[Ca2+]i inhibition of K+ channels in canine pulmonary artery. Novel mechanism for hypoxia-induced membrane depolarization.
Experiments were performed on smooth muscle cells isolated from canine pulmonary artery to identify the type of K+ channel modulated by hypoxia and examine the possible role of [Ca2+]i in hypoxic K+ channel inhibition. Whole-cell patch-clamp experiments revealed that hypoxia (induced by the O2 scavenger, sodium dithionite) reduced macroscopic K+ currents, an effect that could be prevented by strong intracellular buffering of [Ca2+]i. The inhibitory effects of hypoxia were mimicked by acute exposure of cells to caffeine and could be prevented by caffeine pretreatment, suggesting an important obligatory role of [Ca2+]i in hypoxic inhibition of K+ currents. Exposure of cells to low concentrations of 4-aminopyridine (4-AP, 1 mmol/L) prevented hypoxic inhibition of macroscopic K+ currents, whereas low concentrations of tetraethylammonium were without effect, suggesting that the target K+ channel inhibited by hypoxia is a voltage-dependent delayed rectifier K+ channel, which is inhibited by [Ca2+]i. Hypoxia failed to consistently modify the activity of large-conductance (118 picosiemens [pS] in physiological K+) Ca(2+)-activated K+ channels in inside-out membrane patches but reduced open probability of smaller-conductance (25-pS) delayed rectifier K+ channels in cell-attached membrane patches. In inside-out membrane patches, 1 mumol/L Ca2+ added to the cytoplasmic surface significantly reduced open probability of small-conductance (25-pS) 4-AP-sensitive delayed rectifier K+ channels. Whole-cell current measurements using symmetrical K+ to increase driving force for small currents active near the cell's resting membrane potential revealed the presence of a 4-AP-sensitive K+ current that activated near -65 mV and was inhibited by hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)
Post
J M
JM
Department of Physiology, University of Nevada School of Medicine, Reno 89557-0046, USA.
Gelband
C H
CH
Hume
J R
JR
D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D015687:Cell Hypoxia; D004285:Dogs; D004594:Electrophysiology; D066298:In Vitro Techniques; D008564:Membrane Potentials; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D011651:Pulmonary Artery / Q000166:cytology* / Q000378:metabolism / Q000502:physiology*
D015220:Calcium Channels; D015221:Potassium Channels; D002110:Caffeine; D015761:4-Aminopyridine; D002118:Calcium