Troponin I phosphorylation does not increase the rate of relaxation following laser flash photolysis of diazo-2 in guinea-pig skinned trabeculae

In vivo, two effects of beta-adrenergic stimulation in cardiac muscle are phosphorylation of troponin I and an increase in relaxation rate. In vitro, cardiac TnI can be phosphorylated by protein kinase A (PKA). We have used the technique of laser flash photolysis of the calcium chelator diazo-2 to investigate the effect of phosphorylation of TnI on the relaxation rate of skinned trabeculae from the guinea-pig at 12 degrees C. The fibres were phosphorylated by PKA, and double exponential curve fits of the average relaxation transients showed no significant difference between the rate constants of the phosphorylated and control cases. We conclude that TnI phosphorylation has no effect on the rate of relaxation in skinned trabeculae from the guinea-pig following diazo-2 photolysis.