Extracellular cGMP inhibits transepithelial sodium transport by LLC-PK1 renal tubular cells

Both atrial natriuretic peptide (ANP) and sodium nitroprusside (SNP) inhibit tubular sodium reabsorption by generation of guanosine 3',5'-cyclic monophosphate (cGMP). To determine the role of extracellular cGMP in this response, monolayers of porcine renal tubular LLC-PK1 cells mere incubated for 5 min with ANP, SNP, cGMP, or 8-bromo-guanosine 3',5'-cyclic monophosphate (8-BrcCMP) (10 nM to 0.1 mM). Transepithelial sodium transport was measured as amiloride-inhibitable short-circuit current (I-sc). Incubation of cell monolayers with 1 mu M of ANP, cGMP, or 8-BrcGMP inhibited I-sc by >70%, as did SNP at 100 mu M (P < 0.01). Adenosine 3',5'-cyclic monophosphate (0.1 mM) had no significant effect. Incubation of monolayers with 1 mu M LY-83583 (an inhibitor of guanylyl cyclase), 10 mu M probenecid (an organic anion transport inhibitor), or preincubation with 1 mu g/ml nocodazole (a microtubule disrupter) reduced extracellular accumulation of cGMP (P < 0.05) and abolished the SNP-mediated reduction of I-sc. However, addition of these inhibitors did not affect reduction of I-sc by exogenous cGMP. We conclude that SNP inhibits sodium transport by LLC-PK1 monolayers through generation of cGMP but that extrusion of cGMP out the cell is necessary for its effect.