Autocrine A2 in the T-system of ventricular myocytes creates transmural gradients in ion transport: a mechanism to match contraction with load?

Control of cardiac contraction by sodium: Promises, reckonings, and new beginnings

Several generations of cardiac physiologists have verified that basal cardiac contractility depends strongly on the transsarcolemmal Na gradient, and the underlying molecular mechanisms that link cardiac excitation-contraction coupling (ECC) to the Na gradient have been elucidated in good detail for more than 30 years. In brief, small increases of cytoplasmic Na push cardiac (NCX1) Na/Ca exchangers to increase contractility by increasing the myocyte Ca load. Accordingly, basal cardiac contractility is expected to be physiologically regulated by pathways that modify the cardiac Na gradient and the function of Na transporters. Assuming that this expectation is correct, it remains to be elucidated how in detail signaling pathways affecting the cardiac Na gradient are controlled in response to changing cardiac output requirements. Some puzzle pieces that may facilitate progress are outlined in this short review. Key open issues include (1) whether the concept of local Na gradients is viable, (2) how in detail Na channels, Na transporters and Na/K pumps are regulated by lipids and metabolic processes, (3) the physiological roles of Na/K pump inactivation, and (4) the possibility that key diffusible signaling molecules remain to be discovered.

Prolonged AT1R activation induces CaV1.2 channel internalization in rat cardiomyocytes

The cardiac L-type calcium channel is a multi-subunit complex that requires co-assembling of the pore-forming subunit Ca_V1.2 with auxiliary subunits Ca_Vα₂δ and Ca_Vβ. Its traffic has been shown to be controlled by these subunits and by the activation of various G-protein coupled receptors (GPCR). Here, we explore the consequences of the prolonged activation of angiotensin receptor type 1 (AT₁R) over Ca_V1.2 channel trafficking. Bioluminescence Resonance Energy Transfer (BRET) assay between β-arrestin and L-type channels in angiotensin II-stimulated cells was used to assess the functional consequence of AT₁R activation, while immunofluorescence of adult rat cardiomyocytes revealed the effects of GPCR activation on Ca_V1.2 trafficking. Angiotensin II exposure results in β-arrestin₁ recruitment to the channel complex and an apparent loss of Ca_V1.2 immunostaining at the T-tubules. Accordingly, angiotensin II stimulation causes a decrease in L-type current, Ca²⁺ transients and myocyte contractility, together with a faster repolarization phase of action potentials. Our results demonstrate that prolonged AT₁R activation induces β-arrestin₁ recruitment and the subsequent internalization of Ca_V1.2 channels with a half-dose of AngII on the order of 100 nM, suggesting that this effect depends on local renin-angiotensin system. This novel AT₁R-dependent Ca_V1.2-trafficking modulation likely contributes to angiotensin II-mediated cardiac remodeling.

Pressure-overload-induced angiotensin-mediated early remodeling in mouse heart

Our previous work on angiotensin II-mediated electrical-remodeling in canine left ventricle, in connection with a long history of other studies, suggested the hypothesis: increases in mechanical load induce autocrine secretion of angiotensin II (A2), which coherently regulates a coterie of membrane ion transporters in a manner that increases contractility. However, the relation between load and A2 secretion was correlative. We subsequently showed a similar or identical system was present in murine heart. To investigate whether the relation between mechanical load and A2-mediated electrical remodeling was causal, we employed transverse aortic constriction in mice to subject the left ventricle to pressure overload for short-term (1 to 2 days) or long-term (1 to 2 weeks) periods. Heart-to-body weight ratios and cell capacitance measurements were used to determine hypertrophy. Whole-cell patch clamp recordings of the predominant repolarization currents Ito,fast and IK,slow were used to assess electrical remodeling. Hearts or myocytes subjected to long-term load displayed significant hypertrophy, which was not evident in short-term load. However, short-term load induced significant reductions in Ito,fast and IK,slow. Incubation of these myocytes with the angiotensin II type 1 receptor inhibitor saralasin for 2 hours restored Ito,fast and IK,slow to control levels. The number of Ito.fast or IK,slow channels did not change with A2 or long-term load, however the hypertrophic increase in membrane area reduced the current densities for both channels. For Ito,fast but not IK,slow there was an additional reduction that was reversed by inhibition of angiotensin receptors. These results suggest increased load activates an endogenous renin angiotensin system that initially reduces Ito,fast and IK,slow prior to the onset of hypertrophic growth. However, there are functional interactions between electrical and anatomical remodeling. First, hypertrophy tends to reduce all current densities. Second, the hypertrophic program can modify signaling between the angiotensin receptor and target current.

