OBJECTIVE: To investigate the changes in myocardial calcium currents in rats subjected to forced running exercise during acute hypoxia and their association with myocardial injury. METHODS: Forty SD rats were randomized into quiescent group and running group either in normal oxygen (NQ and NR groups, respectively) or in acute hypoxia (HQ and HR groups, respectively). Hypoxia was induced by keeping the rats in a hypobaric oxygen chamber (PaO(2)=61.6kpa) for 4 h a day; the rats in the two running groups were forced to run on running wheels for 4 h each day. Rat ventricular myocytes was isolated by enzymatic digestion for recording action potentials and currents using patch clamp technique, and confocal Ca(2+) imaging was used to monitor intracellular Ca(2+) levels. The expressions of Cav1.2 channel and the cardiac ryanodine receptor (RyR2) were determined using Western blotting. RESULTS: Compared with those in NQ group, the rats in HR group showed significantly decreased SOD activity (P < 0.01), increased h-FABP, hs-CRP and IMA levels (P < 0.05 or 0.01), obvious myocardial pathology, and prolonged APD50 and APD90 (P < 0.05). Of the different stress conditions, forced running in acute hypoxia resulted in the most prominent increase of the densities of ICa, L currents, causing also a significant left shift of the steady state activation curve and a significant right shift of the steady state inactivation curve. Compared with those in NQ group, the rats in NR, HQ and HR groups all exhibited higher rates of spontaneous calcium wave events in the cardiac myocytes, increased frequency of calcium sparks with lowered amplitude, enhanced calcium release amplitude in the ventricular myocytes, and delayed calcium ion reabsorption; in particular, these changes were the most conspicuous in HR group (P < 0.05 or 0.01). There was also a significant increase in the protein levels of Cav1.2 channel and RyR2 receptor in HR group (P < 0.05 or 0.01). CONCLUSIONS: The mechanism of myocardial injury in rats subjected to forced running in acute hypoxia may involve the increase of oxidative stress and calcium current and intracellular calcium overload.