Relations among Transepithelial Sodium Transport, Potassium Exchange, and Cell Volume in Rabbit Ileum HUGH
N. N E L L A N S and S T A N L E Y G. S C H U L T Z
From the Department of Physiology, University of Pittsburgh; School of Medicine, Pittsburgh, Pennsylvania 15261. Dr. Nellans's present address is the Departments of Medicin~ and Physiology, Rochester University, Schools of Medicine and Dentistry, Rochester, New York 14642.
AB ST R AC T The relation between active transepithelial Na transport across rabbit ileum and 4ZK exchange from the serosal solution across the basolateral membranes has been explored. Although 42K influx across the basolateral membranes is inhibited by ouabain and by complete depletion of cell Na, it is not affected when transepithelial Na transport is abolished (i.e. in the presence of an Na-free mucosal solution) or stimulated (i.e. when glucose or alanine is added to the mucosal solution). We are unable to detect any relation between the ouabain-sensitive Na-K exchange mechanism responsible for the maintenance of intracellular Na and K concentrations and active transcellular Na transport. In addition, the maintenance f of cell volume (water content) does not appear to be dependent upon transepithelial Na transport or the ouabain-sensitive Na-K exchange pump. Although the results of these studies cannot be considered conclusive, they raise serious questions regarding the role of the Na-K exchange pump, located at the basolateral membranes, in active transepithelial Na transport and the maintenance of cell volume.
In 1958, K o e f o e d - J o h n s e n and Ussing p r o p o s e d a model for Na transport by isolated f r o g skin that has provided a m a j o r conceptual f r a m e w o r k for the interpretation o f studies on a variety o f epithelia (1, 2). A central feature o f this model is the notion that the same mechanism is responsible for homocellular a n d transceUular Na transport. T h a t is, an active Na-K e x c h a n g e m e c h a n i s m located at the i n n e r m e m b r a n e was p r e s u m e d to be responsible for maintaining the low intracellular Na concentration and the high intracellular K concentration characteristic o f most animal cells, as well as active transcellular Na transport. Some findings on in vitro rabbit ileum are consistent with this notion. T h u s , ouabain inhibits active Na absorption only when it is present in the serosal solution; the presence o f this glycoside in the mucosal solution alone is ineffective (3). A n d , c o n c o m i t a n t with the inhibition o f transepithelial Na transport there is a loss o f cell K a n d a gain o f cell Na (4). F u r t h e r , a u t o r a d i o g r a p h i c studies (5) a n d enzymic analyses o f m e m b r a n e f r a g m e n t s isolated f r o m intestinal cells (6, 7) have localized ouabain binding sites and the ouabain-sensitive, Na-K stimulated A T P a s e to the basolateral m e m b r a n e s ; little or no ouabain binding or N a - K - A T P a s e activity is f o u n d in the b r u s h b o r d e r . T H E J O U R N A L OF G E N E R A L P H Y S I O L O G Y • V O L U M E 6 8 ,
1976 •
pages
441-463
441
442
THE JOURNAL
OF G E N E R A L P H Y S I O L O G Y
" VOLUME
68
• 1976
Rose and Schultz (8) and Frizzell and Schultz (9) have p r e s e n t e d evidence that the active Na extrusion mechanism at the basolateral m e m b r a n e s o f rabbit ileum is rheogenic (or electrogenic) and thus c a n n o t involve a o n e - f o r - o n e exchange o f cell Na for K in the serosal solution. H o w e v e r , this conclusion per se does not exclude the possibility that Na extrusion f r o m the cell is mediated by an Na-K e x c h a n g e m e c h a n i s m since it is well established for erythrocytes (10) and nerve a n d muscle (11) that, in general, the stoichiometry o f the Na-K exchange p u m p is not o n e - f o r - o n e but that the N a : K ratio often exceeds unity. Finally, Schultz and Zalusky (3) r e p o r t e d that active Na t r a n s p o r t by rabbit ileum is not affected when the tissue is bathed for m o r e than 1 h by "nominally" K-free media (
N. N E L L A N S and S T A N L E Y G. S C H U L T Z
From the Department of Physiology, University of Pittsburgh; School of Medicine, Pittsburgh, Pennsylvania 15261. Dr. Nellans's present address is the Departments of Medicin~ and Physiology, Rochester University, Schools of Medicine and Dentistry, Rochester, New York 14642.
