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NEUROHUMORAL CONTROL OF CARDIOVASCULAR FUNCTION

Effect of pregnancy and 5-pregnan-3-ol-20-one on atrial receptor afferent discharge in rats

Department of Physiology, University of Alberta, Edmonton, Alberta T6G 2S2, Canada

Submitted 4 December 2003 ; accepted in final form 11 August 2004

The atrial volume reflex is attenuated in pregnancy. This may be mimicked by chronic administration of 5-pregnan-3-ol-20-one (pregnan). We investigated whether afferent output from sensory receptors may be suppressed at this time. Vagal afferent nerve activity was measured during discrete localized stimulation of the atrial volume receptors by inflation of a balloon at the superior vena caval-right atrial junction. The receptors were classified as high- (HF) or low- (LF) frequency subtypes on the basis of their response to graded atrial distension. Although both HF (regression coefficient = 0.50 ± 0.11 Hz/µl, r² = 0.47, P < 0.001) and LF (regression coefficient = 0.03 ± 0.05 Hz/µl, r² = 0.009, P = 0.613) subtypes could be identified in virgin rats, only LF (regression coefficient = 0.09 ± 0.05 Hz/µl, r² = 0.044, P = 0.099) receptors were found in late-pregnant animals. Similarly, in virgin rats treated chronically with pregnan (500 µg/24 h for 2 days), only LF receptors were identified (regression coefficient = –0.004 ± 0.078 Hz/µl, r² = 0.000, P = 0.962), whereas both subtypes were present in the vehicle-treated animals (HF regression coefficient = 0.626 ± 0.255 Hz/µl, r² = 0.317, P = 0.029; LF regression coefficient = –0.012 ± 0.071 Hz/µl, r² = 0.002, P = 0.866). By contrast, acute intracardiac pregnan (2.6 µg/kg) did not alter vagal afferent nerve activity. In conclusion, stretch-induced discharge of high-frequency atrial receptors is suppressed during pregnancy, whereas that of low-frequency receptors is preserved. This effect may be mimicked by chronic, but not acute, pregnan. We propose that, during pregnancy, pregnan alters the transducer properties of the atrial volume receptors, thus allowing blood volume to increase.

pregnancy; progesterone; cardiopulmonary receptors; baroreceptors; blood volume

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