This project will study microvascular control in skeletal muscle and alterations in this control which occur with exercise training and hypertension. The rat cremaster muscle and isolated vascular tissue will be used as the experimental models. Specifically, I will examine the hypothesis that changes in local PO2, PCO2 and pH can significantly influence the microcirculation by altering vascular smooth muscle sensitivity to norepinephrine and that exercise training increases the response to these local stimuli and this contributes to a decrease in small vessel sensitivity to norepinephrine. Also I hypothesize that exposure to high levels of norepinephrine during each exercise bout will lead to an accommodation of the receptors. Finally, I will evaluate the hypotheses that PO2, PCO2, and exercise training have a significant effect on vascular sensitivity and blood pressure in the spontaneously hypertensive rat (exercise training giving a decrease in blood pressure), but not in the renovascular hypertensive rat. This research program will utilize closed-circuit television microscopy for direct in vivo measurements of the diameters of small arteries, small veins, arterioles and venules in the rat cremaster muscle. The microcirculatory preparation encompasses the advantages of in vivo studies (intact blood and nerve supply) and those of in vitro studies (fine control of tissue pH, PO2, PCO2, temperatures, ion concentrations, drug concentrations etc). Concentration-response curves will be determind under different tissue conditions for topical application of norepinephrine in normotensive, spontaneously hypertensive and renovascular hypertensive rats. Each group of rats will be subdivided into those which have exercised (swimming 1hr/day for six weeks) and those which have not exercised.