Fig. 4. Typical chart recordings for measurement of isometric tension with increasing the concentration of free Ca2+in the absence (control) and presence of 100 μm bupivacaine in skinned ventricular trabeculae. pCa was varied from greater than 9 (relaxed condition) to 4.8 in a stepwise manner. Note that the final tension at pCa 4.8 is similar to that obtained before varying pCa in the absence and presence of bupivacaine, showing reproducibility of tension development. Arrowheads indicate the points at which muscle was transferred to the solution containing a higher concentration of free Ca2+, as indicated. Double arrowheads indicate the points at which muscle was transferred to the relaxing solution. Data taken from the same preparation.