Regulation of Sympathetic Nerve Activity During the Cold Pressor Test in Normotensive Pregnant and Nonpregnant Women

Abstract

T he risk of developing cardiovascular disease is lower in young women when compared with similarly aged men. [1][2][3] However, pregnancy represents a period of increased susceptibility to disordered cardiovascular regulation for women. Pregnant women have an elevated risk for orthostatic intolerance. 4,5 Moreover, ≤8% of pregnancies are associated with the development of de novo hypertension. 6 The cause of such hypertensive pregnancy disorders remains unclear, and as a result, there exist few approaches by which clinicians can predict, screen, or treat the development of the hypertensive disorders of pregnancy (gestational hypertension and preeclampsia). Previous data have shown that the vascular response to the cold pressor test (CPT) is blunted in normal pregnant women compared with women in the nonpregnant, postpartum state. 8 However, an elevated pressor response to the CPT has been observed in women with 9 or who subsequently develop 10 preeclampsia. As such, the CPT has been suggested as a simple screening tool for pregnancy-related hypertensive disorders. The mechanisms driving these divergent responses remain unknown; however, sympathetic nervous system activity is a primary determinant of the pressor response to the CPT. 11Differing vascular responses to the CPT may be mediated by altered sympathetic responses or altered translation of sympathetic activity into a vascular outcome (neurovascular transduction). Studies conducted over the past 2 decades have indicated, albeit not universally, 12 that normotensive Abstract-Baseline neurovascular transduction is reduced in normotensive pregnancy; however, little is known about changes to neurovascular transduction during periods of heightened sympathetic activation. We tested the hypothesis that, despite an exacerbated muscle sympathetic nerve activity (microneurography) response to cold pressor stimulation, the blunting of neurovascular transduction in normotensive pregnant women would result in similar changes in vascular resistance and mean arterial pressure (Finometer) relative to nonpregnant controls. Baseline neurovascular transduction was reduced in pregnant women relative to controls when expressed as the quotient of both total resistance and mean arterial pressure and sympathetic burst frequency (0.

Blunted sympathetic neurovascular transduction during normotensive pregnancy

“…We have also demonstrated that pregnant women (33 ± 4 weeks of gestation) have an augmented SNA response to a cold‐pressor challenge compared to non‐pregnant controls (Usselman et al . ). This occurred despite a similar increase in pressure, supporting the concept that sympathetic signalling is dampened or offset at the level of the vasculature.…”

“…Recently, we demonstrated similar pressor responses to a standard cold challenge in non‐pregnant and pregnant women despite augmented sympathoexcitation during the manoeuvre in pregnant women (Usselman et al . ). These data suggest a reduced transduction of sympathetic nervous system signalling into a vascular outcome.…”

Maternal Responses to Aerobic Exercise in Pregnancy

“…The etiology of the initial drop in resistance is likely due to the elevated bioavailability of nitric oxide 64 and a blunted sympathetic vasoconstrictor responsiveness. 65 The progressive influence of increasing blood volume and cardiac output subsequently contribute to the increase in blood pressure observed during later pregnancy. In a longitudinal assessment, Sady et al 33 demonstrated that women were tested during the late second/ early third trimester and again 2 months postpartum, the increase in pressure during progressive exercise was the same.…”

“…There is an accepted increased dilatory response to shear (FMD) and a decreased responsiveness to sympathetic activation, 65 including decreased circulating catecholamine concentrations in response to exercise. 12,58 Combined this would favor an elevated blood flow in exercising muscles and a decreased total peripheral resistance.…”

Effects of one's sex and sex hormones on sympathetic responses to chemoreflex activation

“…Furthermore, reflex MSNA responses to the cold pressor test (Usselman et al . b ) and to baroreceptor unloading (Okada et al . ) are augmented during pregnancy.…”