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More than 50% of tracer rubidium entering the coronary circulation is actively taken up by the heart muscle cells during a single passage. Rubidium ions must traverse the capillary wall, the interstitial space, and the muscle cell membrane to enter the very large potential rubidium pool within the cells. The aim of this investigation was to determine which, if any, of these steps are rate limiting. The anterior descending branch of the left common coronary artery was perfused at its origin. Pulse injections containing 86Rb+, 22Na+ or 14C-sucrose, and 125I-albumin were made into the perfusion line and timed serial samples of coronary sinus blood were collected and analyzed. A model was designed which incorporates rate constants describing both the exchange of 80Rb+ at the capillary wall and its entry at the muscle cell membrane. Labeled sucrose or 22Na+ was used to determine flow per interstitial fluid volume, a parameter necessary for the application of the model. It was assumed that the interstitial fluid volume available for labeled sucrose or 22Na+ was identical to that available for 86Rb+. The rate constant describing exchange at the capillary wall (the permeability surface product per unit accessible interstitial fluid volume) increased with perfusion, whereas that for uptake by the myocardial cells was relatively constant and independent of flow.

Kinetics of Rubidium Uptake in the Working Dog Heart