Difference in the sensitivity of junctional and longitudinal sarcoplasmic reticulum Ca(2+)-ATPase to ADP

The Ca2+ release mechanism that triggers muscle contraction is still not completely understood. We compared Ca2+ accumulation and acetyl phosphate hydrolysis by the Ca(2+)-ATPases present in the longitudinal and junctional membrane of the sarcoplasmic reticulum of rabbit skeletal muscle and found that Ca(2+)-ATPase is more sensitive to ADP inhibition when the enzyme is located on the junctional membrane than when the enzyme is located on the longitudinal membrane (K0.5 = 144 microM for the junctional enzyme vs K0.5 = 415 microM for the longitudinal enzyme). When the enzyme was solubilized in non-ionic detergent (2% v/v Triton X-100) and tested again using 2 mM AcP as substrate, the difference in ADP sensitivity observed with native preparations disappeared. We conclude that the enzyme is regulated differently depending on its localization on the membrane of the sarcoplasmic reticulum.