Ranges of heart rates and their regulations at various ages (rat)

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Abstract
Maximal heart rates were Induced In anesthetized rats of various ages from fetus to adult, by drive with electrical shocks. Various other agents accelerated the rates by less amounts. Thus, intrathoraclc stimuli to sympathetic nerves Increased the heart rate by about 1/3 of the maximal acceleration. In rats without anesthesia, either catechol-amines or thyroxin accelerated the heart; both together produced additive Increases which at postnatal ages were 90% of the driven rates. Sympa-thettc blockers decelerated the heart even before birth. Parasympathetlc blockers accelerated the heart none or little before 20 days of age. Both blockers together approximately counteracted one another only in adult. The natural heart rate showed 3 waves of increase; one before birth, one just after birth which could not be blocked, and a later one which could be blocked. Regulations of heart rate and the several Influences upon them develop with age as concurrent changes In: intrinsic pacemaking, sensitivity to agents, and tonic influences.