Background and Purpose: This study was designed to determine regional differences and age-related changes in the contribution of ATP-sensitive potassium (K(ATP)) channels to vasodilator responses in the brain stem circulation in vivo. Methods: Changes in diameter of the basilar artery (baseline diameter, 270±5 μm [mean ±SEM]), its large branch (112±5 μm), and its small branch (49±2 μm) in response to K(ATP) channel openers leveromakalim and Y-26763 were measured through a cranial window in anesthetized adult (4 to 6 months) and aged (24 to 26 months) Sprague- Dawley rats. Results: Topical application of leveromakalim and Y-26763 produced concentration-dependent vasodilation that was similar among the three vessel groups in adult rats. In aged rats, dilator responses of the branches, but not of the basilar artery, to the K(ATP) channel openers were smaller than those in adult rats (P<.05). Glibenclamide, a selective K(ATP) channel blocker, almost abolished this vasodilation in both groups of rats. Vasodilator responses to sodium nitroprusside were preserved in aged rats. Conclusions: In adult rats, there is no regional heterogeneity in vasodilator response to K(ATP) channel openers in the brain stem circulation in vivo. In aged Rats, although K(ATP) channel are also functional in the brain stem circulation, dilator response of the microvessels but not of the large arteries to direct activation of K(ATP) channel is impaired.
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