Consequences of specific [3H]ouabain binding to guinea pig left atria and cardiac cell membranes

An analysis of [3H]ouabain binding to electrically stimulated, contracting guinea pig left atria gave the following results. (1) A non-linear Scatchard plot with at least two binding sites: a high-affinity site (KD 1.1 X 10(-6) M) with about 430 receptors/micron2 related to positive inotropy, and a low-affinity site (KD' 2.1 X 10(-4) M) with about 18,000 receptors/micron2, possibly related to (Na+ + K+)ATPase inhibition. A crude left atrial homogenate gave about 530 receptors/micron2. (2) Half-maximal positive inotropic effects occurred at about 4 X 10(-7) M. (3) 86Rb+-uptake was significantly increased at all inotropic ouabain concentrations (10(-7) - 10(-6) M). Toxic concentrations (above 2 X 10(-6) M) inhibited 86Rb+-uptake (half-maximal inhibition at about 5 X 10(-6) M). [3H]Ouabain binding to partly purified guinea pig cardiac cell membranes showed: (a) linear Scatchard plots for (Mg2+, Pi)- and (Na+, ATP, Mg2+)-supported binding (KD 1.18 X 10(-7) M and 1.49 X 10(-7) M, respectively); (b) non-linear Scatchard plots for (Tyrode + ATP)-supported binding (KD 4.7 X 10(-7) M; KD' 6 X 10(-6) M); and (c) half-maximal [3H]ouabain binding occurred at a lower concentration (about 3.2 X 10(-7) M) than half-maximal inhibition of (Na+ + K+)ATPase activity (about 7.2 X 10(-7) M). Thus, we conclude that there may be more than one type of ouabain binding site in guinea pig left atria, and that measurable inhibition of (Na+ + K+)ATPase is not necessarily related to positive inotropy in the guinea pig.