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

Abstract

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 micromol (mg fibre protein)-1 and 19.8 +/- 1.9 nmol min-1 (mg fibre protein)-1, respectively, to 0.210 +/- 0.016 micromol (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 micromol (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 micromol (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 micromol (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.

Denervation produces different single fiber phenotypes in fast- and slow-twitch hindlimb muscles of the rat

“…This trend is consistent with the results of Schulte et al (39) showing that after day 28 of denervation the ratio between SERCA1 and SERCA2a in SOL muscle increases due to a decrease in SERCA2a expression. The SR Ca 2ϩ loading capacity also increases in fibers from denervated SOL muscle (27). This is also consistent with a shift toward the SR characteristics of fast-twitch fibers, which have a severalfold greater SR Ca 2ϩ loading capacity compared with slow-twitch fibers (12).…”

“…After 14 and 28 days of denervation, there is also a net decrease in the total SERCA protein per total muscle protein (39), implying a decrease in the density of SR Ca 2ϩ pumps in denervated SOL muscles. These changes in SR properties would promote increased Ca 2ϩ release due to increased SR Ca 2ϩ capacity and slower reuptake due to reduced SR Ca 2ϩ pump capacity leading to a prolongation of the twitch time course and increased twitch peak in the denervated rat SOL muscle as reported in several studies (11,23,27). Unlike the denervated EDL fibers, the denervated SOL fibers do not display marked changes in the asymmetrical charge movements associated with the voltage sensors (8).…”

Expression of Sarco(endo)Plasmic Reticulum Ca2+‐Atpase Slow (SERCA2) Isoform in Regenerating Rat Soleus Skeletal Muscle Depends on Nerve Impulses

“…Normal and regenerating fibres were chemically skinned as previously described (Salviati et al 1982;Midrio et al 1997;Danieli-Betto et al 2000). Briefly, muscles were tied to a wooden stick and quickly immersed into an ice-cold skinning solution containing (mM): potassium propionate 170, magnesium propionate 2.5, Na 2 ATP 2.5, K 2 EGTA 5 and imidazole buffer 10; pH 7.0.…”

“…2+ uptake measurements Ca 2+ uptake by the sarcoplasmic reticulum was measured at room temperature (22-24°C) using a light-scattering method (Sorenson et al 1980;Salviati et al 1982;Midrio et al 1997). Fibres were mounted in a chamber in a relaxing solution containing (mM): potassium propionate 170, magnesium propionate 2.5, Na 2 ATP 5, K 2 EGTA 5 and imidazole buffer 10; pH 7.0, and stretched to 180 % of slack length to avoid interference in lightscattering measurements caused by actin-myosin interactions.…”

“…The plateau level of light scattering represents the maximum capacity for Ca 2+ uptake of sarcoplasmic reticulum (Sorenson et al 1980;Salviati et al 1983). The slope of the linear part of the curve provides a measure of the rate of Ca 2+ accumulation (Midrio et al 1997). The calibration procedure for converting the lightscattering signal to fibre Ca 2+ concentration was performed by using 45 Ca 2+ (Salviati et al 1983).…”