteins in subcellular fractions of the RIN-m-5F cells were identified using a gel overlay technique. Ca²⁺-dependent binding of ¹²⁵I-calmodulin was observed to cystosolic proteins with apparent M, = 125,110, 56, 52, and 34 K (kilodalton). This binding was completely displaceable with unlabeled calmodulin, whereas only partial displacement was observed when the homologous Ca²⁺binding protein of skeletal muscle, troponin C, was used. Four proteins with M_r similar to the histones bind calmodulin in a Ca²⁺-independent manner. ¹²⁵I-calmodulinxalmodulin binding protein interactions were inhibited in a dose-dependent manner by the anti-calmodulin drug, W13. Little effect was observed with the analogue W12. These data demonstrate that glucose-stimulated insulin secretion is not associated with alterations in total calmodulin levels and suggest that the inhibition of insulin secretion by the anti-calmodulin drug, W13, may be through inhibition of the association of calmodulin to calmodulin-binding proteins in the cell.