An extension of pA principle to the potentiation of drug effects, and its application to the biological assay of some anticholinesterases and cardenolides.

A new scale for the potentiation was introduced and was termed pA1/2. The value can be calculated by using an equation; (Formula: see text), which is similar to that which is used for the calculation of pA2. For the use of the equation, the parallel shift of the dose-response curve and the unchanged maximum response are prerequisites. This experiment showed the validity and the usefulness of calculating the pA1/2 values, in the biological assay. In the assay of anticholinesterases, the potentiating effects of neostigmine or physostigmine on the acetylcholine-induced contraction were examined, by using frog's rectus abdominis. The pA1/2 values obtained in the presence of two different concentrations of anticholinesterases were the same, indicating that pA1/2 is a parameter which is specific to the potentiator. The potentiating effects of some cardenolides on the K-induced contracture were examined, by using the frog's ventricular muscles. The relative potencies of four cardenolides obtained from the pA1/2 values well agreed with those reported earlier. The amount of the cardenolides required for this assay was smaller than required for other methods.