Skeletal muscle is the bodie’s largest organ and the source of locomotion. Muscle contraction is the result of a cascade of events that transfer electrical signals involving surface ion channels to intracellular chemical signals that involve release of Ca2+ ions from intracellular stores. Finally, increases in cytosolic Ca2+ concentrations are transferred to mechanical signals that involve the activation of the motorproteins, actin and myosin, that ultimately result in force production by power strokes of the myosin molecule along the actin filament.
High hydrostatic pressure (HP) acts very distinct on elements in the different steps of this cascade. We have focussed on prolonged high pressure treatments that may irreversibly change the conformational states of the proteins involved such as their function will be permanently compromised even in the decompression phase. Using mouse skeletal muscles that were pressurised for 3 h at 4°C, ‘ex situ’ experiments showed that isometric force production markedly declined for pressures larger than 20 MPa as muscle became tremendously stiff . For larger pressures, cells were usually irreversibly contracted.
During the COST D30 funding period, we found that Na+ and Ca2+ channels which represent the switch for membrane excitability and excitation-contraction coupling were functionally ‘knocked out’ as the surface density of functional channels decreased significantly from 20 MPa as judged by the reduction of peak current amplitudes with unaltered steady-state inactivation [2,3]. Interestingly, intracellular RYR1 release channels were much less affected by HP up to 30 MPa probably due to a more protected environment within intracellular membranes . Using a novel ‘in situ’ high pressure epi-fluorescence/confocal microscopy technique, we could clarify the underlying process of the irreversible contracture induced by HP >20 MPa. A general breakdown of membrane integrity could be excluded. However, we found that HP seemed to induce a sustained leakage of Ca2+ ions from intracellular stores that could partially be buffered by mitochondria. As mitochondria function became more impaired, Ca2+ concentrations began to rise and irreversibly activated the contractile apparatus . For these results, we collaborated with TU Munich within the D30 working group.
As skeletal muscle function and proteins are highly conserved in vertebrates, we wondered whether some adaptive mechanisms must have evolved in deep sea fish to counteract high ambient pressures. During the funding period, a STSM was performed with JAMSTEC, Japan, to conduct electrophysiology experiments in deep sea fish that were salvaged from depths up to 1.000 m. So far, we found that HP seems to change the K+ selectivity of resting membranes, i.e. a decrease with depth, and that deeper fish seem to have higher internal K+ levels .
In a final conclusion, our HP limits found in mammalian muscle closely resemble the depth limits of diving whales. Therefore, muscle from terrestrian mammals studied under HP may serve as a model for muscle function in diving mammals. Further studies in fish species will show whether there is a profound difference between mammalian and fish muscle under HP.
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