β-Adrenergic Receptor Stimulation Increases Unloaded Shortening Velocit of Skinned Single Ventricular Myocytes From Rats

In vitro biochemical experiments have suggested that stimulation of β-adrenergic receptors may increase the rate of crossbridge cycling in mammalian myocardium, but recent attempts to demonstrate a mechanical correlate have yielded conflicting results. To investigate this issue, we measured the effect of isoproterenol (ISO) and cAMP-dependent protein kinase (PKA) on unloaded shortening velocity (V0). V0 is thought to be determined by the rate-limiting step of the crossbridge cycle, ie, the rate of crossbridge detachment from actin, and is therefore an index of the cycling rate. Single rat ventricular myocytes were enzymatically isolated, incubated in Ringerʼs solution without (control) or with 0.1 μmol/L ISO, and then rapidly skinned. Some control cells were subsequently treated with 3 μg/mL PKA for 40 minutes. V was then measured during maximal activation (pCa 4.5) in control, ISO-treated, and PKA-treated cells using the slack-test method. To test the efficacy of the agonist treatments, Ca sensitivity of isometric tension was also assessed for each treatment by determining the [Ca] required for half-maximal tension (ie, pCa50). Both ISO and PKA treatment reduced the Ca sensitivity of isometric tension compared with same-day control cells, in agreement with previous studies in intact and in skinned preparations. VO was increased 38% by ISO treatment and 41% by PKA treatment compared with same-day control cells. P autoradiography showed that troponin I and C protein were the principal proteins phosphorylated by PKA treatment. We conclude that β-adrenergic stimulation increases the rate of crossbridge release from actin, by a mechanism that most likely involves the phosphorylation of troponin I and/or C protein by PKA.