Effect of chronic hypoxia on K+ channels: Regulation in human pulmonary vascular smooth muscle cells

We investigated the effects of chronic hypoxia on the major outward K+ currents in early cultured human main pulmonary arterial smooth muscle cells (HPSMC). Unitary currents were measured from inside-out, outside-out, and cell-attached patches of HPSMC. Chronic hypoxia depolarized resting membrane potential (E-m) and reduced the activity of a charybdotoxin (CTX)- and iberiotoxin-sensitive, Ca2+-dependent K+ channel (K-Ca). The 4-aminopyridine-sensitive and CTX-insensitive channel or the delayed rectifier K+ channel was unaffected by chronic hypoxia. Chronic hypoxia caused a +33- to +53-mV right shift in voltage-dependent activation of K-Ca and a decrease in K-Ca activity at all cytosolic Ca2+ concentrations ([Ca2+](i)) in the range of 0.1-10 mu M. Thus the hypoxia-induced decrease in K-Ca activity was most likely due to a decrease in K-Ca sensitivity to E-m and [Ca2+](i). Chronic hypoxia reduced the ability of nitric oxide (NO .) and guanosine 3',5'-cyclic monophosphate (cGMP) to activate K-Ca. The cGMP-dependent protein kinase-induced activation of K-Ca was also significantly inhibited by chronic hypoxia. In addition, inhibiting channel dephosphorylation with calyculin A caused significantly less increase in K-Ca activity in membrane patches excised from chronically hypoxic HPSMC compared with normoxic controls. This suggests that the mechanism by which hypoxia modulates NO .-induced K-Ca activation is by decreasing the NO ./cGMP-mediated phosphorylation of the channel.