We evaluated the effect of ischemia and reperfusion on sarcoplasmic reticulum Ca uptake in patients subjected to cardiac surgery. Our series included 16 patients (seven female, nine male, age 63 +/- 2 years): five were subjected to aortic valve replacement, five to aortic and mitral valve replacement, six to coronary artery bypass graft. In each case no clinical, electrocardiographic or echocardiographic evidence of perioperative infarction was observed. Biopsies were obtained from the right atrium of each patient before starting extracorporeal circulation, and after the recovery of spontaneous contractile activity, i.e. after cardioplegia-ischemia-reperfusion. The tissue was homogenized, and oxalate-supported Ca uptake, which represents sarcoplasmic reticulum Ca uptake, was measured in the unfractionated homogenate. The assay was performed under basal conditions and in the presence of 900 microM ryanodine, in order to block sarcoplasmic reticulum Ca release channels. Under basal conditions at pCa = 5.85 the rate of sarcoplasmic reticulum Ca uptake averaged 4.76 +/- 0.37 nmol/min per mg of protein in the pre-ischemic samples, and decreased significantly in the post-ischemic samples (3.09 +/- 0.29 nmol/min per mg, P < 0.01). A significant decrease of Ca uptake after ischemia and reperfusion was observed also in the presence of ryanodine (3.53 +/- 0.48 nmol/min per mg) compared to pre-ischemic values (5.98 +/- 0.56 nmol/min per mg, P < 0.01). Additional experiments showed no change in the Ca sensitivity of Ca uptake in the postischemic samples (Kca = 0.48 +/- 0.02 microM, no significant difference after ischemia and reperfusion). In conclusion, active sarcoplasmic reticulum Ca transport was impaired in human atrial myocardium after reversible ischemia and reperfusion.