THE EFFECT OF POTASSIUM AND THE TRANSMURAL POTENTIAL DIFFERENCE ON MAGNESIUM TRANSPORT ACROSS AN ISOLATED PREPARATION OF SHEEP RUMEN EPITHELIUM

Isolated mucosal sheets of sheep rumen were studied in vitro in Ussing chambers using a computer‐controlled voltage clamp. Undirectional Mg 2+ fluxes were measured at high (70 mmol.l −1 ) or low (5 mmol.l −1 ) mucosal concentrations of K + under open‐ or voltage‐clamp conditions. A high mucosal concentration of K + caused a potential difference (p.d.) of 25·1 ± 1·8 mV, blood side positive. The Mg 2+ fluxes were 52·2 ± 6·5 nmol.cm −2 .h −1 from the mucosal to serosal side, J Mg ms , and 11·6 ± 1·4 nmol.cm −2 .h −1 from the serosal to mucosal side, J Mg sm . The net flux, J Mg net , was 40·6 ± 5·9 nmol.cm −2 .h −1 . When, at high mucosal K + concentrations the p.d. was abolished by an external current (short‐circuit conditions) significantly different flux data were obtained: J Mg ms , 76·1 ± 8·3; J Mg sm , 8·0 ± 1·4; and J Mg net 68·1 ± 7·7 nmol.cm −2 .h −1 . The results of the low (control) and high mucosal K + group did not differ significantly when Mg 2+ fluxes were measured under short‐circuit conditions: J Mg ms , 72·1 ± 8·7; J Mg sm , 6·5 ± 2·6; and J Mg net , 65·5 ± 9·0 nmol.cm −2 .h −1 . An increase of the p.d. to 26·6 ± 2·0 mV (control) by an external current led to significant alterations of the fluxes in the control group: J Mg ms , 47·4 ± 3·0; J Mg sm , 12·3 ± 2·3; and J Mg net , 35·2 ± 4·1 nmol.cm −2 .h −1 . Although it is suggested that an increased p.d. caused an enhanced passive flux of Mg 2+ from the serosal to the mucosal side through the shunt pathway, the magnitude of the decrease in J Mg ms cannot solely be explained by a reduction in the passive, putative paracellular part of J Mg ms .