Protein Kinase A Increases the Tension Cost and Unloaded Shortening Velocity in Skinned Rat Cardiac Muscle

To address controversies concerning the effect ofβ-adrenergic stimulation on the rate of cross-bridge cycling in cardiac muscle, we measured Ca2+-induced isometric tension development, unloaded shortening velocity (Vmax) and ATPase activity of demembranated (Triton X-100 skinned) rat right ventricular trabeculae before and after treatment with the catalytic subunit of protein kinase A (PKA), which is known to mimic the action ofβ-adrenergic agonists in demembranated preparations. PKA treatment (1U/μl, 40 min) shifted the pCa-tension relation to the right from 5.41 to 5.26 at pCa50(the [Ca2+] required for half maximal steady state tension) without changing the steepness of the pCa-tension relation and the maximum Ca2+-activated tension; Vmax, as determined by the slack test, was increased for a given pCa value, despite the reduced level of isometric tension. PKA treatment also shifted the pCa-ATPase activity to the right slightly from 5.47 to 5.40 at pCa50(the [Ca2+] required for half maximal ATPase activity), but increased the ATPase activity during a given level of steady isometric tension generation, resulting in a 33% increase of the tension cost (ATPase activity/tension). All the results obtained strongly suggest that, in rat right ventricular trabeculae,β-adrenergic stimulation may increase the rate of cross-bridge cycling by increasing the rate of cross-bridge detachment from actin through a PKA-mediated mechanism, although PKA reduces the Ca2+-sensitivity of the contractile system.