Integrin stimulation induces calcium signalling in rat cardiomyocytes by a NO-dependent mechanism

[Mechanisms of increase and diagnostic role of highly sensitive troponins in arterial hypertension]

Improvement in immunochemical methods for the determination of key biomarkers of acute myocardial infarction has led not only to an improvement in the early diagnosis of acute myocardial infarction, but also to a change in many of our ideas about the biology and diagnostic role of cardiac troponins. Modern (highly and ultrasensitive) laboratory methods for the determination of cardiac troponin molecules in human biological fluids are highly sensitive, which makes it possible to detect even the smallest damage to cardiomyocytes that occur at the early stages of many pathologies of cardiac (coronary heart disease, arterial hypertension, etc.) and extracardiac etiology (renal failure, sepsis, chronic obstructive pulmonary disease and others), as well as under the influence of a number of physiological conditions, including the influence of physical exercises, psychoemotional stress, gender characteristics (higher levels of cardiac troponins in men, compared with women), age characteristics (an increase in the concentration of cardiac troponins with age) and circadian characterisics (prevalence of morning values of cardiac troponins concentration over evening ones). In this regard, the diagnostic capabilities of the use of highly sensitive cardiac troponins have been significantly expanded. One of the promising areas for the use of highly sensitive cardiac troponins includes the assessment of the risk of adverse cardiovascular events both in healthy patients and in patients with various risk factors for their development, one of which can be considered arterial hypertension. This article systematizes the results of clinical studies evaluating the diagnostic role of highly sensitive cardiac troponins in biological fluids (blood serum and urine) in hypertension and discusses in detail the mechanisms of increasing the levels of highly sensitive troponins in this pathological condition.

Clinical and Diagnostic Value of Highly Sensitive Cardiac Troponins in Arterial Hypertension

In modern laboratory diagnostics of cardiovascular diseases (CVD), there is a clear tendency toward an increase in the sensitivity of methods for determining key CVD biomarkers, among which highly sensitive cardiac troponins (hs-Tn) deserve special attention. The introduction of the latter into clinical practice made it possible not only to improve the early diagnosis of acute myocardial infarction but also to open up a number of additional valuable opportunities for the use of hs-Tn, including the assessment of the risk of developing CVD in a healthy population, detection and monitoring of early myocardial injuries in the early stages of CVD development (for example, with ischemic heart disease and arterial hypertension), with noncardiac pathologies (for example, sepsis, chronic obstructive pulmonary disease, chronic renal failure, stroke, cancer, etc), and diagnostics of CVD by using biological fluids that can be obtained by noninvasive methods. This article discusses in detail the diagnostic value of hs-Tn in serum and urine in cases of arterial hypertension. Also, the paper pays considerable attention to the consideration of the mechanisms underlying the increase in hs-Tn in serum and urine in cases of arterial hypertension.

Nitric Oxide and Mechano-Electrical Transduction in Cardiomyocytes

The ability§ of the heart to adapt to changes in the mechanical environment is critical for normal cardiac physiology. The role of nitric oxide is increasingly recognized as a mediator of mechanical signaling. Produced in the heart by nitric oxide synthases, nitric oxide affects almost all mechano-transduction pathways within the cardiomyocyte, with roles mediating mechano-sensing, mechano-electric feedback (via modulation of ion channel activity), and calcium handling. As more precise experimental techniques for applying mechanical stresses to cells are developed, the role of these forces in cardiomyocyte function can be further understood. Furthermore, specific inhibitors of different nitric oxide synthase isoforms are now available to elucidate the role of these enzymes in mediating mechano-electrical signaling. Understanding of the links between nitric oxide production and mechano-electrical signaling is incomplete, particularly whether mechanically sensitive ion channels are regulated by nitric oxide, and how this affects the cardiac action potential. This is of particular relevance to conditions such as atrial fibrillation and heart failure, in which nitric oxide production is reduced. Dysfunction of the nitric oxide/mechano-electrical signaling pathways are likely to be a feature of cardiac pathology (e.g., atrial fibrillation, cardiomyopathy, and heart failure) and a better understanding of the importance of nitric oxide signaling and its links to mechanical regulation of heart function may advance our understanding of these conditions.

P21 (Cdc42/Rac)-activated kinase 1 (pak1) is associated with cardiotoxicity induced by antihistamines

Astemizole, a non-sedating histamine H₁ receptor blocker, is widely known to cause cardiac arrhythmia, which prolongs the QT interval. However, the precise molecular mechanism involved in antihistamine-induced cardiovascular adverse effects other than hERG channel inhibition is still unclear. In this study, we used DNA microarray analysis to detect the mechanisms involved in life-threatening adverse effects caused by astemizole. Rat primary cardiomyocytes were treated with various concentrations of astemizole for 24 h and the corresponding cell lysates were analyzed using a DNA microarray. Astemizole altered the expression profiles of genes involved in calcium transport/signaling. Using qRT-PCR analysis, we demonstrated that, among those genes, p21 (Cdc42/Rac)-activated kinase 1 (pak1) mRNA was downregulated by treatment with terfenadine and astemizole. Astemizole also reduced pak1 protein levels in rat cardiomyocytes. In addition, astemizole decreased pak1 mRNA and protein levels in H9c2 cells and induced an increase in cell surface area (hypertrophy) and cytotoxicity. Fingolimod hydrochloride (FTY720), a pak1 activator, inhibited astemizole-induced hypertrophy and cytotoxicity in H9c2 cells. These results suggest that antihistamine-induced cardiac adverse effects are associated with pak1 expression and function.

