Effect of Increased Cortisol on Heart Rate in Pregnant Ewes

Abstract

In many species pregnancy alters baroreflex control of heart rate. In ovine, as in human pregnancy, both plasma cortisol and progesterone, hormones influencing the baroreflex, are increased. We tested the hypothesis that cortisol alters baroreflex function in sheep, and this effect is greater in pregnancy. 7 late gestation pregnant (P) ewes and 7 nonpregnant (NP) ewes were studied after stepwise infusions (4d each) of saline (SAL), 1 and 2 mg/kg/d cortisol (CORT1 and CORT2); increases in CORT were within the physiologic range. Blood pressure (BP) and heart rate (HR) were measured by telemetry and baroreflex function was evaluated. Resting BP was significantly greater in NP vs P ewes, but there was no effect of CORT. Pregnancy increased HR, and altered the response to CORT; HR was greater in P vs NP ewes, but increased with CORT2 only in P ewes. Analysis of baroreflex function indicated that CORT increased both minimum and maximum HR in P ewes (CORT2 vs SAL: min HR 72.0±4.3 vs 54.7±4.2; 165.4±9.3 vs 130.8±8.7). The results suggest cortisol increases sympathetic cardiac tone, but that this effect requires other factors occurring in the pregnant state. We found that nitric oxide synthase (NOS) expression in brainstem was altered in pregnancy: in P ewes basal nNOS mRNA was increased (at the level of rostral NTS), whereas iNOS was decreased (level of mid‐NTS), consistent with increased sympathetic tone. Supported by HD056228.