It is easy to forget how crowded cells are. The criss-crossing cytoskeletal network is punctuated by organelles and macromolecules so that the only molecules that can freely diffuse between the structures are small molecules. But the organisation of complex cellular environments is determined by cellular function. While all types of muscle produce movement by contraction,the differing speeds and strength of contractions, as well as their voluntary or involuntary control, determines their underlying structure. Research has found, however, that the diffusion of metabolites across muscle fibres occurs in much the same way, whether the muscle comes from lobster, fish or mammals(p. 3377).
Stephen Kinsey and Timothy Moerland follow the movement of metabolites in intact muscle fibres using pulsed-field gradient nuclear magnetic resonance. Using this popular technique, Kinsey and Moerland calculate the displacement of molecules in the fibre by magnetically labelling their position and tracking them over time. `Time is essentially equivalent to space,' explains Kinsey, as diffusion has a constant speed through a given medium. Seeing how diffusion changes with time, however, means that intracellular barriers that impede diffusion can be identified, along with their size. In this case giant muscle fibres from the spiny lobster were used, because of their characteristic size and structure which simplifies the identification of such barriers.
While other groups have made diffusion measurements in muscle, only Kinsey and his collaborators have unambiguously measured the direction. By carefully aligning fibres in the magnetic field, Kinsey and Moerland found that the metabolite arginine phosphate — necessary for ATP transport —diffuses more slowly across the muscle fibre than along its long axis. After eliminating the effect of the large membrane around the fibre, and the mitochondria, which lie around the edge of the fibre, they decided that the only barrier of the right size that could be causing this effect was the sarcoplasmic reticulum, which releases the calcium ions that initiate muscular contraction. Interestingly, the diffusion measurements agree with those from goldfish and mammalian muscle, suggesting that the sarcoplasmic reticulum acts as a common diffusive barrier in many muscle types.
The abdominal muscles in the spiny lobster are used to rapidly escape predators, but the large distances that metabolites have to diffuse across means that the recovery from contraction is slow. Surely this is a disadvantage? `It's a mystery to me why these cells are so large,' says Kinsey. One possibility is that recovery is limited by factors other than diffusion distance. Alternatively, evolution may have reached a balance between the amount of contractile machinery and a porosity that allows metabolite diffusion. However, it may simply be that by weighing in at around one kilogram, these lobsters are simply big enough to defend themselves without having to recover quickly after an encounter with a predator.
Considering a complex whole cell system, it is hardly surprising that such work is generating more questions than answers at this early stage. Kinsey and Moerland want to look further at the contractile recovery system in crustacean muscle, paying special attention to the effects of diffusion. It will be exciting to see whether the general characteristics of diffusion in specialised cells hold up under closer scrutiny.