4.4.1 Electrogenic transport
Effect of mucosal K concentration on transepithelial current
Isolated epithelia were incubated in standard buffer solution on the serosal side and in Na-free buffer on the mucosal side. Increasing the mucosal K concentration from 4 to 100 mmol·l-1 resulted in an increase of current (Δ Isc) across the rumen of goat, sheep and cattle, the jejunum of goat and sheep, as well as across the colon of goat and sheep. Across the abomasum of goat and sheep an increasing K concentration had no effect (Fig. 1).
To investigate, if the increase in current was correlated with the K concentration we measured the Isc at mucosal K concentrations of 4, 25, 50 and 100 mmol·l-1 in goat epithelia. A linear increase (p<0.05) was observed for the rumen (y = 0.01±0.00 x + 0.06±0.03, R²=0.99), the jejunum (y = 0.02±0.00 x - 0.91±0.25, R²=0.87), and the colon (y = 0.03±0.00 x + 0.38±0.28, R²=0.93). In abomasum an increase in K concentration did not change the K current (y = 0.00±0.00 + 4.49±0.20, R²=0.06, p>0.05) (Fig.2).
Effect of Na on the K current
Isolated epithelia were incubated in standard buffer solution in the pre- or absence of Na on the mucosal side (Fig. 3). The mucosal K concentration changed between 4 and 50 mmol·l-1. In the presence of Na, the basal currents were higher across rumen, jejunum and colon than in the absence of mucosal Na. The K dependent current remained in the same order of magnitude across rumen epithelia. In jejunum, the K dependent current disappeared in the presence of Na. In colon, the K dependent current increased in the presence of Na. The currents across abomasal epithelia were slightly higher in the presence of Na with 50 mmol·l-1 K, but did not differ with 4 mmol·l-1K.
4.4.2 K versus Rb currents
To investigate if Rb could be a reliable marker for K transport in the gastrointestinal tract of ruminants we compared K with Rb currents across the different epithelia.
Increasing the Rb concentration from 4 to 100 mmol·l-1 increased the Rb currents in rumen (sheep: Δ Isc 0.70±0.13, N = 9; cattle: Δ Isc 1.22±0.11, N = 8) and colon (sheep: Δ Isc 1.17±0.25, N = 9), as shown for K above. We then compared K induced currents with Rb induced currents, measured in the same epithelia. Averaged over all epithelia and all concentrations the Rb current amounted to 27±7 % (sheep rumen, n = 18, R² = 0.49), 34±5 % (sheep colon, n = 18, R² = 0.70) or 48±6 % (cattle rumen, n = 16, R² = 0.82) of the respective K current (Fig. 4). A direct comparison of the increases in currents induced by an increase in K or Rb concentration within the same epithelia (Δ Isc) did not show any correlation between K and Rb in the rumen of cattle (Fig. 4). Thus, since the Rb current reflected only a part of the K current (or showed no correlation at all) we went on to determine K absorption directly.
4.4.3 K absorption
At a mucosal K concentration of 4 mmol·l-1 the serosal K concentration decreased during the incubation period in rumen and jejunal epithelia, while the mucosal K concentration increased in the same order of magnitude (Fig. 5). In abomasum and colon, changes in the serosal K concentration did not differ from zero during
incubation at 4 mmol·l-1, while the mucosal K concentration increased. A comparison between the first and the second incubation period at 4 mmol·l-1 showed no differences in these changes. At a mucosal K concentration of 100 mmol∙l-1 the serosal K concentration increased in all epithelia (Fig. 6), indicating an increase in K absorption when changing the luminal K concentration from 4 to 100 mmol·l-1. This was accompanied by a decrease in mucosal K concentration. The mucosal decrease in K concentration was 2.4 to 8.5 fold higher than the parallel increase in serosal K concentration with a standard error of 28 to 67 % (rumen: 8.73±5.83 mmol∙l-1, abomasum: 15.45±5.73 mmol∙l-1, jejunum: 9.53±4.74 mmol∙l-1, colon:
22.15±6.25 mmol∙l-1). Therefore, the increase in K absorption due to an increase in mucosal K concentration was calculated from the change in serosal K concentration.
The resulting increases in K absorption from 4 to 100 mmol·l-1 (Δ absorption) were significantly higher in jejunum than in rumen and colon (p<0.05), while the other increases in K absorption did not differ from each other (Fig. 7). The absorption rates at a mucosal K concentration of 100 mmol·l-1 in contrast, were significantly lower in the rumen than in the other epithelia. In rumen and colon, the increase in K absorption was equivalent to the parallel increase in current (Δ Isc). In abomasum and jejunum the increase in K absorption was much higher than the increase in current (Fig. 7). In rumen and colon, where the current appeared to express the K absorption we tested for a linear correlation between Isc and K absorption. In rumen, the increase in K absorption amounted to 80±22 % (R² = 0.32) of the increase in K current. In colon, the increase in K absorption amounted to 69±17 % (R² = 0.46) of the increase in K current (Fig. 8).
4.4.4 Pathways of K transport
To discriminate between paracellular and transcellular pathways of K transport we added blockers of the paracellular pathway and of K channels to the buffer solution (Fig. 9). The mucosal buffer solution contained 100 mmol·l-1K in the absence of Na.
As a blocker of the paracellular pathway TAP was added to the mucosal and serosal buffer solution. TAP decreased the K current across the rumen by 26±8 % (sheep) and by 29±7 % in jejunum (sheep), in abomasum and colon TAP had no effect on the K current.
To block the transcellular pathway barium chloride as a general blocker of most K channels and verapamil as a blocker of luminal K channels in the rumen were added to the mucosal buffer solution. Mucosal addition of barium chloride decreased the K current across rumen by 19±5% after mucosal and by 34±3 % after additional serosal application (cattle). In abomasum the K current decreased by 28±7 % after mucosal addition of barium chloride (sheep). In jejunum mucosal addition of barium chloride decreased the K current by 44±12 % (sheep). Barium chloride did not show any effect in the colon. After mucosal addition of verapamil the K current decreased by 68±4 % in rumen (sheep).