Studies were performed on seven conscious dogs chronically instrumented for measurement of subendocardial segment length in the central area perfused by the left circumflex coronary artery (LCCA) and left anterior descending coronary artery, LCCA flow, and left ventricular pressure. We occluded the LCCA for 2 min using an externally inflatable pneumatic occluder placed around the proximal LCCA. Two-minute LCCA occlusions were repeated hourly for 8 h, 5 days/week. The first LCCA occlusion decreased the ejection phase shortening in the ischemic area from 22.2% +/- 2.5% (SEM) to 3.1% +/- 0.7% (P less than 0.001). Concomitantly, heart rate increased from a resting rate of 76 +/- 4 to 117 +/- 6 beats/min (P less than 0.001) after 2 min of LCCA occlusion, and ejection phase shortening in the nonischemic area decreased from 25.5% +/- 1.3% to 21.6% +/- 1.1% (P less than 0.001) despite increases in end-diastolic segment length (15.31 +/- 1.46 mm to 15.69 +/- 1.52 mm, P less than 0.01). These changes in non-ischemic segment shortening and heart rate during LCCA occlusion were attenuated progressively with improvement tin the regional myocardial function at jeopardy due to the development of collateral circulation, and a significant (P less than 0.001) correlation between both the reduced nonischemic segment shortening and the increased heart rate with the degree of dysfunction in the ischemic region was obtained by linear regression analysis with Fisher's Z transformation.