Sarcomere length dependence of power output is increased after PKA treatment in rat cardiac myocytes

Abstract

The Frank-Starling relationship of the heart yields increased stroke volume with greater end-diastolic volume, and this relationship is steeper after beta-adrenergic stimulation. The underlying basis for the Frank-Starling mechanism involves length-dependent changes in both Ca(2+) sensitivity of myofibrillar force and power output. In this study, we tested the hypothesis that PKA-induced phosphorylation of myofibrillar proteins would increase the length dependence of myofibrillar power output, which would provide a myofibrillar basis to, in part, explain the steeper Frank-Starling relations after beta-adrenergic stimulation. For these experiments, adult rat left ventricles were mechanically disrupted, permeabilized cardiac myocyte preparations were attached between a force transducer and position motor, and the length dependence of loaded shortening and power output were measured before and after treatment with PKA. PKA increased the phosphorylation of myosin binding protein C and cardiac troponin I, as assessed by autoradiography. In terms of myocyte mechanics, PKA decreased the Ca(2+) sensitivity of force and increased loaded shortening and power output at all relative loads when the myocyte preparations were at long sarcomere length ( approximately 2.30 mum). PKA had less of an effect on loaded shortening and power output at short sarcomere length ( approximately 2.0 mum). These changes resulted in a greater length dependence of myocyte power output after PKA treatment; peak normalized power output increased approximately 20% with length before PKA and approximately 40% after PKA. These results suggest that PKA-induced phosphorylation of myofibrillar proteins explains, in part, the steeper ventricular function curves (i.e., Frank-Starling relationship) after beta-adrenergic stimulation of the left ventricle.

Fig. 1.

A: photomicrographs of a cardiac myocyte preparation at long and short sarcomere length (SL). B: myocyte preparation length traces during light load clamps at long and short SL before and after PKA treatment. C: force-velocity and power-load curves showing the SL dependence of loaded shortening and power output before and after PKA. The SL dependence of peak power output was significantly greater after PKA treatment, due primarily to the enhanced power at long SL. D: bar plot of peak normalized power output and peak absolute power output at long and short SL before and after PKA. P/P_{4.5 long SL}, force produced at long sarcomere length with pCa 4.5 solution. Values are means ± SD. *P < 0.05 vs. long SL; #P < 0.05 vs. short SL; §P < 0.05 vs. short SL after PKA.

Fig. 2.

Silver-stained gel and autoradiogram of 1 mg of permeabilized rat cardiac myocytes incubated for 45 min with the catalytic subunit of PKA in the presence of 50 μCi [γ-³²P]ATP or 50 μCi [γ-³²P]ATP alone. PKA treatment increased the phosphorylation of myosin-binding protein C (MyBP-C) and cardiac troponin I (cTnI) in skinned cardiac myocytes.

Fig. 3.

A: force redevelopment traces of a cardiac myocyte preparation at long SL before and after PKA treatment. B: rate constants of force redevelopment (k_tr) at long and short SL before and after PKA treatment. Values are means ± SD. *P < 0.05 vs. long SL; #P < 0.05 vs. short SL. C: slow time base force records after a slack-restretch maneuver. Force redevelopment traces were slower and exhibited a marked transient overshoot after PKA treatment.