A role for nitric oxide within the nucleus tractus solitarii in the development of muscle mechanoreflex dysfunction in hypertension.

Abstract

Evidence suggests that the muscle mechanoreflex, a circulatory reflex that raises blood pressure and heart rate (HR) upon activation of mechanically sensitive afferent fibres in skeletal muscle, is overactive in hypertension. However, the mechanisms underlying this abnormal reflex function have yet to be identified. Sensory input from the mechanoreflex is processed within the nucleus tractus solitarii (NTS) in the medulla oblongata. Within the NTS, the enzymatic activity of nitric oxide synthase produces nitric oxide (NO). This centrally derived NO has been shown to modulate muscle reflex activity and serves as a viable candidate for mediating the mechanoreflex dysfunction that develops in hypertension. We hypothesized that mechanoreflex dysfunction in hypertension is mediated by abnormal alterations in NO production in the NTS. Mechanically sensitive afferent fibres were stimulated by passively stretching hindlimb muscle before and after blocking the endogenous production of NO within the NTS via microdialysis of the NO synthase inhibitor L-NAME (1 and 5 mM) in normotensive Wistar-Kyoto rats and spontaneously hypertensive rats (SHRs). Changes in HR and mean arterial pressure in response to stretch were significantly larger in SHRs compared with Wistar-Kyoto rats prior to L-NAME dialysis. Attenuating NO production via L-NAME in normotensive rats recapitulated the exaggerated cardiovascular response to stretch observed in SHRs. Dialysing L-NAME in SHRs further accentuated the increases in HR and mean arterial pressure elicited by stretch. These findings support the contention that reductions in NO production within the NTS contribute to the generation of abnormal cardiovascular control by the skeletal muscle mechanoreflex in hypertension.