Objective: The aim was to determine whether myofilament Ca2+ sensitivity is altered in skinned cardiac fibres from endotoxin-treated rabbits.
Methods: Endotoxin was injected i.v. in conscious New-Zealand White rabbits at a dose of 0.5 mg/kg (groups I, II, and III) or 1 mg/kg (group IV). A fifth group was used as control. Hearts were excised 4 h (groups I and IV), 24 h (group II), or 5 days (Group III) after injection. Skinned fibres were obtained with chemical (EGTA + Brij 58) treatment of bundles isolated from papillary muscles.
Results: The maximal Ca(2+)-activated force (F0) of skinned cardiac fibres was not different between groups. However, [Ca2+] required to evoke 50% of F0 (Ca50) was higher in the fibres from group I than in controls (1.78 +/- 0.05 vs. 1.53 +/- 0.03 microM; P < 0.05). This effect was dose-dependent (group IV: Ca50 = 2.08 +/- 0.12 microM; P < 0.05 vs. group I), larger after 24 h (group II: Ca50 = 2.12 +/- 0.05 microM; P < 0.05 vs. group I). By 5 days, myofilament Ca2+ sensitivity had returned to normal (group III: Ca50 = 1.54 +/- 0.05 microM).
Conclusion: Myofilament Ca2+ sensitivity is decreased in skinned fibres taken from rabbit myocardium after i.v. endotoxin injection. This effect is dose- and time-dependent, and reversible with time.