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<b>Introduction</b>: High dietary intake of potassium leads a marked increase in sodium absorption1. Simultaneously, the rate of magnesium uptake into ruminal epithelial cells2 and across the ruminal epithelium 3 is reduced, which can lead to grass tetany if high potassium intake continues.
<br>Previous experiments4 suggest that the rumen epithelium expresses a non-selective cation channel that is regulated external divalent cations. In this study, we were interested in investigating if cytosolic Ca++ and Mg++ concentrations alter sodium conductance in ruminal cells.
<br><b>Methods</b>: Experiments were carried out with primary cultures of sheep ruminal epithelial cells using the patch-clamp technique. The effect of divalent cations on channel conductance was examined by varying the concentration of Ca++ and Mg++ in the external and internal solutions. Both potassium gluconate and choline chloride were used.
<br><b>Results</b>: Removal of divalent cations from the external solution induced a highly significant increase in inward current at negative potential levels in the presence of external sodium and potassium, but not choline. This inward current could be blocked by 5 mM barium, but not by 5 mM TEACl. Depending on the pipette solution, removal of external divalents changed reversal potential from –30  3 mV to –20  3 mV (potassium gluconate +Ca++ +Mg++, n = 14, p = 0.001), from –23  4 mV to –16  2 mV (potassium gluconate Ca++ Mg++, n = 14, p = 0.04), from 6  2 mV to 10  2 mV (choline chloride +Ca++ +Mg++, n = 11, p = 0.007) and from 5  2 to 6  1 mV (choline chloride +Ca++ Mg++, n = 13, p = 0.0001). Removal of external Mg++ alone had a smaller effect. Relative permeability PK/PNa of the conductance varied between potassium gluconate and choline chloride pipette solution (2.1 versus 1.4).
<br>Removal of internal Mg++ from the choline chloride pipette solution resulted in a highly significant rise in inward current level in divalent containing external NaCl solution and significant rises in both inward and outward current in divalent free solution, corresponding to a drop in channel rectification.
<br>The component of current that was sensitive to external divalent cations could be calculated by subtracting the current in external divalent–containing NaCl solution from that in divalent-free NaCl solution. Comparison of the data for the different pipette solutions showed that the chloride concentration of the pipette solution had no impact on the divalent-sensitive current, while the conductance level and the degree of inward rectification changed with the concentration of Ca++ and Mg++ in the pipette solution.
<br><b>Conclusions</b>: Ruminal epithelial cells express a channel that conducts both sodium and potassium and opens when either cytosolic or external levels of Ca++ or Mg++ are reduced.
We propose that high ruminal potassium leads to an enhancement of sodium absorption through this channel that is mediated by a drop in cytosolic magnesium. This mechanism should be useful in ensuring osmoregulation after an occasional high-potassium meal. Side effects due to the reduction in magnesium uptake should only become apparent if ruminal potassium concentrations remain high for extended periods of time, a condition unknown in the physiological habitate of the ruminant before the introduction of artificial fertilizing techniques.
<br>1. Sellers AF, Dobson A. Studies on reticulo-rumen sodium and potassium concentration and electrical potentials in sheep. Res Vet Sci 1960;1:95.
<br>2. Schweigel M, Lang I, Martens H. Mg(2+) transport in sheep rumen epithelium: evidence for an electrodiffusive uptake mechanism. Am J Physiol 1999;277:G976-982.
<br>3. Leonhard-Marek S, Martens H. Effects of potassium on magnesium transport across rumen epithelium. Am J Physiol 1996;271:G1034-1038.
<br>4. Lang I, Martens H. Na transport in sheep rumen is modulated by voltage-dependent cation conductance in apical membrane. Am J Physiol 1999;277:G609-618.