Decreased cardiac baroreflex gain during pregnancy in conscious rabbits: Role of parasympathetic and sympathetic pathways

Pregnancy attenuates cardiac baroreflex function. Experiments were performed to determine if this is due to a change in sympathetic (SYM) or parasympathetic (PARA) control of the heart. Rabbits instrumented with nonoccluding aortic and vena caval catheters were studied before (NP) and at the end of pregnancy (P). Baroreflex relationships between arterial pressure and heart rate (HR) were generated using nitroprusside and phenylephrine. Curves were produced before and after treatment with propranolol (PRO) to block SYM, or methscopolamine (MET) to block PARA. Differences in baroreflex curves were determined by comparing (ANOVA) parameters generated by logistic analysis. HR range (difference between highest and lowest HR; bpm) was similar in P (154±9) and NP (172±8) rabbits before treatments. MET decreased (p<0.05) HR range similarly in both groups (to 57±8, P; 85±15, NP; n=3-4). In contrast, after PRO (n=4), HR range was smaller (p<0.05) in P (101±19) compared to NP (134±10) rabbits. Thus, the range of HR controlled by PARA after blockade of SYM was diminished during P. Reflex gain (bpm/mmHg) was higher (p<0.05) in NP (10.3±1.8) than in P (4.4±0.9) rabbits before treatments. PRO decreased (p<0.05) gain to similar levels in both groups (2.8±0.5, NP; 1,8±0.5, P), but MET did not significantly alter gain in either group. These results suggest that SYM is more influential on gain and that pregnancy reduces gain by actions on SYM. In conclusion, pregnancy-induced changes in reflex control of HR range and gain appear to be mediated by alterations in the parasympathetic and sympathetic nervous systems, respectively.