Figure 3.

Effect of EpoTg on Ca²⁺ transport by SR vesicles as compared with that of Tg and O-8–substituted Tg analogs. SR vesicles (0.05 mg of protein/ml) were incubated at 23 ± 0.2 °C with medium containing 50 mm MOPS (pH 7.2), 100 mm KCl, and 5 mm Mg²⁺ together with 0.02 mm ⁴⁵Ca²⁺ and 0.1 mm EGTA buffer (with a pCa of ∼6 to approach the low Ca²⁺ concentration inside a cell) in the absence (DMSO control) or presence of SERCA inhibitor as indicated. The reaction was started after 5 min (A) or 24 h (B) by addition of 0.08 mm MgATP to samples preincubated with no inhibitor (inverted filled triangles), 1 μm EpoTg (upright filled triangles), 5 μm EpoTg (light gray circles), 10 μm EpoTg (black circles), 1 μm Tg (black squares), 1 μm Leu-8ADT (inverted empty triangles), 1 μm βAsp-8ADT (black diamonds), or 1 μm Boc-8ADT (empty circles). Ca²⁺ accumulation by the vesicles was measured after timed intervals on 0.2-ml aliquots by addition of 4 ml of ice-cold incubation buffer and subsequent Millipore filtration. The plots represent the means of three to four experiments with S.D. error bars shown whenever appropriate, i.e. when the S.D. is of a greater magnitude than the size of the symbols.