Cardiac sympathetic nerve control of cardiac output during 24-hour period

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We hypothesized that sympathetic outflows to different vascular beds or organs are specifically and differentially controlled in accordance with the physiological function of an individual vascular bed or organ. Such fine control of the sympathetic nervous system will be revealed by direct measurements of sympathetic outflows to different organs (such as the heart and the kidneys) during voluntary behavior in daily life.The present study was therefore undertaken to determine the responses in cardiac sympathetic nerve activity (CSNA) and renal sympathetic nerve activity (RSNA) during voluntary behavior in conscious cats and to identify the cause-effect relationships between CSNA and cardiac output (CO) and between RSNA and total peripheral vascular resistance (TPR). A pair of electrodes was implanted on the inferior cardiac nerve or the renal nerve and a Doppler flow probe was placed on the ascending aorta. Catheters were inserted into the left carotid artery and jugular vein. Three to … More ten days after the surgery, CSNA or RSNA, ascending aortic blood flow, mean arterial blood pressure (MAP) and heart rate (HR) were simultaneously measured for 5 hour in the unrestrained condition. CO, stroke volume (SV), and TPR were sequentially calculated. The sympathetic nerve and hemodynamic responses to a variety of voluntary behaviors (treadmill exercise, eating, drinking, and sleep) were analyzed.During treadmill exercise, HR and CO increased in proportion to the exercise speed. MAP did not change during exercise with a slow speed of 10-30 m/min and tended to rise gradually during exercise at a faster speed of 40-50 m/min. Thus calculated TPR tended to decrease during exercise. Propranolol blunted the tachycardia and almost abolished the increase in CO during treadmill exercise, suggesting cardiac sympathetic activation. Indeed, we found that CSNA increased during treadmill exercise, in accordance with the responses in HR and CO.It was interesting that RSNA showed no change during exercise at the slow speed and increased at the faster speed. In contrast to treadmill exercise, MAP increased considerably during eating and drinking while HR and CO increased slightly, suggesting a marked increase in TPR.On the other hand, MAP decreased considerably during sleep while HR and CO showed litle changes, suggesting a reduction in TPR.In consistent with the hemodynamic responses, RSNA was augmented during eating and drinking but was inhibited during sleep ; the responses in RSNA were greater than those in CSNA.It is concluded that sympathetic outflows to the heart and the kidneys are separately and differentially regulated during a variety of voluntary behaviors in daily life, suggesting fine control of the sympathetic nervous system by the brain cardiovascular center. Less