AB ST R AC T The relation between active transepithelial Na transport across rabbit ileum and 4ZK exchange from the serosal solution across the basolateral membranes has been explored. Although 42K influx across the basolateral membranes is inhibited by ouabain and by complete depletion of cell Na, it is not affected when transepithelial Na transport is abolished (i.e. in the presence of an Na-free mucosal solution) or stimulated (i.e. when glucose or alanine is added to the mucosal solution). We are unable to detect any relation between the ouabain-sensitive Na-K exchange mechanism responsible for the maintenance of intracellular Na and K concentrations and active transcellular Na transport. In addition, the maintenance f of cell volume (water content) does not appear to be dependent upon transepithelial Na transport or the ouabain-sensitive Na-K exchange pump. Although the results of these studies cannot be considered conclusive, they raise serious questions regarding the role of the Na-K exchange pump, located at the basolateral membranes, in active transepithelial Na transport and the maintenance of cell volume.
In 1958, K o e f o e d - J o h n s e n and Ussing p r o p o s e d a model for Na transport by isolated f r o g skin that has provided a m a j o r conceptual f r a m e w o r k for the interpretation o f studies on a variety o f epithelia (1, 2). A central feature o f this model is the notion that the same mechanism is responsible for homocellular a n d transceUular Na transport. T h a t is, an active Na-K e x c h a n g e m e c h a n i s m located at the i n n e r m e m b r a n e was p r e s u m e d to be responsible for maintaining the low intracellular Na concentration and the high intracellular K concentration characteristic o f most animal cells, as well as active transcellular Na transport. Some findings on in vitro rabbit ileum are consistent with this notion. T h u s , ouabain inhibits active Na absorption only when it is present in the serosal solution; the presence o f this glycoside in the mucosal solution alone is ineffective (3). A n d , c o n c o m i t a n t with the inhibition o f transepithelial Na transport there is a loss o f cell K a n d a gain o f cell Na (4). F u r t h e r , a u t o r a d i o g r a p h i c studies (5) a n d enzymic analyses o f m e m b r a n e f r a g m e n t s isolated f r o m intestinal cells (6, 7) have localized ouabain binding sites and the ouabain-sensitive, Na-K stimulated A T P a s e to the basolateral m e m b r a n e s ; little or no ouabain binding or N a - K - A T P a s e activity is f o u n d in the b r u s h b o r d e r . T H E J O U R N A L OF G E N E R A L P H Y S I O L O G Y • V O L U M E 6 8 ,
1976 •
pages
441-463
441
442
THE JOURNAL
OF G E N E R A L P H Y S I O L O G Y
" VOLUME
68
• 1976
Rose and Schultz (8) and Frizzell and Schultz (9) have p r e s e n t e d evidence that the active Na extrusion mechanism at the basolateral m e m b r a n e s o f rabbit ileum is rheogenic (or electrogenic) and thus c a n n o t involve a o n e - f o r - o n e exchange o f cell Na for K in the serosal solution. H o w e v e r , this conclusion per se does not exclude the possibility that Na extrusion f r o m the cell is mediated by an Na-K e x c h a n g e m e c h a n i s m since it is well established for erythrocytes (10) and nerve a n d muscle (11) that, in general, the stoichiometry o f the Na-K exchange p u m p is not o n e - f o r - o n e but that the N a : K ratio often exceeds unity. Finally, Schultz and Zalusky (3) r e p o r t e d that active Na t r a n s p o r t by rabbit ileum is not affected when the tissue is bathed for m o r e than 1 h by "nominally" K-free media (
