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1 The John B. Pierce Laboratory, New Haven, CT, USA
2 The John B. Pierce Laboratory, New Haven, CT, USA; Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, CT, none
* To whom correspondence should be addressed. E-mail: sssegal{at}jbpierce.org.
Functional hyperemia requires the coordination of smooth muscle cell relaxation along and between branches of the arteriolar network. Vasodilation is conducted from cell-to-cell along the arteriolar wall through gap junction channels comprised of connexin protein subunits. Within skeletal muscle, it is unclear whether arteriolar endothelium, smooth muscle, or both cell layers provide the cellular pathway for conduction. Further, the constitutive profile of connexin expression within the microcirculation is unknown. We tested the hypothesis that conducted vasodilation and connexin expression are intrinsic to the endothelium of arterioles (diameter, 17±1 µm) supplying skeletal muscle fibers in the cremaster of anesthetized C57BL6 mice. Acetylcholine delivered onto an arteriole (500 ms, 1 µA pulse; 1 µm micropipette) produced local dilation of 17±1 µm; conducted vasodilation observed 1 mm upstream was 9±1 µm (n=5). Following light-dye treatment to selectively disrupt endothelium (250 µm segment centered 500 µm upstream; confirmed by loss of local response to ACh while constriction to phenylephrine and dilation to sodium nitroprusside remained intact), conducted vasodilation was nearly abolished (2±1 µm; P<0.05). Whole-mount immunohistochemistry for connexins revealed punctate labeling at borders of arteriolar endothelial cells, with Cx40 and Cx37 in all branches and Cx43 only in the largest branches. Immunoreactivity for connexins was not apparent in smooth muscle or in capillary or venular endothelium, despite robust immunolabeling for 
-actin and PECAM-1, respectively. We conclude that vasodilation is conducted along the endothelium of mouse skeletal muscle arterioles and that Cx40 and Cx37 are the primary connexins forming gap junction channels between arteriolar endothelial cells.
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