Contribution of nitric oxide produced by inducible nitric oxide synthase to vascular responses of mesenteric arterioles in streptozotocin-diabetic rats

Abstract

1. The functional changes in mesenteric arterioles of streptozotocin-induced diabetes were investigated by intravital microscopy. The mesentery was exteriorized from anesthetized rats, spread in a chamber, and superfused with Tyrode solution. All drugs tested were applied to the superfusing Tyrode solution. 2. Compared with age-matched controls, the diabetic rats showed enhanced vascular sensitivity to phenylephrine, an alpha(1)-adrenoceptor agonist. The preincubation of the mesentery with N(G)-nitro-l-arginine (l-NNA), a nitric oxide synthase (NOS) inhibitor, shifted the phenylephrine-concentration-response curves to the left in both the diabetic and control rats. Even in the presence of l-NNA, the sensitivity to phenylephrine was higher in the diabetic rats than in the control. 3. Acetylcholine relaxed the mesenteric arterioles in both groups, but to a significantly greater extent in the control than in the diabetic rats. However, the l-NNA-induced constriction of arterioles did not differ significantly between the groups. In contrast, the amplitude of the constrictions of mesenteric arterioles induced by S-ethylisothiourea, an inducible NOS (iNOS) inhibitor, was significantly greater in the diabetic rats than in the control. 4. Immunostaining of the mesentery with a specific antibody for iNOS revealed iNOS in the microvessels of only the diabetic rats. 5. These results suggest that constrictor responses to alpha(1)-adrenoceptor stimulation are sensitized in the mesenteric arterioles of STZ-diabetic rats, and that iNOS expressed in the arteriolar smooth muscle plays a role in suppressing the basal tone and the reactivity of the arterioles in STZ-diabetic rats.

Figure 1

Concentration–response curves for phenylephrine (Phe)-induced constriction of mesenteric arterioles in the absence (a) and presence (b) of L-NNA in the control and STZ-diabetic rats. Each point represents the mean±s.e.m. of six rats.

Figure 2

Vasodilator responses of mesenteric arterioles to ACh in control and STZ-diabetic rats. Each bar represents the mean±s.e.m of six rats. ^**P<0.01 compared to the control.

Figure 3

Vasoconstrictor responses of mesenteric arterioles to S-ethylisothiourea (EIT), an inducible NO synthase inhibitor, and L-NNA, a nonselective NO synthase inhibitor, in control and STZ-diabetic rats. Each bar represents the mean±s.e.m of seven rats. ^**P<0.01 compared to the corresponding control.

Figure 4

Immunohistochemical localization of iNOS in the mesenteries of control and STZ-diabetic rats. (a) The mesenteries isolated from the control (upper panels) and the STZ-diabetic rats (lower panels) were double-immunolabeled for iNOS (green fluorescence; left panels) and α-smooth muscle (SM) actin (red fluorescence; center panels). (b) The mesenteries isolated from the control rats were double-immunolabeled for iNOS (green fluorescence; left panel) and macrophage (red fluorescence; center panel). The yellowish area in each right panel is produced by the coincidence of both fluorescences.