Year of Publication

2010

Degree Name

Doctor of Philosophy (PhD)

Document Type

Dissertation

College

Medicine

Department

Physiology

First Advisor

Dr. Sean D. Stocker

Abstract

Obesity afflicts more than 30% of the U.S. population and is a major risk factor for the development of hypertension, type II diabetes, and cardiovascular disease. Studies in humans and animals indicate that obesity is associated with increased sympathetic outflow to the vasculature and kidneys. One mechanism postulated to underlie the increase in sympathetic nerve activity (SNA) in obesity is hyperinsulinemia. Little is known regarding the central circuitry underlying elevated SNA and arterial blood pressure (ABP) during hyperinsulinemia and obesity or if sympathoexcitatory circuits are still responsive to insulin in obesity.

Hyperinsulinemic-euglycemic clamps elevate SNA to the hind limb vasculature in lean rodents but obesity is associated with resistance to the peripheral and anorexic effects of insulin. Therefore, the first aim was to determine whether diet-induced obesity causes development of insulin resistance in the central circuits mediating SNA. The sympathoexcitatory response to insulin was still intact in diet-induced obese rats indicating a role for insulin in the elevation in SNA and ABP in obesity.

The second aim of this project was to identify the specific receptors in the rostral ventrolateral medulla (RVLM) that mediate the elevated SNA during hyperinsulinemia. The RVLM provides basal sympathetic tone and maintains baseline ABP. Glutamate is the major excitatory neurotransmitter and glutamate receptors of the RVLM are known to mediate multiple forms of hypertension. Blockade of RVLM NMDA-specific glutamatergic receptors reverses the increased lumbar SNA associated with hyperinsulinemia. In contrast, blockade of angiotensin II type 1 or melanocortin receptors in the RVLM had no effect on the sympathoexcitatory response to insulin.

The goal of the third aim was to identify the cellular mechanisms within RVLM that mediate the elevated SNA and ABP in diet-induced obesity. Blockade of RVLM glutamate receptors reversed the elevated ABP and lumbar SNA associated with diet-induced obesity while it had no effect on rats on a low fat diet or those resistant to weight gain on the high fat diet. Similar to the findings during hyperinsulinemia, blockade of RVLM angiotensin II type 1 or melanocortin receptors had no effect on lumbar SNA or ABP during diet-induced obesity.

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