Estimation of transcapillary transport of palmitate by the multiple indicator dilution technique

From the outflow concentration-time curves for 14C-palmitate, intravascular (131I-albumin) and extracellular (3H-sucrose) tracers, palmitate extraction was estimated in rabbit hearts Langendorff-perfused at a constant flow with nonrecirculated palmitate-albumin Kreb's Ringer buffer. Contamination of 131I-albumin with free 13$1I- (typically 1%) or aggregated albumin (typically 0.1 to 0.5%) greatly alters the shapes of the tails of the curves after 2 albumin transit times, vitiating accurate estimation of cellular permeability or reactions. Buffers were prepared by adding K+-palmitate (made using K2CO3) to albumin solutions. The final concentrations (after dialysing twice and filtering through a 1.2 μ filter) of K+, HCO3, and CO3 were 5.0 mM, 23.5 mM and 0.5 mM respectively, pH was between 7.35 and 7.40 for several hours. The bolus of tracers was prepared by mixing 131I-albumin (dialysed to remove I-, and filtered through a 0.2 μM filter to remove aggregates), K+ [U-14C]palmitate (high specific activity) and 3H-sucrose. Before injection the radioactive bolus is preequilibrated with the perfusate at bolus:perfusate ratio of 1:10. Glacial acetic acid is added to the outflow samples to remove the 14CO2 which, if present in the sample, would be interpreted as increased palmitate back diffusion. The peak extractions of palmitate were about 40% at perfusate palmitate concentrations of 0.02 to 1.0 mM, 0.4 mM albumin, at a flow of 5 mlg-1 2]1, showing capillary permeability-surface area product to be roughly constant. This suggests either than transcapillary palmitate transport is passive or that a transporter interacts with the albumin-palmitate complex