Skinned ventricular fibres: troponin C extraction is species-dependent and its replacement with skeletal troponin C changes Sr²⁺ activation properties

Troponin C (TnC) was extracted from bundles of rat and mouse skinned cardiac ventricular cells by a method similar to that previously used to extract TnC from skinned rabbit skeletal muscle cells (Kerrick et al., J. biol. Chem.260, 15687-93, 1985) and replaced with either bovine cardiac or rabbit fast-twitch skeletal TnC. In contrast, the same TnC extraction conditions will not extract TnC from either bovine or rabbit cardiac skinned fibres. When the extracted TnC was replaced by bovine cardiac TnC the Sr²⁺-activated tension relationship was not altered from control values. In contrast, replacement of the endogenous TnC with exogenous rabbit fast-twitch TnC caused the relationship between Sr²⁺ concentration and tension to shift towards higher concentrations of Sr²⁺. The Sr²⁺-activated tension of rat fibres with fast-twitch TnC was identical to that of rabbit fast-twitch skinned fibres. Partial skeletal TnC substitution in skinned cardiac cells gradually shifted the relationship between tension and [Sr²⁺] to higher Sr²⁺ concentrations and caused the cells to be activated over a wider range of Sr²⁺ concentrations. Thus it appears that the activation of rat cardiac skinned cells by Sr²⁺ is determined by characteristics of the TnC. In contrast, the Sr²⁺ activation of skinned rabbit fast-twitch skeletal fibres containing either cardiac or skeletal TnC is identical, strongly suggesting that protein-protein interactions determined the Sr²⁺-activation properties in these fibres.