Activation and relaxation mechanisms in single muscle fibres.

The effect of Ca2+ on the time course of force generation in frog skinned muscle fibres has been investigated using laser flash photolysis of the caged-calcium, either nitr-5 or DM-Nitrophen. Gradations in the rate and extent of contraction could be achieved by changing the energy of the laser pulse, which varied the amount of caged Ca2+ photolysed and hence the amount of calcium released. The half-time for force development at 12 degrees C was noticeably calcium-sensitive when small amounts of calcium were released (low energy pulses) but did not change appreciably for calcium releases which produced a final tension of more than 50% of the maximal tension at pCa 4.5. This result is unlikely to be due to calcium binding to the regulatory sites of troponin C when on the thin filament, as this process is considered rapid (kon 10(8) M-1 s-1, koff 100 s-1). Our experimental results show that force develops relatively rapidly at intermediate Ca2+ which produce only partial activation (i.e. 50% Pmax or greater). This would not be the case if the affinity of the regulatory sites changes slowly with crossbridge attachment. The kinetics of calcium exchange with the regulatory sites may be much more rapid than crossbridge cycling, so that if calcium binding to a particular functional unit induces crossbridge attachment and force production, the force producing state may be maintained long after calcium has dissociated from that particular functional unit. The relaxation of skinned muscle fibres has also been successfully studied following the rapid uptake of Ca2+ by a photolabile chelator Diazo-2, a photolabile derivative of BAPTA, which is rapidly (> 2000 s-1) converted from a chelator of low Ca2+ affinity (Kd 2.2 microM) to a high affinity chelator (Kd 0.073 microM). We have used single skinned muscle fibres from both frog (actin regulated) and scallop striated muscle (myosin regulated), to study the time course of muscle relaxation. This procedure has enabled us to examine the effects of the intracellular metabolites, ADP, Pi and H+ upon the rate of relaxation.(ABSTRACT TRUNCATED AT 400 WORDS)