The carotid sinus baroreceptor reflex in the pregnant rabbit

Abstract

SUMMARY1. The reflex responses to baroreceptor stimulation have been compared in eight pregnant and eight non-pregnant anaesthetized female rabbits.2. The vascularly isolated, innervated carotid sinus was exposed for 30 sec to a series of non-pulsatile pressures ranging from 30 to 230 mmHg. The contralateral sinus nerve and both aortic nerves were cut. Systemic arterial pressure and heart rate were measured at each sinus pressure.3. The range of arterial pressure change which could be evoked from the isolated innervated sinus was less in the pregnant than in the non-pregnant rabbits. Mean changes were 98 and 61 mmHg respectively. On the other hand changes in heart rate were similar in the two groups (45 and 43 beats/ min respectively).4. The smaller blood pressure response in the pregnant animals resulted from a lesser rise in systemic arterial pressure at low levels of sinus pressure. At high sinus pressures the blood pressure fell to a similar level in both groups of animals.5. Pressure on the great vessels by the gravid uterus was not a factor since there was no consistent difference between the responses obtained with the rabbit lying on its back or on its side.6. Mechanisms which might be responsible for the difference found are discussed.

Reflex response of the rabbit hind‐limb muscle vascular bed to baroreceptor stimulation and its modification by pregnancy.

“…Arterial blood gas tensions. As in our previous studies (Humphreys & Joels,-1974; Humphreyi & Joels, 1977), mean arterial PCO2 was lower in the pregnant animals. Mean arterial pressure.…”

“…Mean arterial pressure. Despite the more extensive operative interference, including laparotomy, required for these experiments, the levels of mean arterial pressure were similar to those recorded in previous studies on pregnant and non-pregnant rabbits (Humphreys & Joels, 1974, 1977.…”

“…Our previous studies have shown, within the limitations of the experimental techniques, that during pregnancy carotid sinus pressure control of heart rate is unaltered (Humphreys & Joels, 1974) and that there is no large change in the carotid sinus pressures corresponding to the threshold and saturation of the reflex control of total peripheral resistance. We have now shown this to be true also of the control by the carotid sinus baroreceptors of the resistance of one particular vascular bed normally considered divorced from the changes produced by pregnancy.…”

Pregnancy and the endocrine regulation of the baroreceptor reflex

“…This change was first noted by Humphreys and Joels (97,98) in an extensive series of studies in anesthetized pregnant rabbits, in which reductions in carotid sinus pressure failed to elicit normal increases in arterial pressure and peripheral vascular resistance. Since then, arterial baroreflex impairment has been documented in several species studied in the conscious state, including dogs (23), rats (24,124), rabbits (21,97,162), goats (152), sheep (104,123), and humans (16,79,122,164,175,193). Reflex responses to atrial stretch are also blunted (51,94,95,103), which may contribute to retention of the expanded blood volume.…”

Arterial pressure maintenance after haemorrhage in the pregnant rabbit.

“…First, both the arterial pressure and systemic vascular resistance are reduced (Humphreys & Joels, 1977 b;Nuwayhid, 1979); secondly, in the sympathectomized vascular bed of the hind limb smaller pressure changes occur in response to changes in blood flow, and this altered relationship persists when the sympathetic innervation is stimulated at fixed frequencies (Humphreys & Joels, 1982); thirdly, and probably as a consequence of the other changes, it has been shown that the increase of arterial pressure which can be evoked by lowering pressure within an isolated, innervated carotid sinus is reduced (Humphreys & Joels, 1974). Furthermore, Ladner, Brinkman, Weston & Assali (1970) have shown in the ewe that close to term the greatly enlarged uterine vascular bed is probably outside the control of the sympathetic vasomotor nerves, though so far this has not been demonstrated in the rabbit.…”