Changes in Membrane Surfaces of Collecting Duct Cells in Potassium Adaptation

Chronic potassium loading results in an increased capacity of the distal nephron to secrete potassium. The cellular mechanism for this adaptation has been correlated to an increase in the activity of sodium-potassium-ATPase. Because adaptation may be dependent on the greater availability of potassium pumps in the basolateral membrane, a stereologic analysis of the membrane surface area was performed to determine whether the apparent increase in pump sites was due to an increase in cell membrane surface. With potssium adaptation, the number of microplicated cells in the outer medulla was reduced from 31 to 18%. There was a marked increase in the basolateral infolding of principal cells, and membrane surface increased by 32%. In papillary collecting duct cells, the basolateral membrane surface was unchanged but the surface density of the luminal membrane increased by 50%. These observations suggest that amplification of the basolateral cell membrane to increase the number of potassium pump sites per cell plays an important role in the mechanism of potassium adaptation in the outer medulla. A different mechanism involving the luminal membrane operates in the papillary collecting duct. Structural alterations in cell membrane surfaces are thus related to the regulation of the epithelial transport of electrolytes.