Role of receptor mechanisms in the adaptive response to habitual exercise.

Many of the physiologic adaptations that occur in response to habitual exercise are associated with changes in neuroendocrine control of specific cell and tissue functions. Because all of the hormones and neurotransmitters important to exercise physiology exert their effects at the cellular level by binding to specific receptors, alterations in receptor properties induced by physical conditioning constitute a potential biochemical mechanism underlying changes in neuroendocrine control that contribute to the altered physiology of the exercise-trained state. This article summarizes existing data concerning the effects of exercise conditioning on receptor properties in several tissues. Considerable evidence suggests that hypersensitivity of adipocyte beta 1-adrenergic receptors to catecholamine stimulation is a response to exercise training, and may contribute to the increased consumption of fat, as opposed to carbohydrate, as fuel for muscular work during submaximal exertion in trained subjects. Conversely, cardiac beta-adrenergic receptors appear to be unaltered by physical conditioning, such that the relative bradycardia of the trained state is not attributable to changes in receptor properties. Training-induced changes in skeletal muscle, vascular and bronchial smooth muscle, platelet and leukocyte adrenergic receptors have also been reported, but the published findings are either inconsistent or of uncertain physiologic significance at the current time.