Modulation of L-type Ca²⁺ channel activity by neuronal nitric oxide synthase and myofilament Ca²⁺ sensitivity in cardiac myocytes from hypertensive rat

Pharmacological nNOS inhibition modified small-conductance Ca2+-activated K+ channel without altering Ca2+ dynamics

Atrial fibrillation (AF) is associated with electrical remodeling processes that promote a substrate for the maintenance of AF. Although the small-conductance Ca²⁺-activated K⁺ (SK) channel is a key factor in atrial electrical remodeling, the mechanism of its activation remains unclear. Regional nitric oxide (NO) production by neuronal nitric oxide synthase (nNOS) is involved in atrial electrical remodeling. In this study, atrial tachyarrhythmia (ATA) induction and optical mapping were performed on perfused rat hearts. nNOS is pharmacologically inhibited by S-methylthiocitrulline (SMTC). The influence of the SK channel was examined using a specific channel inhibitor, apamin (APA). Parameters such as action potential duration (APD), conduction velocity, and calcium transient (CaT) were evaluated using voltage and calcium optical mapping. The dominant frequency was examined in the analysis of AF dynamics. SMTC (100 nM) increased the inducibility of ATA and apamin (100 nM) mitigated nNOS inhibition-induced arrhythmogenicity. SMTC caused abbreviations and enhanced the spatial dispersion of APD, which was reversed by apamin. By contrast, conduction velocity and other parameters associated with CaT were not affected by SMTC or apamin administration. Apamin reduced the frequency of SMTC-induced ATA. In summary, nNOS inhibition abbreviates APD by modifying the SK channels. A specific SK channel blocker, apamin, mitigated APD abbreviation without alteration of CaT, implying an underlying mechanism of posttranslational modification of SK channels.NEW & NOTEWORTHY We demonstrated that pharmacological nNOS inhibition increased the atrial arrhythmia inducibility and a specific small-conductance Ca²⁺-activated K⁺ channel blocker, apamin, reversed the enhanced atrial arrhythmia inducibility. Apamin mitigated APD abbreviation without alteration of Ca²⁺ transient, implying an underlying mechanism of posttranslational modification of SK channels.

Cardiac effect induced by Crotalus durissus cascavella venom: Morphofunctional evidence and mechanism of action

Envenoming, resulting from snake bites, is a global public health problem. The present study was undertaken to investigate the influence of Crotalus durissus cascavella (Cdcas) venom on cardiac activity and the mechanisms of action underlying its effect. To investigate the inotropic and chronotropic effects induced by Cdcas, studies were performed on the left and right atria. A series of tests were conducted to investigate whether the negative inotropic effect, induced by Cdcas, was related to cardiac damage. Cdcas venom (0.1-30 μg/mL) elicited a significant negative inotropic effect. The addition of Cdcas crude venom (7.5, 15 and 30 μg/mL) did not induce significant alterations in cell proliferation, nor in the enzymatic activity of total-CK and CKMB. Ultrastructural evaluation demonstrated that cardiac cells from isoproterenol and Cdcas groups revealed discreet swelling and displaced intermyofibrillar mitochondria with disorganization of the cristae. No change was observed in cardiac electrical activity in perfused isolated rat hearts with Cdcas. In addition, Cdcas reduced contractility in isolated cardiomyocytes from the rat left ventricle. The negative inotropic effect of Cdcas was reduced by l-NAME (100 μM), PTIO (100 μM), ODQ (10 μM) and KT5823 (1 μM), suggesting the participation of NO/cGMP/PKG pathway due to Cdcas. In non-anesthetized rats, Cdcas induced hypotension followed by bradycardia, the latter was also observed by ECG (anesthetized animals). Our results suggest that the negative inotropic effect induced by Cdcas venom is unrelated to cardiac toxicity, at least, at the concentrations tested; and occurs through of NO/cGMP/PKG pathway, likely leading to hypotension and bradycardia when administered in vivo.

