Protein kinase A increases the rate of relaxation but not the rate of tension development in skinned rat cardiac muscle

To clarify the contribution of crossbridge kinetics to the contraction prof ile of cardiac twitch during P-adrenergic stimulation, we studied the rate of tension development and relaxation following laser flash photolysis of c aged compounds in rat-skinned ventricular trabeculae before and after treat ment with the catalytic subunit of protein kinase A (PKA, 0.5 U/mul, 40 min ). Tension development following nitrophenyl (NP)EGTA photolysis was fitted with a single exponential function. The rate constant increased with an in crease in postphotolysis steady tension, and the relation between the rate constant and the tension was not influenced by PKA. The rate of relaxation following diazo-2 photolysis was fitted with a double exponential function. The rate of both initial rapid and subsequent slow relaxation was independ ent of the extent of relaxation. PKA increased the rate of initial rapid re laxation by about twofold, but showed no significant effect on the rate of subsequent slow relaxation. These results suggest that in beta -receptor st imulated rat cardiac muscle, the increased rate of tension development and the facilitated relaxation rate during twitch can be partly explained as be ing due to the combined effects of decreased Ca2+ affinity of troponin C an d increased cycling rate of cross-bridges (subtractive combination for tens ion development and additive combination for tension relaxation).