OBJECTIVE: To investigate the changes in binding features of (3)H -Ryanodine cardiac myocytic nuclei in reperfusion injury in the rat, and the effects of phosphorylation regulation on the binding characteristics. METHODS: Healthy male Wistar rats were randomly divided into ischemia/reperfusion injury (IRI) group and sham-operation group. IRI model was reproduced by ligating the left main coronary artery for 30 minutes followed by reperfusion of the heart for 3 hours, while in the sham-operation group the animals underwent a thoracotomy only for 3.5 hours. Cardiac myocytic nuclei were isolated by sucrose density gradient centrifugation. The maximum binding capacity (Bmax) and the dissociating ratio (Kd) of Ryanodine receptors (RyRs) were measured by radioligand binding analysis as well as Scatchard plot. RESULTS: There was a high affinity of RyRs to bind with (3)H-Ryanodine on the nuclear wall of rat cardiac myocytic nucleus. Compared with that of the sham-operation group, Bmax of RyRs of IRI cardiac myocytic nuclei was decreased by 29% (P<0.01), but there was no difference in Kd between two groups(P>0.05). With phosphorylation by activating the endogenous protein kinase C(PKC) with phorbol myristate acetate (PMA)+phosphatidyl serine(PS), Bmax of both sham and IRI groups were increased markedly (P<0.01), but in the latter group it was less increased (P<0.01). With phosphorylation by Ca(2+)-calmodulin (CaM), Bmax was decreased in both sham-operation and IRI groups (both P<0.05), but was less decreased in the latter(P<0.01). However, both PMA+PS and Ca(2+)-CaM did not change the Kd of nuclear RyR in either group (both P>0.05). CONCLUSION: After IRI, Bmax of (3)H -Ryanodine of cardiac myocytic nuclei is decreased and the impact of PMA+PS and Ca(2+)-CaM on the Bmax is impaired, but the affinity of (3)H -Ryanodine to cardiac myocytic nuclei is not altered under above circumstances.