Voltage-dependent delayed rectifier K+ (KV) channels and Ca(2+)-activated K+ (KCa) channels both play important roles in the regulation of the membrane potential and contractility of vascular smooth muscle cells (SMCs). The expression and function of these K+ channels in cultured vascular SMCs have been extensively studied. The long-term in vitro cell culture would change the properties of K+ channels in SMCs. However, whether the short-term culture could differentially affect the expression and function of KV and KCa channels was not clear. In the present study, both KV and KCa channel currents were identified in freshly dissociated SMCs and in 1-day-cultured SMCs from rat tail arteries. KCa currents were inhibited by iberiotoxin or tetraethylammonium (TEA), and amplified by the calcium ionophore A-23187. Kv currents were inhibited by 4-aminopyridine or beta-dendrotoxin. By using different pharmacological agents and manipulating the calcium concentrations in the recording solutions, it was revealed that in freshly dissociated SMCs the predominant component of total outward K+ currents is KCa current, and KV current a minor component. In contrast, KV current was found to be the predominant component of total outward K+ currents in SMCs primarily cultured with 10% fetal bovine serum at 37 degrees C for 24 h. Differential expression of KV and KCa channels in 1-day-cultured SMCs was thus demonstrated under our experimental conditions. Our results are important for interpreting the electrophysiological properties of vascular SMCs under different cell culture conditions and for understanding the relative contributions of KV and KCa channels to different cellular functions.