Chronic hypoxia up-regulates α_1H T-type channels and low-threshold catecholamine secretion in rat chromaffin cells

α_1H T-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% O₂ 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 Ca²⁺ channels already available at -50 mV with the same gating, pharmacological and molecular features as the α_1H isoform. 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 μm Ni²⁺ 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 m m KCl and spike frequency was drastically reduced by 50 μm Ni²⁺. 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.