Fig. 1.

Intracellular Ca²⁺ regulation in adult ventricular myocytes. (a, top panel) a representative confocal image of rabbit ventricular myocytes loaded with the voltage-sensitive fluorescent dye Di-8-ANEPPS. Di-8-ANEPPS was used to label the T-tubular system. (a, bottom panel) the diagram illustrates the main components of Ca²⁺ release unit (CRU) in ventricular myocytes. A significant fraction of L-type Ca²⁺ channels (LTCC) is localized in the T-tubule (TT), whereas the majority of ryanodine receptors (RyR2) is concentrated in the junctional SR. Ca²⁺-ATPase (SERCA) pumps cytosolic Ca²⁺ back into the SR, and the Na⁺-Ca²⁺ exchanger (NCX) removes Ca²⁺ from the cell. The plasmalemmal Ca²⁺-ATPase and mitochondria play a minor role in cardiac relaxation. (b) Confocal images of diastolic Ca²⁺ spark (top) and an action potential-induced Ca²⁺ transient (bottom). Activation of a single CRU generates a Ca²⁺ spark, whereas simultaneous activation of thousands of these individual release units generates a global Ca²⁺ transient