Molecular Cloning and Characterization of the Intermediate-Conductance Ca ²⁺ -Activated K ⁺ Channel in Vascular Smooth Muscle

Recent evidence suggests that functional diversity of vascular smooth muscle is produced in part by a differential expression of ion channels. The aim of the present study was to examine the role of Ca(2+)-activated K(+) channels (K(Ca) channels) in the expression of smooth muscle cell functional phenotype. We found that smooth muscle cells exhibiting a contractile function express predominantly large-conductance ( approximately 200 pS) K(Ca) (BK) channels. In contrast, proliferative smooth muscle cells express predominantly K(Ca) channels exhibiting a much smaller conductance ( approximately 32 pS). These channels are blocked by low concentrations of charybdotoxin (10 nmol/L) but, unlike BK channels, are insensitive to iberiotoxin (100 nmol/L). To determine the molecular identity of this K(+) channel, we cloned a 1.9-kb cDNA from an immature-phenotype smooth muscle cell cDNA library. The cDNA contains an open reading frame for a 425 amino acid protein exhibiting sequence homology to other K(Ca) channels, in particular with mIK1 and hIK1. Expression in oocytes gives rise to a K(+)-selective channel exhibiting intermediate-conductance (37 pS at -60 mV) and potent activation by Ca(2+) (K(d) 120 nmol/L). Thus, we have cloned and characterized the vascular smooth muscle intermediate-conductance K(Ca) channel (SMIK), which is markedly upregulated in proliferating smooth muscle cells. The differential expression of these K(Ca) channels in functionally distinct smooth muscle cell types suggests that K(Ca) channels play a role in defining the physiological properties of vascular smooth muscle.