The antagonist effects of endrallazine (BQ 22-708), hydrallazine and verapamil have been studied on the contractile responses produced by various agonists on human digital arteries and metacarpal veins obtained post mortem. 2. Endrallazine and hydrallazine antagonized the contractile responses to noradrenaline and serotonin in arteries by shifting the concentration-effect curves to the right and by reducing the maximum responses to both agonists. Neither drug had any effect on responses to BaCl2 in either tissue. 3. The same concentrations of endrallazine and hydrallazine which antagonized responses to noradrenaline and serotonin in arteries, had no effect on concentration-effect curves to those agonists in veins. 4. Hydrallazine was significantly more potent than endrallazine in reducing the maximum response to noradrenaline in arteries, and was less potent than endrallazine in reducing the maximum response to serotonin. 5. Verapamil antagonized the vasoconstrictor effect of all agonists tested in both arteries and veins. 6. It is concluded that in a similar manner to hydrallazine, endrallazine has a direct effect on human vascular smooth muscle at concentrations which are probably similar to those occurring in vivo. Both hydrallazine and endrallazine may produce selective arterio-dilatation by preventing the release of bound calcium from intracellular storage sites, an effect which is different to that of verapamil.
