Autonomic regulation of cardiovascular function in neonates

The degree of participation of the vascular bed supplied by the superior mesenteric artery in autonomic regulation of cardiovascular function during postnatal maturation was evaluated in piglets, ranging in age from birth to two months. Animals were anaesthetized with 0.25-0.5% halothane in a 50% mixture of N2O and O2, paralysed with decamethonium bromide and artificially ventilated to maintain normal arterial blood gases and pH. Mesenteric arterial flow was recorded continuously with an electromagnetic flow transducer; simultaneously aortic pressure, heart rate, and femoral, renal and carotid flows were monitored. Resistance was calculated as the ratio of mean aortic pressure to mean flow. The central neural regulation of the cardiovascular system was altered: (1) by electrical stimulation of medullary vasoactive sites, (2) by changing the input from visceral or somatic afferents and (3) by subjecting the animals to stress, such as haemorrhage or hypoxia. Postnatal development of adrenergic mechanisms was studied with the aid of adrenergic agonist both in vivo and in vitro (helical strips cut from superior mesenteric artery were placed in an organ bath for isometric tension recordings). Recordings of spontaneous efferent splanchnic discharge permitted a more direct examination of autonomic regulation of the mesenteric vascular bed. The results indicate that the vascular bed supplied by the superior mesenteric artery and innervated by the efferent splanchnic nerve is actively involved in the cardiovascular responses to alterations in the cardiovascular regulatory system. Furthermore, this vascular bed may have an inappropriate response to both feeding and stress such that pathological changes in the gastrointestinal tract may result.