Glucose and insulin exert additive ocular and renal vasodilator effects on healthy humans

Aims/hypothesis. There is evidence that insulin and glucose cause renal and ocular vasodilation. There is, however, currently no data on the effect of combined hyperglycaemia and hyperinsulinaemia on the renal and ocular blood flow seen in diabetic patients on insulin therapy. Methods. We carried out two different 3-way crossover studies in healthy subjects (each, n = 9). In study one, hyperglycaemic clamps (5.6 mmol/l, 11.1 mmol/l, 16.7 mmol/l) were carried out during placebo or insulin (dose 1:1 mU/kg/min; dose 2:2 mU/kg/min) infusion. The second study was identical but endogenous insulin secretion was blocked with somatostatin. The renal plasma flow, glomerular filtration rate and pulsatile choroidal blood flow were measured using the paraaminohippurate method, the inulin method and a laser interferometric measurement of fundus pulsation amplitude, respectively. Results. Insulin increased renal plasma flow and fundus pulsation amplitude but not the glomerular filtration rate. Hyperglycaemia increased all the renal and ocular parameters studied. Haemodynamic effects of glucose and insulin were additive when somatostatin was co-administered but not under basal conditions. Conclusions/interpretation. Glucose and insulin can exert additive vasodilator properties on renal and ocular circulation. To find out whether this observation is related to the increased regional perfusion in diabetes longitudinal studies on patients with Type I (insulin-dependent) diabetes mellitus are needed.