The basis of beta adrenergic bronchodilation.

The effects of varying external K and Ca on isometric tension of the tracheal smooth muscle (TSM) of the dog were measured and the relaxant action of isoprenaline IPn) was noted in the same system. The results were similar to those reported for other smooth muscles. Oxygen consumption of the TSM was measured by a reproducible semimicro method simultaneously with mechanical tension under various conditions as above. Depolarized TSM required the energy equivalent of about 0.005 microliter of O2 min -1 . g -1 . t-1 (t = tension in g. cm-2) for sustained contraction. Relaxation of the depolarized TSM by IPn apparently was due to the antagonism of Ca and required extra energy equal to over half that required for contraction. Dibutyryl cyclic adenosine monophosphate failed to relax the depolarized TSM while tissue cyclic adenosine monophosphate level was raised by exposure to IPn. The subcellular energy consuming processes by which IPn antagonizes the contractile effect of Ca in depolarized TSM require further investigation.