Nitric oxide mediates stretch-induced Ca2+ oscillation in smooth muscle

The stretching of smooth muscle tissue modulates contraction via augmentation of Ca2+ transients, but the mechanism underlying stretch-induced Ca2+ transients is still unknown. We found that mechanical stretching and maintenance of mouse urinary bladder smooth muscle strips and single myocytes at the initial length of 30% and 18%, respectively, resulted in Ca2+ oscillations. Experiments indicated that mechanical stretching remarkably increases the production of nitric oxide (NO) as well as the amplitude and duration of muscle contraction. Stretch-induced Ca2+ oscillations and contractility increases were completely abolished by NO inhibitor L-NAME or eNOS gene inactivation. Moreover, exposure of eNOS knockout myocytes to exogenous NO donor induced Ca2+ oscillations. The stretch-induced Ca2+ oscillations were greatly inhibited by selective IP3R inhibitor xestospongin C and partially inhibited by ryanodine. Moreover, the stretch-induced Ca2+ oscillations were also suppressed by LY294002, but not by the soluble guanylyl cyclase (sGC) inhibitor ODQ. These results suggest that myocytes stretching and maintenance at a certain length resulted in Ca2+ oscillations that is NO dependent and sGC/cGMP independent and results from the activation of PI(3)K in smooth muscle.

Integrins and integrin-associated proteins in the cardiac myocyte

Integrins are heterodimeric, transmembrane receptors that are expressed in all cells, including those in the heart. They participate in multiple critical cellular processes including adhesion, extracellular matrix organization, signaling, survival, and proliferation. Particularly relevant for a contracting muscle cell, integrins are mechanotransducers, translating mechanical to biochemical information. Although it is likely that cardiovascular clinicians and scientists have the highest recognition of integrins in the cardiovascular system from drugs used to inhibit platelet aggregation, the focus of this article will be on the role of integrins specifically in the cardiac myocyte. After a general introduction to integrin biology, the article will discuss important work on integrin signaling, mechanotransduction, and lessons learned about integrin function from a range of model organisms. Then we will detail work on integrin-related proteins in the myocyte, how integrins may interact with ion channels and mediate viral uptake into cells, and also play a role in stem cell biology. Finally, we will discuss directions for future study.

Remanent cell traction force in renal vascular smooth muscle cells induced by integrin-mediated mechanotransduction

It was previously demonstrated in isolated renal vascular smooth muscle cells (VSMCs) that integrin-mediated mechanotransduction triggers intracellular Ca(2+) mobilization, which is the hallmark of myogenic response in VSMCs. To test directly whether integrin-mediated mechanotransduction results in the myogenic response-like behavior in renal VSMCs, cell traction force microscopy was used to monitor cell traction force when the cells were pulled with fibronectin-coated or low density lipoprotein (LDL)-coated paramagnetic beads. LDL-coated beads were used as a control for nonintegrin-mediated mechanotransduction. Pulling with LDL-coated beads increased the cell traction force by 61 ± 12% (9 cells), which returned to the prepull level after the pulling process was terminated. Pulling with noncoated beads had a minimal increase in the cell traction force (12 ± 9%, 8 cells). Pulling with fibronectin-coated beads increased the cell traction force by 56 ± 20% (7 cells). However, the cell traction force was still elevated by 23 ± 14% after the pulling process was terminated. This behavior is analogous to the changes of vascular resistance in pressure-induced myogenic response, in which vascular resistance remains elevated after myogenic constriction. Fibronectin is a native ligand for α(5)β(1)-integrins in VSMCs. Similar remanent cell traction force was found when cells were pulled with beads coated with β(1)-integrin antibody (Ha2/5). Activation of β(1)-integrin with soluble antibody also triggered variations of cell traction force and Ca(2+) mobilization, which were abolished by the Src inhibitor. In conclusion, mechanical force transduced by α(5)β(1)-integrins triggered a myogenic response-like behavior in isolated renal VSMCs.

