DEMONSTRATION OF RAPID ENTRY AND A CELLULAR-BINDING SPACE FOR SALICYLAMIDE IN PERFUSED-RAT-LIVER - A MULTIPLE INDICATOR DILUTION STUDY

The kinetics of influx, efflux and removal of salicylamide under stead y-state conditions (input concentration, 9-870 mu M) were studied with the single pass erythrocyte-perfused rat liver in the absence of albu min (12 ml min(-1)). A substantial distribution of salicylamide into r ed blood cells (red cell/plasma ratio, 3.5) was observed. During stead y state, a bolus dose containing multiple indicators {Cr-51-labeled re d cells (vascular space marker), [H-3] sucrose (interstitial space mar ker), D2O (cellular space marker) and added [C-14]salicylamide tracer} was injected into the portal vein. The steady-state hepatic salicylam ide extraction ratio decreased from 0.99 to 0.4 over the concentration range used. The extraction ratio for bulk salicylamide was similar to 1 minus the integral of the fractional outflow recovery of unchanged tracer [C-14]salicylamide (or [1-F] where F is the availability). Mode ling of the indicator dilution outflow data revealed an extremely rapi d (flow-limited) influx and efflux for salicylamide that was independe nt of the partitioning of salicylamide into red blood cells. The decre ase in extraction ratio was due solely to saturation of the metabolic processes, shown previously to be sulfation, glucuronidation and hydro xylation. The sequestration rate constant, representing the pooled con stant for all of the metabolic pathways, decreased from 0.45 to 0.035 sec(-1) with increase in concentration. From the spectrum of its chang e with concentration, calculated values for a corresponding apparent V -max and K-m were 17.5 nmol sec(-1) ml(-1) cellular water and 27 mu M, respectively. The cellular distribution space for salicylamide was ex ceedingly large, 20 times the size of the cellular water space, at tra ce levels, and fell to a constant level (5 times the cellular water sp ace) with increasing concentration. The phenomenon, first observed for n-propanol (Goresky et al., Am, J. Physiol. 244: G215-G244, 1983a), i s explained by a nonsaturable (partition coefficient, 3.8-5) and a sat urable binding (binding site concentration, 352-98 mu M) component in the tissue. The binding effects inverted the order of elution of the p arent and metabolite profiles; the outflow emergence of metabolites be gan earlier than that of the parent compound, salicylamide.