Uneven blunting of chronotropic baroreflexes in obese Zucker rats

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.