Oertzenweg 19 b
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of the omasum, mainly its absorptive function. It has been shown in vivo that Na, HCO3 and SCFA in addition to other electrolyte are absorbed from the omasum. The transport of Na takes place via two parallel mechanisms: (a) The electrogenic transport which is probably represented by the short circuit current (Isc) and is enhanced by the removal of divalent cations and abolished by Na substitution and ouabain (Schultheiß and Martens, 1999). (b) The electroneutral one which is mediated via Na+/II+ exchange. The electroneutral component of Na ion transport system exhibits interactions with Cl- and HCO3 - and it is reduced by mucosal addition of 1 imnol/l amiloride (Martens and Gäbel, 1988). However, amiloride does not completely abolish electroneutral Na ion transport which might indicate that (i) the concentration of amiloride was not sufficient to abolish completely NHE, that (ii) an amiloride-resistant NHE or that (iii) a further mechanism(s) of electroneutral Na transport exists. It has been shown that Jsm Na in the omasum is higher than total tissue conductance (Gt) which may indicate that part of Na back flow is transported transcellular via an electroneutral mechanism (Martens and Gäbel, 1988). The transport of SCFA in the omasum has not been yet characterised. It was therefore, the aim of the current study to further characterise electroneutral Na transport and SCFA transport and to establish a model for the interaction between Na, Cl, and SCFA transport. The findings of the study: 1. Na transport a. Na electroneutral transport 1. Lowering mucosal pH from 7.4 to 6.4 significantly enhanced Na flux rates. 2. Amiloride at the mucosal side decreased Na transport in a concentration dependent manner. However, high concentration of amiloride (1.5 mmol/l) did not cause further reduction in Na transport and did not completely abolish Na electroneutral transport. This may indicate that 1 mmol/l is enough and that another mechanism of electroneutral Na transport exists. 3. Bicarbonate substitution or 0.1 mmol/l ethoxyzolamide at the mucosal side did not affect Na flux rates, excluding a direct interaction between HC03 - and Na. 4. Luminal substitution of Cl ion significantly decreased Na transport rates and supports the assumption of a direct link between Na and Cl transport in the omasum. 5. This hypothesis was confirmed by addition of hydrochlorothiazide (HCTZ).