Activity of Muscle Sympathetic Neurons during Normotensive Pregnancy

In pathological populations, elevated sympathetic activity is associated with increased activity of individual sympathetic neurons. We used custom action potential detection software to analyze multi-unit sympathetic activity in 18 normotensive pregnant women (third trimester; 33 ± 5 weeks) and 19 non-pregnant women at rest and a subset (10 and 13 respectively) during a cold pressor challenge. Although the number of action potentials per burst, and number of active amplitude based "clusters" were not different between groups the total number of sympathetic action potentials per minute was higher in pregnant women at rest. Individual clusters were active predominately once per burst suggesting they represent single neurons. Action potentials occurred in closer succession in normotensive pregnant (inter-spike interval 36 ± 10 ms) versus non-pregnant women (50 ± 27 ms; P<0.001) at rest. Pregnant women had a lower total peripheral resistance (11.7 ± 3.0 mmHg/L/min) than non-pregnant women (15.1 ± 2.7 mmHg/L/min; P<0.001) indicating a blunted neurovascular transduction. The cold pressor reduced the number of action potentials per burst in both groups due to shortening of the R-R interval in conjunction with increased burst frequency, total neural firing per minute was unchanged. Thus, elevated sympathetic activity during normotensive pregnancy is specific to increased incidence of multi-unit bursts. This is likely due to decreased central gating of burst output as opposed to generalized increases in central drive. These data also reinforce the concept that pregnancy appears to be the only healthy state of chronic sympathetic hyperactivity that we are aware of.

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