Sympathetic neural recruitment strategies following acute intermittent hypoxia in humans.

We examined the effect of acute intermittent hypoxia (IH) on sympathetic neural firing patterns and the role of the carotid chemoreceptors. We hypothesized exposure to acute IH would increase muscle sympathetic nerve activity (MSNA) via an increase in action potential (AP) discharge rates and within-burst firing. We further hypothesized any change in discharge patterns would be attenuated during acute chemoreceptor deactivation (hyperoxia). MSNA (microneurography) was assessed in 17 healthy adults (11 male/6 female; 31 ± 1 yr) during normoxic rest before and after 30 min of experimental IH. Prior to and following IH, participants were exposed to 2 min of 100% oxygen (hyperoxia). AP patterns were studied from the filtered raw MSNA signal using wavelet-based methodology. Compared with baseline, multiunit MSNA burst incidence (P < 0.01), AP incidence (P = 0.01), and AP content per burst (P = 0.01) were increased following IH. There was an increase in the probability of a particular AP cluster firing once (P < 0.01) and more than once (P = 0.03) per burst following IH. There was no effect of hyperoxia on multiunit MSNA at baseline or following IH (P > 0.05); however, hyperoxia following IH attenuated the probability of particular AP clusters firing more than once per burst (P < 0.01). Acute IH increases MSNA by increasing AP discharge rates and within-burst firing. A portion of the increase in within-burst firing following IH can be attributed to the carotid chemoreceptors. These data advance the mechanistic understanding of sympathetic activation following acute IH in humans.