The effects of sulfamethoxazole (SMX), trimethoprim (TMP) and their mixture in a ratio of 5 : 1 (SMX-TMP) on the circulatory and respiratory systems were studied. They had no noteworthy pharmacodynamic action on the circulatory and respiratory systems of rabbit except for the following effects which were observed after large doses.
1) Blood pressure : A slight and transient hypertensive effect was observed after intravenous administration of SMX. On the other hand, slight and transient hypotensive effects were observed after both oral and intravenous administration of TMP and SMX-TMP.
2) Respiration : TMP and SMX-TMP caused slight and transient acceleration of respiration when administered intravenously.
3) ECG : A slight tachycardia was observed after both oral and intravenous administration of TMP and SMX-TMP.
4) Heart : Movement of heart in situ was slightly suppressed by 'intravenous administration of SMX, TMP and SMX-TMP. The spontaneous beating rate of isolated right atrial strip of guinea-pig was decreased by SMX and TMP, and the amplitude was slightly increased by TMP.
5) Blood vessels : TMP had a slight vasodilator action on the perfused blood vessels of hind leg. The hypotensive effect of TMP may be attributed to this action.
