Divalent cations reduce the electrogenic transport of monovalent cations across rumen epithelium

Abstract.

The rumen epithelium of sheep and goats showed an increase in short circuit current (Isc) and transepithelial conductance (gt) upon mucosal removal of divalent cations. A divalent-sensitive Isc and gt were present in Na⁺, K⁺ or Rb⁺ buffer, but nearly abolished in mucosal NMDG⁺ (N-methyl-D-glucamine) buffer. High K buffer, addition of BaCl₂ or of ouabain on the serosal side also reduced or abolished the divalent-sensitive Isc. Mucosal Ca²⁺ was more potent in blocking Isc, but had the same potency as Mg²⁺ in blocking gt. A prolonged mucosal deprivation of Mg²⁺ ions increased gt, potential difference and basal as well as the Ca²⁺-sensitive Isc. Mucosal addition of Mg²⁺ had a smaller effect on gt after serosal preincubation with Ba. The data suggest that rumen epithelial cells exhibit an apical non-selective cation conductance, which permits the passage of monovalents in the mucosal absence of divalents. The development of a divalent-sensitive Isc in Na buffer requires Na⁺/K⁺ pumps and K⁺ recycling through Ba²⁺-sensitive K⁺ conductances on the basolateral side. This Isc is blocked by extracellular Ca²⁺ and both extracellular and intracellular Mg²⁺ ions. A prolonged deprivation of mucosal Mg²⁺ alone seems to affect intracellular Mg²⁺ in this Mg²⁺-absorbing tissue.