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377 DEVELOPMENTAL INCREASE IN THE CARDIAC CHRONOTROPIC RESPONSE TO ISOPROTERENOL (IS)

The ovine chronotropic response to injection of catecholamines is reported to increase during gestation, exhibit denervation supersensitivity, but is similar to the adult after birth. This study examined the developmental changes in the heart rate response to beta adrenergic stimulation with IS. Dose response studies were carried out in chronically catheterized, sheep - 5 near-term fetuses, 4 newborn, and 5 pregnant adults. IS was infused for 15-30 min until a steady state response was obtained for each dosage of the series. Results are shown below.These studies indicate that the ovine chronotropic response to beta adrenergic stimulation increases not only during gestation but from the newborn period to adulthood. This may represent a maturational increase in cardiac beta receptor number, affinity or responsiveness.