An investigation of "absolute plasma level effect relationships" and "absolute cardioselectivity" with respect to beta-adrenoceptor blockade.

In a group of 6 healthy (but physically unfit) volunteers an investigation was made of whether there was a true plasma level effect relationship and absolute "cardioselectivity" with respect to beta-adrenoceptor blockade due to practolol. To this end three treatments were given orally, namely placebo, practolol 100 mg and practolol 400 mg. Cardiac beta-adrenoceptor blockade was assessed by measuring the inhibition of exercise heart rate and pulmonary beta-adrenoceptor blockade was assessed by measuring the reduction in exercise peak flow rate, up to 24 h after each dose. Plasma practolol levels were determined spectrophotometrically. From the results obtained there was no reason to doubt the existence of a valid relationship between cardiac beta-adrenoceptor blocking activity and the plasma concentration of practolol. It was also concluded that even in non-asthmatics, practolol does not possess absolute "cardioselectivity" since significant reductions in peak flow rate were discernible during vigorous exercise, particularly when the large dose had been given.