The effects of 3-methylxanthine (3-MX), a major metabolite of theophylline in man, were studied and compared with those of theophylline in isolated preparations of human and guinea-pig airways, isolated guinea-pig hearts, and in anaesthetized cats. 3-MX was pharmacologically active, producing effects similar to those of theophylline. The guinea-pig and human airways preparations were relaxed; the rate and force of contraction of guinea-pig hearts were increased. In anaesthetized cats, 3-MX decreased the respiratory overflow and arterial blood pressure, and increased the heart rate. Theophylline was always more potent (between 1 and 5 times) than 3-MX. It is suggested that 3-MX may contribute to the effects of theophylline during long-term treatment.
