Drug Elimination by Renal Route: Tubular Secretion

Following glomerular filtration, blood that contains unfiltered drug molecules travels through the efferent arterioles and enters into the peritubular capillaries of the proximal tubule.

Different carriers facilitate the active secretion of drugs from the peritubular capillaries into the tubular fluid.

The organic anion transporter transfers acidic drugs, into the renal tubule cells, against an electrochemical gradient.

The drugs exit into the urine through ATP-dependent efflux transporters localized in the apical membrane.

The organic cation transporter transfers basic drugs down the gradient and into the urine through P-glycoproteins expressed on the apical membrane.

Since these transporters are nonselective, different drugs can compete with one another for the same transport system. 

This feature is used to prolong the action of penicillin by co-administering probenecid. As both drugs compete for the same acid transporter,  tubular secretion of penicillin can be retarded, which prolongs its circulation in the blood.