Role of neuronal nitric oxide synthase on cardiovascular functions in physiological and pathophysiological states

This review describes and summarizes the role of neuronal nitric oxide synthase (nNOS) on the central nervous system, particularly on brain regions such as the ventrolateral medulla (VLM) and the periaqueductal gray matter (PAG), and on blood vessels and the heart that are involved in the regulation and control of the cardiovascular system (CVS). Furthermore, we shall also review the functional aspects of nNOS during several physiological, pathophysiological, and clinical conditions such as exercise, pain, cerebral vascular accidents or stroke and hypertension. For example, during stroke, a cascade of molecular, neurochemical, and cellular changes occur that affect the nervous system as elicited by generation of free radicals and nitric oxide (NO) from vulnerable neurons, peroxide formation, superoxides, apoptosis, and the differential activation of three isoforms of nitric oxide synthases (NOSs), and can exert profound effects on the CVS. Neuronal NOS is one of the three isoforms of NOSs, the others being endothelial (eNOS) and inducible (iNOS) enzymes. Neuronal NOS is a critical homeostatic component of the CVS and plays an important role in regulation of different systems and disease process including nociception. The functional and physiological roles of NO and nNOS are described at the beginning of this review. We also elaborate the structure, gene, domain, and regulation of the nNOS protein. Both inhibitory and excitatory role of nNOS on the sympathetic autonomic nervous system (SANS) and parasympathetic autonomic nervous system (PANS) as mediated via different neurotransmitters/signal transduction processes will be explored, particularly its effects on the CVS. Because the VLM plays a crucial function in cardiovascular homeostatic mechanisms, the neuroanatomy and cardiovascular regulation of the VLM will be discussed in conjunction with the actions of nNOS. Thereafter, we shall discuss the up-to-date developments that are related to the interaction between nNOS and cardiovascular diseases such as hypertension and stroke. Finally, we shall focus on the role of nNOS, particularly within the PAG in cardiovascular regulation and neurotransmission during different types of pain stimulus. Overall, this review focuses on our current understanding of the nNOS protein, and provides further insights on how nNOS modulates, regulates, and controls cardiovascular function during both physiological activity such as exercise, and pathophysiological conditions such as stroke and hypertension.

Neuronal nitric oxide synthase regulation of calcium cycling in ventricular cardiomyocytes is independent of Cav1.2 channel modulation under basal conditions

Neuronal nitric oxide synthase (nNOS) is considered a regulator of Cav1.2 L-type Ca2+ channels and downstream Ca2+ cycling in the heart. The commonest view is that nitric oxide (NO), generated by nNOS activity in cardiomyocytes, reduces the currents through Cav1.2 channels. This gives rise to a diminished Ca2+ release from the sarcoplasmic reticulum, and finally reduced contractility. Here, we report that nNOS inhibitor substances significantly increase intracellular Ca2+ transients in ventricular cardiomyocytes derived from adult mouse and rat hearts. This is consistent with an inhibitory effect of nNOS/NO activity on Ca2+ cycling and contractility. Whole cell currents through L-type Ca2+ channels in rodent myocytes, on the other hand, were not substantially affected by the application of various NOS inhibitors, or application of a NO donor substance. Moreover, the presence of NO donors had no effect on the single-channel open probability of purified human Cav1.2 channel protein reconstituted in artificial liposomes. These results indicate that nNOS/NO activity does not directly modify Cav1.2 channel function. We conclude that-against the currently prevailing view-basal Cav1.2 channel activity in ventricular cardiomyocytes is not substantially regulated by nNOS activity and NO. Hence, nNOS/NO inhibition of Ca2+ cycling and contractility occurs independently of direct regulation of Cav1.2 channels by NO.

Voltage-Dependent Sarcolemmal Ion Channel Abnormalities in the Dystrophin-Deficient Heart

Mutations in the gene encoding for the intracellular protein dystrophin cause severe forms of muscular dystrophy. These so-called dystrophinopathies are characterized by skeletal muscle weakness and degeneration. Dystrophin deficiency also gives rise to considerable complications in the heart, including cardiomyopathy development and arrhythmias. The current understanding of the pathomechanisms in the dystrophic heart is limited, but there is growing evidence that dysfunctional voltage-dependent ion channels in dystrophin-deficient cardiomyocytes play a significant role. Herein, we summarize the current knowledge about abnormalities in voltage-dependent sarcolemmal ion channel properties in the dystrophic heart, and discuss the potentially underlying mechanisms, as well as their pathophysiological relevance.

