CONSCIOUS OBESE RATS HAVE IMPAIRED REFLEX BRADYCARDIA AND ENHANCED NOREPINEPHRINE SENSITIVITY

Male Sprague-Dawley rats fed a condensed milk diet were classified as either ''obesity susceptible'' (OS) or ''obesity resistant'' (OR) base d on body weight increases attained after 12 wk. Overall caloric intak e in OS rats was higher than in chow-fed controls, and OS rats were he avier than chow-fed controls or OR rats. There were no significant dif ferences in blood glucose, serum insulin, ventricular weight, basal bl ood pressure, or heart rate. Presser responses recorded after combined blockade with atropine and propranolol to eliminate reflex effects we re identical for vasopressin, but those to norepinephrine were larger in OS than in OR rats, whereas those to angiotensin were larger in OS than in control rats. When baroreflex sensitivity was assessed using i ntravenously infused sodium nitroprusside or phenylephrine to alter sy stemic arterial pressure, differences in reflex tachycardia were equiv ocal, but reflex bradycardia was clearly inhibited in OS rats. These r esults show that, although basal blood pressure was unaffected in OS r ats, their impaired reflex bradycardia along with enhanced presser res ponsiveness to norepinephrine could predispose them to subsequent deve lopment of hypertension.