Nitric oxide inhibition simulates the enhancement of alpha 1 agonist-induced vasoconstriction in diabetes

We have previously reported that endothelium-dependent, nitric oxide (NO)-mediated vasorelaxation is impaired in diabetic mesenteric arteries. We hypothesized that vasoconstrictor responses should therefore be enhanced. The purpose of this study was to determine whether diabetic mesenteric arteries exhibit increased vasoconstrictor responses, and to investigate if these changes are receptor and/or NO mediated. Thirty age-matched male Sprague-Dawley rats were divided into control (C) and diabetic (D, streptozotocin: 60 mg/kg) groups and studied after 4 weeks. Terminal branches of ileal mesenteric arteries (300 +/- 9 microns) were isolated, pressurized, and superfused with modified Krebs solution. Changes in vessel internal diameter were measured and dose-response curves (DRC) for each vasoactive agent were determined. Each vessel was initially constricted with 40 mM of KC1 to determine maximal vasoconstriction. Phenylephrine (Phe, 10(-8)-10(-4) M) and UK14304 (10(-9)-10(-5) M) were used to determine alpha 1- and alpha 2-receptor responses, respectively. Similar studies were performed in the presence of N omega-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), a competitive inhibitor of NO synthase. Maximal response (Max), area under the curve (AUC), and vessel sensitivity (ED50) for each DRC were calculated. Comparisons among groups were made using analysis of variance and Student's t test with Bonferroni correction. There were no differences in vasoconstrictor responses induced by KCl (C: 82 +/- 2% vs D: 80 +/- 1%). alpha 1-vasoconstrictor responses to Phe were enhanced in diabetes with significantly higher Max (96 +/- 2% vs 83 +/- 3%), and AUC (1.92 +/- 0.09 vs 1.56 +/- 0.08), but no difference in ED50. The addition of L-NAME enhanced only Phe-induced vasoconstrictor response significantly in control rats. Thus, differences in Phe-induced vasoconstrictor responses between C and D were abolished in the presence of L-NAME. alpha 2-vasodilator responses induced by UK14304 were similar between C and D and unaffected by L-NAME. alpha 1-, but not alpha 2-, vasoconstrictor responses are enhanced in streptozotocin-induced diabetic rats. These enhanced responses can be duplicated by treatment of control vessels with L-NAME.