From this point on, as the plasma PAH concentration is increased, PAH clearance continues to decrease and, at very high plasma concentrations, the PAH clearance begins to level off and approaches the inulin clearance asymptotically. This occurs because the amount of plasma cleared of PAH comes to be more and more the result of the amount of PAH filtered. However, although creatinine is freely filtered, it is also secreted to a small extent by the organic cation transport system in the renal tubules. However, this difference is small and the convenience of using endogenous creatinine, which does not have to be infused, more than makes up for this small error. This transporter has recently been cloned.
Energy is expended directly only in the transport of sodium out of the cells.
The exact form is not known and may vary among species. Organic anion secretion occurs primarily in the S2 segment of the proximal tubule. These now appear to be different modes of function of the same transporter which has recently been identified, cloned, and sequenced. Energy is expended directly only in the transport of sodium out of the cells.