Relationship between theophylline-induced relaxation and excitation-contraction coupling in intestinal smooth muscle.

The effects of 1.0 to 10.0 mM theophylline on excitation-contraction (E-C) coupling in the taenia coli of the guinea-pig were observed with the sucrose-gap technique. Theophylline at all concentrations simultaneously inhibited spontaneous, calcium-dependent action potentials and muscle contractions. At the higher doses, theophylline depolarized the membrane in spontaneously active preparations. In the presence of high potassium (60 mM) the spike activity was blocked only by 10.0 mM theophylline. High potassium made the spikes more resistant to the drug than were those in spontaneously active preparations. The phasic and tonic components of the potassium-contracture were progressively decreased by increasing concentrations of theophylline and were completely blocked by the 10.0 mM concentration. The drug had no effect on the potassium-induced membrane depolarization observed during the potassium-contracture. On the basis of these observations, it is proposed that theophylline inhibits spike and contractile activity in taenia coli by interfering with the availability of calcium which has been postulated to be necessary for maintenance of these functions.