Increased muscle glycogen content is associated with increased capacity torespond to T-system depolarisation in mechanically skinned skeletal musclefibres from the rat

The ability of mechanically skinned muscle fibres from the rat to respond t o T-system depolarisation was studied ill relation to muscle glycogen conte nt. Muscle glycogen was altered by incubating extensor digitorum longus (ED L) muscles in Krebs solution without glucose or in Krebs solution with gluc ose (10 mM) and insulin (20 U(.)1(-1)). The glycogen content of muscles sto red without glucose was rather stable between 30 and 480 min (11.27 +/-0.39 mu mol(.)g(-1)), while the muscles stored with glucose and insulin maintai ned an elevated and stable level of glycogen (23.48 +/-1.67 mu mol.g(-1)) b etween 100 and 360 mill. Single mechanically skinned fibres from paired mus cles, incubated in either glucose-free Krebs or in Krebs with glucose and i nsulin, were subjected to cycles of T-system depolarisation-repolarisation in a controlled environment (8 mM ATP, 10 mM creatine phosphate, 1 mM Mg2+, pH 7.10) and the force response was monitored until the force had declined to 50% of the maximum response (50% rundown). Fibres from muscles with a h igher glycogen content reached 50% rundown after a larger number of depolar isations and displayed consistently larger average response capacity values , calculated as the sum of the force responses to 50% rundown divided by th e maximum Ca2+-activated force response in that fibre. Thus skinned fibres originating from muscles with a higher glycogen content have an increased a bility to respond to T-system depolarisation when the effect of metabolite accumulation is minimised and the function of glycogen acting as an energy source is by-passed. This provides direct support to the hypothesis that gl ycogen has a protective role in maintaining fibre excitability.