Fig. 8. Differential effects of endocytosed and overexpressed S100A1 on sarcolemmal and sarcoplasmic Ca²⁺-cycling. (A) Simplified scheme of intracellular Ca²⁺-fluxes in a ventricular cardiomyocyte. Endogenous S100A1 (hatched oval) is located at the SR where it interacts with SERCA2a and RyR2. (1) Electrical depolarization of the transverse tubule membrane (T-tubulus) activates inward Ca²⁺-flux through L-type voltage-gated Ca²⁺-channels (LCC), which (2) triggers the release of Ca²⁺ from SR stores via ryandine receptors (RyR2). As a result, contractile filaments are activated. (3) For relaxation to occur, the cytosolic [Ca²⁺]_i must decline. This process is mainly mediated by the SR Ca²⁺-ATPase (SERCA2a), which resequesters cytosolic Ca²⁺ in the SR. (4) At the sarcolemma, Ca²⁺ is extruded primarily via the sodium-calcium exchanger (NCX). (B) Extracellularly added S100A1 is internalized and subsequently routed to the endosomal compartment, where it increases the activity of PLC and PKC (both associated with the endosomal compartment). Through activation of this signaling pathway, internalized S100A1 eventually modulates intracellular Ca²⁺-flux through an enhanced sarcolemmal Ca²⁺-extrusion via NCX. The increased Ca²⁺-extrusion leads to a decreased SR Ca²⁺-load. (C) Effects of overexpressed S100A1 on the intracellular Ca²⁺-cycling. Overexpressed S100A1 is located at the SR, where it associates with the SR-regulatory proteins SERCA2a and RyR2. As a result, intracellular Ca²⁺-cycling is enhanced leading to an increased SR Ca²⁺-uptake and SR Ca²⁺-load. This, in turn, gives rise to an enhanced Ca²⁺-induced SR Ca²⁺-release (CICR).