Abstract
Strong inwardly rectifying K(+) (K(IR)) channels that contribute to maintaining the resting membrane potential are encoded by the Kir2.0 family (Kir2.1-2.4). In smooth muscle, K(IR) currents reported so far have the characteristics of Kir2.1. However, Kir2.4, which exhibits unique characteristics of barium block, has been largely overlooked. Using patch-clamp techniques, we characterized K(IR) channels in cultured human pulmonary artery smooth muscle (HPASM) cells and compared them to cloned Kir2.1 and Kir2.4 channels. In a physiological K(+) gradient, inwardly rectifying currents were observed in HPASM cells, the magnitude and reversal potential of which were sensitive to extracellular K(+) concentration. Ba(2+) (100 microM ) significantly inhibited inward currents and depolarized HPASM cells by approximately 10 mV. In 60 mM extracellular K(+), Ba(2+) blocked K(IR) currents in HPASM cells with a 50% inhibitory concentration of 39.1 microM at -100 mV compared to 3.9 microM and 65.6 microM for Kir2.1 and Kir2.4, respectively. Cloned Kir2.4 and K(IR) currents in HPASM cells showed little voltage dependence to Ba(2+) inhibition, which blocked at a more superficial site than for Kir2.1. Single-channel recordings revealed strong inwardly rectifying channels with an average conductance of 21 pS in HPASM cells, not significantly different from either Kir2.1 (19.6 pS) or Kir2.4 (19.4 pS). Reverse-transcription polymerase chain reaction detected products corresponding to Kir2.1, Kir2.2 and Kir2.4 but not Kir2.3. We demonstrate that cultured HPASM cells express K(IR) channels and suggest both Kir2.1 and Kir2.4 subunits contribute to these channels, although the whole-cell current characteristics described share more similarity with Kir2.4.