Direct Evidence for Microdomain-Specific Localization and Remodeling of Functional L-Type Calcium Channels in Rat and Human Atrial Myocytes (original) (raw)

I n the heart, L-type calcium channels (LTCCs) are essential in determining the electric and mechanical properties of cardiac muscle. 1 In adult ventricular myocytes, LTCCs are predominantly located in the transverse tubules (T-tubules), 2 where they form dyadic complexes with calcium-sensing and-release units, the ryanodine receptors (RyR2s) on the opposing junctional sarcoplasmic reticulum membrane (SR). A well-developed network of ventricular T-tubules ensures that the electric impulse is conducted into the cell interior, where Ca 2+ influx can trigger the opening of RyR2 and subsequent release of SR Ca 2+ stores. Atrial myocytes are believed to lack an elaborate T-tubule network 3-5 and their Ca 2+ signaling is substantially different from that in ventricular myocytes. 6-8 Lack of a regular T-tubular system has been thought to affect the distribution of LTCCs and give rise to the unique Ca 2+ signaling in atrial myocytes. 6,9,10 Clinical Perspective on p 2384 A number of important LTCC subpopulations have been identified in ventricular myocytes that associate with unique Background-Distinct subpopulations of L-type calcium channels (LTCCs) with different functional properties exist in cardiomyocytes. Disruption of cellular structure may affect LTCC in a microdomain-specific manner and contribute to the pathophysiology of cardiac diseases, especially in cells lacking organized transverse tubules (T-tubules) such as atrial myocytes (AMs). Methods and Results-Isolated rat and human AMs were characterized by scanning ion conductance, confocal, and electron microscopy. Half of AMs possessed T-tubules and structured topography, proportional to cell width. A bigger proportion of myocytes in the left atrium had organized T-tubules and topography than in the right atrium. Super-resolution scanning patch clamp showed that LTCCs distribute equally in T-tubules and crest areas of the sarcolemma, whereas, in ventricular myocytes, LTCCs primarily cluster in T-tubules. Rat, but not human, T-tubule LTCCs had open probability similar to crest LTCCs, but exhibited ≈40% greater current. Optical mapping of Ca 2+ transients revealed that rat AMs presented ≈3-fold as many spontaneous Ca 2+ release events as ventricular myocytes. Occurrence of crest LTCCs and spontaneous Ca 2+ transients were eliminated by either a caveolae-targeted LTCC antagonist or disrupting caveolae with methyl-βcyclodextrin, with an associated ≈30% whole-cell I Ca,L reduction. Heart failure (16 weeks post-myocardial infarction) in rats resulted in a T-tubule degradation (by ≈40%) and significant elevation of spontaneous Ca 2+ release events. Although heart failure did not affect LTCC occurrence, it led to ≈25% decrease in T-tubule LTCC amplitude. Conclusions-We provide the first direct evidence for the existence of 2 distinct subpopulations of functional LTCCs in rat and human AMs, with their biophysical properties modulated in heart failure in a microdomain-specific manner.