[Effects of methysergide on pulmonary circulation].

The effects of methysergide (a serotonin antagonist) on the pulmonary circulation were investigated experimentally. Twenty-eight mongrel dogs were anesthetized with sodium pentobarbital, and thoracotomy was carried out. A micro-tip pressure transducer was inserted into the pulmonary trunk and an electromagnetic flow probe was placed around it. Aortic and left atrial pressure were also measured. Three different doses (100, 250 and 1000 micrograms/kg) of methysergide and a dose of 3 micrograms/kg of serotonin were infused rapidly into the right atrium through a catheter inserted transvenously. Then, all parameters were continuously recorded. Furthermore, moduli of pulmonary vascular input impedance were calculated from pulmonary pressure and flow waves by means of Fourier analysis. The administration of each dose of methysergide resulted in an increase in pulmonary arterial pressure and resistance and a decrease in heart rate. With the highest dose of methysergide, the aortic pressure was decreased moderately. No increase in pulmonary vascular resistance was observed even after serotonin administration under the treatment with methysergide(1000 micrograms/kg). Although the characteristic impedance did not change significantly with methysergide, the impedance modulus at 0-frequency and low frequencies increased slightly in magnitude. The shift of the frequency of the impedance minimum was slight in comparison with the case of serotonin. These findings were similar to those obtained with serotonin, in general, The results have shown that methysergide produces serotonin-like effects on the pulmonary circulation, and have suggested that it may stimulate serotonin receptors in the pulmonary vascular smooth muscles as a partial agonist. It is desirable that serotonin antagonist have little effect by itself on the pulmonary circulation, in order to determine the role of serotonin in some pulmonary circulatory disturbances.