Amiloride-inhibitable Na+ conductance in rat proximal tubule

 Previous single-channel recordings from the luminal membrane of the rabbit proximal tubule have revealed amiloride-inhibitable Na+ channels of a characteristic conductance range. The present study aimed to pursue this issue in rat proximal tubule. Control rats were compared to those put on a low-Na+ diet or pretreated by triamcinolone injections (s.c.). Stimulation of Na+ absorption by glucocorticoids was verified by examining the transepithelial voltage in Ussing chamber studies of the distal colon. The membrane voltage (Vm) of isolated, in-vitro-perfused proximal tubule segments was measured in patch-clamp and impalement studies. It was found that amiloride hyperpolarized Vm significantly by 2.1 ± 0.9 mV (n = 26) in tubules of control rats, by 3.9 ± 0.7 mV (n = 12) in rats put on a low-Na+ diet and by 3.7 ± 1.0 mV (n = 17) in rats treated with glucocorticoids. The effect of amiloride was concentration dependent with a half-maximal effect at < 1 μmol/l. RT-PCR techniques were used to search for the presence of the α-, β- and γ-subunits of the epithelial Na+ channel in isolated oximal tubule segments. The presence of the respective mRNAs was verified. These data indicate that: (1) amiloride-inhibitable Na+ channels are present in rat proximal tubules; (2) the Na+ conductance may be upregulated by Na+ deprivation but is still very limited when compared to total cell conductance; (3) therefore, the contribution of Na+-channel-mediated absorption to total proximal Na+ absorption is probably small.