The kinetics of rigor onset in beef muscle fibres

Force and stiffness were investigated as beef muscle fibre preparations entered rigor mortis. The results show that single fibres enter rigor rapidly (exhibit a small t(10-90)) whilst fibre bundles exhibit a much slower rate of entry into rigor (larger t(10-90)). The mean 'rigor time' in both cases is, however, similar. The difference in kinetics is probably due to the fact that individual fibres within a bundle enter rigor at different times. Effectively anaerobic fibre bundles treated with iodoacetate exhibit a t(10-90) almost as short as do single fibres when similarly treated, suggesting that much of the inferred heterogeneity of response amongst fibres in a bundle arises from differences between fibres in the ability of glycolysis to sustain 'relaxing' concentrations of ATP. Fibres have been chemically 'skinned' after entering the rigor state. Measurements of the relationship between stiffness and tension in the skinned fibres indicate that there are two readily distinguishable rigor 'states' depending upon whether rigor is entered from the relaxed of from the contracting state. By contrast, the rigor state achieved by intact fibres is similar to that attained in skinned fibres via the contracted state, suggesting that the 'natural' rigor state is preceded by a calcium-activated contraction. Interconversion of the two rigor states generated in vitro requires an intermediary relaxed state.