Hemodynamic consequences of endogenous hyperinsulinemia in obese rats withventromedian hypothalamus lesions.

The rat with ventromedian hypothalamus lesions (VMH) is characterized by ma ssive obesity, hyperinsulinemia, increase in parasympathetic tonus and symp athetic depression. The aim of this study was to examine in this model the hemodynamic changes and the baroreflex response and to compare the data wit h the evaluation of beta adrenergic sensitivity. In VMH rats and Sham operated rats hemodynamic parameters were followed unt il 8 weeks after operation. Heart rate (HR) and blood pressure (BP) were mo nitored each week during 24 hours by a telemetric system, a catheter being implanted in aorta. In VMH, HR was significantly lower by the first week (p =0.02) and until the last measurement, Systolic BP increased progressively in the two groups but was higher in VMH only at 8 weeks (p=0.03). Compared with Sham rats, 5 days after operation, the percentage of HR acceleration i n response to atropine and isoprenaline was significantly higher in VMH, wh ereas HR response to sodium nitroprussiate was similar in the two groups, P lasma epinephrine and norepinephrine levels were significantly higher in VM H rats. The density of cardiac beta receptors decreased from 15 days to 3 m onths after operation, similarly in VMH and Sham rats. The affinity of card iac beta receptors remained stable during the same period and very similar in VMH and Sham rats. This study suggests that in VMH rats 1. bradycardia results mainly from an increase in parasympathetic tone: 2. the increase in reflex tachycardia des cribed in normal rats after insulin infusion needs a normal activity of the sympathetic nervous system; 3. catecholamine levels may be increased despi te sympathetic depression, probably as a result of an increase in adrenomed ullary secretion possibly due to endogenous hyperinsulinemia; 4. the lack o f hypertension in this model including a massive obesity is likely to resul t from the proper vasodilatory effect of insulin.