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Rat Kidney Plasma Membranes; Brushborder; Basal Infoldings; Mg++−Na+−K+-ATPase; Transepithelial Sodium Transport
Abstract:
Plasma membrane fractions of rat kidney cortex were subdivided by centrifugation on a continuous and a discontinuous sucrose gradient and by carrier free continuous electrophoresis. In the different fractions the activity of alkaline phosphatase and aminopeptidase, enzymes which are present in the brushborder membrane, as well as Mg++-ATPase, Na+−K+-ATPase, 5′-nucleotidase, acid phosphatase and glucose-6-phosphatase were determined.
The distribution of alkaline phosphatase, aminopeptidase and 5′-nucleotidase is identical, indicating the localization of these enzymes in the brushborder membrane. Na+−K+-ATPase does not show an identical distribution with any of the enzymes studied.
Using carrier free continuous electrophoresis fractions can be obtained which are enriched in alkaline phosphatase by a factor of 15 when compared to the cortex homogenate, whereas the specific activity of Na+−K+-ATPase is reduced to one third of the starting material. On the other hand fractions can be isolated in which the specific activity of Na+−K+-ATPase is 16 times higher than in the homogenate. No enrichment of alkaline phosphatase occurs in these fractions.
It is therefore concluded that the Na+−K+-ATPase is not present in the brushborder membrane nor in the lysosomes or endoplasmatic reticulum. The most probable localization of the Na+−K+-ATPase are the basal infoldings of the plasma membranes of the cells.
