Recirculation fraction of the activator Ca2+: index of the extent of Ca2+ loading of rat myocardium during ischemia-reperfusion.

The recirculation fraction (RF) of the activator Ca2+ of the cardiac muscle is an index of the fraction of the internally released Ca2+ sequestered by the sarcoplasmic reticulum during each contraction-relaxation cycle. Estimates of the RF were obtained by the slope method during the decline of the post-rest potentiation in the isolated aorta-perfused rat heart. Normalized contractile force P(max) of the second post-interval beat, B2, was plotted as a function of the first post-interval beat, B1, and fitted by a linear regression line. The correlation coefficient (r2) and the slope of the line were computed. Under the control experimental perfusion with oxygenated (95% O2-5% CO2) Krebs-Henseleit buffer ([Ca2+]0 1.25 mM, 34 degrees C, ph 7.40), the slope of the line, representing the RF of the rat left ventricle, was 0.73 +/- 0.01 (mean +/- SE) (r2 0.95 +/- 0.01). Increasing the stimulation frequency from 1 to 3.3 Hz produced a negative inotropic effect and significantly reduced the RF, to 0.17 +/- 0.02. Positive inotropic interventions significantly increased the RF, to 0.95 +/- 0.05 with [Ca2+]0 4 mM and to 0.92 +/- 0.04 with a 30% reduction in [Na+]0, whereas inhibition of Ca2+ release from the sarcoplasmic reticulum by ryanodine (1 microM) perfusion significantly reduced the RF, to 0.68 +/- 0.05 from the control ([Ca2+]0 2.5 microM) value of 0.81 +/- 0.05. These findings indicate that RF is a good index of the inotropic status of the rat heart. The time course of changes in RF after graded ischemia-reperfusion indicated a significant increase in the reperfusion RF between 30 and 60 min of ischemia accompanied by a significant rise in left ventricular end-diastolic pressure (LVEDP) and a significant fall in P(max), indicating an irreversible phase of the injury. During the reversible phase ( < 30 min) of the ischemia-reperfusion injury, no significant changes in RF were detected. It was concluded that RF, as derived from the simple interval-force relationship, is a good predictor of the reversible and irreversible phases of the myocardial ischemia-reperfusion injury and index of the extent of Ca2+ loading of the sarcoplasmic reticulum.