The Importance of Calcium Ions for the Regulation of Guanosine 3':5'-Cyclic Monophosphate Levels

Guanosine 3‧:5‧-cyclic monosphosphate (cyclic GMP) levels in the ductus deferens of the rat were increased 2- to 3-fold by acetylcholine (10-1000 μM) or by 125 mM KCl, while adenosine 3‧:5‧-cyclic monophosphate (cyclic AMP) levels were not changed. After incubation for 30 min in the absence of Ca⁺⁺, cyclic GMP control levels were decreased by 85% and were not affected by acetylcholine or KCl. The readdition of Ca⁺⁺ (1.8 mM) for 3 min to Ca⁺⁺-deprived tissue partially restored basal cyclic GMP levels and the effects of acetylcholine and KCl. The addition of Sr⁺⁺ (3.6 mM) or of Ba⁺⁺ (1.8 or 10 mM) also caused an increase in basal cyclic GMP in Ca⁺⁺-deprived tissue. Cyclic AMP levels were not significantly changed under any of these conditions. The addition of the phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine (0.1 mM), to ductus deferentes increased the amount of cyclic AMP about 50% and that of cyclic GMP about 2-fold. The later effect also depended on the presence of Ca⁺⁺. 1-Methyl-3-isobutylxanthine (0.1 mM) increased cyclic GMP and cyclic AMP levels in slices of rat submaxillary glands. Methacholine increased cyclic GMP if added in the presence of methyl isobutylxanthine. Cyclic GMP control levels and the effect of methyl isobutylxanthine were unchanged by Ca⁺⁺ omission, but the effect of methacholine was abolished. These findings indicate that calcium ions are important for the control of cyclic GMP levels in these tissues.