Depolarization in response to hypoxia

Type I cell depolarization appears to be a critical step in O2 transduction and therefore is a major potential site for maturational changes. Type I cells in vitro (37) and in situ depolarize in response to hypoxia and, in the absence of a type I cell depolarization, hypoxia does not induce catecholamine secretion from the carotid... 

HERG-like channels was suggested (37). These channels are active at resting membrane potential, and the inhibition of these channels has been proposed to initiate the depolari2ation of glomus cells. In summary, it is still controversial how glomus cells are depolarized in response to hypoxia. 

Figure 2 Hypoxia inhibits K+ channels in several 02-sensitive tissues, (a) Whole-ceU patch-clamp experiments show K" " current inhibition in response to hy poxia in rat resistance PASMCs. The complex I ETC blocker rotenone and the Kv blocker 4-aminopyridine (4-AP) also inhibit K" current, (b) Using the current clamp mode, resistance PASMCs depolarize in response to hypoxia. (From Ref. 29.) (c,d,e) Hypoxia inhibits outward K" " current in isolated cells from the NEB, CB, and PC-12 cells. (From Refs. 39,105,106.) Inset Proposed mechanism for HPy featuring the effector portion of the pathway.

Taken together, these studies show that activation of the Pyk2 protein tyrosine kinase is one of the early events in response to hypoxia in the Oi-responsive PC 12 cell hne. This effect occurs in a calcium-dependent manner and is therefore likely to be mediated by the rapid depolarization and calcium influx that occur in response to hypoxia in these excitable cells. 

Ca channels. Therefore, a likely hypothesis for maturational changes in the response to hypoxia is that hypoxia-induced Vm depolarization is small in cells of newborns and increases with age. 

As noted above, in the unanesthetized animal, acute systemic hypoxia immediately following chemodenervation or inhalation of carbon monoxide (to produce systemic hypoxia without stimulating arterial chemoreceptors) elicits a tachpneic response. Dillon and Waldrop (47), using whole-cell patch recording in tissue slices, have shown that neurons in the caudal hypothalamus depolarize and increase firing rates when exposed to hypoxia. In addition, direct stimulation of this region in the intact animal elicits tachypnea. They propose that this site is the locus of the tachypneic response to hypoxia in the acutely denervated animal. 

Figure 1 Effect of reduced O2 on potassium (K ) current (Ik.), (a) Superimposed current traces recorded during control period (C, normoxia, ISOmmHg), after steady-state inhibition by hypoxia (H, OmmHg), and after return to normoxic conditions (R, ISOmmHg). Cells were depolarized to +50 mV for 800 msec from a holding potential of —90 mV (b) Time course of the hypoxic inhibition of outward current (same cell as in panel a). Sampling rate was 0.1 Hz. Steady-state responses to changes in P02 were attained < 1 min after exposure, (c) Current-voltage relationships recorded during control period (C) and after steady-state inhibition by hypoxia (H). Cell voltage was ramped from —60 to +60 mV over 5 sec and repeated every 10 sec.

Schematically, hypoxia and H" " bind to membrane heme and interact with K channels (not the pore) followed by membrane depolarization. This membrane phenomenon will take place in a split seeond. Down the eascade, HIF-a is stabilized, which can form a heterodhner with eonstitutively expressed HIF-)S in the cytosol, activating HIF-1. This then is translocated to the nucleus and binds to hypoxia response elements in inducible genes. In normoxia, HIF-a is formed continually and is oxidatively modified and then is degraded by the proteasomal pathway. Ferrous iron is needed at two points, in the generation and degradation of HIF-a, although HIF does not contain Fe " ". Without Fe " ", these reactions will not occm (see Refs. 28,29).

The initial sensors and subsequent signal transduction systems of hypoxic vasoconstriction (HPV) remain an area of intense investigation (21,22). At the cellular level, pulmonary artery smooth muscle (PASM) contraction depends on an increase in cytosolic calcium from the extracellular space as well as release from intracellular stores, and membrane depolarization due to closure of K+ channels. Many argue that the mitochondria is a primary oxygen sensor such that electron transport chain inhibitors can specifically inhibit HPV and/or prevent the hypoxia-specific response. Reactive oxygen species are also implicated in HPV. Two different models are proposed one describes an increase in mitochondrial ROS mediated via increased intracellular calcium release, and the second describes a decrease in mitochondrial ROS mediated via inhibition of the Kv channel (Figure 8.2). 

Figure 13 (A) Nicotinic ACh receptor in NEB cells of neonatal hamster lung. ACh-iaduced inward current under normoxia and hypoxia conditions (p02 = 20 mmHg. Holding potential was —60 mV). (B) Application of nicotine evoked an inward current (a). Holding potential was —60 mV Effects of holding potential on inward currents evoked by 50 pM nicotine. Each plotted point is the mean peak inward current amphtude taken from between five and eight cells at each holding potential, (c) Nicotine evoked a membrane potential depolarization, (d) The peak currents evoked at each concentration are expressed relative to the peak current evoked by 50 mM nicotine and plotted against the log [nicotine] mean response taken from five to eight cells. The experimental data were fitted by the Hill equation with a Hill coefficient of 0.9 and EC50 = 4 pM.

Further studies by Sun and Reis with the in vitro medullary slice preparation provide additional evidenee for a direet excitatory effect of hypoxia on these neurons (69). In this redueed preparation, the RVLM sympathoexeitatory neurons retain exeitability by hypoxia and foeal NaCN. When synaptie transmission is blocked by applieation of tetrodotoxin (TTX), both hypoxia and NaCN eontinue to elieit neuronal membrane depolarization while the membrane response is abolished by applieation of Co +, a nonseleetive Ca +-ehannel bloeker (68-70). These observations argue for intrinsie hypoxic chemosensitivity of these sympathoexcitatory neurons of the RVLM. 

---