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Despite its importance for health and feed-intake of the ruminant, the mechanisms that restore ruminal osmolarity after a hypertonic meal largely remain obscure. Thus, an osmotic challenge with non-ionic agents stimulates influx of water into the rumen, while potassium salts lead to a compensatory stimulation of sodium absorption. This observation is surprising, since the depolarization of the apical membrane by potassium should lead to a decrease in the driving force for postive cations, as is observed experimentally and clinically in the case of magnesium. Since the conductance of the apical membrane of the rumen rises with depolarization, existence of a voltage-dependent channel as an uptake mechanism for sodium has been postulated. This study was performed to gain more information about this pathway.
Methods: Isolated cells from the ruminal epithelium were studied using the patch-clamp technique.
Results: Cells were filled with a standard solution with potassium gluconate, Ca/EGTA and a concentration of magnesium in the upper range (0.9 mM) of concentrations observed physiologically in cells of the ruminal epithelium. Cells were initially superfused NaCl Ringer with physiological concentrations of calcium and magnesium. Replacement of sodium with KCl solution resulted in a significant increase in inward current (by 345 +/- 66%, n = 15, p = 0.007) and a depolarization of the reversal potential (by 33 +/- 5 mV, n = 15, p = 0,000007), corresponding to effects on intact ruminal tissue. Conversely, replacement of sodium with choline in the external solution had no effect on inward current or reversal potential (n = 15, p = 0.09), suggesting that permeability for sodium is negligible.
In a second series of experiments, cytosolic magnesium concentration was reduced by perfusion with magnesium free potassium gluconate solution. Cells were superfused with the same physiological NaCl Ringer. Inward current (p = 0.03) and reversal potential (-20 +/- 3 mV, n = 15) were significantly higher than in the first group of cells (-31 +/- 2 mV, n = 66 and p = 0.01). When NaCl was replaced with choline chloride bath solution, inward current dropped significantly to 75 +/- 4 % of the original level (n = 6, p = 0.002) while reversal potential changed from – 21 +/- 4 mV to –32 +/-3 mV (n = 6, p = 0.001). Thus, reduction of cytosolic magnesium concentration stimulates influx of sodium in ruminal epithelial cells superfused with physiological NaCl Ringer with normal concentrations of divalent cations.
In parallel to observations in isolated epithelium of the rumen, inward current and reversal potential rose significantly when divalent cations were removed from the outside solution. The rise in inward current could be partially blocked by Ca (1.7 mM and higher), Mg (0.9 mM and higher), Ba (5 mM), verapamil (100 microM) and quinidine (100 microM). Amiloride (1 mM) or flufenamic acid ( 10 microM) had no effect.
Conclusion: Influx of sodium into ruminal epithelial cells occurs via a non-selective cation channel that is regulated by changes in the cytosolic concentration of magnesium. We suggest that an elevation of the concentration of ruminal potassium leads to depolarization of the apical membrane, decrease of magnesium absorption, lowering of cytosolic magnesium, and increase in conductance for sodium. This mechanism may have evolved as useful for osmoregulation after an occasional high potassium load in the natural habitat of the ruminant, with complications arising only if high potassium intake persists.