The release of Ca++ from the terminal cisternae through the RYR and its reuptake into the longitudinal portion of the SR by SERCA is depicted in Fig. 12.8. Sarcalumenin, a low-affinity Ca++-binding protein, is present in the longitudinal tubules of the SR and nonjunctional regions of the terminal cisternae. It is believed to be involved in transferring Ca++ from the sites of Ca++ uptake in the longitudinal tubules to the sites of Ca++ release in the terminal cisternae. Research indicates that sarcalumenin enhances Ca++ uptake by SERCA by buffering luminal Ca++ near the pump.

SERCA's activity is regulated by endogenous micropeptides: phospholamban, sarcolipin, and myoregulin. These micropeptides decrease the Ca sensitivity of Ca uptake, as shown in Fig. 12.11. Phosphorylation of phospholamban by protein kinase A in slow-twitch skeletal muscle has been found to increase Ca transport in the SR, similar to the effect seen in the heart. Phospholamban and sarcolipin are present in slow-twitch muscle, while myoregulin is found in both fast and slow-twitch muscle.

During the transport of Ca++, SERCA exchanges two Ca++ ions for two H+ ions, effectively pumping H+ out of the SR. This is illustrated in Fig. 12.10, which shows the molecular structure and relationship between the Dihydropyridine Receptor (DHPR) in the T Tubule Membrane and the Ryanodine Receptor (RYR) in the Sarcoplasmic Reticulum (SR) Membrane. Triadin, an associated SR protein, may participate in the interaction of RYR and DHPR. Calsequestrin, a low-affinity Ca++-binding protein, helps accumulate Ca++ in the terminal cisternae.

The interaction between actin and myosin in skeletal muscle contraction is regulated by the thin filament. The movement of the associated tropomyosin molecule toward the cleft of the actin filament exposes myosin binding sites, allowing a cross-bridge to form and generate tension, as explained in the "Cross-Bridge Cycling: Sarcomere" section.
Fig. 12.8 ) reflects release of Ca++ from the terminal cisternae via the RYR and reuptake primarily into the longitudinal portion of the SR by SERCA. The low-affinity Ca++-binding protein sarcalumenin is present throughout the longitudinal tubules of the SR and nonjunctional regions of the terminal cisternae and is thought to be involved in the transfer of Ca++ from sites of Ca++ uptake in the longitudinal tubules to sites of Ca++ release in the terminal cisternae. Results of studies suggest that sarcalumenin increases Ca++ uptake by SERCA, at least in part by buffering luminal
Ca++ near the pump.
The endogenous micropeptides phospholamban, sarcolipin, and myoregulin have been shown to regulate the activity of SERCA by decreasing the Ca sensitivity of Ca uptake (
Fig. 12.11 ). Protein kinase A–dependent phosphorylation of phospholamban in slow-twitch skeletal muscle has been reported to increase Ca transport in the SR, similar to the effect of phospholamban phosphorylation in the heart. Phospholamban and sarcolipin are present in slow-twitch muscle, whereas myoregulin is present in both fastand slow-twitch muscle.
*During the transport of Ca++ , SERCA exchanges two Ca++ ions for two H+ ions (i.e., H+ is pumped out of the SR).
•Fig. 12.10. Molecular Structure and Relationships Between the Dihydropyridine Receptor (DHPR) in the T Tubule Membrane and the Ryanodine Receptor (RYR) in the Sarcoplasmic Reticulum (SR) Membrane. TriadinisanassociatedSRproteinthatmayparticipateintheinteractionofRYRandDHPR.Calsequestrinisalow-affinityCa++-bindingproteinthathelpsaccumulateCa++ intheterminalcisternae.Seetextfordetails.COOH,carboxylicacid.(FromRossiAE,DirksenRT.Sarcoplasmicreticulum:thedynamiccalciumgovernorofmuscle.Muscle Nerve. 2006;33:715.) movement of the associated tropomyosin molecule toward
Actin-Myosin Interaction: the cleft of the actin filament. This movement of tropo myosin exposes myosin binding sites on the actin filament As noted, contraction of skeletal muscle requires an increase and allows a cross-bridge to form and thereby generate in intracellular [Ca++]. Moreover, the process of contraction tension (see section “
Cross-Bridge Cycling: Sarcomere is regulated by the thin filament. As shown in