INFLUENCE OF GLOBAL-ISCHEMIA ON THE SARCOLEMMAL ATPASES IN THE RAT-HEART

To elucidate the effect of global ischemia on the energy utilizing pro cesses, regarding the molecular principles, the kinetic and thermodyna mic properties of the sarcolemmal ATPases were investigated in the rat heart. The activation energy for hydrolysis of ATP during ischemia wa s higher when the reaction was catalyzed by Ca-ATPase or Mg-ATPase. Fo r the Na,K-ATPase reaction, no changes in the activation energy were o bserved. With respect to the enzyme kinetics, ischemia in a time-depen dent manner induced important alterations in K-M and V-max values of N a,K-ATPase, Ca-ATPase and Mg-ATPase. The V-max value decreased signifi cantly already after 15 min of ischemia, and it also remained low afte r 30, 45 and 60 min for all 3 enzymes. The significant diminution of K -M values occurred later in the 30th min for Ca-ATPase, in the 45th mi n for Na,K-ATPase. The observed drop in K-M indicates the increase in the affinity of the enzymes to substrate, suggesting thus the adaptati on to ischemic conditions on the molecular level. This effect could be attributed to some conformational changes of the protein molecule in the vicinity of the ATP-binding site developing after longer duration of ischemia.