Lack of effect of sodium nitroprusside on insulin-mediated blood flow and glucose disposal in the elderly

Insulin increases skeletal muscle blood flow in healthy young subjects by a nitric oxide (NO)-dependent mechanism. Impairment of this mechanism may co ntribute to the insulin resistance of normal aging, a state characterized b y reduced endothelial production of NO, an attenuated effect of insulin on skeletal muscle blood flow, and resistance to insulin-mediated glucose upta ke (IMGU). We tested the hypothesis that the NO donor sodium nitroprusside (SNP) would augment insulin-mediated vasodilation and thus increase IMGU in healthy elderly subjects. Experiments were performed with young (n = 9; ag e, 25 +/- 1 years; body mass index [BMI], 24 +/- 1 kg/m(2)) and old (n = 10 ; age, 78 +/- 2 years: BMI, 25 +/- 1 kg/m(2)) healthy subjects. Each group underwent two studies in random order. In one study (control), insulin was infused using the euglycemic clamp protocol for 240 minutes at a rate of 40 mU/m(2)/min (young) and 34 mU/m(2)/min (old). In the other study (SNP), SN P was coinfused with insulin from 120 to 240 minutes. At regular intervals in each study, blood samples were obtained and calf blood flow was measured using venous occlusion plethysmography. Glucose and insulin values were si milar in control and SNP studies in both age groups. In the young, SNP had no effect on blood flow to the calf, but its action in calf resistance vess els augmented insulin-mediated vasodilation, since incremental calf vascula r conductance was greater during SNP infusion (control v SNP, 0.027 +/- 0.0 02 v 0.040 +/- 0.008 mL/100 mL/min/mm Hg, P < .0001). However, SNP had no e ffect on insulin-mediated glucose disposal. In the elderly, SNP reduced the blood flow to the calf, but this was countered by its effect on calf resis tance vessels such that vascular conductance was unaffected (control v SNP, 0.012 +/- 0.003 v 0.011 +/- 0.003 mL/100 mL/min/mm Hg, P = nonsignificant [NS]). Steady-state (180 to 240 minutes) glucose disposal (control v SNP, 7 .47 +/- 0.47 v 6.54 +/- 0.56 mg/kg/min, P < .01) rates were significantly l ower during SNP infusion. In summary, systemic infusion of SNP did not incr ease insulin-mediated glucose disposal in either young or old subjects. Thu s, the present findings do not support the concept that increasing NO avail ability will enhance glucose disposal in either age group. However, because the incremental increases in IMGU during SNP infusion paralleled the chang es in blood supply to the calf rather than calf vascular conductance, any p otential benefits on NO delivery in elderly subjects may have been offset b y the direct or reflex effects of systemic hypotension. Other stimuli to NO production that do not cause hypotension must be tested before this therap eutic strategy can be considered as a potential means for enhancing the met abolic actions of insulin in the elderly. Copyright (C) 2000 by W.B. Saunde rs Company.