COEXISTENCE OF 2 TYPES OF CA2-ACTIVATED K+ CHANNELS IN RAT RENAL ARTERIOLES()

Single-channel K+ currents were characterized in vascular smooth muscl e cells freshly isolated from preglomerular arterioles (15-40 mu m OD) of the rat. Under conditions of symmetrical K+ (145 mM), two types of single-channel K+ currents with unitary slope conductances of 68 +/- 2.6 and 251 +/- 4.9 pS were recorded from excised inside-out membrane patches. The open state probability (NPo) of these two types of K+ cha nnels was voltage sensitive and the channels were highly selective for K+ over Na+. Elevation of intracellular calcium concentration ([Ca2+] (i)) from 0.1 to 0.5 mu M on the cytoplasmic face of inside-out patche s increased the frequency of opening and the NPo of both the 68-pS and the 251-pS K+ channels. Application of ATP (0.1-1 mM) to the internal surface of inside-out patches had no effect on the activities of both channel types. Internally applied Ba2+ (1 mM) blocked both of these c hannels. Externally applied tetraethylammonium (0.1-0.3 mM) or charybd otoxin (50 nM) blocked both the 68-pS and the 251-pS K+ channels. Exte rnally applied apamin (50 nM), however, selectively blocked the 68-pS K+ channel but had no effect on the frequency of opening of the 251-pS K+ channel. Apamin also reduced macroscopic K+ current recorded from voltage-clamped rat renal arteriolar muscle cells by 25-30%. These res ults indicate the coexistence of two types of Ca2+-activated K+ channe ls in the membranes of vascular smooth muscle cell isolated from renal preglomerular arterioles of the rat that differ in unitary conductanc es and pharmacological properties.