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

¹ Department of Physiology, School of Veterinary Medicine, Bischofsholer Damm 15, D - 3000 Hannover 1, F.R.G.

Isolated mucosal sheets of sheep rumen were studied in vitro in Ussing chambers using a computer-controlled voltage clamp. Undirectional Mg²⁺ 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 Mg²⁺ 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 Mg²⁺ 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 Mg²⁺ 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.

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