Effects of phosphorylation of troponin I and C protein on isometric tension and velocity of unloaded shortening in skinned single cardiac myocytes from rats

Effects on isometric tension generation and maximum velocity of unloaded shortening after exposure to cAMP-dependent protein kinase (PKA) were investigated in rat enzymatically isolated, tritonized ventricular myocytes. Exposure of myocytes to PKA in the presence of 32P]ATP resulted in phosphorylation of troponin I and C protein. Ca2+ sensitivity of isometric tension was assessed as pCa50, ie, the Ca2+] at which tension was 50% of maximum, and was lower after PKA treatment (pCa50 5.58) than before PKA treatment (pCa50 5.74). This suggests beta-adrenergic stimulation of the heart and subsequent increases in PKA activity and phosphorylation of troponin I and C protein lead to a significant decrease in tension-generating ability at a given submaximum Ca2+]. Unloaded shortening velocity was determined by measuring the time required to take up various amounts of slack imposed at one end of the cardiac myocyte preparation. Unloaded shortening velocity during maximum activation was 2.88 +/- 0.11 muscle lengths per second (mean +/- SEM) before PKA exposure and 2.86 +/- 0.13 muscle lengths per second after PKA exposure. Unloaded shortening velocity during 40% of maximum activation was 1.91 +/- 0.25 muscle lengths per second before PKA exposure and 2.17 +/- 0.15 muscle lengths per second after PKA exposure. The absence of an effect of PKA on unloaded shortening velocity in skinned ventricular myocytes suggests that beta-adrenergic stimulation of myocardium either does not affect myofilament velocity of shortening or alters velocity of shortening by a non-PKA-dependent process.