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

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 approximate to 40% greater current. Optical mapping of Ca2+ transients revealed that rat AMs presented approximate to 3-fold as many spontaneous Ca2+ release events as ventricular myocytes. Occurrence of crest LTCCs and spontaneous Ca2+ transients were eliminated by either a caveolae-targeted LTCC antagonist or disrupting caveolae with methyl-beta-cyclodextrin, with an associated approximate to 30% whole-cell I-Ca,I-L reduction. Heart failure (16 weeks post-myocardial infarction) in rats resulted in a T-tubule degradation (by approximate to 40%) and significant elevation of spontaneous Ca2+ release events. Although heart failure did not affect LTCC occurrence, it led to approximate 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.