Opposite roles of cAMP and cGMP on volume loss in muscle cells

It is controversial whether changes in adenosine 3',5'-cyclic monophosphate (cAMP) and in the cAMP-to-guanosine 3',5'-cyclic monophosphate (cGMP) ratio are involved with cell swelling and in the activation of volume-regulatory mechanisms. We examined whether these nucleotides are involved in cell volume regulation in skeletal muscle. Isolated (intact and internally perfused) barnacle muscle cells were used because these cells, when exposed to a hyposmotic environment, undergo an extracellular Ca2+ (Cao)-dependent regulatory volume decrease (RVD). Using intact cells we found that dibutyryl cAMP and forskolin significantly promoted RVD in cells exposed to Cao-free solutions and that dibutyryl cGMP significantly inhibited RVD in cells exposed to Cao-containing solutions. In perfused cells in which the intracellular free Ca2+ concentration ([Ca2+]i) was heavily buffered [with 8 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA)], cAMP induced a volume loss that was inhibited by presence of cGMP. Furthermore, if perfused cells were exposed to hyposmotic conditions, they swelled and underwent RVD provided that [Ca2+]i buffering was low (with 2 mM EGTA). This effect was inhibited by presence of the cAMP antagonist, [R]-p-adenosine 3',5'-cyclic monophosphorothioate.