Characterization of the large-conductance Ca-activated K channel in myocytes of rat saphenous artery

We used the patch-clamp method to characterize the BK channel in freshly is olated myocytes from the saphenous branch of the rat femoral artery. Single -channel recordings revealed that the BK channel had a conductance of 187 p S in symmetrical 150 mM KCl, was blocked by external tetraethylammonium (TE A) with a K-D(TEA) of approx. 300 muM at +40 mV, and by submicromolar chary bdotoxin (CTX). The sensitivity of the BK channel to Ca was especially high (K-D(ca) approx. 0.1 muM at +60 mV) compared to skeletal muscle and neuron al tissues. We also investigated the macroscopic K current, which under cer tain conditions is essentially sustained by BK channels. This conclusion is based on the findings that the macroscopic current activated upon depolari zation follows a single exponential time course and is virtually fully bloc ked by 100 nM CTX and 5 mM external TEA. We made use of this occurrence to assess the voltage and Ca dependence of the macroscopic BK current. In inta ct myocytes, the BK channel showed a strong and voltage-dependent reduction of the outward current (62% at +40 mV), most likely due to block by intrac ellular Ba and polyamines. The results obtained from macroscopic and unitar y current indicate that approx. 2.5% of the BK channels are active under ph ysiological conditions, sustaining approx. 20 pA of outward current. Given the high input resistance of these cells, few BK channels are required to o pen in order to cause a significant membrane hyperpolarization, and thus fu nction to limit the contraction resulting from acute increases in intravasc ular pressure, or in response to hypertensive pathologies.