Function dependent changes in the subcellular distribution of high energy phosphates in fast and slow rat skeletal muscles.

Function dependent changes in the subcellular distribution of ATP, ADP, creatine phosphate (CrP) and creatine (Cr) in rat fast-twitch gastrocnemius and slow-twitch soleus muscles were studied by fractionation of freeze-clamped and freeze-dried tissue in non-aqueous solvents. During 5 min of isotonic contraction of gastrocnemius muscles the mitochondrial content of total creatine [sigma(CrP + Cr)] decreases by 9.5 nmol/mg total protein whereas there is an increase in extramitochondrial total creatine by 12.3 nmol/mg total protein, indicating a net transfer of approximately 10 nmol total creatine/mg total protein/5 min across the mitochondrial inner membrane. During short-term stimulation (6 s) of gastrocnemius muscles the socalled "additionally-bound ADP" correlates not only with force (Hebisch et al. 1984) but also with filament overlap. This confirms the previous suggestion that "additionally-bound ADP" represents actomyosin-ADP-complexes. Following long-term stimulation (10 s), the rate of decay of force is at least two orders of magnitude faster than that of "additionally bound ADP". This indicates a decrease of actomyosin-ADP complexes due to formation of myosin-ADP complexes. Short-term stimulation (6 s) of slow-twitch soleus muscles does not lead to any force-dependent change in the content of "additionally-bound ADP", similar to the finding in long-term contracting gastrocnemius muscles. Denervation of soleus muscles leads to a decrease in "additionally-bound ADP" to values comparable to those found in resting fast-twitch gastrocnemius muscles.