Effects of Potassium Chloride and Smooth Muscle Relaxants on Tension and Cyclic Nucleotide Levels in Rat Myometrium

Ten minutes after KCl-depolarization of rat myometrial strips, at which time the muscles were in a state of sustained contracture, tissue levels of adenosine 3′,5′-cyclic monophosphate (cyclic AMP) were increased by approximately 40% over relaxed controls, and levels of guanosine 3′,5′-cyclic monophosphate (cyclic GMP) were decreased by 40%. At this point both nitroglycerin (4 × 10−4 M) and papaverine (2 × 10−5 M) were capable of relaxing the depolarized muscles without significantly increasing cyclic AMP levels. Isoproterenol, in concentrations from 5 × 10−9 M to 10−6 M, relaxed the depolarized muscles and significantly increased tissue levels of cyclic AMP. However, the magnitudes of the cyclic AMP increases seen after the lower concentrations of isoproterenol were small relative to the increases observed during KCl-contracture alone. For example, the 40% elevation of cyclic AMP seen 10 min after KCl-depolarization did not cause the muscles to relax, whereas 5 × 10−9 M isoproterenol caused relaxation with an increase in cyclic AMP levels of only 16% over depolarized controls. It was concluded that changes in total tissue levels of cyclic AMP were not responsible for the uterine relaxation caused by nitroglycerin, papaverine or isoproterenol in these experiments. Cyclic GMP levels in the depolarized muscles were not significantly changed by isoproterenol or papaverine but were increased approximately 80% by nitroglycerin.The above results are not consistent with the previously suggested roles for cyclic GMP and cyclic AMP as mediators of smooth muscle contraction and relaxation, respectively.