The postnatal development of adrenoceptor responses in isolated papillary muscles from rat.

The postnatal development of positive inotropic responses in ventricular tissue was investigated using isolated rat papillary muscles. Responses to exogenous noradrenaline or isoprenaline were already fully developed in preparations from newborn rats. These newborn preparations were also supersensitive to noradrenaline. Responses to field stimulation of intramural nerves or to tyramine were very low in preparations from animals of up to two weeks in age, but had increased to near adult values by 3 weeks of age. This increase in responsiveness was correlated with a progressive increase in sympathetic innervation revealed by fluorescence histochemistry. It is concluded that beta-receptors are present on the ventricular myocardium at birth in the rat, but that positive inotropic responses to sympathetic nerve stimulation do not develop until between 2 and 3 weeks of age.