Intracellular sodium and calcium homeostasis during hypoxia in dopamine neurons of rat substantia nigra pars compacta

We investigated the hypoxia-induced disturbance of cytosolic sodium concentration ([Na⁺](i)) and of cytosolic calcium concentration ([Ca²⁺](i)) in dopamine neurons of the substantia nigra pars compacta in rat midbrain slices, by combining whole cell patch-clamp recordings and microfluorometry. Transient hypoxia (3-5 min) induced an outward current (118.7 ± 15.1 pA, mean ± SE; V(H) = -60 mV). The development of this outward current was associated with an elevation in [Na⁺](i) and in [Ca²⁺](i). The hypoxia-induced outward current as well as the elevations in [Na⁺](i) and [Ca²⁺](i) were not affected by the ionotropic and metabotropic glutamate receptor antagonists D-amino-phosphonovalerate (50 μM), 6-nitro-7-sulfamoyl-benzo [f]quinoxaline-2,3-dione (10 μM) and S- (α)-methyl-4-carboxyphenylglycine (500 μM). Tolbutamide, a blocker of ATP- dependent K⁺ channels, depressed the hypoxia-induced outward current but did not affect the increases in [Na⁺](i) or [Ca²⁺](i). Increasing the concentration of ATP in the internal solution from 2 to 10 mM strongly reduced the hypoxia-induced outward current but did not reduce the rise in [Na⁺](i). Decreasing the concentration of extracellular Na⁺ to 19.2 mM depressed the hypoxia-induced outward current and resulted in a decrease in resting [Na⁺](i). Under this condition hypoxia still increased [Na⁺](i), albeit to levels not exceeding those of resting [Na⁺](i) observed under control conditions. We conclude that 1) a major component of the hypoxia- induced outward current of these cells is caused by a depletion of intracellular ATP in combination with an increase in [Na⁺](i), 2) that the [Na⁺](i) and [Ca²⁺](i) responses are not mediated by glutamate receptors, 3) that the [Na⁺](i) and [Ca²⁺](i) responses are not depressed by activation of sulfonylurea receptors, and 4) that the rise in [Na⁺](i) induced by short-lasting hypoxia is not due to a ATP depletion-induced failure of Na⁺ extrusion.