Capacities for regulation of heart rate in fetal, infant, and adult rats

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Abstract
In anesthetized pregnant rats, fine-wire electrodes were inserted into fetuses 14-21 days of age; electrocardiograms were recorded. What influences can control the heart rate at stages before and after nerves are functional in the heart? Percentage changes of heart rate are reported. Uterine contraction, needle prick, hypoxia, and hypercapnia transiently decelerated the heart at the earliest preneural stages. Norepinephrine injection into fetus accelerated the heart. In older fetuses, isopropylnorepinephrine and sometimes epinephrine also accelerated the heart. The heart rate therefore became susceptible to more influences. At birth, responses to needle prick and to hypoxia began to reverse, acceleration becoming the rule. Epinephrine and norepinephrine then induced transient decelerations; hyperoxia induced deceleration and an off-effect acceleration. In infants rats, all the responses were exaggerated compared with both those in fetuses and those in adults. Adult rats still responded to all the above agents, however. Especially in the age range from fetus to infant the capacities for regulation of heart rate augmented; a multiple-factor relation may be used to express the augmentation.