The mechanisms underlying protection of ischemic myocardium by reperfusion with solutions containing reduced concentrations of calcium (CA) were studied in isolated vascularly perfused rabbit interventricular septa at 37 degrees C. After 45 min of total ischemia adenosine triphosphate (ATP), and phosphocreatine (PC) contents fell to 6.8 +/- 0.82, and 12.4 +/- 2.0 micrometer/g dry tissue (dt) (+/- S.E.M.). After 5 min of reperfusion with 0.75 mM Ca these values had recovered significantly to 10.3 +/- 1.0, and 33.3 +/- 3.6, (micrometer/g dt), while reperfusion with 2.5 mM Ca produced 5.9 +/- 1.0 and 22.0 +/- 2.6 (micrometer/g dt). The significantly greater recovery of ATP and PC after 0.75 mM Ca reperfusion persisted after an additional 25 min of reperfusion with 2.5 mM Ca when compared to septa reperfused for the entire 30 min with 2.5 mM Ca. When mechanical work was reduced by cessation of stimulation during the first 5 min of reperfusion, ATP, PC and total creatine (TC) recovery were significantly improved in muscles exposed to 2.5 mM Ca but showed no additional improvement in muscles reperfused with 0.75 mM Ca. A further reduction to 100 micrometers Ca reperfusion in quiescent muscles did significantly improve ATP, PC and TC recovery. Quiescence achieved with 16.0 mM K impaired ATP, PC and TC recovery. This effect was reversed by 100 micrometers CA reperfusion. Measurements of mitochondrial oxygen consumption, respiratory control and ADP/O showed results parallel to the ATP, PC and TC determinations. Both mechanical work and CA itself influence mitochondrial respiration, ATP, PC and TC content after ischemia. As previously demonstrated for mechanical function, metabolic recovery can be improved by interventions limited to the first 5 min of reperfusion.