Chronic hypoxia up‐regulates α1HT‐type channels and low‐threshold catecholamine secretion in rat chromaffin cells

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Abstract
α1HT‐type channels recruited by β1‐adrenergic stimulation in rat chromaffin cells (RCCs) are coupled to fast exocytosis with the same Ca2+dependence of high‐threshold Ca2+channels. Here we show that RCCs exposed to chronic hypoxia (CH) for 12–18 h in 3% O2express 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 Ca2+channels already available at −50 mV with the same gating, pharmacological and molecular features as the α1Hisoform. Chronic hypoxia had no effects on cell size and high‐threshold Ca2+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 Ca2+. 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 μmNi2+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 mmKCl and spike frequency was drastically reduced by 50 μmNi2+. 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.