Figure 4. (A) Representative trace demonstrating the dose-dependent inhibitory effect of thiopental on uptake of Ca2+by the sarcoplasmic reticulum (SR). Aliquots of SR vesicles (8 [micro sign]g) were preincubated in buffer alone or in the presence of thiopental at the concentrations indicated for 10 min at 37 [degree sign]C. Uptake of Ca2+into SR vesicles was triggered by addition of adenosine triphosphate to the cuvette (arrow) and measured continuously as a decrease in the 340/380 ratio. The total amount of Ca2+released from the SR vesicles in response to caffeine (20 mM) was unaltered by thiopental. (B) Summarized data for the effects of thiopental on uptake of Ca2+in isolated SR vesicles. Results are expressed as a percent of maximal initial rate (no treatment). *Significant change from control (P < 0.05). n = 4 determinations/6 hearts for each concentration of thiopental.