Increased constitutive nitric oxide production by whole body periodic acceleration ameliorates alterations in cardiomyocytes associated with utrophin/dystrophin deficiency

Duchenne Muscular Dystrophy (DMD) cardiomyopathy is a progressive lethal disease caused by the lack of the dystrophin protein in the heart. The most widely used animal model of DMD is the dystrophin-deficient mdx mouse; however, these mice exhibit a mild dystrophic phenotype with heart failure only late in life. In contrast, mice deficient for both dystrophin and utrophin (mdx/utrn^-/-, or dKO) can be used to model severe DMD cardiomyopathy where pathophysiological indicators of heart failure are detectable by 8-10weeks of age. Nitric oxide (NO) is an important signaling molecule involved in vital functions of regulating rhythm, contractility, and microcirculation of the heart, and constitutive NO production affects the function of proteins involved in excitation-contraction coupling. In this study, we explored the efficacy of enhancing NO production as a therapeutic strategy for treating DMD cardiomyopathy using the dKO mouse model of DMD. Specifically, NO production was induced via whole body periodic acceleration (pGz), a novel non-pharmacologic intervention which enhances NO synthase (NOS) activity through sinusoidal motion of the body in a headward-footward direction, introducing pulsatile shear stress to the vascular endothelium and cardiomyocyte plasma membrane. Male dKO mice were randomized at 8weeks of age to receive daily pGz (480cpm, Gz±3.0m/s², 1h/d) for 4weeks or no treatment, and a separate age-matched group of WT animals (pGz-treated and untreated) served as non-diseased controls. At the conclusion of the protocol, cardiomyocytes from untreated dKO animals had, respectively, 4.3-fold and 3.5-fold higher diastolic resting concentration of Ca²⁺ ([Ca²⁺]_d) and Na⁺ ([Na⁺]_d) compared to WT, while pGz treatment significantly reduced these levels. For dKO cardiomyocytes, pGz treatment also improved the depressed contractile function, decreased oxidative stress, blunted the elevation in calpain activity, and mitigated the abnormal increase in [Ca²⁺]_d upon mechanical stress. These improvements culminated in a significant reduction in circulating cardiac troponin T (cTnT) and an extension of the median lifespan of dKO mice from 16 to 31weeks. Treatment with L-NAME (NOS inhibitor) significantly decreased overall lifespan and abolished the cardioprotective properties elicited by pGz. Our results provide evidence that enhancement of NO synthesis by pGz can ameliorate cellular dysfunction in dKO cardiomyocytes and may represent a novel therapeutic intervention in DMD cardiomyopathy patients.

Nitric oxide signalling and neuronal nitric oxide synthase in the heart under stress

Nitric oxide (NO) is an imperative regulator of the cardiovascular system and is a critical mechanism in preventing the pathogenesis and progression of the diseased heart. The scenario of bioavailable NO in the myocardium is complex: 1) NO is derived from both endogenous NO synthases (endothelial, neuronal, and/or inducible NOSs [eNOS, nNOS, and/or iNOS]) and exogenous sources (entero-salivary NO pathway) and the amount of NO from exogenous sources varies significantly; 2) NOSs are located at discrete compartments of cardiac myocytes and are regulated by distinctive mechanisms under stress; 3) NO regulates diverse target proteins through different modes of post-transcriptional modification (soluble guanylate cyclase [sGC]/cyclic guanosine monophosphate [cGMP]/protein kinase G [PKG]-dependent phosphorylation, S-nitrosylation, and transnitrosylation); 4) the downstream effectors of NO are multidimensional and vary from ion channels in the plasma membrane to signalling proteins and enzymes in the mitochondria, cytosol, nucleus, and myofilament; 5) NOS produces several radicals in addition to NO (e.g. superoxide, hydrogen peroxide, peroxynitrite, and different NO-related derivatives) and triggers redox-dependent responses. However, nNOS inhibits cardiac oxidases to reduce the sources of oxidative stress in diseased hearts. Recent consensus indicates the importance of nNOS protein in cardiac protection under pathological stress. In addition, a dietary regime with high nitrate intake from fruit and vegetables together with unsaturated fatty acids is strongly associated with reduced cardiovascular events. Collectively, NO-dependent mechanisms in healthy and diseased hearts are better understood and shed light on the therapeutic prospects for NO and NOSs in clinical applications for fatal human heart diseases.

