Excretion and metabolism of nicotinic acid by the avian kidney

The renal tubular transport and metabolism of nicotinic acid (NA) were investigated using the Sperber technique in unanesthetized hens. Infusion of [¹⁴C]NA into the avian renal portal circulation at 10^-10 mol/kg/min revealed that the ¹⁴C label was actively transported into urine at a rate 74% that of simultaneously infused tetraethylammonium. Increases in NA infusion rates enhanced ¹⁴C label transport so that it eventually equalled the excretion rate of tetraethylammonium. At a NA infusion rate of 10^-8 mol/kg/min, this transport was not affected by probenecid, pyrazinoic acid or p-aminohippurate. Above infusion rates of 10^-7 mol/kg/min, saturation of ¹⁴C label transport was reached. Electrophoretic analysis of the ¹⁴C label excreted in urine at NA infusion rates less than 10^-7 mol/kg/min revealed a single, unidentified metabolite. At infusion rates greater than 10^-7 mol/kg/min, both the radiolabeled metabolite and [¹⁴C]NA were found in the urine. We conclude that NA is transported by a specific mechanism into the avian tubule cell where it is metabolized. As the body's load of this vitamin increases, NA is excreted first in the form of a metabolite and then as both metabolite and unutilized NA.