Hyperinsulinemia instead of insulin resistance induces baroreflex dysfunction in chronic insulin-infused rats

Background: The present study was undertaken to compare the effects of chronic hyperinsulinemia with or without insulin resistance on the autonomic control of heart rate (HR) in rats. Methods: Male Sprague-Dawley rats were implanted subcutaneously with insulin (3 mU/kg . min) or vehicle-filled osmotic minipumps for 8 weeks. Insulin-infused rats were further divided into insulin resistant (IR) and insulin sensitive (IS) groups according to the results of the homeostasis model assessment method and euglycemic hyperinsulinemic clamp study. Autonomic function in HR control was indicated by arterial baroreflex sensitivity (BRS) after a bolus injection of phenylephrine or sodium nitroprusside. Results: Compared with those in control group, plasma insulin levels were elevated about threefold and 1.5-fold in the IR and IS groups at the end of week 8, respectively. Blood glucose level remained basal in the IR group, but was significantly lower in the IS group. The elevated mean arterial pressure (MAP) observed in IR was not exhibited in IS. The HR and BRS in reflex tachycardia were significantly increased in the IR and IS groups, but the BRS in reflex bradycardia was not different among all rats. Propranolol eliminated the tachycardia and enhanced BRS responses in both groups. Methylatropine further accelerated tachycardia and diminished the enhanced BRS in the IR group. However, in IS, the enhanced BRS remained after methylatropine was given. The intrinsic HR was similar among all groups. The baseline MAP, HR, and BRS in reflex tachycardia were significantly correlated to plasma insulin levels but not to the Si value, an index of insulin sensitivity. Conclusions: The present results demonstrate that hyperinsulinemia but not insulin resistance is a dominant contributing factor to the development of arterial baroreflex abnormalities in this chronic hyperinsulinemic model, which may simultaneously enhance sympathetic nerve activity and possibly vagal withdrawal if insulin resistance coexisted.