Effect of isoproterenol on relation between maximal rate of contraction and maximal rate of relaxation

The relaxing effect of two positive inotropic interventions was studied in the isolated rat heart beating at constant rate and perfused at constant coronary flow. Experiments were performed under isotonic or isometric conditions during heart perfusions with Ca²⁺, 3.5x10^-3M, and with isoproterenol, 10^-7 to 10^-8M. Changes induced in maximal velocity of contraction (+L or +T) and maximal velocity of relaxation (-L or -T) were analyzed as the ratio between both maximal velocities (+L/-L or +T/-T). For similar increments in the amount of the apex-to-base shortening or in developed tension during Ca²⁺ or isoproterenol perfusions, the effect on the maximal velocity of relaxation differed for each intervention. High Ca²⁺ perfusion increased velocities of contraction and relaxation proportionally without significant changes in their ratio. Isoproterenol increased -L or -T more than +L or +T and the ratio showed a significant fall both in isotonic and isometric experiments (+L/-L, -0.33±0.11 and +T/-T, -0.38±0.09; P<0.01). The relaxing effect of the beta agonist seems to be related to the activation of the protein kinase system since perfusions with dibutyryl 3',5'-cAMP had a greater effect on the relaxation of the hearts.