Troponin-based regulation of sarcomere length-dependence of activation in skinned porcine ventricular muscle

Abstract

Recent findings indicate that the sarcomere length (SL)-dependence of activation is modulated partly via a reduction in interfilament lattice spacing due to titin-based passive force. We tested the hypothesis that the Frank-Starling mechanism of the heart is regulated at the thin filament level, by the Ca²⁺ regulatory protein, troponin. We used skinned porcine ventricular muscle. Reconstitution of the preparations with fast skeletal troponin (from rabbit psoas muscle) increased Ca²⁺ sensitivity and attenuated SL-dependent increase in Ca²⁺ sensitivity. SDS-PAGE analysis showed that endogenous cardiac troponin subunits were clearly replaced with skeletal counterparts and titin was not degraded upon troponin exchange. Pimobendan, increased Ca²⁺ sensitivity similar to sTn; however this compound did not affect SL-dependent activation, suggesting that a simple increase in Ca²⁺ binding to troponin C does not underlie the attenuation of SL-dependent activation by sTn. Also, sTn diminished osmotic compression-induced increases in Ca²⁺ sensitivity. We found that the Ca²⁺-desensitizing effect of Pi was attenuated in sTn-reconstituted preparations, suggesting that cross-bridge formation is accelerated upon sTn reconstitution. It is likely that the proportion of detached cross-bridge is decreased as cross-bridge formation is accelerated. Therefore, sTn may attenuate SL-dependent activation via reduction in the proportion of detached cross-bridges that can potentially produce active force. [J Physiol Sci. 2007;57 Suppl:S92]