Cellular mechanisms of digoxin transport and toxic interactions in the kidney.

Renal tubular secretion of digoxin appears to be one of the main ports of elimination of the glycoside from the body. Because of its narrow therapeutic window and severe toxicity, the mechanisms of tubular handling of digoxin are important. Moreover, several drugs which are commonly administered with digoxin, including quinidine, spironolactone, verapamil and amiodarone have been shown to decrease renal clearance of digoxin without affecting GFR. We studied the handling of digoxin using in vitro and in vivo approaches. The handling of the glycoside by the brush border suggests passive reabsorption which is not enhanced by commonly coadministered drugs. Digoxin binding to the antiluminal (basal) membrane suggests that the secretion of the glycoside may not involve the pharmacologic receptor, the Na+, K+, ATPase. Using the multiple indicator dilution technique, we could directly show the two steps of secretion of digoxin: Its sequestration from the postglomerular circulation, and its appearance in the urine after transtubular transport. Digoxin transport is not inhibited by a cationic or anionic molecule (PAH and tolazoline). It is possible that digoxin is secreted by a yet unidentified transport mechanism.