Effects of Purinergic Stimulation, CFTR and Osmotic Stress on Amiloride-sensitive Na⁺ Transport in Epithelia and Xenopus Oocytes

Abstract

Both stimulation of purinergic receptors by ATP and activation of the cystic fibrosis transmembrane conductance regulator (CFTR) inhibit amiloride-sensitive Na⁺ transport and activate Cl⁻ secretion. These changes in ion transport may well affect cell volume. We therefore examined whether cell shrinkage or cell swelling do affect amiloride-sensitive Na⁺ transport in epithelial tissues or Xenopus oocytes and whether osmotic stress interferes with regulation of Na⁺ transport by ATP or CFTR. Stimulation of purinergic receptors by ATP/UTP or activation of CFTR by IBMX and forskolin inhibited amiloride-sensitive transport in mouse trachea and colon, respectively, by a mechanism that was Cl⁻ dependent. When exposed to a hypertonic but not hypotonic bath solution, amiloride-sensitive Na⁺ transport was inhibited in mouse trachea and colon, independent of the extracellular Cl⁻ concentration. Both inhibition of Na⁺ transport by hypertonic bath solution and ATP were additive. When coexpressed in Xenopus oocytes, activation of CFTR by IBMX and forskolin inhibited the epithelial Na⁺ channel (ENaC) in a Cl⁻dependent fashion. However, both hypertonic and hypotonic bath solutions showed only minor effects on amiloride-sensitive conductance, independent of the bath Cl⁻ concentration. Moreover, CFTR-induced inhibition of ENaC could be detected in oocytes even after exposure to hypertonic or hypotonic bath solutions. We conclude that amiloride-sensitive Na⁺ absorption in mouse airways and colon is inhibited by cell shrinkage by a mechanism that does not interfere with purinergic and CFTR-mediated inhibition of ENaC.