Ventilation inhibits sympathetic action potential recruitment even during severe chemoreflex stress

This study investigated the influence of ventilation on sympathetic action potential (AP) discharge patterns during varying levels of high chemoreflex stress. In seven trained breath-hold divers (33±12 yrs), we measured muscle sympathetic nerve activity (MSNA) at baseline, during preparatory rebreathing (RBR), and during i) functional residual capacity apnea (FRC_Apnea) and ii) continued RBR. Data from RBR were analyzed at matched (i.e., to FRC_Apnea) hemoglobin saturation (HbSat) levels (RBR_Matched) or more severe levels (RBR_End). A third protocol compared alternating periods (30s) of FRC and RBR (FRC-RBR_ALT). Subjects continued each protocol until 85% volitional tolerance. AP patterns in MSNA (i.e., providing the true neural content of each sympathetic burst) were studied using wavelet-based methodology. First, for similar levels of chemoreflex stress (both HbSat: 71±6%; P=NS), RBR_Matched was associated with reduced AP frequency and APs per burst compared to FRC_Apnea (both P<0.001). When APs were binned according to peak-to-peak amplitude (i.e., into clusters), total AP clusters increased during FRC_Apnea (+10±2; P<0.001), but not RBR_Matched (+1±2; P=NS). Second, despite more severe chemoreflex stress during RBR_End (HbSat: 56±13 vs. 71±6%; P<0.001), RBR_End was associated with a restrained increase in the APs per burst (FRC_Apnea: +18±7; RBR_End: +11±5) and total AP clusters (FRC_Apnea: +10±2; RBR_End: +6±4) (both P<0.01). During FRC-RBR_ALT, all periods of FRC elicited sympathetic AP recruitment (all P<0.001), whereas all periods of RBR were associated with complete withdrawal of AP recruitment (all P=NS). Presently, we demonstrate that ventilation per se restrains and/or inhibits sympathetic axonal recruitment during high, and even, extreme, chemoreflex stress.

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