CGMP AND ATRIAL-NATRIURETIC-FACTOR REGULATE CELL-VOLUME OF RABBIT ATRIAL MYOCYTES

Atrial natriuretic factor (ANF) reduces the volume of atrial myocytes by inhibiting Na+/K+/2Cl(-) cotransport. We determined the role of cGMP and cAMP in ANF-induced shrinkage by using digital video microscopy to measure cell volume; volumes are reported relative to control. ANF (1 mu mol/L) reversibly reduced atrial cell volume from 1.0 to 0.915+/-0.005 (mean+/-SEM). This effect was mimicked by 10 mu mol/L 8-bromo-cGMP (8-Br-cGMP), which decreased myocyte volume to 0.894+/-0.007 with an ED(50) of 0.99+/-0.05 mu mol/L. In contrast, 100 mu mol/L 8-bromo-cAMP (8-Br-cAMP) did not affect volume, and activating the cAMP pathway with 100 mu mol/L 8-Br-cAMP did not alter the volume decrease caused by 8-Br-cGMP or ANF. Inhibition of Na+/K+/2Cl(-) cotransport with bumetanide (1 mu mol/L) also reduced cell volume and prevented further shrinkage on subsequent exposure to 9-Br-cGMP. Similarly, 8-Br-cGMP (10 mu mol/L) prevented further shrinkage by ANF. Block of Na+-H+ exchange, a participant in volume regulation in other cells, did not alter the response to 8-Br-cGMP. More evidence implicating cGMP was obtained by altering its metabolism. LYS3583 (10 mu mol/L), a guanylate cyclase inhibitor, blocked ANF-induced cell shrinkage. Zaprinast (100 mu mol/L), a cGMP-specific phosphodiesterase inhibitor, markedly potentiated the effect of a threshold concentration of ANF (0.01 mu mol/L). The actions of ANF, LY83583, and zaprinast on cGMP levels were verified by radioimmunoassay. These data strongly support the idea that the cGMP cascade is the intracellular signaling pathway responsible for ANF-induced atrial cell shrinkage. ANF activates guanylate cyclase, and the subsequent rise in cGMP inhibits Na+/K+/2Cl(-) cotransport. This reduces the uptake of osmolytes, and cell shrinkage follows. In addition, LY83583 caused a small swelling and zaprinast caused a small shrinkage in the absence of other agents. This suggests that physiological levels of cGMP partially inhibit Na+/K+/2Cl(-) cotransport and help set isosmotic cell volume. It is proposed that ANF-induced elevation of atrial cGMP and the ensuing cell shrinkage serve as a negative-feedback mechanism limiting stretch-induced ANF secretion.