Heart rate of the preweanling rat and its autonomic control

Abstract

The heart rate of the rat at birth is about 300 beats/min, increases to 360 beats/min within 3 days, and remains there until the animal is about 10 days old. From 10 to 21 days the heart rate again increases to a maximum of about 500 beats/min. When reserpine is used to selectively block adrenergic sympathetic function the rise in heart rate that occurs during the first 3 days is only slightly modified, but the rise in heart rate from 10 days onward is completely prevented. Atropine, however, has no effect on heart rate at all ages tested. When newborn rats are kept at a body temperature of 37 C and then at 40 C, the average heart rate increases 7%. Three-day-old animals have a 19% increase, whereas 6-day-old animals show a 46% increase. When nerve-growth-factor antiserum is used to destroy sympathetic function, the expected hyperthermic tachycardia does not appear. It is concluded that in the first 6 days of the rat's life sympathetic control over the heart rate is minimal, but from then on the sympathetic nervous system continuously exerts a positive chronotropic effect. The parasympathetic nervous system does not seem to be involved.