Background: The presence of substantial mitral annular area reduction prior to ventricular systole suggests an atrial influence on annular dynamics. We implanted radiopaque myocardial markers in normal sheep to investigate the effects of acute LV ischemia on atrial contraction, mitral annular area (MAA), and mitral regurgitation. Methods: Six adult sheep had radiopaque markers placed on the left ventricle, mitral annulus, left atrium (LA) and the central edge of both mitral leaflets. After 6-8 days, animals were studied with biplane videofluoroscopy and transesophageal echocardiography before and during sequential balloon occlusion of the LAD, distal LCX, and proximal LCX coronary artery. MAA and LA area were calculated from the corresponding markers. Percent reduction in these parameters was calculated between maximum in late diastole and minimum in early systole. MR (0-4) was assessed by TEE. Results: LAD occlusion did not alter LA area reduction (14.8±8.4% and 14.8±5.8% for pre-ischemia and ischemia, respectively; p=0.9) or pre-systolic MAA reduction (68±25% and 69±14%; p=0.9) whereas dLCX occlusion resulted in a mild decrease in the former (16.4±3.4% and 12.4±6.2%; p=.04) with no change in the latter (79±18% and 70±23%; p=0.3). Neither occlusion resulted in MR. pLCX occlusion, however, significantly decreased LA area reduction (16.5±3.2% and 5.6±4.0%; p=0.005) and pre-systolic MAA reduction (79±16% and 35±23%; p=0.01) and resulted in increased end-diastolic MAA (7.09±1.50cm² and 8.46±2.01cm²; p=.006), delayed valve closure from end-diastole (52±26 ms and 92±23 ms; p=0.03) and mitral regurgitation (0.5±0.5 and 2.0±1.1; p=0.007). Conclusion: Atrial contraction is closely coupled to pre-systolic mitral annular area reduction. Decreased atrial contractile function, as observed during acute posterolateral ischemia, is linked to diminished pre-systolic mitral annular reduction, a larger mitral annular size at end-diastole, and mitral regurgitation.