Effect of ouabain on Ca45 kinetics in the myocardium determined by the technique of diffusible indicator-dilution

1. The purpose of this research was two-folds a) to investigate the relationship of the positive inotropic effect of ouabain to an action on calcium kinetics, and b) to test the applicability of indicator dilution techniques to studies of distribution and kinetics of small ions and molecules in multicompartment systems. 2. The hypothesis was made that ouabain increases the influx of calcium into a small labile pool situated on or within the cell membrane and/or associated structures and that this pool was involved in the coupling of excitation to contraction. 3. Several models were constructed to describe the calcium exchange system in the myocardium. The simplest model to which the data could be fitted was a catenated three-compartment model. The hypothetical small labile pool was situated between the extracellular and intracellular compartments. 4. Experiments were performed in the heart-lung preparation of the goat. Radioiodinated serum albumin was used as an indicator of intravascular space, radiosulfate as an indicator of extracellular space, radioantipyrine as an indicator of total tissue water, and radiocalcium as an indicator of calcium distribution and kinetics. Indicators were injected into the left coronary artery; indicator-containing blood was sampled from the great coronary vein. The work load on the heart was changed by increasing reservoir height or peripheral resistance. The positive inotropic effect of ouabain was assessed from changes in cardiac work at standard reservoir height and from shifts in Starling curves. 5. The diffusible indicator-dilution method of Martin and Yudilevich was extended to include kinetic analyses for intracellular compartments. 6. The indicator-dilution technique provided a value for extracellular space (.23) which was in the range of that determined by other workers with classical methods. It provided a value for tissue water (.54) which was lower than that usually obtained; this may have been the result of too slow a return of indicator to the venous blood to distinguish the indicator from background radioactivity. An appreciable fraction of radiosulfate and radiocalcium appeared to be retained in the intravascular space. Three pools of calcium could be distinguished by the method. It was concluded that the indicator-dilution technique is a useful tool for the evaluation of flux rates and constants when the exchange is rapid but not when it is slow. Advantages and disadvantages of the method were discussed, especially with reference to uptake-washout methods. 7. The kinetics of calcium in the interstitial space and labile pool were readily defined with the aid of an analog computer by adjustment of rate constants in the model until curves described by the computer matched those determined experimentally. The kinetics of calcium in the large, slowly exchanging intracellular compartment could not be defined, so that it was necessary to assign an arbitrary value to the flux rates between this and the labile compartment in order to evaluate kinetics in the labile compartment. It was shown that a ten-fold difference in the arbitrary values assigned had an insignificant effect on the kinetics in the labile pool. 8. An attempt was made to assess the relationship between calcium kinetics and the force of myocardial contraction. When cardiac work was increased by raising reservoir height, no significant effect on calcium kinetics was found. When cardiac work was increased by raising peripheral resistance, the exchange of calcium in the labile pool was accelerated and the calcium content was increased. 9. Ouabain increased calcium content in the labile pool and the net calcium flux, mainly by an increase in influx. In the majority of experiments this was accompanied by an increase in the force of myocardial contraction. There was a moderate correlation between increase in net calcium flux and increase in cardiac work. The results are considered to support the hypothesis that ouabain increases the influx of calcium into a small, labile pool which is involved in the coupling of excitation to contraction.