Arterial baroreflex during pregnancy and renal sympathetic nerve activity during parturition in rabbits.

The arterial baroreflex control of renal sympathetic nerve activity (RSNA) was evaluated in nine term pregnant (P) and 12 nonpregnant (NP) conscious New Zealand White rabbits. In an additional four P rabbits, the RSNA response to spontaneous parturition was measured. The blood pressure (BP)-RSNA relationship was generated by sequential inflations of aortic and vena caval perivascular occluders. Rest BP (P: 61 +/- 2 vs. NP: 73 +/- 2 mmHg) and the centering point of the baroreflex (P: 57 +/- 2 vs. NP: 70 +/- 2 mmHg) were lower (P < 0.05) in term pregnancy. Baroreflex range (P: 246 +/- 14% vs. NP 263 +/- 24% of rest RSNA) was not affected by pregnancy. However, maximal reflex gain was moderately depressed (-44%) in P rabbits (P: -15 +/- 1 vs. NP: -27 +/- 4% of rest RSNA/mmHg; P < 0.05) due to a significant reduction in the slope coefficient. Delivery of a fetus was associated with strong renal sympatho-excitation. Peak RSNA averaged 80 +/- 37% of smoke-elicited RSNA or 1,221 +/- 288% of rest RSNA (mean +/- SD). These results suggest that, in contrast to rat pregnancy, depressed arterial baroreflex control of RSNA in rabbit pregnancy is due primarily to a reduction in maximal gain rather than a reduction in the maximal sympathetic response to hypotension.