There is ample evidence that potassium channel openers protect the ischaemic myocardium. Although the protective mechanism is unknown, indirect evidence suggests that potassium channel openers reduce calcium influx during ischaemia which may explain their protective effects. However, recently discovered potassium channel openers such as BMS-180448 are cardioprotective without displaying classical indications of calcium lowering. The current study was designed to provide direct evidence that potassium channel openers delay or prevent increased intracellular free calcium in the myocardium during ischaemia and reperfusion. Cytosolic calcium concentrations were directly measured in perfused rat hearts during global ischaemia by 19F-NMR of the calcium chelator 5F-BAPTA. The cytosolic Ca2+ concentration in vehicle-treated hearts increased from a pre-ischaemia average of 310 +/- 40 nM to 1000 +/- 130 nM during 25 min of ischaemia, followed by partial recovery to 530 +/- 100 nM during 19 min of reperfusion. In contrast, the cytosolic Ca2+ concentration in hearts treated with potassium channel openers BMS-180448 and cromakalim remained low throughout ischaemia, changing from pre-ischaemia averages of 270 +/- 30 nM to 230 +/- 60 nM and from 240 +/- 20 nM to 170 +/- 30 nM during 25 min of ischaemia, respectively. The cytosolic Ca2+ concentrations in these hearts increased to 440 +/- 110 nM in BMS-180448 treated hearts and 290 +/- 60 nM in cromakalim treated hearts during the first 6 min of reperfusion, and were 460 +/- 60 nM for BMS-180448 and 600 +/- 70 nM for after cromakalim 19 min of reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)