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

Abstract

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, JMgms, and 11·6 ± 1·4 nmol.cm−2.h−1 from the serosal to mucosal side, JMgsm. The net flux, JMgnet, 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: JMgms, 76·1 ± 8·3; JMgsm, 8·0 ± 1·4; and JMgnet 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: JMgms, 72·1 ± 8·7; JMgsm, 6·5 ± 2·6; and JMgnet, 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: JMgms, 47·4 ± 3·0; JMgsm, 12·3 ± 2·3; and JMgnet, 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 JMgms cannot solely be explained by a reduction in the passive, putative paracellular part of JMgms. © 1987 The Physiological Society