MUSCLE CHEMOREFLEX ALTERS CAROTID-SINUS BAROREFLEX RESPONSE IN HUMANS

The arterial baroreflex opposes presser responses to muscle ischemia ( muscle chemoreflex). Our experiments sought to quantify the unknown ef fects of muscle chemoreflex on carotid sinus baroreflex (CSB) sensitiv ity. We generated CSB stimulus-response (S-R) curves by pulsatile appl ication (triggered by each electrocardiogram R wave) of positive and n egative neck pressure (from 60 to -80 mmHg in 20-mmHg steps of 20 s ea ch) in seven normal young men. S-R curves were obtained at rest (uprig ht), during the last :I min of ergometer exercise (150 W), and at the first minute of postexercise recovery with leg circulation free (contr ol). A second study repeated the same procedures, except that leg circ ulation was occluded 20 s before the end of exercise to elicit muscle chemoreflex, and occlusion was maintained during recovery measurements (similar to 3- to 4-min duration). S-R curves for CSB were shifted up ward and rightward (25 mmHg) to higher arterial blood pressure (BF) by exercise and less so (10 mmHg) in recovery (free leg flow). Postexerc ise occlusion (muscle chemoreflex) raised BP and shifted S-R curves ab ove exercise curves. CSB gain rose from -0.26 +/- 0.06 (control) to -0 .44 +/- 0.08 (occlusion) during positive neck pressure application and was reduced from -0.14 +/- 0.04 to zero (-0.04 +/- 0.03) during negat ive neck pressure. Heart rate responses during postexercise muscle che moreflex were not significantly different from control. Results reveal a nonlinear summation of CSB and muscle chemoreflex effects on BP. BP -raising capability of muscle chemoreflex enhances CSB responses to hy potension but overpowers baroreflex opposition to hypertension.