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Abstract:
Until recently the kidney was not among the organs favored for research on the molecular mechanisms of transport processes. The key experiments for exploring the molecular events of sugar and amino acid transport, for instance, were performed on intestinal and Ehrlich ascites tumor cells. The interest of the scientist studying molecular transport in the kidney rose rapidly, however, since the organ served as a source for transport enzymes such as the Na-K-ATPase or the glucose binding protein and especially since it became possible to obtain closed vesicles from either cell side of the proximal tubule. With these vesicles the transcellular transport steps could be studied separately without interference by cellular metabolism. Previously or simultaneously performed microperfusion and electrophysiological studies served as a basis for, or were at least complementary to, experiments with tubular membrane vesicles. At once some similarities with the corresponding intestinal transport processes became apparent. Although our knowledge is growing rapidly so that many more answers can be expected in the near future, the main principle for transtubular hexose and amino acid transport is already evident: co-transport with sodium (secondary active transport) at one cell side. The transport step at the other cell side proceeds also by a carrier, but with a different specificity.
The situation with organic acids, if they are not transported by nonionic diffusion, and with the organic bases is less clear, but a similar principle, possibly a whole chain of countertransport processes, may also hold for them.
Because in this review, results gained largely from double-perfused kidney tubules and from membrane vesicles are reported, the corresponding methods and their potency should be discussed.
