Calcium Biology of the Transverse Tubules in Heart

Abstract: Ca2+ sparks in heart muscle are activated on depolarization by the influx of Ca2+ through dihydropyridine receptors in the sarcolemmal (SL) and transverse tubule (TT) membranes. The cardiac action potential is thus able to synchronize the [Ca2+]i transient as Ca2+ release is activated throughout the cell. Increases in the amount of Ca2+ within the sarcoplasmic reticulum (SR) underlie augmented Ca2+ release globally and an increase in the sensitivity of the ryanodine receptors (RyRs) to be triggered by the local [Ca2+]i. In a similar manner, phosphorylation of the RyRs by protein kinase A (PKA) increases the sensitivity of the RyRs to be activated by local [Ca2+]i. Heart failure and other cardiac diseases are associated with changes in SR Ca2+ content, phosphorylation state of the RyRs, [Ca2+]i signaling defects and arrhythmias. Additional changes in transverse tubules and nearby junctional SR may contribute to alterations in local Ca2+ signaling. Here we briefly discuss how TT organization can influence Ca2+ signaling and how changes in SR Ca2+ release triggering can influence excitation‐contraction (EC) coupling. High speed imaging methods are used in combination with single cell patch clamp experiments to investigate how abnormal Ca2+ signaling may be regulated in health and disease. Three issues are examined in this presentation: (1) normal Ca2+‐induced Ca2+ release and Ca2+ sparks, (2) abnormal SR Ca2+ release in disease, and (3) the triggering and propagation of waves of elevated [Ca2+]i.

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