No relationship between enzyme activity and structure of nucleotide binding site in sarcoplasmic reticulum Ca2+-ATPase from short-term stimulated rat muscle

Aim: We examined whether structural alterations to the adenine nucleotide binding site (ANBS) within sarcoplasmic (endo) reticulum Ca2+-ATPase (SERCA) would account for contraction-induced changes in the catalytic activity of the enzyme as assessed in vitro. Methods: Repetitive contractions were induced in rat gastrocnemius by electrical nerve stimulation. Measurements of sarcoplasmic reticulum properties were performed on control and stimulated muscles immediately after or at 30 min after the cessation of 5-min stimulation. In order to examine the properties at the ANBS, the binding capacity of SERCA to fluorescence isothiocyanate (FITC), a competitive inhibitor at the ANBS, was analysed in microsomes. Results: Short-term electrical stimulation evoked a 23.9% and 32.6% decrease (P < 0.05) in SERCA activity and in the FITC binding capacity, respectively, in the superficial region of the muscle. Whereas SERCA activity reverted to normal levels during 30-min recovery, a restoration of the FITC binding capacity did not occur. Conclusion: The discordant changes between the enzyme activity and the FITC binding suggest that, at least during recovery after exercise, changes in SERCA activity may not correlate closely with structural alterations to the ANBS within the enzyme.