Specific characteristics of sympathetic fibres innervating the heart of young rats.

The noradrenaline content of the heart of albino rats was determined spectrofluorometrically by the trihydroxyindole method after adsorption to aluminium oxide. The concentration (mg/kg tissue) changed during postnatal life as follows: at 10 and 18 days--0.13, 24 days--0.18, 35 days--0.21, 45 days--0.34, over 3 months--0.68. We determined whether the low transmitter concentration in the heart tissue of the young was reflected in the function of the sympathetic fibres innervating and sinoatrial node. We stimulated the region of the sinoatrial node electrically in isolated atria and compared the positive chronotropic reaction of preparations from 15-day-old young and adult rats to various forms of stimulation (bursts of 1-5 s, frequencies of 10-100 Hz, voltage 10-25 V, duration of individual pulses 0.02-0.08 ms). The reactions of young and adult rats were all identical except for two: 1. with the shortest pulses (0.02 ms), the sympathetic innervation of the young hardly reacted at all, while adult animals had a marked positive chronotropic reaction; 2. with 5-s bursts, young rats reacted significantly more strongly than adult animals. The first of these differences could be related to lower excitability of infant rat nerve fibres, while insufficient reuptake of noradrenaline in the heart of infant rats could have accounted for the second.