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1 Department of Physiology, Medical College of Georgia, Augusta, GA, USA
2 Vascular Biology Center, Medical College of Georgia, Augusta, GA, USA
3 Department of Physiology, Medical College of Georgia, Augusta, GA, USA; Vascular Biology Center, Medical College of Georgia, Augusta, GA, USA
* To whom correspondence should be addressed. E-mail: dstepp{at}mcg.edu.
Obese Zucker rats (OZR) are mildly hypertensive with an apparently elevated sympathetic vasomotor tone compared to lean Zucker rats (LZR). Studies have also suggested enhanced adrenergic pressor reactivity in OZR but assumed comparable baroreflexes or blood volumes / body weight to LZR. In 15-wk old OZR and LZR, we measured plasma volume and vascular reactivity to norepinephrine (NE) and phenylephrine (PE) with doses evaluated by body weight and plasma volume. Plasma volume measured by dye dilution (Evans Blue; 200 µl of 0.5%) showed OZR had comparable blood volumes to LZR, but lower blood volume / body weight (3.4±0.2 / 100g) than LZR (5.7±0.2 ml/100 g, p<0.05). Ganglionic blockade (mecamylamine, 4 mg/kg) in isoflurane-anesthetized rats produced larger decreases in arterial pressure in OZR compared to LZR (52±2 vs. 46±2 mmHg). Pressor responses to NE (0.01-10 µg/kg) were exaggerated with doses analyzed by body weight, but not analyzed by drug quantity. Pressor responses to PE (1-24 µg/kg) showed no difference with doses analyzed by body weight, but analyzed by drug quantity OZR showed a slight decrease in pressor reactivity. PE-induced increases in vascular resistance were exaggerated in the hindlimb circulation of OZR, normal in the renal circulation and attenuated in the mesenteric circulation. The timing of the peak pressor response to PE corresponded with the increase in mesenteric vascular resistance, followed by rises in hindlimb and renal resistance. These data suggest that systemic adrenergic pressor reactivity is not enhanced in OZR despite exaggerated vascular reactivity in the hindlimb of the OZR.
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