Abstract 2802: Staurosporine inhibits Frequency Dependent Myofilament Desensitization in Intact Rabbit Trabeculae.
Increases in heart rate change both contractility (force-frequency relationship or FFR) and relaxation rate (frequency dependent acceleration of relaxation or FDAR). These relationships are altered in heart failure and have been shown to contribute to the impaired contractility and/or relaxation. We have recently shown that myofilament calcium sensitivity decreases with frequency and this mechansim may contribute to FDAR. Phosphoprotein analysis revealed that both Troponin I and Myosin Light Chain-2 became more phosphorylated when frequency increased. We set out to test whether phosphorylation of these myofilaments are involved in the underlying molecular mechanism responsible for FDAR. Ultra-thin right ventricular trabeculae were isolated from New Zealand White rabbit hearts and iontophorically loaded with the calcium indicator bis-fura-2. Twitch force-calcium relationships and steady state force-[Ca2+]i relationships were measured at frequencies 1 and 4 Hz at 37°C using potassium induced contractures. Staurosporine (100 nM), a non-specific Ser-Thr kinase inhibitor, or vehicle (DMSO) were included in the superfusion solution before and during the contractures. The EC 50 of the force-calcium relationship shifted significantly 0.35 pCa units to the right upon an increase in frequency under control conditions (vehicle only, P<0.01). Staurosporine eliminated this shift such that there was virtually no change in sensitivity (0.06 pCa units to the left with an increase in frequency, P=n.s.). More specific kinase inhibitors such as KN93 (CaMKII) did not alter the normal shift in sensitivity observed with an increase in frequency. We conclude that a serine-threonine kinase is responsible for frequency dependent myofilament desensitization, and while we can eliminate CaMKII as the sole contributor, we cannot currently eliminate any of the other major possible candidates (PKC, PKG, PKA or a combination several).