Abstract
The effects of norepinephrine on the heart rate were studied in 1-, 3-, 6-, and 20-week-old rats before and after blockade of hyperpolarization-activated currents (I f , I h ). In animals with mature sympathetic regulation of cardiac activity (6- and 20-week-old animals), I f blockage decreased the severity of norepinephrine-induced tachycardia. In newborn rats lacking sympathetic innervation of the heart, norepinephrine only slightly affected heart rate before and after I f blockage. In 3-week-old animals, I f blockage after norepinephrine pretreatment increased tachycardia.
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