Relationship between contraction and cyclic GMP levels of guinea pig taenia coli

The relationship between isometric tension and the increase in cyclic GMP level associated with the initial phase of drug-induced contraction of taenia coli was studied. Maximal contraction induced by 100 micrometers carbachol or 124 mM KCl occurred in 30 sec and was associated with an increase in cyclic GMP levels of at least ten fold. However, 100 micrometers serotonin or low concentrations of carbachol (30 microM) produced nearly maximal contraction but only 10--20% of the maximal elevation in cyclic GMP induced by high concentrations of KCl. Furthermore, in the presence of a low concentration of Ca2+, or verapamil, both 124 mM KCl and 100 micrometers carbachol increased cyclic GMP with little elevation in isometric tension. Thus, large contractions were obtained with only small increases in cyclic GMP, and under other conditions, large increases in cyclic GMP were obtained with little increase in isometric tension. The results suggest that the initial increase in cyclic GMP level caused by these agents is not directly dependent on Ca2+ in the cellular pool containing the contractile proteins.