Exercise training normalizes wall-to-lumen ratio of the gracilis muscle arterioles and reduces pressure in spontaneously hypertensive rats

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Abstract
To investigate mechanisms underlying the training-induced blood pressure-lowering effect we analyzed the hemodynamic responses and morphometric changes of the skeletal muscle microcirculation of spontaneously hypertensive (SHR) and normotensive Wistar–Kyoto (WKY) rats during an exercise training program. Training (50–60% VO2 max) was performed on a treadmill for 13 weeks and control groups were kept sedentary over the same period of time. Trained and sedentary rats were chronically instrumented for hindlimb flow and arterial pressure (AP) recordings under conscious unrestrained conditions. Gracilis and myocardial muscle samples were obtained for morphometric analysis after transcardiac perfusion of fixative. SHR, when compared to WKY presented an elevated blood pressure, an increased relative hindlimb vascular resistance, capillary rarefaction in both gracilis and myocardium and an increased wall-to-lumen ratio of gracilis arterioles. Training increased significantly both capillary density and capillary/fiber ratio in the gracilis and myocardium of WKY and SHR groups, causing a complete reversal of capillary rarefaction in trained SHR. In SHR, training also reduced resting blood pressure and caused normalization of both relative hindlimb vascular resistance and gracilis arterioles wall-to-lumen ratio. Regression analysis revealed strong positive correlation between hindlimb vascular resistance and mean AP (MAP) and between arterioles wall-to-lumen ratio and MAP. The results suggest that low-intensity training can significantly reduce pressure in SHR while normalizing both the arteriole morphology and the resistance of the skeletal muscle microcirculation.