Neuronal regulation of the development of the alpha-adrenergic chronotropic response in the rat heart.

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Abstract
During development, there are changes in the response of automatic cardiac fibers to alpha-adrenergic agonists. In neonatal rat ventricle, in vitro phenylephrine (1 X 10(-8) M) induces an increase in automatic rate from 115 +/- 12 (mean +/- SEM) to 168 +/- 10 beats/min, P less than 0.05. In contrast, in adult rat ventricle, the rate decreases from 36 +/- 8 to 12 +/- 12 beats/min, P less than 0.05. At both ages, the response is attenuated by the alpha 1-antagonist, prazosin (1 X 10(-6) M). We used cultures of neonatal rat myocytes to determine whether maturation of innervation contributes to the ontogeny of this response. All non-innervated cultures showed a positive chronotropic response to alpha-stimulation; phenylephrine (1 X 10(-8) M) increased the rate from 40 +/- 2 to 52 +/- 2 beats/min, P less than 0.05. In contrast, 60% of the myocytes innervated with sympathetic neurons showed a decrease in rate in response to phenylephrine, from 78 +/- 6 to 67 +/- 6 beats/min, P less than 0.05. The negative chronotropic effect of phenylephrine did not result from the release of acetylcholine or adenosine, or the inhibition of presynaptic norepinephrine release by phenylephrine. Furthermore, exposure to neuronal norepinephrine is not responsible for the alteration in muscle cell responsiveness. In conclusion, we have demonstrated the modulation of the myocardial response to alpha-adrenergic stimulation by the occurrence of innervation in tissue culture. This provides an explanation for the previously identified ontogenetic change in alpha-adrenergic effects on intact cardiac fibers from excitation to inhibition.