Acute phosphatidylinositol 4,5 bisphosphate depletion destabilizes sarcolemmal expression of cardiac L-type Ca channel Ca1.2.

Ca1.2 channels are critical players in cardiac excitation-contraction coupling, yet we do not understand how they are affected by an important therapeutic target of heart failure drugs and regulator of blood pressure, angiotensin II. Signaling through G-coupled AT1 receptors, angiotensin II triggers a decrease in PIP, a phosphoinositide component of the plasma membrane (PM) and known regulator of many ion channels. PIP depletion suppresses Ca1.2 currents in heterologous expression systems but the mechanism of this regulation and whether a similar phenomenon occurs in cardiomyocytes is unknown. Previous studies have shown that Ca1.2 currents are also suppressed by angiotensin II. We hypothesized that these two observations are linked and that PIP stabilizes Ca1.2 expression at the PM and angiotensin II depresses cardiac excitability by stimulating PIP depletion and destabilization of Ca1.2 expression. We tested this hypothesis and report that Ca1.2 channels in tsA201 cells are destabilized after AT1 receptor-triggered PIP depletion, leading to their dynamin-dependent endocytosis. Likewise, in cardiomyocytes, angiotensin II decreased t-tubular Ca1.2 expression and cluster size by inducing their dynamic removal from the sarcolemma. These effects were abrogated by PIP supplementation. Functional data revealed acute angiotensin II reduced Ca1.2 currents and Ca transient amplitudes thus diminishing excitation-contraction coupling. Finally, mass spectrometry results indicated whole-heart levels of PIP are decreased by acute angiotensin II treatment. Based on these observations, we propose a model wherein PIP stabilizes Ca1.2 membrane lifetimes, and angiotensin II-induced PIP depletion destabilizes sarcolemmal Ca1.2, triggering their removal, and the acute reduction of Ca1.2 currents and contractility.