Effects of chronic exercise on muscarinic receptor-mediated vasodilation in rats

Previous studies have demonstrated that vascular responses to acetylcholine (ACh), an endothelium-dependent vasodilator, are enhanced in exercise-trained animals. In order to see if chronic exercise upregulates endothelial muscarinic (M) receptor, the subtype of M receptors responsible for ACh-induced vasorelaxation in the thoracic aorta of male Wistar rats was characterized first, then a receptor assay was performed. These animals were divided into exercise and control groups. The trained rats ran on a treadmill with a moderate intensity for 60 min per day, 5 days per week. After 10 weeks of training, rats were decapitated and their thoracic aortae were isolated. The subclass of M receptor in endothelium was pharmacologically identified on the basis of selective affinity of antagonists; ie, pirenzepine for M1, gallamine for M2, and 4-diphenylacetoxy-N-methylpiperidine methiodide for M3. Our results showed that in the thoracic aorta of Wistar rats, 1) ACh-induced vasorelaxation was mediated by M3 receptor; 2) chronic exercise enhanced ACh-evoked vasodilating responses. However, this alteration was not caused by receptor upregulation, as maximal binding sites and affinity of M3 receptor were not changed by chronic exercise. Other possible mechanisms need to be further studied.