ALTERED HEART-RATE BAROREFLEX DURING PREGNANCY - ROLE OF SYMPATHETIC AND PARASYMPATHETIC NERVOUS SYSTEMS

Two studies were performed to determine whether the attenuation of bar oreflex control of heart rate during late pregnancy in conscious rabbi ts is due to changes in parasympathetic (Para) or sympathetic (Sym) co ntrol of the heart. In the first, baroreflex relationships between art erial pressure and heart rate were generated before and after treatmen t with propranolol (Pro) to block Sym or with methscopolamine (Meth) t o block Para. Each rabbit was studied in both the pregnant and nonpreg nant state. Pregnancy decreased maximum baroreflex gain from 14.9 +/- 4.0 to 4.8 +/- 0.9 beats . min(-1) . mmHg(-1) (P < 0.01) and decreased heart rate range from 177 +/- 6 to 143 +/- 10 beats/min (P < 0.01), p rimarily by increasing minimum heart rate (114 +/- 6 to 134 +/- 8 beat s/min; P < 0.01). The difference between pregnant and nonpregnant rabb its in baroreflex gain was not altered by Meth but was abolished by Pr o, suggesting that it is due to decreased Sym control of the heart. Th e elevated minimum heart rate of pregnancy persisted after Pro, but wa s abolished by Meth, suggesting that it is mediated by decreased Para control of the heart. In the second study, isolated buffer-perfused he arts from pregnant and nonpregnant rabbits were treated with increasin g doses of isoproterenol (0.3-300 mM) or acetylcholine (0.3-10,000 mu M), and the heart rate responses were determined. Hearts from pregnant rabbits were more sensitive to isoproterenol (P < 0.05), but less res ponsive to acetylcholine (P < 0.05). In conclusion, pregnancy-induced decreases in cardiac reflex gain and range appear to be mediated by al terations in Sym and Para, respectively. The change in Sym occurs prox imal to the heart, whereas the decreased contribution of Para may be d ue, at least in part, to decreased sensitivity of the heart to acetylc holine.