Cell shrinkage is an incipient hallmark of apoptosis in a variety of cell types. The apoptotic volume decrease has been demonstrated to attribute, in part, to K+ efflux; blockade of plasmalemmal K+ channels inhibits the apoptotic volume decrease and attenuates apoptosis. Using combined approaches of gene transfection, single-cell PCR, patch clamp, and fluorescence microscopy, we examined whether overexpression of Bcl-2, an anti-apoptotic oncoprotein, inhibits apoptosis in pulmonary artery smooth muscle cells (PASMC) by diminishing the activity of voltage-gated K+ (Kv) channels. A human bcl-2 gene was infected into primary cultured rat PASMC using an adenoviral vector. Overexpression of Bcl-2 significantly decreased the amplitude and current density of Kv currents (I(Kv)). In contrast, the apoptosis inducer staurosporine (ST) enhanced I(Kv). In bcl-2-infected cells, however, the ST-induced increase in I(Kv) was completely abolished, and the ST-induced apoptosis was significantly inhibited compared with cells infected with an empty adenovirus (-bcl-2). Blockade of Kv channels in control cells (-bcl-2) by 4-aminopyridine also inhibited the ST-induced increase in I(Kv) and apoptosis. Furthermore, overexpression of Bcl-2 accelerated the inactivation of I(Kv) and downregulated the mRNA expression of the pore-forming Kv channel alpha-subunits (Kv1.1, Kv1.5, and Kv2.1). These results suggest that inhibition of Kv channel activity may serve as an additional mechanism involved in the Bcl-2-mediated anti-apoptotic effect on vascular smooth muscle cells.