Maxi‐K channel (BK(C) (a)) activity veils the myogenic tone of mesenteric artery in rats

Arterioles and small arteries change their tone in response to transmural pressure changes, called myogenic tone (MT). In comparison to the branches of cerebral arteries (CA) showing prominent MT, the third branches of mesenteric arteries (MA) with similar diameters show weaker MT. Here, we aimed to analyze the electrophysiological differences responsible for the weaker MT in MA (MT(MA)) than MT in CA (MT(CA)). We measured ionic current using patch clamp in isolated MA smooth muscle cells (MASMCs) and CA smooth muscle cells (CASMCs) of rats. MT was analyzed using video analysis of pressurized small arteries. Quantitative‐PCR (q‐PCR) and immunofluorescence confocal microscopy were used to compare the mRNA and protein expression level of big‐conductance Ca(2+)‐activated K(+) channel (BK(C) (a)) subunits (Slo1α and Sloβ1). Whole‐cell patch clamp study revealed higher density of voltage‐operated Ca(2+) channel current (I(C) (aV)) in the MASMCs than in CASMCs. Although voltage‐gated K(+) channel current (I(K) (v)) was also higher in MASMCs, treatment with Kv inhibitor (4‐aminopyridine) did not affect MT(MA). Interestingly, BK(C) (a) current density and the frequency of spontaneous transient outward currents (STOCs) were consistently higher in MASMCs than in CASMCs. Inside‐out patch clamp showed that the Ca(2+)‐sensitivity of BK(C) (a) is higher in MASMCs than in CASMCs. Iberiotoxin, a selective BK(C) (a) inhibitor, augmented MT(MA) by a larger extent than MT(CA). Although q‐PCR analysis did not reveal a significant difference of mRNAs for Slo1α and Sloβ1, immunofluorescence image suggested higher expression of Slo1α in MASMCs than in CASMCs. Despite the large I(C) (aV) density, the high activities of BK(C) (a) including the more frequent STOCs in MASMCs veils the potentially strong MT(MA).