Demonstration of rapid entry and a cellular binding space for salicylamide in perfused rat liver: a multiple...

The kinetics of influx, efflux and removal of salicylamide under steady-state conditions (input concentration, 9-870 microM) were studied with the single pass erythrocyte-perfused rat liver in the absence of albumin (12 ml min-1). A substantial distribution of salicylamide into red blood cells (red cell/plasma ratio, 3.5) was observed. During steady state, a bolus dose containing multiple indicators (51Cr-labeled red cells (vascular space marker), [3H]sucrose (interstitial space marker), D2O (cellular space marker) and added [14C]salicylamide tracer) was injected into the portal vein. The steady-state hepatic salicylamide 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 [14C]salicylamide (or [1-F] where F is the availability). Modeling of the indicator dilution outflow data revealed an extremely rapid (flow-limited) influx and efflux for salicylamide that was independent of the partitioning of salicylamide into red blood cells. The decrease in extraction ratio was due solely to saturation of the metabolic processes, shown previously to be sulfation, glucuronidation and hydroxylation. The sequestration rate constant, representing the pooled constant for all of the metabolic pathways, decreased from 0.45 to 0.035 sec-1 with increase in concentration. From the spectrum of its change with concentration, calculated values for a corresponding apparent Vmax and Km were 17.5 nmol sec-1 ml-1 cellular water and 27 microM, respectively. The cellular distribution space for salicylamide was exceedingly large, 20 times the size of the cellular water space, at trace levels, and fell to a constant level (5 times the cellular water space) with increasing concentration. The phenomenon, first observed for n-propanol (Goresky et al., Am. J Physiol. 244: G215-G244, 1983a), is explained by a nonsaturable (partition coefficient, 3.8-5) and a saturable binding (binding site concentration, 352-98 microM) component in the tissue. The binding effects inverted the order of elution of the parent and metabolite profiles; the outflow emergence of metabolites began earlier than that of the parent compound, salicylamide.