Insulin Resistance and Microvascular Blood Flow in Spinal Cord Injury

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Cutaneous microvascular blood flow is regulated by multiple mechanisms, including that by insulin and by the sympathetic nervous system (SNS). Insulin is the principal hormone responsible for the disposal and storage of glucose in skeletal muscle, in part by the re-direction of blood flow through the rhythmic dilatation or contraction of arterioles. In insulin-sensitive individuals, this "vasomotion" is thought to involve the activation of the vascular smooth muscle, with vasodilatation occurring through nitric oxide and vasoconstriction through the SNS and endothelin-1. A tonic upregulation of SNS activity and increased vasoconstrictor action of insulin may be a contributor to the development of hypertension, decreased peripheral blood flow, and endothelial dysfunction in the general population, especially in individuals with hyperinsulinemia and diabetes mellitus. In persons with spinal cord injury (SCI), a disproportionately high prevalence of insulin resistance and diabetes mellitus has been reported. We postulate that insulin resistance, in combination with the added consequence of SNS impairment below the neurological level of injury, contribute to hemodynamic dysregulation and a variety of medical complications, including pressure ulcer formation and decreased wound healing. Recently, our group demonstrated that the sub-lesional blood perfusion response to iontophoresis with insulin is blunted in euinsulinemic persons with motor-complete SCI compared to demographics-matched neurologically-intact control subjects. To confirm and extend our preliminary finding and to provide additional insight into its implications, we propose to perform an open-label, non-randomized, placebo-controlled, parallel-group intervention, observational trial to determine the hemodynamic actions of insulin in individuals with complete motor lower extremity paralysis due to SCI and either systemic insulin sensitivity or insulin resistance. Subjects will participate in a screening visit to determine their eligibility and insulin sensitivity (i.e., categorized as being insulin-sensitive or insulin-resistant). Eligible individuals will return for participation in our study to determine skin blood flow by iontophoresis with vasoactive agents or application of heat to the extremities. Measurements will be performed simultaneously with provocation (i.e., with either heat or insulin or acetylcholine iontophoresis) being performed on the ipsilateral extremity and no provocative intervention (i.e., either no heat or placebo iontophoresis) in parallel and simultaneously on the contralateral extremity. On a separate visit, all subjects will repeat the iontophoresis with acetylcholine, which is the gold-standard to induce endothelium-dependent vasodilatation of the microvasculature. The respective outcomes from iontophoresis with insulin will be compared and correlated to systemic insulin sensitivity (as determined by an intravenous glucose tolerance test with insulin administration) (Primary Aim). The peak microvascular perfusion responses to vasodilatation by iontophoresis with acetylcholine to that with insulin will be compared (Secondary Aim). In participants with SCI , the findings from the neurologically intact upper extremity will be compared to those of the neurologically impaired lower extremity (Tertiary Aim). A group of neurologically-intact subjects who are matched for group assignment (i.e., insulin-sensitive or insulin resistant) will serve as age- and gender-matched controls to the participants with SCI.