Sodium pump evokes high density pump currents in rat midbrain dopamine neurons

Abstract

Patch pipettes contained various concentrations of Na⁺ ([Na⁺]_pip) in order to record strophanthidin‐sensitive currents under voltage clamp in dopamine neurons in slices of rat substantia nigra and ventral tegmental area. When [Na⁺]_pip was 40 mm and the external K⁺ concentration ([K⁺]_o) was 2.5 mm, strophanthidin (10 μm) evoked 461 ± 121 pA of inward current. This effect was concentration dependent, with an EC₅₀ of 7.1 ± 2.6 μm. At potentials of −60 to −120 mV, strophanthidin‐induced currents were not associated with significant changes in chord conductance. Strophanthidin (10 μm) evoked 234 ± 43 pA of inward current when [Na⁺]_pip was 0.6 mm, and 513 ± 77 pA when [Na⁺]_pip was 80 mm. Despite higher pump currents with greater [Na⁺]_pip, the strophanthidin EC₅₀ was not significantly different for any of six different [Na⁺]_pip. Sodium pump currents were half‐maximal when the [Na⁺]_pip was about 1.3 mm. Maximum pump current was estimated at 830 pA (29 μA cm⁻²) at concentrations of intracellular Na⁺ that were assumed to be saturating (50–100 mm). Strophanthidin currents were smaller in a reduced [K⁺]_o (EC₅₀= 0.2 mm). These data show that intracellular Na⁺ loading evokes relatively large pump currents. Our results are consistent with the physiological role of the sodium pump in burst firing in midbrain dopamine neurons