Post-transcriptional Control of Human maxiK Potassium Channel Activity and Acute Oxygen Sensitivity by Chronic Hypoxia

Abstract

Various cardiorespiratory diseases (e.g. congestive heart failure, emphysema) result in systemic hypoxia and patients consequently demonstrate adaptive cellular responses which predispose them to conditions such as pulmonary hypertension and stroke. Central to many affected excitable tissues is activity of large conductance, Ca 2؉ -activated K ؉ (maxiK) channels. We have studied maxiK channel activity in HEK293 cells stably co-expressing the most widely distributed of the human ␣-and ␤-subunits that constitute these channel following maneuvers which mimic severe hypoxia. ] i could sustain an acute hypoxic inhibitory response. Chronic hypoxia caused no change in ␣-subunit immunoreactivity with Western blotting but evoked a 3-fold increase in ␤-subunit expression. These observations were fully supported by immunocytochemistry, which also suggested that chronic hypoxia augmented ␣/␤-subunit colocalization at the plasma membrane. Using a novel nuclear run-on assay and RNase protection we found that chronic hypoxia did not alter mRNA production rates or steady-state levels, which suggests that this important environmental cue modulates maxiK channel function via post-transcriptional mechanisms.Crucial to the cellular and physiological response to acute perturbation of systemic and/or pulmonary O 2 levels is the rapid inhibition of K ϩ channels by hypoxia (see Ref. 1 for recent review). Thus, acute modulation of ion channel activity is central to the homeostatic mechanisms that underlie chemosensing in carotid body (2-4), neuroepithelial body (5, 6) (and its immortalized cellular counterpart, H146 cells, Ref. 7) and (8 -11) systemic vascular smooth muscle (12). Although somewhat controversial, ion channel inhibition has also been implicated in hypoxic vasoconstriction in the pulmonary circulation (13).In addition, such O 2 sensitivity is believed to play a significant role in modulation of excitability in several cellular components of the mammalian nervous system (14 -17).Although O 2 -sensitive tissues express a wide variety of channel types, central to the cellular mechanism of acute O 2 sensing in several is hypoxic suppression of large conductance Ca 2ϩ -activated K ϩ (maxiK) channels. Indeed, hypoxic inhibition of native maxiK channel activity has been demonstrated in carotid body (4, 18, 19), pulmonary arteriolar smooth muscle (20), chromaffin cells (21), and central neurons (20,22). Although their contribution to carotid body, chromaffin cell, and central neuronal function is well supported, some controversy still surrounds their involvement in pulmonary vasoconstriction (15) and there is good evidence for both delayed rectifier (23) and tandem P domain K ϩ channels in the response (24); the latter observation is fully supported by our recent reports of O 2 sensitivity of the recombinant human tandem P domain channels, hTASK1 (25), and hTASK3 (26).Tissue specificity notwithstanding, we have recently demonstrated at the single channel level that a recombinant human maxiK channel can be rapidly and reversibly in...

The influence of chronic hypoxia upon chemoreception

“…Chronic hypoxia is reported to decrease K + channel density in glomus cells from chronically hypoxic neonatal rats and this would increase excitability (Hempleman, 1995). Chronic hypoxia is reported to increase expression and current density of Ca 2+ activated K + channels (maxi-K channels) in a HEK293 cell culture system (Hartness et al, 2003). However, it is difficult to extrapolate between organs or model cell systems because regulatory proteins required for O 2 -sensitivity and Kv subunits are differentially expressed in different cell types.…”

Neuronal Depolarization Controls Brain-Derived Neurotrophic Factor-Induced Upregulation of NR2C NMDA Receptor via Calcineurin Signaling

“…In situ hybridization of parasagittal cerebellar sections (10 m) was performed with use of the digoxigenin-labeled RNA probe as described previously (Yamazaki et al, 2004). PCR-based nuclear run-on assays were performed as described previously (Rolfe and Sewell, 1997;Hartness et al, 2003).…”

Hypoxic Pulmonary Hypertension (HPH) and Iptakalim, a Novel ATP-sensitive Potassium Channel Opener Targeting Smaller Arteries in Hypertension

“…Chronic hypoxia affects the activity of large-conductance, Ca 2+ -activated K + (maxiK or BK Ca ) channels. However, chronic hypoxia does not alter mRNA production rates or steady-state levels, which suggests that this important environmental cue modulates maxiK channel function via post-transcriptional mechanisms (26). Inwardly rectifying K + (K ir ) channels regulate membrane potential in smooth muscle cells from several types of resistance arteries and may be responsible for external K + -induced vasodilatation.…”