SODIUM-PUMP EVOKES HIGH-DENSITY PUMP CURRENTS IN RAT MIDBRAIN DOPAMINE NEURONS

1. Patch pipettes contained various concentrations of Na+ ([Na+](pip)) in order to record strophanthidin-sensitive currents under voltage cl amp in dopamine neurons in slices of rat substantia nigra and ventral tegmental area. 2. When [Na+](pip) was 40 mM and the external K+ conce ntration ([K+](o)) was 2.5 mM, strophanthidin (10 mu M) evoked 461 +/- 121 pA of inward current. This effect was concentration dependent, wi th an EC50 of 7.1 +/- 2.6 mu M. At potentials of -60 to -120 mV, strop hanthidin-induced currents were not associated with significant change s in chord conductance. 3. Strophanthidin (10 mu M) evoked 234 +/- 43 pA of inward current when [Na+](pip) was 0.6 mM, and 513 +/- 77 pA whe n [Na+](pip) was 80 mM. Despite higher pump currents with greater [Na](pip), the strophanthidin EC50 was not significantly different for an y of six different [Na+](pip). 4. Sodium pump currents were half-maxim al when the [Na+](pip) was about 1.3 mml. Maximum pump current was est imated at 830 pA (29 mu A cm(-2)) at concentrations of intracellular N a+ that were assumed to be saturating (50-100 mM). 5. Strophanthidin c urrents were smaller in a reduced [K+](o) (EC50 = 0.2 mM). 6. These da ta show that intracellular Na+ loading evokes relatively large pump cu rrents. Our results are consistent with the physiological role of the sodium pump in burst firing in midbrain dopamine neurons.