The effects of isoprenaline and phenylephrine on oxygen consumption in isolated smooth muscle.

1. Simultaneous measurements of oxygen consumption and mechanical activity have been made in isolated preparations of guinea-pig taenia coli, rabbit abdominal aorta and the longitudinal muscle of rabbit duodenum.2. Mean resting oxygen consumption was greater in mechanically-active preparations (taenia coli and duodenum) than in the aorta which showed no spontaneous activity.3. The relaxation produced by isoprenaline in guinea-pig taenia coli and rabbit duodenum was accompanied by a dose-dependent fall in oxygen consumption.4. The relaxation produced by phenylephrine in guinea-pig taenia coli was accompanied by a dose-dependent fall in oxygen consumption whereas, in the rabbit duodenum, the transient fall in tension was not accompanied by any significant change in oxygen consumption.5. The increase in tension produced by phenylephrine in rabbit abdominal aorta was accompanied by an increase in oxygen consumption.6. Changes in mechanical activity and oxygen consumption produced by isoprenaline and phenylephrine were antagonized by propranolol and phentolamine respectively.7. It is concluded that the variations in oxygen consumption associated with spontaneous or drug-induced mechanical changes are simple reflections of altered mechanical activity. The possibility of additional direct metabolic actions of isoprenaline and phenylephrine is discussed.