Mechanism of tissue Ca2+ gain during reoxygenation after hypoxia in rabbit myocardium

The mechanism of increased tissue Ca2+ uptake during reoxygenation after hypoxia was studied in isolated, arterially perfused rabbit septum maintained at 27 degrees C or 37 degrees C. Tissue 47Ca2+, 85Sr2+, or 133Ba2+ uptake was measured by a juxtaposed gamma-probe and a counter. At 27 degrees C, Ba2+ flux across the sarcolemma is similar to that of Ca2+ and Sr2+, but Ba2+ is not taken up by sarcoplasmic reticulum or mitochondria. Therefore 133Ba2+ flux studies were used to delineate the effects of hypoxia and reoxygenation on sarcolemmal permeability to divalent cations. In muscles maintained at 27 degrees C, reoxygenation after 40 min of hypoxia caused significant increases in both 47Ca2+ and 85Sr2+ uptakes. In contrast, there was no change in tissue 133Ba2+ uptake, 133Ba2+ efflux, determined from 133Ba2+ washout studies, was also unchanged. When the sarcolemma was disrupted by perfusing the muscle with a solution containing phospholipase C, tissue 133Ba2+ uptake as well as 47Ca2+ and 85Sr2+ uptakes increased. Moreover an increase in 133Ba2+ efflux was observed after phospholipase infusion. Addition of an inhibitor or an uncoupler of mitochondrial respiration [sodium cyanide (5 X 10(-3) M) or dinitrophenol (5 X 10(-4) M), respectively] in the perfusate caused significant decreases in reoxygenation-induced tissue Ca2+ gain. In muscles perfused with a solution that did not contain permeant anions capable of proton donation (Tris buffer without HCO3(-) and H2PO4(-)), tissue CA2+ gain during reoxygenation was significantly reduced. Perfusion with Tris buffer also caused greater recovery of mechanical function and myocardial ATP concentration during reoxygenation. In muscles maintained at 37 degrees C, both tissue 47Ca2+ and 133Ba2+ uptakes increased during reoxygenation after 40 min of hypoxia. Isolated mitochondria accumulated both Ca2+ and Ba2+ at 37 degrees C. These data suggest that the reoxygenation-induced tissue Ca2+ uptake is primarily caused by an active uptake by mitochondria and that the increase in mitochondrial Ca2+ uptake can occur without any changes in sarcolemmal permeability to divalent cations (Ca2+, Sr2+, or Ba2+). The data also suggest that the increased mitochondrial Ca2+ uptake is responsible, at least in part, for the impaired recovery of myocardial mechanical and cellular function after hypoxia.