The activation of stress-activated protein (SAP) kinase may lead to an induction of apoptosis that is responsible for part of the cardiomyocyte death in reperfusion injury. The objective of the present study was to investigate the mechanism by which magnesium tanshinoate B (MTB), a bioactive compound isolated from Danshen, prevents apoptosis in cardiomyocytes in the ischemic/reperfused heart. Isolated adult rat hearts were perfused by the Langendorff mode with medium containing MTB prior to the induction of normothermic global ischemia. At the end of the 30-min ischemic period, the heart was reperfused with the same medium with or without MTB for an additional 20 min. In the MTB-treated ischemic/reperfused heart, the number of apoptotic nuclei was reduced by 2.5-fold in comparison to that in untreated ischemic/reperfused controls [23 +/- 4 vs 57 +/- 7 (mean +/- SD) TUNEL-positive cells, respectively, N = 3-4, P < 0.001]. SAP kinase activity was elevated 1.7-fold in ischemic/reperfused rat hearts [35.6 +/- 3.8 vs 21.2 +/- 3.3 (control) (mean +/- SEM) relative densitometric units, N = 4-6, P < 0.05]. Treatment with MTB abolished this elevation in SAP kinase activity (25.0 +/- 5.2 relative densitometric units), which was also decreased by 40% in the nucleus. When the heart was subjected to ischemia alone, there was no significant change in SAP kinase activity in the presence or absence of MTB. MTB did not appear to affect the p38 mitogen-activated protein kinase activity in this model system. In conclusion, MTB was shown to have cardioprotective activity against apoptosis, probably through the inhibition of SAP kinase activity.