Calcineurin signaling in the heart: The importance of time and place

The calcium-activated protein phosphatase, calcineurin, lies at the intersection of protein phosphorylation and calcium signaling cascades, where it provides an essential nodal point for coordination between these two fundamental modes of intracellular communication. In excitatory cells, such as neurons and cardiomyocytes, that experience rapid and frequent changes in cytoplasmic calcium, calcineurin protein levels are exceptionally high, suggesting that these cells require high levels of calcineurin activity. Yet, it is widely recognized that excessive activation of calcineurin in the heart contributes to pathological hypertrophic remodeling and the progression to failure. How does a calcium activated enzyme function in the calcium-rich environment of the continuously contracting heart without pathological consequences? This review will discuss the wide range of calcineurin substrates relevant to cardiovascular health and the mechanisms calcineurin uses to find and act on appropriate substrates in the appropriate location while potentially avoiding others. Fundamental differences in calcineurin signaling in neonatal verses adult cardiomyocytes will be addressed as well as the importance of maintaining heterogeneity in calcineurin activity across the myocardium. Finally, we will discuss how circadian oscillations in calcineurin activity may facilitate integration with other essential but conflicting processes, allowing a healthy heart to reap the benefits of calcineurin signaling while avoiding the detrimental consequences of sustained calcineurin activity that can culminate in heart failure.

Na+/Ca2+ exchange and Na+/K+-ATPase in the heart

This paper is the third in a series of reviews published in this issue resulting from the University of California Davis Cardiovascular Symposium 2014: Systems approach to understanding cardiac excitation–contraction coupling and arrhythmias: Na⁺ channel and Na⁺ transport. The goal of the symposium was to bring together experts in the field to discuss points of consensus and controversy on the topic of sodium in the heart. The present review focuses on cardiac Na⁺/Ca²⁺ exchange (NCX) and Na⁺/K⁺-ATPase (NKA). While the relevance of Ca²⁺ homeostasis in cardiac function has been extensively investigated, the role of Na⁺ regulation in shaping heart function is often overlooked. Small changes in the cytoplasmic Na⁺ content have multiple effects on the heart by influencing intracellular Ca²⁺ and pH levels thereby modulating heart contractility. Therefore it is essential for heart cells to maintain Na⁺ homeostasis. Among the proteins that accomplish this task are the Na⁺/Ca²⁺ exchanger (NCX) and the Na⁺/K⁺ pump (NKA). By transporting three Na⁺ ions into the cytoplasm in exchange for one Ca²⁺ moved out, NCX is one of the main Na⁺ influx mechanisms in cardiomyocytes. Acting in the opposite direction, NKA moves Na⁺ ions from the cytoplasm to the extracellular space against their gradient by utilizing the energy released from ATP hydrolysis. A fine balance between these two processes controls the net amount of intracellular Na⁺ and aberrations in either of these two systems can have a large impact on cardiac contractility. Due to the relevant role of these two proteins in Na⁺ homeostasis, the emphasis of this review is on recent developments regarding the cardiac Na⁺/Ca²⁺ exchanger (NCX1) and Na⁺/K⁺ pump and the controversies that still persist in the field.

Angiotensin II Type 1 Receptor-Mediated Electrical Remodeling in Mouse Cardiac Myocytes

We recently characterized an autocrine renin angiotensin system (RAS) in canine heart. Activation of Angiotensin II Type 1 Receptors (AT1Rs) induced electrical remodeling, including inhibition of the transient outward potassium current Ito, prolongation of the action potential (AP), increased calcium entry and increased contractility. Electrical properties of the mouse heart are very different from those of dog heart, but if a similar system existed in mouse, it could be uniquely studied through genetic manipulations. To investigate the presence of a RAS in mouse, we measured APs and Ito in isolated myocytes. Application of angiotensin II (A2) for 2 or more hours reduced Ito magnitude, without affecting voltage dependence, and prolonged APs in a dose-dependent manner. Based on dose-inhibition curves, the fast and slow components of Ito (Ito,fast and IK,slow) appeared to be coherently regulated by [A2], with 50% inhibition at an A2 concentration of about 400 nM. This very high K0.5 is inconsistent with systemic A2 effects, but is consistent with an autocrine RAS in mouse heart. Pre-application of the microtubule destabilizing agent colchicine eliminated A2 effects on Ito and AP duration, suggesting these effects depend on intracellular trafficking. Application of the biased agonist SII ([Sar1-Ile4-Ile8]A2), which stimulates receptor internalization without G protein activation, caused Ito reduction and AP prolongation similar to A2-induced changes. These data demonstrate AT1R mediated regulation of Ito in mouse heart. Moreover, all measured properties parallel those measured in dog heart, suggesting an autocrine RAS may be a fundamental feedback system that is present across species.