Functional properties of myofibrils from chronically ischemic canine myocardium were evaluated. Ischemia was produced by tight stenosis of left anterior descending artery (LAD), followed by 40 min acute ischemia with prior preconditioning. Animals of the first group were sacrificed after 8 weeks. In the second group, angioplasty of LAD was performed after 8 weeks of ischemia and animals were kept alive for other 4 weeks. Control animals were sham operated. Activity and kinetic parameters of myofibrillar Ca2+-stimulated Mg2+-ATPase were measured in myofibrils isolated from anterior and posterior parts of all hearts. We did not find any differences in maximal velocity (Vmax), half-maximal activation constant for calcium (K(Ca2+)50) and cooperativity coefficient (n(hill)) of myofibrils from different experimental groups as compared to controls, either at pH 7, pH 6.5 (acidosis) or pH 7.5 (alkalosis). K(Ca2+)50 increased in medium simulated acidosis (12.6-33.5 times) and n(hill) decreased significantly in all groups as compared with values obtained at pH 7. These results indicate that activity and Ca2+-sensitivity of myofibrillar Mg2+-ATPase remain unchanged despite deteriorated heart function 8 weeks after LAD obstruction. Experiments have confirmed that Ca2+-stimulated-ATPase from canine heart myofibrils responded to pH decrease by a decreased sensitivity to Ca2+ and a decreased cooperativity. However, sensitivity of the enzyme to the pH changes is unaltered by 8 weeks of chronic ischemia.