Homeostatic Regulation of Resting Heart Rate in Fetal Lambs

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Abstract
Fetal lambs in the last 3rd of gestation were delivered by Cesarean section, the placental circulation maintained, and breathing prevented. Carotid blood pressure was recorded during rest or during temporary occlusion of the 2 umbilical arteries or umbilical veins. Observations were made when the cardiac innervation was intact, when the vagi were cut, when carotid sinuses were excluded, when the stellate ganglia were removed, and when various combinations of these manipulations were employed. With innervation intact, resting heart rate was over 190 beat/min. in most cases. There was no relation to fetal age, or time in the expt. Bilateral vagotomy caused a decrease in blood pressure and heart rate. Stimulation of peripheral cut ends of the vagi caused cardiac inhibition. Vagotonia (nerves intact) could be induced by causing mild circulatory distress. This was unmodified by exclusion of carotid sinus effects or the stellate ganglia. When the heart was fully denervated, circulatory distress, including anoxic bradycardia, was followed by marked tachycardia. This recovery response is abolished by adrenalectomy. It appears, then, that the resting heart is operating free of vagal or sympathetic tone. The former is induced by imposing a light load on the circulation, but excessive distress mobilizes a sympathotonic response, including endogenous secretion of epinephrine.
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