Modulation of noradrenaline-induced vasoconstriction in isolated perfused mesenteric arterial beds from obese Zucker rats in the presence and absence of insulin

Abstract

The genetically obese Zucker rat (fa/fa) is an insulin-resistant animal model with early-onset severe hyperinsulinemia that eventually develops mild hypertension. Thus, it represents a model in which the effect of hyperinsulinemia – insulin resistance associated with hypertension on vascular reactivity can be examined. The purpose of this study was to investigate the contribution of endogenous nitric oxide (NO) and prostaglandins to reactivity to noradrenaline (NA) in the presence and absence of insulin in mesenteric arterial beds (MAB) from 25-week-old obese Zucker rats and their lean, gender-matched littermates. In the absence of insulin, bolus injection of NA (0.9–90 nmol) produced a dose-dependent increase in perfusion pressure in MAB from both lean and obese rats. Although there was no significant difference in NA pD2(–log ED50) values, the maximum response of MAB from obese rats to NA was slightly but significantly reduced compared with that of MAB from lean rats. The nitric oxide synthase inhibitor NG-monomethyl-L-arginine (L-NMMA, 300 µM) enhanced and indomethacin (20 µM) inhibited pressor responses to NA in MAB from both obese and lean rats. Perfusion with insulin (200 mU/L, a level similar to that in obese rats in vivo) potentiated only the responses of the obese MAB to the two lowest doses of NA tested (0.9 and 3 nmol). In the presence of L-NMMA, insulin further potentiated the NA response in MAB from obese rats. Indomethacin, the prostaglandin H2/thromboxane A2receptor antagonist SQ 29548 (0.3 µM), and the nonselective endothelin-1 (ET-1) receptor antagonist bosentan (3 µM) all abolished insulin potentiation of the NA response in obese MAB. These data suggest that concurrent release of NO and vasoconstrictor cyclooxygenase product(s) in MAB from both obese and lean Zucker rats normally regulates NA-induced vasoconstrictor responses. Furthermore, insulin increases the release of contracting cyclooxygenase product(s) and enhances reactivity to low doses of NA in MAB from obese rats. The effects of insulin may be partially mediated by ET-1 via ET receptors and are buffered to some extent by concomitant NO release. This altered action of insulin may play a role in hypertension in this hyperinsulinemic – insulin-resistant model.Key words: hyperinsulinemia, insulin resistance, hypertensive Zucker obese rat, mesenteric arterial bed, noradrenaline.