Alternative mechanisms for the potentiation of the relaxation evoked by isoprenaline in aortae from young and aged rats.

The action of cinnarizine and flunarizine on the relaxant effect of isoprenaline was studied in the isolated rat aorta and was compared with the action of papaverine. The results show that isoprenaline-induced relaxation and K+ contraction were reduced. On the contrary, in the presence of papaverine, isoprenaline relaxation and K+ contraction were reduced. On the contrary, in the presence of papaverine, isoprenaline relaxation was enhanced by cinnarizine and flunarizine at concentrations which were lower that those required for inducing an inhibition of the contraction evoked by K+ in the presence of 1.25 mM CaCl2; with concentrations higher than 10(-8) M both isoprenaline relaxation was enhanced and K+ contraction was depressed in a dose-dependent manner. In aged rats, isoprenaline-induced relaxation was tachyphylactic. The tachyphylaxis was prevented by cinnarizine and flunarizine. The present observations indicate that several processes might be responsible for potentiating isoprenaline action.