Ryanodine binding sites measured in small skeletal muscle biopsies

Effects of Congestive Heart Failure on Ca 2+ Handling in Skeletal Muscle During Fatigue

Skeletal muscle weakness and decreased exercise capacity are major symptoms reported by patients with congestive heart failure (CHF). Intriguingly, these skeletal muscle symptoms do not correlate with the decreased heart function. This suggests that CHF leads to maladaptive changes in skeletal muscles, and as reported most markedly in slow-twitch muscles. We used rats at 6 weeks after infarction to measure expression of key proteins involved in SR Ca 2+ release and uptake in slow-twitch soleus muscles. We also measured force and myoplasmic free [Ca 2+ ] ([Ca 2+ ] i ) in intact single fibers of soleus muscles. CHF rats showed clear signs of severe cardiac dysfunction with marked increases in heart weight and left ventricular end-diastolic pressure compared with sham operated rats (Sham). There were small, but significant, changes in the content of proteins involved in cellular Ca 2+ handling in CHF muscles: slight increases in SR Ca 2+ release channels (ie, the ryanodine receptors) and in SR Ca 2+ -ATPase. Tetanic force and [Ca 2+ ] i were not significantly different between CHF and Sham soleus fibers under resting conditions. However, during the stimulation period there was a decrease in tetanic force without changes in [Ca 2+ ] i in CHF fibers that was not observed in Sham fibers. The fatigue-induced changes recovered rapidly. We conclude that CHF soleus fibers fatigue more rapidly than Sham fibers because of a reversible fatigue-induced decrease in myofibrillar function.

Enhanced sarcoplasmic reticulum Ca(2+) release following intermittent sprint training

To evaluate the effect of intermittent sprint training on sarcoplasmic reticulum (SR) function, nine young men performed a 5 wk high-intensity intermittent bicycle training, and six served as controls. SR function was evaluated from resting vastus lateralis muscle biopsies, before and after the training period. Intermittent sprint performance (ten 8-s all-out periods alternating with 32-s recovery) was enhanced 12% (P < 0.01) after training. The 5-wk sprint training induced a significantly higher (P < 0.05) peak rate of AgNO(3)-stimulated Ca(2+) release from 709 (range 560-877; before) to 774 (596-977) arbitrary units Ca(2+). g protein(-1). min(-1) (after). The relative SR density of functional ryanodine receptors (RyR) remained unchanged after training; there was, however, a 48% (P < 0.05) increase in total number of RyR. No significant differences in Ca(2+) uptake rate and Ca(2+)-ATPase capacity were observed following the training, despite that the relative density of Ca(2+)-ATPase isoforms SERCA1 and SERCA2 had increased 41% and 55%, respectively (P < 0.05). These data suggest that high-intensity training induces an enhanced peak SR Ca(2+) release, due to an enhanced total volume of SR, whereas SR Ca(2+) sequestration function is not altered.

Elevated levels of endogenous adenosine alter metabolism and enhance reduction in contractile function during low-flow ischemia: associated changes in expression of Ca(2+)-ATPase and phospholamban

Adenosine has several potentially cardioprotective effects including vasodilatation, reduction in heart rate and alterations in metabolism. Adenosine inhibits catecholamine-induced increase in contractile function mainly through inhibition of phosphorylation of phospholamban (PLB), the main regulatory protein of Ca(2+)-ATPase in sarcoplasmic reticulum (SR), and during ischemia it reduces calcium (Ca2+) overload. In this study we examined the effects of endogenous adenosine on contractile function and metabolism during low-flow ischemia (LFI) and investigated whether endogenous adenosine can alter expression of the Ca(2+)-ATPase/PLB-system and other Ca(2+)-regulatory proteins. Isolated blood-perfused piglet hearts underwent 120 min 10% flow. Hearts were treated with either saline, the adenosine receptor blocker (8)-sulfophenyl theophylline (8SPT, 300 micromol/l) or the nucleoside transport inhibitor draflazine (1 micromol/l). During LFI, 8SPT did not substantially influence metabolic or functional responses. However, draflazine enhanced the reduction in heart rate, contractile force and MVO(2), with less release of H+ and CO2. Before LFI there were no significant differences between groups for any of the proteins (Ca(2+)-ATPase, ryanodine-receptor, Na+/K(+)-ATPase) or mRNAs (Ca(2+)-ATPase, PLB, calsequestrin, Na+/Ca(2+)-exchanger) measured. At end of LFI mRNA-level of PLB was higher in draflazine-treated hearts compared to both other groups (P<0.01 vs both). Also, at end of LFI protein-level of Ca(2+)-ATPase was lower in draflazine-treated hearts (P<0.05 vs both), and a parallel trend towards a lower mRNA-level was seen (P=0.11 vs saline and P=0.43 vs 8SPT). During LFI tissue Ca2+ tended to rise in saline- and 8SPT-treated hearts but not in draflazine-treated hearts (at end of LFI, P=0.01 vs 8SPT). We conclude that the amount of adenosine normally produced during LFI does not substantially influence function and metabolism. However, increased endogenous levels by draflazine enhance downregulation of function and reduce signs of anaerobic metabolism. At end of LFI associated changes in expression of PLB and Ca(2+)-ATPase were seen. The functional significance was not determined in the present study. However, altered protein-levels might influence Ca(2+)-handling in sarcoplasmic reticulum and thus affect contractile force and tolerance to ischemia.

Intracellular calcium signalling in striated muscle cells

Calcium signalling in cells is dependent on a communication between channels/ transporters in two membrane structures: the cell membrane and the membranes of endo- and sarcoplasmic reticula (ER/SR). In general, cytosolic Ca2+ can be raised by influx of calcium over the cell membrane through three types of channels: voltage-, receptor-, and store-operated channels (VOCs, ROCs and SOCs). This small Ca2+ influx is most often amplified by a Ca2+ release from the ER/SR through two types of channels: the IP3-receptor and the ryanodine receptor (RyR), which are huge proteins identified and cloned in recent years. We focus on the 'synaptic' connection between VOCs (L-type calcium channels) and RyRs of the SR in heart and skeletal muscle. Depolarization of the cell membrane (an action potential) opens the VOC and moves it in the membrane. One VOC triggers opening of a certain number of underlying RyRs that together release a quantum of calcium from the SR, a calcium spark. The communication between the VOC and RyRs is probably achieved primarily by a mechanical link in skeletal muscle (voltage-controlled calcium release), and by the small inward calcium flux through the VOC in the heart (calcium-induced calcium release, CICR). Conditions as different as heart failure, myasthenia gravis, malignant hyperthermia, and skeletal muscle fatigue, may be examples of deteriorated control or function of the RyR.