The apical cell membrane of cultured monolayers of collecting duct principal cells was investigated with the patch-clamp technique to study single ion channels. A broad spectrum of channel events was observed which complicated the analysis. Although principal cells absorb mainly Na+ ions and although most patches must have contained an Na+ conductance as evidenced by asymmetric clamp currents in the presence of amiloride, single Na+ channel events could not be identified with certainty, possibly because single-channel conductance was too low (less than or equal to 5 pS). Instead, non-selective cation channels of 21.2 +/- 4.0 pS were frequently observed. They were impermeable to choline and anions but did not discriminate between Na+ and K+. These channels, however, do not appear to participate in active Na+ absorption. Different types of K+ channels were observed: a high-conductance Ca2(+)-activated K+ channel and a bursting low-conductance K+ channel. Since the former channel has been denied a role in K+ secretion/absorption in native collecting ducts, the latter might be involved. In addition three types of Cl- channels have been observed which will be described separately. At least one of those, a 30-pS outwardly rectifying Cl- channel appears to allow small amounts of Cl- ions to be absorbed across principal cells.

Animals↗

PubMedDate not supplied

What are the driving forces for the proximal tubular H+ and Ca++ transport? The electrochemical gradient for Na+ and/or ATP.

The H+ ion secretion in the proximal tubule as revealed by the reabsorption of the glycodiazine buffer vanishes when the ambient solutions are sodium-free. The same holds for other Na+-dependent transport processes such as Ca++, phosphate, glucose and amino acid reabsorption. If Na+ transport is blocked by ouabain the latter transport processes are abolished, the secretion of H+ ions, however, remains unchanged suggesting H+ to be not exclusively driven by active Na+ transport. These observations agree with electrical measurements which show an electrogenic component of H+ secretion to exist in rat proximal tubule. In experiments with isolated membrane vesicles an electroneutral Na+/H+-exchange mechanism could be demonstrated in the brush border membrane and an ATP-driven Ca++ pumpt as well as Na+-Ca++ countertransport in the baso-lateral cell membrane. These data suggest that both, the Na+ gradient and ATP, are used to drive H+ ion secretion across the luminal brush border and Ca++ reabsorption across the baso-lateral cell side. The biochemical nature of the various systems and their relative importance for the transepithelial ion movement remain to be elucidated.

Adenosine Triphosphate↗

PubMedDate not supplied

Progress in microelectrode techniques for kidney tubules.

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Animals↗