Ca-sensitive Na transport in sheep omasum

Na transport across a preparation of sheep omasum was studied. All tissues exhibited a serosa-positive short-circuit current (I-sc), with a range of 1-4 mu eq.h(-1).cm(-2). A Michaelis-Menten-type kinetic was found between the Na concentration and the I-sc (Michaelis-Menten constant for transport of Na = 6.7 mM; maximal transport capacity of Na = 4.16 mu eq.h(-1).cm(-2)). Mucosal amiloride (1 mM), phenamil (1 or 10 mu), or serosal aldosterone (1 mu M for 6 h) did not change I-sc. Removal of divalent cations (Ca and Mg) enhanced I-sc considerably from 2.61 +/- 0.24 to a peak value of 11.18 +/- 1.1 mu eq.h(-1).cm(-2). The peak Isc (overshoot) immediately declined to a plateau I-sc of similar to 6-7 mu eq.h(-1).cm(-2). Na flux measurements showed a close correlation between changes in I-sc and Na transport. Transepithelial studies demonstrated that K, Cs, Rb, and Li are transported, indicating putative nonselective cation channels, which are inhibited by divalent cations (including Ca, Mg, Sr, Ba) and by (trivalent) La. Intracellular microelectrode recordings from the luminal side clearly showed changes of voltage divider ratio when mucosal divalent cations were removed. The obtained data support the assumption of a distinct electrogenic Na transport mechanism in sheep omasum.