Action of papaverine and verapamil on norepinephrine and calcium evoked contraction of vascular and non vascular smooth muscle

Direct evidence for the interaction between papaverine and alpha 2-adrenergic receptors in canine platelets

The effects of papaverine on specific [3H]-yohimbine binding to canine platelet alpha 2-adrenergic receptors and on the platelet aggregation were assessed and compared with those of verapamil. Both compounds concentration-dependently inhibited [3H]-yohimbine binding with KI values for respective compounds of 0.39 +/- 0.05 microM (n = 3) and 15 +/- 0.19 microM (n = 3). In the presence of either compound KD values in Scatchard analysis of the equilibrium ligand binding increased in concentration-dependent manner, whereas Bmax did not change, indicating competitive inhibition of the ligand binding by these compounds. (-)-Epinephrine (3 microM) potentiation of adenosine diphosphate (ADP, 0.1 microM) aggregation was inhibited by papaverine with IC50 of 11 +/- 3.6 microM (n = 4). In the same experiments verapamil inhibited the platelet aggregation with lower IC50 (3.1 +/- 0.87 microM, n = 4) in comparison with that for papaverine. These results suggest that papaverine, like verapamil, inhibits physiological response of canine platelets through alpha-adrenergic receptor stimulation by direct interaction with the receptors.

The influence of verapamil and papaverine on the calcium- and epinephrine-induced responses of isolated guinea-pig atria

The effects of papaverine and verapamil on Ca2+ - and epinephrine evoked responses were investigated on spontaneously beating atria of the guinea-pig. Papaverine showed a non competitive antagonism against epinephrine but was ineffective against Ca2+; verapamil antagonized both the action of epinephrine and Ca2+ in a different manner but at the same concentrations. The pharmacological prerequisite to classify an anti-Ca2+ substance as a blocker of Ca2+ entry (that is the ability to selectively antagonize the Ca2+ -induced contraction relative to those of norepinephrine in vascular smooth muscle) was not met by the results obtained in the cardiac tissue. The behaviors of verapamil and papaverine with regard to Ca2+ response suggest another criterion for classifying a substance as a Ca2+ entry blocker based on the ability or inability to inhibit the response of the atrial preparation to Ca2+.