The multidrug resistance P-Glycoprotein (PGP), the product of the human MDR1 and the murine mdr1 or the mdr3 genes is a membrane protein that extrudes chemotherapeutic drugs. It can also transport hydrophobic peptides, steroids and a number of lipophilic substrates. It is expressed in normal organs, including the kidney, where it is thought to mediate excretion of xenobiotics. The purpose of this research is to reach a global understanding of the function of PGP in the kidney. This goal will be achieved through out a series of experiments designed to explore the role of PGP in the handling of exogenous substrates and of two potential endogenous substrates. The following hypotheses are to be tested: 1) That the kidney plays an important role in the excretion of xenobiotics, and that xenobiotic competitors for PGP-mediated transport modulate extrusion of endogenous substrates. 2) That the presence of exogenous substrates (blockers) inhibits the transepithelial transport of potential endogenous substrates. 3) That functional PGP is expressed in the apical domain of proximal tubule cells. 4) That PGP expressed in the apical membrane excrete hydrophobic xenobiotics and hydrophobic endogenous substrates. 5) That xenobiotics inhibit PGP-mediated transport or other substrates, potentially causing cell damage. 6) That prostaglandin E2 (PGE2) is a PGP endogenous substrate in mesangial cells. 7) That active Vitamin D3 is an endogenous substrate in proximal tubule cells. Experiments extend from studies in the whole animal, to the function of tubular segments, the study of transepithelial and cellular transport and, finally, to the direct assessment of transport mechanisms in a simple experimental system, namely yeast membrane vesicles expressing PGP. The methods include clearance, micropuncture, quantitative-fluorescence, radioactive-tracer and molecular-biology techniques. The results of these experiments will provide new information needed to understand the role of PGP in the kidney in the transport of endogenous (PGE2 and active Vitamin D3) and exogenous substrates, and their interactions in mesangial and proximal tubule cells.