THE EFFECT OF STATIC EXERCISE ON RENAL SYMPATHETIC-NERVE ACTIVITY IN CONSCIOUS CATS

1. Renal sympathetic nerve activity (RNA), heart rate (HR), arterial blood pressure (AP), and force development were measured simultaneously during voluntary static (isometric) exercise performed by conscious cats. The cats were operantly trained to press a bar with one forelimb. When the force applied to the bar exceeded a predetermined value (threshold), a sound was emitted by a buzzer for audio-feedback. If the cat continued to produce the appropriate force for a period of 26-55 s, food was given as a reward. 2. A total of eighty-nine exercise trials were performed by seven cats. The peak force applied to the bar was 468 +/- 28 g (mean +/- S.E.M.). RNA, HR, and AP increased significantly from the control value during static exercise by 102 +/- 14%, 23 +/- 2 beats/min, and 11 +/- 1 mmHg, respectively. 3. The increase in RNA had both an initial and a late component. The initial component occurred at or immediately before the onset of force development and lasted for 10 s, while the late component gradually increased 14 s after the onset of static exercise and was sustained until the exercise was terminated. 4. HR also increased at the beginning of static exercise with a similar time course as RNA. Then, HR returned to the control value and remained at that level during the remainder of exercise. The increase in AP was delayed by 10 s from the initial increase in RNA and then continued to rise throughout the period of exercise. 5. The sound of the buzzer was emitted during rest to determine any influence of anticipation or conditioning on the response. RNA and AP increased slightly, but HR did not change. The increases in RNA and AP were smaller than the increases obtained during static exercise. Thus, the increases in RNA, HR and AP during static exercise appeared to be associated with the exercise itself and not due to anticipation and/or conditioning. 6. When AP was elevated by a bolus injection of noradrenaline, RNA during rest was almost abolished and the increase of RNA during static exercise was markedly inhibited. Thus the arterial baroreflex significantly influences RNA both during rest and during static exercise. 7. This study suggest that the initial increases in RNA and HR at the beginning of static exercise in conscious cats are caused by descending input from higher brain centres and not by afferent feedback signals from muscle receptors or by arterial baroreceptors. In contrast, the late increases in RNA and AP during static exercise tend to be related to muscle metabolism and a feedback signal from the exercising muscle may play a role.