Chronic hypoxia up-regulates alpha1H T-type channels and low-threshold catecholamine secretion in rat chromaffin cells.
alpha(1H) T-type channels recruited by beta(1)-adrenergic stimulation in rat chromaffin cells (RCCs) are coupled to fast exocytosis with the same Ca(2+) dependence of high-threshold Ca(2+) channels. Here we show that RCCs exposed to chronic hypoxia (CH) for 12-18 h in 3% O(2) express comparable densities of functional T-type channels that depolarize the resting cells and contribute to low-voltage exocytosis. Following chronic hypoxia, most RCCs exhibited T-type Ca(2+) channels already available at -50 mV with the same gating, pharmacological and molecular features as the alpha(1H) isoform. Chronic hypoxia had no effects on cell size and high-threshold Ca(2+) current density and was mimicked by overnight incubation with the iron-chelating agent desferrioxamine (DFX), suggesting the involvement of hypoxia-inducible factors (HIFs). T-type channel recruitment occurred independently of PKA activation and the presence of extracellular Ca(2+). Hypoxia-recruited T-type channels were partially open at rest (T-type 'window-current') and contributed to raising the resting potential to more positive values. Their block by 50 microm Ni(2+) caused a 5-8 mV hyperpolarization. The secretory response associated with T-type channels could be detected following mild cell depolarizations, either by capacitance increases induced by step depolarizations or by amperometric current spikes induced by increased [KCl]. In the latter case, exocytotic bursts could be evoked even with 2-4 mm KCl and spike frequency was drastically reduced by 50 microm Ni(2+). Chronic hypoxia did not alter the shape of spikes, suggesting that hypoxia-recruited T-type channels increase the number of secreted vesicles at low voltages, without altering the mechanism of catecholamine release and the quantal content of released molecules.
Carabelli
V
V
Department of Neuroscience, NIS Center of Excellence, CNISM Research Unit, 10125 Torino, Italy.
Marcantoni
A
A
Comunanza
V
V
de Luca
A
A
Díaz
J
J
Borges
R
R
Carbone
E
E
D000818:Animals; D002118:Calcium / Q000378:metabolism; D020747:Calcium Channels, T-Type / Q000378:metabolism*; D002395:Catecholamines / Q000378:metabolism*; D002478:Cells, Cultured; D019439:Chromaffin Cells / Q000378:metabolism*; D003676:Deferoxamine; D005089:Exocytosis / Q000502:physiology*; D000860:Hypoxia / Q000378:metabolism*; D051793:Hypoxia-Inducible Factor 1 / Q000378:metabolism; D008564:Membrane Potentials / Q000502:physiology; D011189:Potassium Chloride / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D017262:Siderophores; D013997:Time Factors; D015854:Up-Regulation
D020747:Calcium Channels, T-Type; D002395:Catecholamines; D051793:Hypoxia-Inducible Factor 1; D017262:Siderophores; D011189:Potassium Chloride; D003676:Deferoxamine; D002118:Calcium
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