In vivo demonstration of maturational changes of the chronotropic response to alpha-adrenergic stimulation.
In vitro studies suggest that neonates and adults may have different cardiac chronotropic responses to alpha-adrenergic stimulation. To investigate these differences in vivo, three groups of dogs were studied. Group I = 12 puppies, ages 3-7 days; group II = 12 puppies ages 8-15 days, and group III = seven adult dogs. Heart rate and blood pressure determinations were made in the control setting and then after combined beta-adrenergic and parasympathetic blockade (propranolol 0.6 mg/kg and bilateral vagotomies). Alpha-stimulation was then achieved with phenylephrine given in doses of 0.5, 1.0, and 10.0 micrograms/kg/min. A second, high dose of propranolol (1.0 mg/kg intravenously) was administered after the highest phenylephrine infusion dosage to assure complete beta-blockade. Finally, alpha-blockade was achieved with phentolamine (groups I and II: 0.5 mg intravenously; group III: 5.0 mg intravenously). An alpha-mediated positive chronotropic effect was observed in 42 and 100% of subjects in groups I and II, respectively, but never observed in the adults. Whereas alpha-blockade with phentolamine resulted in a large decrease in heart rate of all puppies (groups I and II), it had no effect on adults. Blood pressure responses were similar in all three groups. Thus, there are important maturational changes in the chronotropic response to alpha-adrenergic stimulation and blockade demonstrable in the intact neonatal canine.
McCormack
J
J
Department of Pediatrics, University of Miami School of Medicine, FL 33101.
Gelband
H
H
Villafañe
J
J
Xu
H
H
Stolfi
A
A
Pickoff
A S
AS
D000375:Aging / Q000502:physiology*; D000818:Animals; D000831:Animals, Newborn / Q000502:physiology; D001794:Blood Pressure / Q000187:drug effects; D004285:Dogs; D006339:Heart Rate / Q000187:drug effects*; D010275:Parasympathetic Nervous System / Q000502:physiology; D010646:Phentolamine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D013268:Stimulation, Chemical; D014628:Vagotomy
D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D011433:Propranolol; D010646:Phentolamine