Natriuretic Peptides and Diadenosine Polyphosphates Modulate pH Regulation of Rat Mesangial Cells
Modulation of cell proliferation has often been thought to be connected to changes in the activity of pH-regulatory transporters and consequently intracellular pH (pH<sub>i</sub>). The influence of natriuretic peptides, diadenosine polyphosphates, adenosine and ATP as well as platelet-derived growth factor (PDGF) on pH<sub>i</sub> regulation of cultured rat mesangial cells was examined with the pH-sensitive dye 2′,7′-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. The inhibitors of Na<sup>+</sup>/H<sup>+</sup> exchange, amiloride and HOE694, blocked pH<sub>i</sub> recovery completely in the absence of and by approximately 50% in the presence of HCO<sub>3</sub><sup>–</sup>/CO<sub>2</sub>. Natriuretic peptides (ANP, BNP, CNP, urodilatin) completely inhibited pH<sub>i</sub> recovery in the absence of and by approximately 40% in the presence of HCO<sub>3</sub><sup>–</sup>/CO<sub>2</sub>. These effects were abolished by the cGMP-dependent protein kinase inhibitor KT5823. Diadenosine polyphosphates (Ap3A-Ap6A), ATP and adenosine also inhibited pH<sub>i</sub> recovery completely in the absence of and partially (30–40%) in the presence of HCO<sub>3</sub><sup>–</sup>/ CO<sub>2</sub>. The effect of adenosine was abolished in the presence of the cAMP-dependent protein kinase inhibitor KT5720, and that of Ap5A by the protein kinase C inhibitor calphostin C. PDGF activated acid extrusion in these cells by approximately 40%. From the four cloned isoforms of the Na<sup>+</sup>/H<sup>+</sup> exchanger in the rat, only transcripts of NHE-1 were found in these mesangial cell cultures using RT-PCR analysis. These data suggest that in these rat mesangial cells the Na<sup>+</sup>/H<sup>+</sup> exchanger, specifically the NHE-1 isoform, accounts for around 50% of pH<sub>i</sub> recovery from an acid load under physiological conditions, and that Na<sup>+</sup>/H<sup>+</sup> exchange stimulated by acidification can be inhibited by activation of PKG, PKA, and PKC and stimulated by PDGF after acute exposition to these agonists.