Measurement of antagonist affinity for purine receptors of drugs producing concomitant phosphodiesterase blockade: the use of pharmacologic resultant analysis.

It was observed experimentally that both theophylline and isobutylmethylxanthine (IBMX) produced surmountable antagonism of the agonist effects of 2-chloroadenosine on purine receptors in rat vas deferens and guinea pig atria. In the case of IBMX, there was a statistically significant difference between the Schild regressions in the two tissues, ostensibly indicating a possible purine receptor heterogeneity with respect to the binding of this antagonist. However, the analysis was complicated by the fact that both theophylline and IBMX produced positive inotropic responses in guinea pig atria (presumably by inhibition of cardiac phosphodiesterase), making the calculation of dose ratios subjective and ambiguous. To determine whether the phosphodiesterase blocking property of theophylline and IBMX was interfering with the observation of purine receptor antagonism in guinea pig atria, a new technique described as pharmacologic resultant analysis was utilized to measure the purine receptor blocking properties of theophylline and IBMX in guinea pig atria in the presence of phosphodiesterase blockade. Based on the principle of additive dose ratios for two antagonists competing for one receptor, pharmacologic resultant analysis measures the effects of a test antagonist (in this case theophylline or IBMX) on the competitive blockade of a reference antagonist; for these studies, the reference antagonist was 8-sulfophenyltheophylline. Under these circumstances, the direct effects of the test antagonist (positive inotropy) are expressed throughout the experiment and cancel with the normal null methods used for measurement of competitive antagonism. Using this technique, it was found that the potencies of both theophylline and IBMX on the purine receptors in rat vas deferens and guinea pig atria were identical. The use of this method in the delineation of multiple drug activities is discussed.