CALCIUM-ACTIVATED AND STRETCH-INDUCED FORCE RESPONSES IN 2 BIOCHEMICALLY DEFINED MUSCLE-FIBER TYPES OF THE NORWAY LOBSTER

Mechanical properties of thin (<80 mu m) myofibrillar bundles from sin gle rehydrated freeze-dried fibres of the superficial abdominal flexor muscle of the lobster Nephrops norvegicus have been measured, and sub sequently 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 S-1: 6.0-7.5 mu m, type S-2: 8.0-10.9 mu m). Differences (means+/-SD, average of s even fibres of each type) were observed in the kinetics for Ca2+ activ ation (half time of force development (ms); S-1: 416 +/- 174; S-2: 762 +/- 199 plus a delay of 280 +/- 130) and relaxation (half time of for ce decay (ms); S-1: 162 +/- 75, S-2: 257 +/- 53), for Ca2+ sensitivity of force generation (-log [Ca2+] for half maximal activation; S-1: 5. 40 +/- 0.12; S-2: 5.55 +/- 0.08), and of the kinetics of stretch activ ation (delay of the peak of stretch-induced force increase (ms); S-1: 91 +/- 30; S-2: 493 +/- 436). From these results and partly also in co mbination with previously obtained mechanical data on intact fibres it can be concluded (1) that S-2 fibres are specialized for long-lasting force maintenance whereas S-1 fibres are adapted for slow movements; (2) intrinsic myofibrillar kinetics is not the main time-limiting fact or for either activation or relaxation of intact fibres under physiolo gical conditions; (3) processes which precede crossbridge cycling seem to be the main time-limiting factors for the Ca2+ activation of the m yofibrils.