Prominent role of K(Ca)3.1 in endothelium-derived hyperpolarizing factor-type dilations and conducted responses in the microcirculation in vivo

The activation of endothelial Ca2+-dependent K+-channels, K(Ca)3.1 (IKCa), and K(Ca)2.3 (SKCa) has been proposed to be a prerequisite for endothelial hyperpolarization, which subsequently hyperpolarizes and relaxes smooth muscle [endothelium-derived hyperpolarizing factor (EDHF)-type dilation] and initiates conducted dilations. Although EDHF is the main mediator of acetylcholine (ACh)-induced dilation in the murine skeletal microcirculation, the differential contribution of K(Ca)3.1 and K(Ca)2.3 is not known. We assessed agonist-induced and conducted dilations as well as endothelial hyperpolarization in the cremaster microcirculation of K(Ca)3.1-deficient (K(Ca)3.1-/-) and wild-type mice (wt) in vivo after blockade of NO and prostaglandins. Compared with wt, resting tone was enhanced by similar to 25% in arterioles of K(Ca)3.1-/- mice. ACh-induced dilations in K(Ca)3.1-/- mice were virtually abolished at low and intermediate concentrations and a remaining dilation at 10 mu mol/L ACh was abrogated by blockade of K(Ca)2.3 with UCL1684. Sodium nitroprusside- and adenosine-induced dilations were similar in wt and K(Ca)3.1-/-. Focal application of ACh induced dilations at the local site in both genotypes, which conducted along the vessel. However, the amplitude of the dilation decreased with distance only in K(Ca)3.1-/-. Blockade of K(Ca)2.3 in wt did not affect conducted dilations. A K(Ca)3.1 opener induced a conducting dilation in wt but not in K(Ca)3.1-/-. Membrane potential recordings in vivo demonstrated endothelial hyperpolarization in response to ACh in both genotypes; however, the hyperpolarization was severely impaired in K(Ca)3.1-/- (Delta membrane potential: -3 +/- 1 vs. -14 +/- 2 mV). We conclude that K(Ca)3.1 is of major importance for endothelial hyperpolarization and EDHF-type responses in skeletal muscle arterioles, and its deficiency is not compensated by K(Ca)2.3. Sole activation of K(Ca)3.1 is capable of initiating conducted responses, and K(Ca)3.1 may contribute to the propagation of the signal, although its presence is not mandatory.