What are the driving forces for the proximal tubular H+ and Ca++ transport? The electrochemical gradient for Na+ and/or ATP.

The H+ ion secretion in the proximal tubule as revealed by the reabsorption of the glycodiazine buffer vanishes when the ambient solutions are sodium-free. The same holds for other Na+-dependent transport processes such as Ca++, phosphate, glucose and amino acid reabsorption. If Na+ transport is blocked by ouabain the latter transport processes are abolished, the secretion of H+ ions, however, remains unchanged suggesting H+ to be not exclusively driven by active Na+ transport. These observations agree with electrical measurements which show an electrogenic component of H+ secretion to exist in rat proximal tubule. In experiments with isolated membrane vesicles an electroneutral Na+/H+-exchange mechanism could be demonstrated in the brush border membrane and an ATP-driven Ca++ pumpt as well as Na+-Ca++ countertransport in the baso-lateral cell membrane. These data suggest that both, the Na+ gradient and ATP, are used to drive H+ ion secretion across the luminal brush border and Ca++ reabsorption across the baso-lateral cell side. The biochemical nature of the various systems and their relative importance for the transepithelial ion movement remain to be elucidated.