Early effects of denervation on sarcoplasmic reticulum properties of slow-twitch rat muscle

The Ca2+ release activity of the sarcoplasmic reticulum (SR) in chemically skinned single slow-twitch fibres from control, 2-day and 7-day denervated rat soleus muscle was studied. Histochemical fibre type composition of the whole muscle, electrophysiological properties and the Ca2+ sensitivity of tension development by single muscle fibres were also studied, All the data were correlated with contractile properties of the in vitro muscle. In the 2-day denervated muscle the SR Ca2+ capacity and the rate of Ca2+ uptake decreased from the control values of 0.384 +/- 0.030 mu mol (mg frbre protein)(-1) and 19.8 +/- 1.9 nmol min(-1) (mg frbre protein)(-1), respectively, to 0.210 +/- 0.016 mu mol (mg fibre protein)(-1) and 13.5 +/- 0.9 nmol min(-1) (mg fibre protein)(-1); the calculated amount of Ca2+ released upon stimulation by caffeine decreased from the control value of 0.148 to 0.078 mu mol (mg fibre protein)(-1). In the 7-day denervated muscle, the SR Ca2+ capacity and the rate of Ca2+ uptake increased to 0.517 +/- 0.06 mu mol (mg fibre protein)(-1) and 21.6 +/- 2.3 nmol min(-1) (mg fibre protein)(-1), respectively; the calculated amount of Ca2+ released increased to 0.217 mu mol (mg fibre protein)(-1). Both contraction time and tension of the isometric twitch decreased in 2-day denervated and increased in 7-day denervated muscles. Electrophysiological and histochemical changes, as well as changes in the Ca2+ sensitivity of the muscle fibres did not show any apparent correlation with mechanical changes. It is therefore concluded that the SR plays a prominent role in the early changes of contraction time and tension following denervation.