An index for comparing the inhibitory action of vasodilators

An index for comparing the inhibitory effects of vasodilators was developed to gain insight into their mechanism of action on vascular smooth muscle. Rat aortic strips were bathed in Krebs bicarbonate solution and were initially contracted to a stable tension by either phenylephrine or barium chloride. A vasodilator was then added and the remaining tension was noted; this was repeated for cumulative concentrations of vasodilator. At each concentration of vasodilator, the percent reduction in phenylephrine-induced tension (Phe) was compared to the percent reduction in barium-induced tension (Ba) and was expressed as a ratio (Phe/Ba). This ratio clearly separated verapamil and nifedipine (ratio less than 1), which block calcium influx, from papaverine (ratio = 1) which promotes calcium sequestration regardless of the source of calcium, and from dantrolene (ratio greater than 1) which interferes with intracellular calcium mobilization. This index provides a method for comparing the action of those agents presently classified as non-receptor specific vasodilators which act directly on vascular smooth muscle.