The effect of static exercise on renal sympathetic nerve activity in conscious cats.

Abstract

SUMMARY1. 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 much 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 suggests that the initial increases in RNA and HR at the beginning

Gadolinium does not blunt the cardiovascular responses at the onset of voluntary static exercise in cats: a predominant role of central command

“…The peak force during static exercise in conscious cats corresponded to ϳ17% of their body weight. Since the maximally developed force was nearly 1.0 kg in the present and previous studies (5,9,24,26), the cats seemed to produce ϳ60% of the maximal voluntary force at the most, and the intensity of static exercise appeared mild or moderate. Thus the exercise intensity might be insufficient to initiate the muscle mechanoreflex, the contribution of which remains to be studied with a higher intensity of static exercise.…”

“…Although these limitations must be taken into account, the present finding that gadolinium did not at all alter the time course and magnitude of the cardiovascular responses at the onset of voluntary static exercise in conscious cats suggests a role of central command in producing the initial cardiovascular adaptation at least in the presence of gadolinium. The central command hypothesis is supported by previous findings from Matsukawa and colleagues (24,25,43) demonstrating that cardiac and renal sympathetic efferent nerve activities start to increase immediately before or at the onset of static and dynamic exercise, which in turn contributes to tachycardia and pressor response in several seconds. The rapid sympathetic nerve responses are hardly explained by the muscle mechanoreflex.…”

“…The cats were operantly conditioned to perform static exercise as previously described in detail (5,9,22,24,26). They were trained to sit quietly in a transparent plastic box (width 35 ϫ height 40 ϫ depth 50 cm) with a small window (width 5 ϫ height 7 cm), extend a forelimb through the window, and press a bar for 10 -40 s while maintaining a sitting posture.…”

“…Therefore, either central command or the muscle mechanoreflex is considered as a candidate mechanism responsible for the initial cardiovascular adaptation at the onset of exercise. We have reported that cardiac and renal sympathetic efferent nerve activities abruptly increase immediately before or at the onset of voluntary muscle excursion, suggesting that central command has a predominant role in regulating the cardiovascular system during exercise (23)(24)(25). On the other hand, passive mechanical stretch of skeletal muscle, which is assumed to predominantly stimulate mechanoreceptors, increases HR, AP, and respiration in anesthetized animals and humans (8,39).…”

Exaggerated Renal Vasoconstriction During Exercise in Heart Failure Patients

“…In animals, the mechanoreflex has been shown to modulate the renal vascular bed. 22,23 For example, in chloralose-anesthetized cats, mechanical stimulation of the triceps surae muscle produces an immediate reflex increase in efferent renal sympathetic nerve activity. 22 In normal humans, we have reported that central command and/or mechanoreceptors help mediate reflex renal vasoconstriction during static handgrip exercise.…”