Glucose 6-phosphate alters rat skeletal muscle contractile apparatus and sarcoplasmic reticulum function

We investigated the effects of glucose 6-phosphate (G6P) on skeletal muscle contractile apparatus and sarcoplasmic reticulum (SR) function. Using rat extensor digitorum longus fibres, the presence of 5 mM G6P decreased the Ca2+ sensitivity of both force production and actomyosin ATPase (AM-ATPase) activity. Conversely, maximal Ca(2+)-activated force was unaffected while maximal AM-ATPase activity was increased by 37%. In SR vesicles isolated from rat gastrocnemius, G6P markedly altered Ca2+ handling. It increased Ca(2+)-stimulated Ca(2+)-ATPase activity but depressed the net rate of Ca2+ uptake. This latter effect appears to be due to G6P-stimulated Ca2+ release. When G6P was added to Ca(2+)-loaded vesicles, a small, transient release of Ca2+ was elicited. In addition, G6P lowered the threshold for Ca(2+)-induced Ca2+ release but depressed the net rates of both AgNO3- and caffeine-induced releases. It is possible that the accumulation of G6P during muscular activity may adversely affect muscle force production and contribute to the fatigue process via its action on the contractile apparatus and SR.