Effects of bepridil and nifedipine on regional myocardial contractility during ischaemia in anaesthetized dogs.

Subtotal reduction (60%-70%) of blood flow in the circumflex artery in anaesthetized open-chest dogs caused a long-lasting increase (222%) in coronary vascular resistance and stable changes in myocardial segmental contractility (% delta L), as measured by the method of piezoelectric crystals. % delta L increased by 31.2% in the healthy zone while hypokinesia occurred in the moderately ischaemic zone (-50.8%). In the severely ischaemic zone there was a paradoxical lengthening (bulge or dyskinesia) of 7.3% during systole. Bepridil (2.5 mg.kg-1, IV) decreased heart rate (-15.6%) and systolic (-20.8%) and diastolic (-31.6%) blood pressure, and increased total coronary blood flow (+40.7%). % delta L was increased in the healthy zone (+20.4%) and in the moderately ischaemic zone (+48.2%). Bepridil completely inhibited the bulge in the severely ischaemic zone. Nifedipine (0.02 mg.kg-1, IV) greatly reduced systolic (-31.3%) and diastolic (-40.7%) blood pressure as well as coronary blood flow (-30.4%), without changing heart rate. A delayed increase in % delta L occurred in the healthy zone (+8.4%) and in the moderately ischaemic zone (+38.5%). In the severely ischaemic zone, there was no improvement of the bulge. The observed differences in contractility between the two calcium antagonists are discussed in terms of their haemodynamic differences. Improvement of myocardial segmental contractility, if it occurred, may have been due to bradycardia, slight decrease in contractility, as measured by LV dP/dt max/P, increased total coronary blood flow, decreased afterload, so long as it was not too great, and anti-ischaemic properties of the compound.