O2 sensing in chromaffin cells: new duties for T-type channels

T-type Cav3 channels are voltage-gated Ca2+ channels that are able to sustain key physiological functions such as low-threshold spikes generation, neuronal and cardiac pacemaking, muscle contraction, hormone release, cell growth and differentiation. This mainly derives from the unique property of T-type channels that activate at rather negative voltages (∼ −60 mV). These channels are ubiquitous in most excitable tissues and their expression at sufficient densities lowers the threshold of action potential generation with a consequent increase of cell excitability. The chromaffin cells of adult adrenal medulla seemed to escape this general rule, but recent works have shown that during chronic hypoxia or under β-adrenergic stimulation, chromaffin cells also acquire sufficiently high densities of Cav3.2 T-type channels, which lower the resting membrane potential and sustain low-threshold spike activity (Carbone et al. 2006; Carabelli et al. 2007). Recruitment of T-type channels is not a unique feature of chromaffin cells. PC12 cells also express Cav3.2 channels during exposure to chronic hypoxia, the effect being mediated by hypoxia-inducible factors (HIFs) (Del Toro et al. 2003).