THE ANTAGONISTIC EFFECT OF K-O(-OUABAIN ON THE NA+ PUMP CURRENT OF SINGLE-RAT AND GUINEA-PIG CARDIAC-CELLS() AND DIHYDRO)

1. The antagonistic effect of extracellular potassium ions (K-o(+)) an d dihydro-ouabain (DHO) on the Na+-K+ pump current (I-p) was studied i n isolated ventricular cells. 2. The myocytes were isolated from rats and guinea-pigs, two species with different sensitivity towards cardia c glycosides. I-p measurements were performed at 32-34 degrees C by me ans of whole-cell recording. The membrane potential was held at -20 mV throughout. 3. The DHO concentration ([DHO]) required for half-maxima l I-p inhibition (apparent K value, K-D') amounted to 2.4 x 10(-3) and 1.4 x 10(-5) M for rat and guinea-pig myocytes, respectively, at 5.4 mM K-o(+). 4. The data suggest one-to-one binding of DHO to the Na+-K pump and a smaller association rate constant, as well as a larger dis sociation rate constant, for binding of DHO in the rat cells. 5. I-p a ctivation by K-o(+) tvas nearly identical in myocytes of both species and was measured to be half-maximal at similar to 1 mM K-o(+). Half-ma ximal I-p activation by K-o(+) remained essentially unchanged, but I-p decreased in media containing [DHO] near the respective K-D' at 5.4 m M K-o(+). 6. The concentration-response curve of I-p inhibition by DHO was shifted to higher [DHO] at higher [K+](o). K-D' increased corresp ondingly. The slope of the curve was unaffected. 7. I-p and K-D' displ ayed a similar dependence on [K+](o). 8. K-D' was larger in Na+-free t han in Na+-containing media under conditions in which the activation o f I-p by K-o(+) was nearly the same. 9. It is concluded that the antag onism between K-o+ and DHO, with regard to the activation of I-p, is n on-competitive. A possible mechanism of the antagonism is discussed. T he mechanism implies binding of K-o(+) and DHO to different conformati onal states of the Na+-K+ pump which are temporarily exposed to the ex ternal face of the sarcolemma in the pump cycle. The DHO-bound states do not participate in the generation of I-p.