DRUG-ACTION OF RITODRINE ON THE SARCOPLASMIC-RETICULUM CA2-ATPASE FROM SKELETAL-MUSCLE()

Ritodrine inhibits the steady-state Ca2+-ATPase activity of isolated s arcoplasmic reticulum vesicles in a dose-dependent manner, The observe d K-0.5 value for inhibition (similar to 3 mM) gives proof of a low-af finity interaction. Ritodrine also interferes with the steady-state Ca 2+ transport ability decreasing the maximal rate without modification of the Ca2+ or ATP affinity for the enzyme, This is consistent with an absence of competition for the transport and the catalytic sites. Ana lysis of the catalytic and transport cycle shows that: (i) ritodrine i nhibits the phosphorylation partial reaction, This is supported by pre -steady-state kinetic experiments of Ca2+ transport and also by the te mperature dependence of the phosphoenzyme level. (ii) At high ritodrin e concentrations the dephosphorylation step becomes rate-limiting. Thi s is suggested by the biphasic profile (V-shape) of phosphoenzyme accu mulation at different ritodrine concentrations, This was also confirme d by chase experiments of radioactive phosphoenzyme decomposition at s teady state. These data reveal a complex pattern of inhibition involvi ng two sites for interaction with low and high ritodrine concentration s. It is envisaged that ritodrine does not exert any direct effect on the smooth muscle sarcoplasmic reticulum Ca2+-ATPase when used in the treatment of preterm birth. (C) 1995 Academic Press,Inc.