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

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Abstract
1 The ionic dependence of [3H]-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 [3H]-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 [3H]-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 [3H]-metaraminol transport in slices in which Na+ pumping had been prevented by 10−5m ouabain, 4° 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).