It is well established that transforming growth factor-beta1 (TGF-beta1) can induce the transformation of fibroblasts to myofibroblasts. The molecular mechanisms of the phenotypic change remain unknown. The effect of TGF-beta1 on the expression of K+ channels in cultured rat vascular fibroblasts was investigated by using the patch-clamp technique and quantitative RT-PCR. In fibroblasts, the only voltage-dependent outward K+ current that can be electrophysiologically detected is non-inactivating. In myofibroblasts, induced by the treatment of fibroblasts with TGF-beta1, we report the emergence of an additional transient outward K+ current The TGF-beta1-induced outward current is inhibited by 4-aminopyridine. K-v2.1, the transcript for a non-inactivating potassium channel gene, was detected by quantitative RT-PCT in both cultured fibroblasts and myofibroblasts. In contrast, the transcript of the transient I-A gene, K-V4.1, can be detected only in myofibroblasts. The results suggest that TGF-beta1-induced phenotypic transformation of vascular fibroblasts to myofibroblasts is accompanied by the induction of I-A channels. (C) 2002 Elsevier Science (USA). All rights reserved.