IMPROVED ISOVOLUMETRIC RELAXATION IN CANINE REPERFUSED MYOCARDIUM AFTER BETA-1 ADRENERGIC-STIMULATION

Objective: Beta-1 adrenergic stimulation with the partial agonist xamoterol improves the systolic recovery of reperfused ("stunned") myocardium. The effects of this agent on isovolumetric relaxation and diastolic ventricular filling were evaluated. Methods: In 15 open chest dogs, the distal left circumflex coronary artery was occluded for 15 min and then reperfused. Eight dogs served as placebo controls, and seven received xamoterol (100 mu-g.kg-1 intravenously) at 10 min reperfusion. Diastole was divided into an isovolumetric relaxation phase and a subsequent filling phase, and wall excursion velocities were calculated. Results: In the control group isovolumetric wall thinning velocity in the reperfused myocardium recovered only slowly, from -5.67(SD 5.29) mm.s-1 at 10 min reperfusion to -4.84(6.35) mm.s-1 at 30 min reperfusion and 2.99(7.97) mm.s-1 at 8 h reperfusion. Xamoterol increased isovolumetric wall thinning velocity in the reperfused myocardium, from -7.78(6.25) mm.s-1 at 10 min reperfusion to 3.43(6.09) mm-s-1 at 30 min reperfusion, and 12.04(5.66) mm.s-1 at 8 h reperfusion, whereas wall thinning velocity during the filling phase was unaffected. At 8 h reperfusion the difference between the groups was still significant. When compared with the effects on systolic wall thickening velocity, the impairment of isovolumetric wall thinning velocity during occlusion and the improvement due to xamoterol were both more marked. These functional differences were not reflected by the two indices of global diastolic function, peak negative dP/dt and relaxation constant-tau. Conclusions: Beta-1-adrenergic stimulation with xamoterol improves isovolumetric relaxation in reperfused myocardium more markedly than systolic contraction, but it has no direct effect during diastolic filling.