Relaxing effect of insulin in renal arteries from diabetic rats

Abnormal renal vasomotor tone exists in the early stages of diabetes mellit us. Insulin has been proposed to modulate renal function and to possess vas odilatory effects. The present study was initiated in order to evaluate the direct effect of insulin on isolated renal arteries. Twelve insulin-treate d streptozotocine diabetic rats with diabetes for 50 days were compared wit h 15 weight-matched control rats. The contractile responses to 60 mM K+ and 10(-4) M noradrenaline, and the insulin- (0.8-6.4 I.U./ml) induced relaxat ion of vessels precontracted with noradrenaline, were similar in diabetic a nd control rats. There was a tendency towards greater relaxation in diabeti c (71%) than in control rats (54%). N-w-nitro-L-arginine methyl ester (L-NA ME) (10-4 M) given before noradrenaline tended to attenuate the insulin-ind uced relaxation, while addition of L-arginine (10(-6) M) to L-NAME attenuat ed the relaxation in diabetic but increased it in control rats (P < 0.05). The effect of insulin was tested further in control rats and was not influe nced by administration of a single dose (10(-6) M) of indomethacin or propr anolol given instead of L-NAME. The effect of a single dose of methylene-bl ue, given before noradrenaline, was tested in control rats in varying doses between 2 x 10(-6) and 2 x 10(-4) M. In the highest concentration it made no difference whether insulin was given or not and there was a similar rela xing effect in diabetic and control arteries. In conclusion, the present st udy showed that insulin per se has a relaxing effect on renal arteries. The re was a tendency to greater relaxation in diabetic than in control rats, a n effect which was attenuated by in-vitro-pretreatment with L-NAME as well as with L-NAME and L-arginine in diabetic vessels, while relaxation was inc reased in control vessels. This may indicate that the effect of insulin may be mediated through nitric oxide in diabetic but not in control rats. The effects of insulin in control vessels were not modified in vitro by indomet hacin, propranolol or methylene-blue. (C) 1999 Elsevier Science B.V. All ri ghts reserved.