FIGURE 5.

The effect of PKA on the rate of muscle relaxation in skinned cardiac fibers from WT (A) and R21C (B) mice. The rates of muscle relaxation in the absence (solid lines) and presence (dashed lines) of PKA phosphorylation were normalized to the initial steady-state force before the UV flash (as described under “Experimental Procedures”). The muscle relaxed as Ca²⁺ was chelated by Diazo-2 that was converted to its high Ca²⁺ affinity form. A, the average rate of relaxation for WT fibers increased from t_½ of 40.88 ± 2.52 ms to 27.57 ± 1.89 ms (n = 13 and 12, respectively; p < 0.01) after incubating the fiber with PKA. B, the t_½ values for R21C fibers before and after PKA treatment are not statistically different from each other (37.99 ± 3.55 and 35.55 ± 2.51, n = 16 and 22, respectively); nor are they significantly different from those of WT fibers in the absence of PKA.