Central command blunts sensitivity of arterial baroreceptor-heart rate reflex at onset of voluntary static exercise

Abstract

Matsukawa, Kanji, Hidehiko Komine, Tomoko Nakamoto, and Jun Murata. Central command blunts sensitivity of arterial baroreceptor-heart rate reflex at onset of voluntary static exercise. Am J Physiol Heart Circ Physiol 290: H200 -H208, 2006. First published August 19, 2005 doi:10.1152/ajpheart.00013.2005.-We have reported that baroreflex bradycardia by stimulation of the aortic depressor nerve is blunted at the onset of voluntary static exercise in conscious cats. Central command may contribute to the blunted bradycardia, because the most blunted bradycardia occurs immediately before exercise or when a forelimb is extended before force development. However, it remained unknown whether the blunted bradycardia is due to either reduced sensitivity of the baroreflex stimulus-response curve or resetting of the curve toward a higher blood pressure. To determine this, we examined the stimulus-response relationship between systolic (SAP) or mean arterial pressure (MAP) and heart rate (HR) at the onset of and during the later period of static exercise in seven cats (n ϭ 348 trials) by changing arterial pressure with infusion of nitroprusside and phenylephrine or norepinephrine. The slope of the MAP-HR curve decreased at the onset of exercise to 48% of the preexercise value (2.9 Ϯ 0.4 beats ⅐ min Ϫ1 ⅐ mmHg Ϫ1 ); the slope of the SAP-HR curve decreased to 59%. The threshold blood pressures of the stimulus-response curves, at which HR started to fall due to arterial baroreflex, were not affected. In contrast, the slopes of the stimulus-response curves during the later period of exercise returned near the preexercise levels, whereas the threshold blood pressures elevated 6 -8 mmHg. The maximal plateau level of HR was not different before and during static exercise, denying an upward shift of the baroreflex stimulus-response curves. Thus central command is likely to attenuate sensitivity of the cardiac component of arterial baroreflex at the onset of voluntary static exercise without shifting the stimulus-response curve.gain; resetting; exercise pressor reflex; conscious cat ARTERIAL BLOOD PRESSURE (AP) is sensed by arterial baroreceptors in the carotid sinus and aortic arch regions, whose activities transmit the beat-to-beat changes in AP to the central nervous system and promote the baroreflex responses of autonomic efferent nerve activities to the heart and blood vessels. For example, if arterial baroreceptors are stimulated in proportion to a rise in AP, reflex bradycardia is elicited to restore AP to the control level. However, because heart rate (HR) and AP increase simultaneously during static and dynamic exercise in humans and conscious animals, it has been thought that arterial baroreflex function is modulated during exercise (1, 2, 14, 21, 36). The stimulus-response curve of the arterial baroreflex has been studied in the steady state of exercise. Melcher and Donald (23), when studying carotid baroreflex function with the isolated carotid sinuses in conscious dogs, found that the stimulus-response curve was displaced...

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.…”

“…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.…”

“…Matsukawa and colleagues (22,26,34) have recently provided clear evidence that central command acts upon the arterial baroreflex circuit within the brainstem and modulates its central property at the start of static exercise. The baroreflex bradycardia elicited by aortic nerve stimulation was blunted immediately before static exercise in conscious cats, even in the absence of muscular exertion, and at the onset of spontaneous muscle contraction in decerebrate cats; in contrast, the depressor response to aortic nerve stimulation was not affected by static exercise (22,34).…”

Central Command Regulation of Circulatory Function Mediated by Descending Pontine Cholinergic Inputs to Sympathoexcitatory Rostral Ventrolateral Medulla Neurons

“…16 Recent evidence indicates that the cardiac baroreflex is transiently inhibited at exercise onset, which may facilitate immediate vagal withdrawal. 55 In conclusion, our data indicate that the cholinergic projection from the PPT to the RVLM is an integral component of the central command pathway that regulates circulatory function during exercise and possibly other arousal or behavioral states. …”

Autonomic Adjustments to Exercise in Humans

“…In contrast, the magnitude of the fall in HR in response to aortic depressor nerve stimulation in decerebrate cats was similar at rest and the onset of both electrically evoked muscle contraction and muscle stretch (227). However, the bradycardic response to aortic depressor nerve stimulation was transiently attenuated at the onset of volitional isometric exercise in conscious cats, but restored later in exercise (177,200,227). Taken together these findings suggest that the activation of central command and/or skeletal muscle afferents can inhibit baroreflex regulation of HR via the parasympathetic nerves at the onset of exercise.…”