PHYSICAL-FITNESS, MUSCLE MORPHOLOGY, AND INSULIN-STIMULATED LIMB BLOOD-FLOW IN NORMAL SUBJECTS

The response of limb blood flow to insulin is highly variable even in normal subjects. We examined whether physical fitness or differences i n muscle morphology contribute to this variation. Maximal aerobic powe r, muscle fiber composition and capillarization, and the response of f orearm glucose extraction and blood flow to a sequential hyperinsuline mic euglycemic clamp (serum insulin 374 +/- 10, 816 +/- 23, and 2,768 +/- 78 pmol/l) were determined in 16 normal males (age 25 +/- 1 yr, bo dy mass index 24 +/- 1 kg/m(2)). Maximal aerobic power correlated posi tively with the proportion of type I fibers (r = 0.67, P < 0.01) and n egatively with the proportion of type IIb fibers (r = -0.73, P < 0.01) . Fiber composition but not blood flow correlated significantly with f orearm and whole body glucose uptake. All doses of insulin significant ly increased forearm blood flow, maximally by 123 +/- 21%. The ratio o f capillaries per fiber was significantly correlated with basal and in sulin-stimulated blood flow (0.58-0.76, P < 0.05-0.01). Mean arterial blood pressure and the insulin-induced increase in blood flow were inv ersely correlated (r = -0.59, P < 0.05). We conclude that variation in glucose extraction is significantly determined by muscle fiber compos ition, whereas variation in insulin-stimulated blood flow is closely a ssociated with muscle capillarization.