Evidence for Na⁺ independent active secretion of K⁺ and HCO ⁻₃ by rat salivary duct epithelium

Summary

In order to elucidate whether or not active secretion of potassium and bicarbonate by the rat submaxillary duct epithelium operates independently of sodium reabsorption, Na⁺ transport was blocked by amiloride, which is known to inhibit Na⁺ entry from lumen into cell.

With 10⁻⁴ M amiloride in HCO ⁻₃ -Ringer at the luminal side, the transepithelial electrical potential difference approached zero, the Na⁺ conductance of the luminal cell membrane was drastically reduced, and the K⁺ conductance was significantly reduced. Net K⁺ secretion was reduced by 80%, whereas net HCO ⁻₃ secretion was significantly increased. The remaining 20% of net K⁺ secretion proceeded at zero net Na⁺ transport and in the absence of significant chemical and electrical potential differences between lumen and interstitium of the duct. This active component of net K⁺ secretion was accompanied by an equal rate of active HCO ⁻₃ secretion.

These findings confirm the independence of this active secretion of K⁺ and HCO ⁻₃ from Na⁺ transport. They indicate an electrically neutral secretion of K⁺ and HCO ⁻₃ , probably by the postulated luminal K⁺−H⁺-exchange mechanism. The 80% of net K⁺ secretion, which were abolished by amiloride together with Na⁺ reabsorption, seem to be functionally coupled with Na⁺ transport. The linkage of K⁺-to-Na⁺ is probably mediated by a luminal carrier exchanging Na⁺ for K⁺ and H⁺.