Spinal autonomic afferents in elicitation of tachycardia in volume infusion in the dog

Intravenous infusion of blood (36 ml/kg body wt) elicited tachycardia in artificially ventilated anesthetized dogs with intact autonomic innervation and in dogs with cardiac beta-receptor blockade. In contrast, infusion elicited bradycardia in dogs with section of the spinal cord at C6-C7, and in dogs with combined spinal section and cardiac beta-receptor blockade. The control heart rate was less than or equal to 110 beats/min in all the animals. The presence of infusion-induced tachycardia in dogs with beta-receptor blockade, i.e., dogs in which cardiac sympathetic efferents were blocked, and its absence in dogs with combined spinal section and beta-receptor blockade, i.e., dogs in which spinal autonomic afferents plus cardiac sympathetic efferents were blocked, may be the result of an additional interruption of spinal autonomic afferents by spinal section. It is concluded that tachycardia elicited by infusion may be partly due to a reflex with its afferent pathway in the spinal cord and its efferent pathway in the vagus nerves.