Simulatory effect of porcine insulin on noradrenaline secretion in guinea‐pig ileum myenteric nerve terminals

The effect of insulin on the release of noradrenaline (NA) from nerve terminals was investigated in isolated ileal synaptosomes of guinea‐pig. Release was determined as the amount of NA, quantified by h.p.l.c.‐electrochemical detection, from samples incubated with insulin minus that in parallel blanks treated with some volume of vehicle. Porcine insulin stimulated the secretion of NA in a concentration‐dependent manner from 0.01 i.u. ml −1 , while the value of lactate dehydrogenase in the incubated medium was not influenced by insulin. The presence of insulin receptors in this preparation was illustrated by immunoblotting with insulin receptor monoclonal antibodies. The release of NA by insulin was reduced by guanethidine and bretylium and it was markedly lowered in the samples obtained from guinea‐pigs that had received an intraperitoneal injection of DSP‐4, the noradrenergic neurotoxin. Tetrodotoxin attenuated the action of insulin at concentrations sufficient to block sodium channels. The depolarizing effect of insulin on the membrane potential was also illustrated by a concentration‐dependent increase in the fluorescence of bisoxonol, a potential‐sensitive dye. The action of insulin was attenuated by removal of calcium chloride from the bathing medium. The induction of calcium ion influx by insulin into the synaptosomes is supported by the inhibitory effects of the calcium channel blockers ω‐conotoxin GVIA (for the N‐type channels) and nifedipine (for the L‐type channels). These findings suggest that insulin can stimulate NA release from noradrenergic terminals via activation of calcium influx.British Journal of Pharmacology (1997) 121 , 15–20; doi: 10.1038/sj.bjp.0701091