1 Islets from normal mice were used to test the acute effects of genistein, a potent tyrosine kinase inhibitor, on stimulus-secretion coupling in pancreatic B-cells. 2 Genistein produced a concentration-dependent (10-100 mu M), reversible, increase of insulin release. This effect was marginal on basal release or in the presence of non-metabolized secretagogues, and much larger in the presence of glucose or other nutrients. The increase in insulin release caused by 100 mu M genistein was abolished by adrenaline or omission of extracellular Ca2+. It was not accompanied by any rise of cyclic AMP, inositol phosphate or adenine nucleotide levels. 3 Although genistein slightly inhibited ATP-sensitive KC channels, as shown by Rb-86 efflux and patch-clamp experiments, this effect could not explain the action of the drug on insulin release because the latter persisted when ATP-sensitive K+ channels were all blocked by maximally effective concentrations of glucose and tolbutamide. Genistein was also effective when ATP-sensitive K+ channels were opened by diazoxide and the B-cell membrane depolarized by 30 mM K, but ineffective in the presence of diazoxide and normal extracellular K. 4 Genistein paradoxically decreased Ca2+ influx in beta-cells, as shown by the inhibition of glucose-induced electrical activity, by the inhibition of Ca2+ currents (perforated patches) and by the lowering of cytosolic [Ca2+](i) (fura-2 technique). Genistein thus increases insulin release in spite of a lowering of [Ca2+](i) in beta-cells. 5 Daidzein, an analogue of genistein reported not to affect tyrosine kinases, was slightly less potent than genistein on K+ and Ca2+ channels, but increased insulin secretion in a similar way. Three other tyrosine kinase inhibitors, tyrphostin A47, herbimycin A and an analogue of erbstatin variably affected insulin secretion. 6 Genistein exerts a number of heretofore unrecognized effects. The unusual mechanisms, by which genistein increases insulin release in spite of a decrease in beta-cell [Ca2+](i) and without activating known signalling pathways, do not seem to result from an inhibition of tyrosine kinases.
