CA2+-free perfusion of rat heart reveals a (Na+ + K+)ATPase form highly sensitive to ouabain.

Abstract

The sensitivities of the cardiac (Na+ + K+)ATPases to inhibition by ouabain have been shown to correlate with the pharmacologically active digitalis concentrations in the various source species1. The rat heart enzyme is generally considered to be insensitive to ouabain with half-maximal inhibition at ∼6 × 10−5 M ouabain2,3 and a KD for binding3–7 of 1–6 × 10−5 M. However, recent data5–7 showed that rat cardiac sarcolemma preparations can bind ouabain with high affinity (KD 1–3 × 10−7M) with no detectable concomitant enzyme inhibition in vitro but producing a sustained positive inotropic effect in vivo. In the present study, we measured the (Na+ + K+)ATPase inhibition by ouabain in sarcolemma-enriched fractions obtained from rat hearts relaxed by perfusion with Ca2+-free solutions. In these conditions, two enzyme forms having high and low sensitivities to ouabain were found (half-maximal inhibitions with 1.2×10−8 and 6×10−5M, respectively). Their relative proportion is close to 1. Confirming previous data2–7, the highly sensitive form was not detected in preparations from hearts maintained at a physiological calcium level. This heterogeneous population of functional (Na+ + K+)ATPases suggests that, in rat heart, the positive inotropic effect of low doses of ouabain in vivo5–9 would act through the inhibition9 of the enzyme activity highly sensitive to ouabain as detected in vitro.