The carotid baroreflex modifies the pressor threshold of the muscle metaboreflex in humans

Abstract

The purpose of the present study was to test our hypothesis that unloading the carotid baroreceptors alters the threshold and gain of the muscle metaboreflex in humans. Ten healthy subjects performed a static handgrip exercise at 50% of maximum voluntary contraction. Contraction was sustained for 15, 30, 45, and 60 s and was followed by 3 min of forearm circulatory arrest, during which forearm muscular pH is known to decrease linearly with increasing contraction time. The carotid baroreceptors were unloaded by applying 0.1-Hz sinusoidal neck pressure (oscillating from +15 to +50 mmHg) during ischemia. We estimated the threshold and gain of the muscle metaboreflex by analyzing the relationship between the cardiovascular responses during ischemia and the amount of work done during the exercise. In the condition with unloading of the carotid baroreceptors, the muscle metaboreflex thresholds for mean arterial blood pressure (MAP) and total vascular resistance (TVR) corresponded to significantly lower work levels than the control condition (threshold for MAP: 795 ± 102 vs. 662 ± 208 mmHg and threshold for TVR: 818 ± 213 vs. 572 ± 292 kg·s, < 0.05), but the gains did not differ between the two conditions (gain for MAP: 4.9 ± 1.7 vs. 4.4 ± 1.6 mmHg·kg·s·100 and gain for TVR: 1.3 ± 0.8 vs. 1.3 ± 0.7 mmHg·l·min·kg·s·100). We conclude that the carotid baroreflex modifies the muscle metaboreflex threshold in humans. Our results suggest the carotid baroreflex brakes the muscle metaboreflex, thereby inhibiting muscle metaboreflex-mediated pressor and vasoconstriction responses. We found that unloading the carotid baroreceptors shifts the pressor threshold of the muscle metaboreflex toward lower metabolic stimulation levels in humans. This finding indicates that, in the normal loading state, the carotid baroreflex inhibits the muscle metaboreflex pressor response by shifting the reflex threshold to higher metabolic stimulation levels.

Sex differences in the ventilatory and cardiovascular response to supine and tilted metaboreflex activation

“…Therefore, our findings of an attenuated pressor response to metaboreflex activation in the upright position support the concept that activation of the arterial baroreflex acts as a “brake” for the metaboreflex (as proposed by (Ichinose et al. ) and (Calbet et al. )).…”

“…Using either a neck pressure device (Ichinose et al. ) or the seated position (Teixeira et al. ) previous studies have suggested that unloading of baroreceptors should increase the function of the metaboreflex.…”

“…Similarly, Ichinose et al. () found that unloading of the carotid baroreflex via a neck pressure device decreases the threshold of activation of the metaboreflex (i.e., greater activation with less stimulus). Since Katayama et al.…”

“…This increase in sympathetic output in upright tilt due to baroreceptor resetting could lead to inhibition of the metaboreflex in the upright posture as Ichinose et al. () suggested that the carotid baroreflex functions to “brake” the metaboreflex and inhibit the pressor response.…”

“…Recent studies have found interactions between the baroreflexes and the metaboreflex; however, these studies have included mostly (90%), or exclusively, male participants (Ichinose et al. ; Katayama et al. ; Teixeira et al.…”