H-LINE( SECRETION IS INHIBITED BY CLOSTRIDIAL TOXINS IN AN INNER MEDULLARY COLLECTING DUCT CELL)

Renal epithelial cell H+ secretion is an exocytic-endocytic phenomenon . In the inner medullary collecting duct (IMCD) cell line, which we ha ve utilized as a model of renal epithelial cell acid secretion, me fou nd previously that-acidification increased exocytosis and alkalinizati on increased endocytosis. It is likely, therefore, that the rate of pr oton secretion is regulated by the membrane insertion and retrieval of proton pumps. There is abundant evidence from studies in the nerve te rminal and the chromaffin cell that vesicle docking, membrane fusion, and discharge of vesicular contents (exocytosis) involve a series of i nteractions among so-called trafficking proteins. The clostridial toxi ns, botulinum and tetanus, are proteases that specifically inactivate some of these proteins. In these experiments we demonstrated, by immun oblot and immunoprecipitation, the presence in this IMCD cell line of the specific protein targets of these toxins, synaptobrevin/vesicle-as sociated membrane proteins (VAMP), syntaxin, and synaptosomal-associat ed protein-25 (SNAP-25). Furthermore, we showed that these toxins mark edly inhibit the capacity of these cells to realkalinize after an acid load. Thus these data provide new insight into the mechanism for H+ s ecretion in the IMCD.