Calcium-activated and stretch-induced force responses in two biochemically defined muscle fibre types of the Norway lobster.

Mechanical properties of thin (< 80 microns) myofibrillar bundles from single rehydrated freeze-dried fibres of the superficial abdominal flexor muscle of the lobster Nephrops norvegicus have been measured, and subsequently the protein content of these fibres has been analysed by SDS-PAGE. Two slow fibre phenotypes can be distinguished on the basis of their myofibrillar assemblages and sarcomere length (type S1: 6.0-7.5 microns, type S2: 8.0-10.9 microns). Differences (means +/- SD, average of seven fibres of each type) were observed in the kinetics for Ca2+ activation (half time of force development (ms); S1: 416 +/- 174; S2: 762 +/- 199 plus a delay of 280 +/- 130) and relaxation (half time of force decay (ms); S1: 162 +/- 75, S2: 257 +/- 53), for Ca2+ sensitivity of force generation (-log [Ca2+] for half maximal activation; S1: 5.40 +/- 0.12; S2: 5.55 +/- 0.08), and of the kinetics of stretch activation (delay of the peak of stretch-induced force increase (ms); S1: 91 +/- 30; S2: 493 +/- 436). From these results and partly also in combination with previously obtained mechanical data on intact fibres it can be concluded (1) that S2 fibres are specialized for long-lasting force maintenance whereas S1 fibres are adapted for slow movements; (2) intrinsic myofibrillar kinetics is not the main time-limiting factor for either activation or relaxation of intact fibres under physiological conditions; (3) processes which precede crossbridge cycling seem to be the main time-limiting factors for the Ca2+ activation of the myofibrils.