Effects of ADP on sarcoplasmic reticulum function in mechanically skinned skeletal muscle fibres of the rat

Abstract

The sarcoplasmic reticulum (SR) Ca2+ content (expressed in terms of endogenous SR Ca2+ content under physiologically resting conditions and measured from caffeine‐induced force responses) and the effective rates of the SR Ca2+ pump and SR Ca2+ leak (measured from the temporal changes in SR Ca2+ content) were determined in mechanically skinned skeletal muscle fibres of the rat at different [ADP] (< 0.10 μm to 1.04 mm). The estimated SR Ca2+ pump rate at 200 nm Ca2+ did not change when [ADP] increased from below 0.10 μm to 10 μm but decreased by about 30 % when [ADP] increased from 10 μm to 1.04 mm. The rate constant of SR Ca2+ leak increased markedly with rising [ADP] when [Ca2+] in solution was 200 nm (apparent dissociation constant KdADP= 64 ± 27 μm). Decreasing the [Ca2+] in solution from 200 nm to < 10 nm significantly increased the leak rate constant at all [ADP]. The SR Ca2+ leak rate constant could be significantly reduced by blocking the SR Ca2+ pump with 2,5‐di(tert‐butyl)‐1,4‐hydroquinone (TBQ). The decrease in the SR Ca2+ pump rate and the increase in the rate constant of SR Ca2+ leak when the [ADP] increased from < 0.10 μm to 1.04 mm caused a 4.4‐fold decrease in SR Ca2+ loading ability at 200 nm Ca2+. The results can be fully explained by a mechanism whereby the presence of ADP causes a marked increase in the ADP‐sensitive fraction of the phosphorylated pump protein, which can act as a Ca2+‐Ca2+ exchanger and demonstrates that ADP is an important modulator of SR function in skeletal muscle.

Mitochondrial and sarcoplasmic reticulum abnormalities in cancer cachexia: Altered energetic efficiency?

“…If we take into consideration the fact that cancer patients have an increased energy expenditure [33], which contributes to weight loss, it becomes clear the importance that SERCA pumps may have in cancer cachexia. In addition, previous studies have shown that an increased cytosolic ADP concentration, due to an increased SERCA activity, leads to Ca 2+ leakage from the SR in fast-twitch muscles, effectively reducing the capacity of SERCA to maintain an adequate Ca 2+ intracellular gradient, and thus leading to increased energetic inefficiency [34].…”

“…into mitochondria can worsen the catabolic condition in cachexia, potentiate the ADP accumulation [34], Ca 2+ leak and heat production as represented in the Figure 4. The constant Ca 2+ leakage from SR and disruption of mitochondrial functions amplify subsequently the degradation signals, triggering proteolytic processes through calpain, cathepsins and Ub-Proteasome systems, and apoptotic signals performed by PTP formation through ANT1 and Bax [48].…”

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

“…In a mechanically skinned fiber preparation, the relative levels of Ca 2ϩ loading in the SR are indicated by the relative areas under the force responses elicited upon exposure of the preparation to the caffeine-and-low-Mg 2ϩ SR Release solution (25). In a previous study from one of our laboratories (12), we showed that typically fast-twitch EDL muscle fibers and slowtwitch SOL fibers display quite different SR Ca 2ϩ -loading properties.…”

Fiber Types in Mammalian Skeletal Muscles

“…Among the splicing variants, the isoform SERCA2a is specific for muscle cells, while SERCA2b is a "housekeeping" isoform, ubiquitously expressed in all cells (507). When studied in muscle fibers (512), SERCA isoforms show a pronounced diversity in calcium leakage and ADP sensitivity, which may impair the refilling of the SR and become relevant during muscle fatigue when ADP concentration rises. Slow fibers show a smaller ADP-induced reduction in SR calcium-pump rate and a lower increase in SR calcium-leak rate, thus contributing to the greater resistance to low-frequency fatigue typical of slow fibers (511) (see below).…”