The relationship between antibronchospasmic, tachycardiac, or hypokalaemic effects and plasma concentration of L‐isoproterenol (ISP) hydrochloride was investigated in guinea‐pigs in vivo. ISP was infused at the rate of 10, 30, 50, 100, and 300 ng kg−1 min−1. The antibronchospasmic effect was expressed as the attenuation of methacholine‐induced bronchospasm. The EC50 values of ISP for antibronchospasmic and tachycardiac effects were 5.12 nM and 3.95 nM, respectively. Although they were comparable to the values reported in vitro (7.23–0.358 nM, 1.77 nM), the concentration response relationship of ISP for antibronchospasmic effect was quite steep with a slope factor of more than six. Moreover, a decrease in plasma potassium level was not clearly detected. The experimental procedure in our present study was useful for evaluating antibronchospasmic and tachycardiac effects of beta‐agonists.
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