Choice and concentration of contractile agent influence responses of rat aorta to vascular relaxant drugs.

Concentration-response (relaxation) curves to diltiazem, forskolin, isobutylmethylxanthine (IBMX), procaterol, isoprenaline and sodium nitrite were obtained on isolated ring preparations of rat aorta contracted either submaximally or maximally with noradrenaline or KCl. Diltiazem was more potent on KCl-contracted than on noradrenaline-contracted preparations whether the preparations were submaximally or maximally contracted. The other relaxant drugs were at least 20-fold less potent against KCl than against noradrenaline, but only on maximally contracted preparations. On submaximally-contracted preparations there was no potency difference between preparations contracted with these two contractile agents. Increasing the KCl concentration had a marked influence on the location of the concentration-response (relaxation) curves to all the drugs except diltiazem. This influence was different for drugs that act via cyclic AMP and those that act via cyclic GMP. It is concluded that both the choice of contractile agent (noradrenaline or KCl) and the concentration (especially of KCl) influence relaxant responses of rat aorta to vasodilator drugs.