The distribution of 3H-labelled cardenolides between isolated guinea-pig atrial tissue and circulating, oxygenated whole blood.

1. An experimental method was developed that allowed the incubation of isolated organs in circulating whole blood. The circulating blood was oxygenated with a specially designed disc-oxygenator and drawn through the system by means of a roller pump.2. The method proved suitable for guinea-pig isolated atria, rabbit duodenum and to a lesser extent for chronically denervated rat diaphragm. Isolated atria could be kept for several hours. Various parameters of the circulating blood (haemolysis, pH, O(2) saturation, concentration of electrolytes) remained satisfactory for at least 5 hr. The method proved convenient for pharmacological and kinetic studies on isolated organs, suspended in whole blood of the corresponding species. The organs showed normal spontaneous mechanical activity and also responded to electrical stimuli and various drugs.3. The uptake of (3)H-labelled ouabain, digoxin, digitoxin and digitoxigenin was studied in guinea-pig isolated atria, suspended in circulating blood. The uptake reached equilibrium after 60-90 min. With respect to the total serum radio-activity the "apparent" tissue/medium (T/M) ratios obtained for the four drugs were within the range 0.4-1.1. If, however, the amount of free, non protein bound drug was taken as a base for the calculations, the following "true" T/M ratios were obtained: (3)H-ouabain 0.45; (3)H-digoxin 1.6; digitoxin 8.8; digitoxigenin 8.4. These values are virtually the same as those obtained with atria, suspended in an aqueous medium. Obviously, the uptake of (3)H-cardenolides from whole blood is determined by the amount of free non-protein-bound drug.4. (3)H-digitoxin and (3)H-digitoxigenin were taken up by guinea-pig erythrocytes to a small extent. No measurable amounts of (3)H-ouabain and (3)H-digoxin were taken up by erythrocytes.