Mechano- and chemoreceptor modulation of renal sympathetic nerve activity at birth in fetal sheep

Abstract

Physiological responses at birth include increases in heart rate (HR), blood pressure, sympathetic nerve activity, and circulating vasoactive peptides. The factors mediating these responses are not known. To test the hypothesis that afferent input from peripheral mechanoreceptors (arterial and cardiopulmonary baroreceptors) and chemoreceptors contribute to the sympathoexcitatory and hormonal responses at birth, we studied the effects of sinoaortic denervation (SAD) and SAD with vagotomy (Vx) on changes in HR, mean arterial blood pressure (MABP), renal sympathetic nerve activity (RSNA), and catecholamine, arginine vasopressin (AVP), and ANG II levels at birth in term sheep. One hour after delivery by cesarean section, RSNA increased by 168 ± 49 and 192 ± 32% (relative to fetal values) in SAD and SAD-Vx animals, respectively. Significant increases in HR (18 ± 5 and 20 ± 6%) and MABP (24 ± 4 and 20 ± 5%) were also observed 1 h after delivery in SAD and SAD-Vx lambs, respectively. These responses are similar to those seen in intact sheep delivered at the same gestational age. AVP levels markedly increased after birth (19.8 ± 6.7 to 136.1 ± 75.9 pg/ml) in SAD-Vx lambs, whereas SAD animals displayed no change in AVP concentrations. Plasma ANG II also did not change after birth in either group, although levels were consistently higher ( P < 0.01) in SAD compared with SAD-Vx animals. In the presence of SAD, Vx resulted in significantly greater plasma levels of norepinephrine, although levels did not change after birth in either group. The epinephrine responses at birth were similar in both groups of animals. The present data suggest that afferent input from peripheral chemoreceptors and mechanoreceptors contributes little to the hemodynamic and sympathetic responses after delivery by cesarean section. On the other hand, these peripheral mechanisms appear to be involved in modulating endocrine responses at birth.

Effects of fetal ovine adrenalectomy on sympathetic and baroreflex responses at birth

“…On the other hand, we achieved plasma cortisol levels well within the physiological range present immediately before delivery. Because newborn systemic hemodynamic measurements in ADX ϩ F animals were similar to those reported by others and us in intact animals, we suggest that adequate levels of circulating glucocorticoids are of primary importance in cardiovascular adaptation at birth (23,31,32). Antenatal administration of glucocorticoids augments cardiovascular performance in prematurely delivered lambs, despite these animals' having lower plasma epinephrine levels than controls (22,30).…”

The Nervous Mechanism of Sympathetic over Activity Induced by Chronic Intermittent Hypoxia

“…When any chemical stimulation acts on peripheral chemoreceptor, such as hypoxic condition, glomus cells in the CB depolarize and release multiple putative neurotransmitters, including acetylcholine, serotonin, ATP, substance P and so on [46], then the afferent information along the carotid body sinus nerve is sent to brainstem firstly integration station, the Nucleus Tractus Solitarius (NTS) [47], activating primary neurons and then the secondary, reaching to other regulating areas such as the RVLM [48], in which containing sympathetic vascularactivated and cardio-sympathetic neurons, inducing contraction in the skin, the kidney, and other vessels but relaxation in important tubes to assure heart and brain possess sufficient blood flow. In this scenario, sympathetic nerve may present overactivity resulting from increased chemoreceptor sensitivity.…”