AJP - Heart and Circulatory Physiology, Vol 256, Issue 2 417-H421, Copyright © 1989 by American Physiological Society
Uneven blunting of chronotropic baroreflexes in obese Zucker rats
D. L. Barringer and R. D. Bunag
Department of Pharmacology, College of Health Sciences and Hospital, University of Kansas Medical Center, Kansas City 66103.
We compared reflex chronotropic responses to intravenously infused drugs in
three groups of age-matched normotensive female rats, namely,
Sprague-Dawley, lean Zucker, and obese Zucker. Initial mean pressures did
not differ between rat groups, but heart rates tended to be lower in obese
Zucker rats. Baroreflex impairment was already evident, because heart rate
responses to infused phenylephrine (reflex bradycardia) or sodium
nitroprusside (reflex tachycardia) were consistently weaker in obese Zucker
than in other rats. Regardless of rat grouping, subsequent cholinergic
blockade with atropine elevated, whereas beta-adrenergic blockade with
propranolol lowered, basal heart rates without affecting mean pressure.
Reflex heart rate responses were all appreciably reduced after either type
of autonomic blockade, and although the extent of inhibition varied between
rat groups, the residual heart rate responses remaining after blockade were
nonetheless always smaller in obese than in lean rats. This difference
suggests that efferent sympathetic and parasympathetic mechanisms normally
responsible for mediating heart rate reflexes were unevenly blunted in
obese Zucker rats.
