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 Mv 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 micromol/l. RT-PCR techniques were used to search for the presence of the alpha-, beta- and gamma-subunits of the epithelial Na+ channel in isolated proximal tubule segments. The presence of the respective mRNAs was verified. These data indicate that: (1) amiloride-inhibitable Na+ channels are present in rate proximal tubules; (2) the Na+ conductance may be up-regulated 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.
Willmann
J K
JK
Physiologisches Institut der Albert-Ludwigs-Universitat, Hermann-Herder-Strasse 7, D-79104 Freiburg i. Br., Germany.
Bleich
M
M
Rizzo
M
M
Schmidt-Hieber
M
M
Ullrich
K J
KJ
Greger
R
R
D000584:Amiloride / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D005260:Female; D007687:Kidney Tubules, Proximal / Q000187:drug effects*; D018408:Patch-Clamp Techniques; D051381:Rats; D015222:Sodium Channels / Q000187:drug effects*
D015222:Sodium Channels; D000584:Amiloride