Vascular involvement in skeletal muscle metabolism

Perfused hindlimb preparations have been used to investigate vasoconstrictormediated control of skeletal muscle metabolism, with particular emphasis on the regulation of oxygen consumption (V0 2) as an index of muscle nonshivering thermogenesis (NST). The ability of a group of molecules known as vanilloids to modulate muscle V02 was investigated using hindlimb preparations of hooded Wistar rats. Both naturally-occurring and synthetic vanilloids were examined. Infused vanilloids gave dose-dependent V02 changes in association with increased perfusion pressure (PP). Vanilloid V02 concentration-response curves were biphasic, lower concentrations stimulating and higher concentrations inhibiting V02. Nitrovasodilators demonstrated an association between the V02 changes and vasoconstriction, whilst a- and 13-adrenergic antagonists showed that neither adrenergic receptors nor secondary catecholamine release were responsible for the increased V02. The observed effects may have been due to specialised vanilloid receptors. The data in fact supported two vanilloid receptor subtypes; the putative higher affinity (VNI) receptor mediated increased V02 and vasoconstriction, and was dependent on the presence of oxygen and external Ca2+. The putative lower affinity (VN2) receptor mediated an inhibition of V02 with vasoconstriction, but the vasoconstriction was independent of external Ca2+ or 02 presence. A range of vanilloid structural analogues were synthesised and used to construct a structure-activity profile for hindlimb thermogenic action. A distinct set of structural features required for thermogenic activity (a pharmacophore) was defined. However, there was no clear distinction between the pharmacophore for thermogenesis and the structural features deduced by others to be necessary for antinociceptive action in sensory neurone studies. Complete separation of the responses attributed to the putative dual vanilloid receptors was not observed, although there was some evidence of partial selectivity. The concept of vascular metabolic control in muscle was further examined in a series of comparative perfusion studies. The first study established a viable technique for perfining bird lower limbs. Since birds are reported to be devoid of brown adipose tissue, the perfused chicken lower limb was an appropriate model for examining the potential of skeletal muscle, via vascular metabolic control, as a major contributor to NST. Infused catecholamines increased PP and gave biphasic V02 concentrationresponse curves. Low dose V02 stimulation was blocked by prazosin and nitrovasodilation, but was unaffected by propranolol. The demonstration of potential muscle NST in another taxon raised the possibility of vascular thermogenic control being a widespread and perhaps a fundamental NST mechanism. In a further comparative study, genetically obese (fa/fa) Zucker rats were used primarily to examine the hypothesis that the obesity was related to a defect in vascular metabolic control. Differences in basal and noradrenaline-mediatedV0 2 related to lower muscle content and higher fat content in the obese hindlimb. 5-HT-mediated V02 inhibition was significantly greater in age-matched lean (Fa/? ) hindlimbs, even when the data were expressed in terms of muscle mass. This may indicate a reduced potential for vascular metabolic control with possible implications for the whole-body energy balance of the obese phenotype. In a separate series of experiments measuring glucose uptake, perfused obese hindlimbs were found to be markedly insulin-resistant relative to lean counterparts. The possibility of a link between the impaired insulin effectiveness and altered haemodynamic function is discussed. The studies undertaken underline the potential significance of altering regional nutrient and hormone access in regulating skeletal muscle metabolism, and in particular support a critical role for the vasculature in the control of skeletal muscle thermogenesis.