A calcium-sensitive, elektrogenic Na transport was studied in isolated preparations of sheep rumen epithelium with the Ussing chamber method.
1. Removal of divalent cations (Ca2+ \+ Mg2+) from the mucosal solution enhances the Isc considerably. This increase of Isc is reversibly blocked by repeated addition of Ca2+ (2 mM) to the mucosal solution. 2. The determination of Na flux rates with and without Ca2+ in the mucosal solution revealed that the Ca-sensitiv Isc accounts for increase of JnetNa which supports the assumption that removal of Ca2+ from the mucosal solution causes an activation of a Ca-sensitive, electrogenic Na transport. In the absence of Ca2+ increasing Na concentrations enhanced Isc without saturation. 3. The Ca-sensitive change of Isc is accompanied by a corresponding alteration of Gt. 4. Removal of Ca2+ from the mucosal solution did not change fluxes of mannitol as a marker for paracelluler permeability. 5. The putative, Ca-sensitive Na conductance in the luminal membran is very unspecific. Total replacement of Natrium in the mukosal solution by Rb, K+, Cs+ or Li+ caused in all cases a Ca-sensitive current: Rb+ > Na+ > Cs+ > K+ > Li+. 6. The Ca-sensitive Isc is inhibited by mikromolar Ca concentrations (Km = 0,28µM). 7. Mucosal addition of amilorid (1 - 100 µM) did not change Ca-sensitive Isc nor serosal addition of aldosterone (1µM). 8. Theophylline decreases the Isc in the presence and in the absence of mucoal divalent cations.
The obtained data support the assumption that removal of divalent cations from the mucosal solution activates a nonspecific cation conductance (NSCC) which remarkably resembles NSCC in other epithelia from the gastro-intestinal-tract.