ABSTRACTThe effects of pharmacologically induced changes in myocardial properties on diastolic mitral valve mechanics were studied in five open-chest dogs. After the induction of complete atrioventricular block, the dogs were subjected to a protocol of programmed pacing. During prolonged diastolic pauses, programmed atrial contractions were induced at progressively increasing coupling intervals. Echocardiographically determined mitral valve reopening time was established for each coupling interval in the control state as well as under the influence of calcium or verapamil. Compared with control, calcium caused an increase in myocardial tension from 23.8 ± 3.0 to 30.0 ± 4.6 g/cm2(mean ± SD, p < .005) as well as an increase in mean septal contraction and relaxation velocities from 142 ± 25.9 and 144 ± 15.2 mm/sec to 188 ± 21.7 and 174 + 19.5 mm/sec, respectively (each p < .001). Conversely, verapamil caused a decrease in mean myocardial tension from 23.8 ± 3.0 to 19.4 ± 5.3 g/cm2(p < .001) and in mean septal contraction and relaxation velocities from 142 ± 25.9 and 144 ± 15.2 mm/sec to 112 ± 32.7 and 112 ± 21.6 mm/sec, respectively (each p < .001). At every coupling interval, calcium significantly (p < .01 to .001) prolonged, whereas verapamil significantly shortened (p < .01 to .001), mitral reopening time compared with the control state. These pharmacologically induced changes in mitral valve mechanics occurred despite variations in left ventricular volume, as well as left ventricular and left atrial pressures that under normal conditions exert opposite effects on mitral valve mechanics. Thus, in addition to affecting active systolic and early diastolic myocardial properties, calcium and verapamil also caused marked changes in passive late diastolic myocardial properties as demonstrated by late diastolic alterations in mitral valve mechanics. This experimental preparation offers a unique method for the investigation of late diastolic myocardial properties.