Influence of vagal cooling on cardiac output in normal and beta-blocked exercising dogs
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To study the relative influence of parasympathetic and sympathetic innervation on the early adaptation of cardiac output (CO) to exercise, we determined the time constant and amplitude of the CO change in dogs following a stepwise increase in treadmill velocity. The animals were studied during control conditions, beta-blockade, vagal blockade and combined beta-blockade and vagal blockade. To measure CO, an electromagnetic flow probe was implanted around the ascending aorta. Vagal activity was blocked with coolers, implanted around the cervical vagosympathetic trunks.
The time constant during beta-blockade (12.1 s) was not different from the control situation (11.4 s), but during vagal cooling it increased significantly (16.2 s), and with combined vagal cooling and beta-blockade it rose to 20.7 s. Thus the increase in cardiac output with exercise is accelerated most by the loss of vagal tone and to a lesser degree by sympathetic activation.
The amplitude of the change in CO during control was 112%. Heart rate (HR) rose by 74% and stroke volume (SV) by 22%. Beta-blockade lowered the initial CO but did not alter the percentage increase. Vagal cooling, with or without beta-blockade, caused an increased initial HR but did not influence basal CO because of a concomitant reduction in SV. Exercise now increased HR less (21% and 30%, respectively) and SV more (52% and 52%) but the increase in CO did not change significantly (87% and 97%).
Both the high percentage increase in CO (87%) and the time constant during combined beta-blockade and vagal blockade (20.7 s), which is far less than previously found in dogs with surgically denervated hearts (29.5 s), suggest the existence of a non-vagal, non-beta-adrenergic innervation of the heart.
Key wordsCardiac output regulation Exercise Vagal cooling Adrenergic beta-receptor blockade Time constant
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