Cardiac inotropy, lusitropy, and Ca2+ handling with major metabolic substrates in rat heart

Fatty acid (FA)-dependent oxidation is the predominant process for energy supply in normal heart. Impaired FA metabolism and metabolic insufficiency underlie the failing of the myocardium. So far, FA metabolism in normal cardiac physiology and heart failure remains undetermined. Here, we evaluate the mechanisms of FA and major metabolic substrates (termed NF) on the contraction, relaxation, and Ca²⁺ handling in rat left ventricular (LV) myocytes. Our results showed that NF significantly increased myocyte contraction and facilitated relaxation. Moreover, NF increased the amplitudes of diastolic and systolic Ca²⁺ transients ([Ca²⁺]_i), abbreviated time constant of [Ca²⁺]_i decay (tau), and prolonged the peak duration of [Ca²⁺]_i. Whole-cell patch-clamp experiments revealed that NF increased Ca²⁺ influx via L-type Ca²⁺ channels (LTCC, I_Ca-integral) and prolonged the action potential duration (APD). Further analysis revealed that NF shifted the relaxation phase of sarcomere lengthening vs. [Ca²⁺]_i trajectory to the right and increased [Ca²⁺]_i for 50 % of sarcomere relengthening (EC₅₀), suggesting myofilament Ca²⁺ desensitization. Butanedione monoxime (BDM), a myosin ATPase inhibitor that reduces myofilament Ca²⁺ sensitivity, abolished the NF-induced enhancement of [Ca²⁺]_i amplitude and the tau of [Ca²⁺]_i decay, indicating the association of myofilament Ca²⁺ desensitization with the changes in [Ca²⁺]_i profile in NF. NF reduced intracellular pH ([pH_i]). Increasing [pH]_i buffer capacity with HCO₃/CO₂ attenuated Δ [pH]_i and reversed myofilament Ca²⁺ desensitization and Ca²⁺ handling in NF. Collectively, greater Ca²⁺ influx through LTCCs and myofilament Ca²⁺ desensitization, via reducing [pH]_i, are likely responsible for the positive inotropic and lusitropic effects of NF. Computer simulation recapitulated the effects of NF.

Assessment of Myofilament Ca2+ Sensitivity Underlying Cardiac Excitation-contraction Coupling

Heart failure and cardiac arrhythmias are the leading causes of mortality and morbidity worldwide. However, the mechanism of pathogenesis and myocardial malfunction in the diseased heart remains to be fully clarified. Recent compelling evidence demonstrates that changes in the myofilament Ca(2+) sensitivity affect intracellular Ca(2+) homeostasis and ion channel activities in cardiac myocytes, the essential mechanisms responsible for the cardiac action potential and contraction in healthy and diseased hearts. Indeed, activities of ion channels and transporters underlying cardiac action potentials (e.g., Na(+), Ca(2+) and K(+) channels and the Na(+)-Ca(2+) exchanger) and intracellular Ca(2+) handling proteins (e.g., ryanodine receptors and Ca(2+)-ATPase in sarcoplasmic reticulum (SERCA2a) or phospholamban and its phosphorylation) are conventionally measured to evaluate the fundamental mechanisms of cardiac excitation-contraction (E-C) coupling. Both electrical activities in the membrane and intracellular Ca(2+) changes are the trigger signals of E-C coupling, whereas myofilament is the functional unit of contraction and relaxation, and myofilament Ca(2+) sensitivity is imperative in the implementation of myofibril performance. Nevertheless, few studies incorporate myofilament Ca(2+) sensitivity into the functional analysis of the myocardium unless it is the focus of the study. Here, we describe a protocol that measures sarcomere shortening/re-lengthening and the intracellular Ca(2+) level using Fura-2 AM (ratiometric detection) and evaluate the changes of myofilament Ca(2+) sensitivity in cardiac myocytes from rat hearts. The main aim is to emphasize that myofilament Ca(2+) sensitivity should be taken into consideration in E-C coupling for mechanistic analysis. Comprehensive investigation of ion channels, ion transporters, intracellular Ca(2+) handling, and myofilament Ca(2+) sensitivity that underlie myocyte contractility in healthy and diseased hearts will provide valuable information for designing more effective strategies of translational and therapeutic value.