Effects of tetrandrine on calcium transport, protein fluorescences and membrane fluidity of sarcoplasmic reticulum.

To understand whether the molecular mechanism of Tetrandrine (Tet)'s pharmacological effects is concerned with sarcoplasmic reticulum calcium transport so as to be involved in myocardial contractility, we observed the effects of Tet on calcium transport and membrane structure of rabbit skeletal muscle sarcoplasmic reticulum vesicles (SR) and rat cardiac sarcoplasmic reticulum vesicles (CSR). Calcium uptake was monitored with a dual-wavelength spectrophotometer. Protein conformation and fluorescence polarization were measured by fluospectrophotometric method and membrane lipids labelled with fluorescence probes for SR, respectively. 128 micromol l(-1) Tet reduced the initial rate of calcium uptake to 59% of control 6 min after reaction. Tet un-competitively inhibited SR Ca(2+), Mg(2+)-ATPase activity, causing the stoichiometric ratio of SR Ca(2+)/ATP to decrease to 1.43 from 2.0 of control. Inhibitory rates on SR Ca(2+),Mg(2+)-ATPase by Tet were reduced from 60% in the absence of phosphate to 50% in the presence of phosphate and reduced from 92% in 1 mmol l(-1) ATP to 60% in 5 mmol l(-1) ATP. Tet markedly reduced SR intrinsic protein fluorescence, while it slightly decreased the thiol(SH)-modified protein fluorescence of SR labelled with N-(3-pyrene)-maleimide. Tet slightly increased fluorescence polarization in the middle and deep layers of SR membrane lipids labelled with 7- or 12-(9-anthroyloxy) stearic acid (AS) probes, whereas it did not change that of SR labelled with 1, 6-diphenyl-1,3,5-hexatrine (DPH). These results revealed that prevention of SR calcium uptake by Tet was due to inhibition of the SR calcium pump Ca(2+),Mg(2+)-ATPase, changes in spatial conformation of the pumps protein molecules and a decrease in the extent of motion of membrane lipid molecules, thus altering the regulation of [Ca(2+)](i) and myocardial contractility.