Autonomic control of cardiovascular performance and whole body O2 delivery and utilization in swine during treadmill exercise

Objective: The present study determined the role of the autonomic nervous system (ANS) in the regulation of systemic and pulmonary circulation and of O-2 delivery and utilization in swine at rest and during graded treadmill exercise. Methods: Instrumented swine (n = 12) were subjected to treadmill exercise (1-5 km/h) under control conditions and in the presence of single and combined beta-adrenergic, alpha-adrenergic and muscarinic (M) receptor blockade. Results: Exercise produced a four-fold increase in body O-2 consumption, due to a doubling of both cardiac output and the arterio-mixed-venous O-2 content difference. The latter resulted from an increase in O-2 extraction, from 45+/-1% at rest to 74+/-1% at 5 km/h, as the O-2 carrying capacity [haemoglobin concentration (Hb)] increased by only similar to 10%. The increase in cardiac output resulted from a doubling of the heart rate and a small (<10%) increase in stroke volume. The mean aortic pressure (MAP) was unchanged, implying a 50% decrease in systemic vascular resistance (P less than or equal to 0.05). In contrast, exercise had no significant effect on pulmonary vascular resistance. The sympathetic division of the ANS controlled O-2 delivery via beta-adrenoceptors (heart rate and contractility) and Hb concentration via alpha-adrenoceptor-mediated splenic contraction. In addition, the sympathetic division modulated systemic vascular tone via alpha- and beta-adrenoceptors, but also exerted a vasodilator influence on the pulmonary circulation via beta-adrenoceptors. The parasympathetic division controlled O-2 delivery in part directly (heart rate) and in part indirectly via inhibition of beta-adrenoceptor activity (heart rate and contractility), even during heavy exercise. In addition, the parasympathetic division exerted a direct vasodilator influence on the pulmonary, but not on the systemic, circulation. Conclusions: Thus, in swine, in a manner similar to that in humans, both the sympathetic and parasympathetic division of the ANS contribute to cardiovascular homeostasis during exercise up to levels of high intensity. (C) 1998 Elsevier Science B.V. All rights reserved.