Baroreceptor reflexes in new‐born rabbits

Abstract

This paper describes an investigation of the baroreceptor reflexes from the carotid sinus and aortic arch in the new-born rabbit. Bauer (1939) concluded from measurement of the heart rate in rabbits that the depressor reflex was not activated during asphyxia until the 30th day from birth, and the carotid sinus reflex until the 40th day. He also stated that 'faradic stimulation of the depressor nerves and carotid sinus regions in rabbits produces a reflex inhibition of the heart as early as the 11th and 14th days, respectively '. Both Bauer (1939) and Barcroft (1946) inferred from these and other observations in Bauer's paper that the threshold pressures for the baroreceptor reflexes were high, 65 mm Hg for the depressor and 80 mm Hg for the carotid sinus reflex; the normal blood pressure of a new-born rabbit did not reach these levels until several weeks from birth.The implication is that in the new-born rabbit the cardiovascular system is not under the control of the baroreceptors unless the blood pressure rises above the threshold levels quoted above. The object of the experiments now to be described was to determine by more direct methods whether the baroreceptor reflexes are functioning in the rabbit during the first few days of life at the low arterial pressure which is then normally present. METHODSObservations were made on seventeen adult, nine young (3-weeks-old or more, 0-2-1.0 kg body weight) and seventy-five new-born (0-15 days) rabbits, anaesthetized witb pentobarbitone and/or urethane. Of the new-born rabbits fifty-five were given pentobarbitone (10-15 mg/kg intraperitoneally, supplemented with additional pentobarbitone as necessary), and the remainder received urethane (1.2 g/kg intraperitoneally). Older rabbits were given pentobarbitone (20-25 mg/kg intravenously) followed by urethane (1.2 g/kg subcutaneously). The trachea was cannulated. In most experiments rectal temperature was recorded by a thermocouple, and was maintained at 38 + 10C by application of external heat as necessary. This was found to be particularly desirable in very young rabbits.Pres*ure changes applied to the carotid 8inus. In new-born rabbits the left carotid sinus was isolated from the rest of the circulation. The cardiac end of the right or left common carotid artery was cannulated to record systemic blood pressure. The cephalic end of the left common carotid artery was connected by a polyethylene tube to a pressure bottle filled with saline and to a manometer. The left external carotid was tied, together with all branches of the common carotid above the polyethylene tube and the internal carotid distal to the sinus. Static and pulsatile pressures were applied to the carotid sinus. Pulsations were

Afferent Fibers of Cardiovascular and Pulmonary Origins and Their Intrabulbar Connections (A Review)

“…They are already active in newborn and three-week-old rabbits, and action current studies reveal as in adults a linear relation between discharge frequencies in the aortic depressor nerve and the height of the systemic blood pressure. 43) Electrophysiological evidence of baro-receptors in the pulmonary artery have been presented for the dog26,28) and the cat.11) In the cat after denervation of the sino-aortic baroreceptor areas action currents may be led off in the right vagus nerve, which are synchronous with changes in pulmonary artery pressure . The discharge occurs during the systolic rise of the pulmonary arterial blood pressure and clamping of the pulmonary artery causes an abrupt decrease of the discharge rate.…”

Respiratory and Cardiovascular Changes During Asphyxia and Resuscitation of Fœtal and Newborn Rabbits

“…Experimentally, interest in the systemic circulation during foetal or newborn asphyxia has mostly been in relation to reflex activity [Bauer, 1938;Downing, 1960;Mott, 1965]. On the other hand, the pulmonary circulation has been widely studied, and has been shown to be highly reactive during asphyxia and resuscitation [Ardran et al, 1952;Dawes et al, 1953].…”

Function of Vascular Smooth Muscle and Its Sympathetic Innervation in the Newborn Dog*

“…The status of the subject may be illustrated by examining studies on baroreceptor mechanisms in the neonate. Some investigators have indicated that these reflex adjustments are not fully operative in the newborn (1-3), whereas others believe them to be quite functional (4)(5)(6)(7). It is important to realize that most of these investigations have been of a qualitative nature and do not imply that quantitatively control of the newborn circulation is the same as in the adult.…”