Freshly isolated bovine coronary endothelial cells do not express the BK_Ca channel gene

Recent reports have suggested that different types of Ca²⁺-activated K⁺ channels may be selectively expressed either in the vascular endothelial cells (ECs) or smooth muscle cells (SMCs) of a single artery. In this study, we directly compared mRNA, protein and functional expression of the high-conductance Ca²⁺-activated K⁺ (BK_Ca) channel between freshly isolated ECs and SMCs from bovine coronary arteries. Fresh ECs and SMCs were enzymatically isolated, and their separation verified by immunofluorescent detection of α-actin and platelet/endothelium cell adhesion molecule (PECAM) proteins, respectively. Subsequently, studies using a sequence-specific antibody directed against the pore-forming α-subunit of the BK_Ca channel only detected its expression in the SMCs, whereas PECAM-positive ECs were devoid of the α-subunit protein. Additionally, multicell RTPCR performed using cDNA derived from either SMCs or ECs only detected mRNA encoding the BK_Ca α-subunit in the SMCs. Finally, whole-cell recordings of outward K⁺ current detected a prominent iberiotoxin-sensitive BK_Ca current in SMCs that was absent in ECs, and the BK_Ca channel opener NS 1619 only enhanced K⁺ current in the SMCs. Thus, bovine coronary SMCs densely express BK_Ca channels whereas adjacent ECs in the same artery appear to lack the expression of the BK_Ca channel gene. These findings indicate a cell-specific distribution of Ca²⁺-activated K⁺ channels in SMCs and ECs from a single arterial site.