Renal handling and effects of [3H]digoxin and interactions with quinidine in the avian kidney

Isolation and characterization of a digoxin transporter and its rat homologue expressed in the kidney

Digoxin, which is one of the most commonly prescribed drugs for the treatment of heart failure, is mainly eliminated from the circulation by the kidney. P-glycoprotein is well characterized as a digoxin pump at the apical membrane of the nephron. However, little is known about the transport mechanism at the basolateral membrane. We have isolated an organic anion transporter (OATP4C1) from human kidney. Human OATP4C1 is the first member of the organic anion transporting polypeptide (OATP) family expressed in human kidney. The isolated cDNA encodes a polypeptide of 724 aa with 12 transmembrane domains. The genomic organization consists of 13 exons located on chromosome 5q21. Its rat counterpart, Oatp4c1, is also isolated from rat kidney. Human OATP4C1 transports cardiac glycosides (digoxin, K(m) = 7.8 microM and ouabain, K(m) = 0.38 microM), thyroid hormone (triiodothyronine, K(m) = 5.9 microM and thyroxine), cAMP, and methotrexate in a sodium-independent manner. Rat Oatp4c1 also transports digoxin (K(m) = 8.0 microM) and triiodothyronine (K(m) = 1.9 microM). Immunohistochemical analysis reveals that rat Oatp4c1 protein is localized at the basolateral membrane of the proximal tubule cell in the kidney. These data suggest that human OATP4C1/rat Oatp4c1 might be a first step of the transport pathway of digoxin and various compounds into urine in the kidney.

Peptidergic and serotoninergic innervation of the rat dura mater

The peptidergic and serotoninergic innervation of the rat dura mater was investigated by reacting dural wholemounts immunohistochemically with antibodies to calcitonin gene-related peptide (CGRP), substance P (SP), neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP), and serotonin (5-HT). CGRP and SP innervations of the dura were robust and the patterns of distribution of these neuropeptides were essentially the same. The majority of the fibers were perivascular and distributed to branches of the anterior and middle meningeal arteries and to the superior sagittal and transverse sinuses. Other CGRP/SP fibers appeared to end "free" within the dural connective tissue. NPY-immunoreactive fibers were extremely numerous and also distributed heavily to the branches of the meningeal arteries, the venous sinuses, and to the dural connective tissue. The pattern of NPY innervation resembled in many ways that of CGRP/SP; however, NPY innervation of the sinuses was greater and NPY perivascular fibers supplying the meningeal arteries formed more intimate contacts with the walls of the vessels. The pattern of VIP innervation was, in general, similar to that observed for the three previous neuropeptides; however, the overall density was considerably less. Small to moderate numbers of serotoninergic nerve fibers were observed in some, but not all, of the duras processed for 5-HT. The latter fibers were almost exclusively perivascular in distribution. Dural mast cells were prominently stained in the 5-HT preparations because of their serotonin content. Mast cells were also labeled in a nonspecific fashion in some of the tissues reacted immunohistochemically for neuropeptides; some of them were located in close apposition to passing nerve fibers. This study represents, to our knowledge, the first comprehensive work on the peptidergic and serotoninergic innervation of the mammalian dura mater. The results should increase our understanding of the roles that these fibers play in normal dural physiology and of their potential interactions in the pathogenesis of vascular headache.

Clinical pharmacokinetic significance of the renal tubular secretion of digoxin

Tubular secretion appears to be a major route of the renal elimination of digoxin. Secretion of the drug by the tubules is modulated by renal blood flow, by a number of commonly coadministered drugs (e.g. quinidine, spironolactone, verapamil, amiodarone), and by age. The maximal transport capacity does not appear to be achieved with clinically relevant concentrations. The tubular transport of digoxin does not appear to be associated with the anionic or cationic transport systems, nor the Na+/K+-ATPase receptor. Further studies are needed to elucidate the exact mechanisms involved in the transtubular movement of the glycoside.