A ubiquitous non-selective cation channel in the mouse renal tubule with variable sensitivity to calcium

Abstract

Basolateral membranes of microdissected collagenase-treated fragments of renal tubules from the mouse were examined using the cell-attached and the cell-free variants of the patch-clamp technique. With a K⁺-rich solution in the pipette, a highly active, inwardly rectifying K⁺ channel was observed on intact cells of the cortical collecting tubule (CCT). The mean inward and outward conductances were 38.5±3.1 pS and 17.3±1.8 pS, respectively (n=4). In contrast, cell-attached patches were usually inactive when a Na⁺-rich solution filled the patch pipette. However, another type of channel with a conductance of 20–30 pS exhibited a sparse activity in 4/20 CCT. In excised, inside-out patches, the most frequent channel in CCT had an ohmic unit conductance of 27.1±1.2 pS (n=17), excluded anions (P _Cl /P _Na=0.09), discriminated little between NH₄ ⁺, K⁺ and Na⁺ (P _NH4 /P _Na=1.5;P _K /P _Na=0.9), and was much less permeable to Ca²⁺ and Ba²⁺ than to Na⁺ (P _Ca /P _Na=0.09;P _Ba /P _Na≈0). The cation channel was moderately voltagedependent, showing a decreased open probability (P _o) at negative voltages. It was activated by internal calcium (threshold: 1 μmol/l–0.1 mmol/l calcium), and inhibited by the adenine nucleotides ATP, ADP and AMP with half-maximal inhibition ofP _o at 1.2 umol/l AMP. As in other cell models, 3′,5′-dichlorodiphenylamine-2-carboxylic acid blocked channel activity when added to the internal surface of the membrane patch. Extending our study to other parts of the renal tubule, we found that the basolateral membranes of the proximal (pars recta), distal convoluted, connecting and outer medullary collecting tubules, the thin descending limb and the medullary thick ascending limb all contained a similar Ca- and ATP-sensitive cation channel. The calcium sensitivity varied from one part to another.