Effects of Na+ and K+ on the uptake of metaraminol by rabbit ventricular slices.

1. The ionic dependence of [(3)H]-metaraminol transport by rabbit ventricular slices was studied.2. Transport was Na(+) dependent. Choline, Li(+), K(+), Rb(+) or Cs(+) could not be substituted for Na(+).3. Transport was K(+) dependent. Rb(+) and Cs(+), but not Li, could be substituted for K(+), their relative potencies being K(+) = or >Rb(+)>Cs(+)>>Li(+). Higher concentrations of K(+), Rb(+), Cs(+) and Li inhibited [(3)H]-metaraminol transport.4. The inhibitory effects on transport of either a marked reduction in the concentration of Na(+) or of omission of K(+) were rapidly reversible on exposure of slices to Krebs solution.5. The inhibitory effect of ouabain on [(3)H]-metaraminol transport was markedly time dependent, being significantly increased by preincubation with ouabain. The inhibitory action of ouabain was not affected, however, by the Na(+) concentration present during preincubation.6. An inwardly directed Na(+) gradient did not increase [(3)H]-metaraminol transport in slices in which Na(+) pumping had been prevented by 10(-5)M ouabain, 4 degrees C, omission of K(+) or by metabolic inhibition.7. These findings provide additional evidence that the Na(+) and K(+) activated adenosine triphosphatase may participate in the transport of metaraminol and related amines. In the absence of Na(+) pumping, induced Na(+) gradients were unable to produce transport of amine as would be predicted by the co-transport model proposed by Bogdanski & Brodie (1969).