Changes in sensitivity to histamine of guinea pig cardiac muscles during postnatal development

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Abstract
Histamine stimulates the heart by interacting with cardiac histamine receptors. The postnatal changes in histamine sensitivity were investigated with spontaneously beating right atria and electrically deriven left atria and right ventricular papillary muscles from 0, 5 and 10 day old and adult guinea pigs. The positive chronotropic response to histamine in right atria was antagonized by cimetidine, but not by chlorpheniramine at any age. Chlorpheniramine did not antagonize the positive inotropic effect of histamine and 2-(2-pyridyl)ethylamine in the immature left atria, but it blocked the positive inotropic effect in the adult; cimetidine had no effect. The positive inotropic effect of histamine in right ventricular muscles was not affected by chlorpheniramine in immature right ventricular muscles but was antagonized in the adult. In immature left atria and right ventricular muscles, there apparently is no H1-receptor system mediating the positive inotropic effect of histamine; as age advances, this system begins to mediate the positive inotropic effect. In immature left atria, non-H1 and non-H2 receptors exist and mediate the positive inotropic effect of histamine.