Incubation with PKA accelerated myofibrillar relaxation half-time from ~150 to ~90 ms in wild-type mice but had no effect in transgenic mice lacking phosphorylatable cardiac TnI.
The acceleration of myofibrillar relaxation rate by PKA is driven by the phosphorylation of troponin I rather than myosin binding protein-C, mediated partly by faster crossbridge cycle kinetics.
Phosphorylation of cardiac myofibrils by cAMP-dependent protein kinase (PKA) can increase the intrinsic rate of myofibrillar relaxation, which may contribute to the shortening of the cardiac twitch during beta-adrenoceptor stimulation. However, it is not known whether the acceleration of myofibrillar relaxation is due to phosphorylation of troponin I (TnI) or of myosin binding protein-C (MyBP-C). To distinguish between these possibilities, we used transgenic mice that overexpress the nonphosphorylatable, slow skeletal isoform of TnI in the myocardium and do not express the normal, phosphorylatable cardiac TNI: The intrinsic rate of relaxation of myofibrils from wild-type and transgenic mice was measured using flash photolysis of diazo-2 to rapidly decrease the Ca(2+) within skinned muscles from the mouse ventricles. Incubation with PKA nearly doubled the intrinsic rate of myofibrillar relaxation in muscles from wild-type mice (relaxation half-time fell from approximately 150 to approximately 90 ms at 22 degrees C) but had no effect on the relaxation rate of muscles from the transgenic mice. In parallel studies with intact muscles, we assessed crossbridge kinetics indirectly by determining f(min) (the frequency for minimum dynamic stiffness) during tetanic contractions. Stimulation of beta-adrenoceptors with isoproterenol increased f(min) from 1.9 to 3.1 Hz in muscles from wild-type mice but had no effect on f(min) in muscles from transgenic mice. We conclude that the acceleration of myofibrillar relaxation rate by PKA is due to phosphorylation of TnI, rather than MyBP-C, and that this may be due, at least in part, to faster crossbridge cycle kinetics.
Kentish et al. (2001) studied this question. Protein Kinase A (PKA) and isoproterenol vs. Transgenic mice lacking phosphorylatable cardiac TnI was evaluated on Intrinsic rate of myofibrillar relaxation and crossbridge kinetics. Incubation with PKA accelerated myofibrillar relaxation half-time from ~150 to ~90 ms in wild-type mice but had no effect in transgenic mice lacking phosphorylatable cardiac TnI.