Modulation of recombinant T-type Ca2+ channels by hypoxia and glutathione

T-type Ca2+ channels are expressed in a wide variety of central and peripheral neurons and play an important role in neuronal firing and rhythmicity. Here we examined the effects of hypoxia on the recently cloned T-type Ca2+ channel alpha (1G), alpha (1H) and alpha (11) subunits, stably expressed in HEK 293 cells. In cells expressing the human alpha (1H) or the rat alpha (11) subunit, Ca2+ channel currents were inhibited reversibly by hypoxia (PO2<110 mmHg). The degree of inhibition was more marked in cells expressing the <alpha>(1H) subunit. This hypoxic inhibition was not voltage dependent. In cells expressing the rat alpha (1G) subunit, hypoxia caused no detectable reduction in Ca2+ channel activity. Regardless of the channel type examined, hypoxia was without effect on the kinetic properties of the Ca2+ current (activation, inactivation and deactivation) or on steady-state inactivation. Ca2+ current through the alpha (1H) subunit was enhanced by the reducing agent reduced glutathione (GSH; 2 mM) and inhibited by oxidised glutathione (GSSG; 2 mM). In contrast, Ca2+ current through the alpha (1G) subunit was unaffected by GSH. In alpha (1H) cells, neither GSH nor GSSG had any effect on the ability of hypoxia to reduce Ca2+ current amplitudes. Thus, different members of the T-type Ca2+ channel family are differently regulated by hypoxia and redox agents. Hypoxic regulation of the alpha (1H) subunit appears to be independent of changes in levels of the intracellular redox couple GSSG:GSH.