ErbB/integrin signaling interactions in regulation of myocardial cell-cell and cell-matrix interactions

Neuregulin (Nrg)/ErbB and integrin signaling pathways are critical for the normal function of the embryonic and adult heart. Both systems activate several downstream signaling pathways, with different physiological outputs: cell survival, fibrosis, excitation-contraction coupling, myofilament structure, cell-cell and cell-matrix interaction. Activation of ErbB2 by Nrg1β in cardiomycytes or its overexpression in cancer cells induces phosphorylation of FAK (Focal Adhesion Kinase) at specific sites with modulation of survival, invasion and cell-cell contacts. FAK is also a critical mediator of integrin receptors, converting extracellular matrix alterations into intracellular signaling. Systemic FAK deletion is lethal and is associated with left ventricular non-compaction whereas cardiac restriction in adult hearts is well tolerated. Nevertheless, these hearts are more susceptible to stress conditions like trans-aortic constriction, hypertrophy, and ischemic injury. As FAK is both downstream and specifically activated by integrins and Nrg-1β, here we will explore the role of FAK in the heart as a protective factor and as possible mediator of the crosstalk between the ErbB and Integrin receptors. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

A potential role for integrin signaling in mechanoelectrical feedback

Certain forms of heart disease involve gross morphological changes to the myocardium that alter its hemodynamic loading conditions. These changes can ultimately lead to the increased deposition of extracellular matrix (ECM) proteins, such as collagen and fibronectin, which together work to pathologically alter the myocardium's bulk tissue mechanics. In addition to changing the mechanical properties of the heart, this maladaptive remodeling gives rise to changes in myocardium electrical conductivity and synchrony since the tissue's mechanical properties are intimately tied to its electrical characteristics. This phenomenon, called mechanoelectrical coupling (MEC), can render individuals affected by heart disease arrhythmogenic and susceptible to Sudden Cardiac Death (SCD). The underlying mechanisms of MEC have been attributed to various processes, including the action of stretch activated channels and changes in troponin C-Ca(2+) binding affinity. However, changes in the heart post infarction or due to congenital myopathies are also accompanied by shifts in the expression of various molecular components of cardiomyocytes, including the mechanosensitive family of integrin proteins. As transmembrane proteins, integrins mechanically couple the ECM with the intracellular cytoskeleton and have been implicated in mediating ion homeostasis in various cell types, including neurons and smooth muscle. Given evidence of altered integrin expression in the setting of heart disease coupled with the associated increased risk for arrhythmia, we argue in this review that integrin signaling contributes to MEC. In light of the significant mortality associated with arrhythmia and SCD, close examination of all culpable mechanisms, including integrin-mediated MEC, is necessary.

Identification of troponin I and actin, alpha cardiac muscle 1 as potential biomarkers for hearts of electrically stimulated chickens

Methods

In this study, proteomics methods have been used to study the effects of different currents and voltages used to stun chickens. Protein profiles of chicken hearts were constructed to detect differences in protein expression and modification. The different voltages studied were 10 V, 40 V and 70 V, while the currents examined were 0.25 A, 0.5 A, and 0.75 A. The profiles obtained from these stunning conditions were compared to the non-stunned (0 A, 0 V) sample.

Results

Proteomics analyses using 2D Platinum ImageMaster 6.0 and Matrix-assisted laser desorption/ionization with time-of-flight mass spectrometry (MALDI-TOF) identified troponin I and alpha cardiac muscle actin 1 in the electrically stimulated heart samples. The overexpression of the proteins was further confirmed at the transcriptional level by Real Time PCR.

Conclusion

The results from MALDI-TOF and Real Time PCR agreed; therefore, this method for identifying biomarkers of electrically stimulated chicken hearts provides a novel approach for differentiation the hearts of increased electrically stimulated chickens from those of non-stunned chickens.

Integrin alpha9beta1 mediates enhanced cell migration through nitric oxide synthase activity regulated by Src tyrosine kinase

Integrins are important mediators of cell adhesion and migration, which in turn are essential for diverse biological functions, including wound healing and cancer metastasis. The integrin alpha9beta1 is expressed on numerous mammalian tissues and can mediate accelerated cell migration. As the molecular signaling mechanisms that transduce this effect are poorly defined, we investigated the pathways by which activated integrin alpha9beta1 signals migration. We found for the first time that specific ligation of integrin alpha9beta1 rapidly activates Src tyrosine kinase, with concomitant tyrosine phosphorylation of p130Cas and activation of Rac-1. Furthermore, activation of integrin alpha9beta1 also enhanced NO production through activation of inducible nitric oxide synthase (iNOS). Inhibition of Src tyrosine kinase or NOS decreased integrin-alpha9beta1-dependent cell migration. Src appeared to function most proximal in the signaling cascade, in a FAK-independent manner to facilitate iNOS activation and NO-dependent cell migration. The cytoplasmic domain of integrin alpha9 was crucial for integrin-alpha9beta1-induced Src activation, subsequent signaling events and cell migration. When taken together, our results describe a novel and unique mechanism of coordinated interactions of the integrin alpha9 cytoplasmic domain, Src tyrosine kinase and iNOS to transduce integrin-alpha9beta1-mediated cell migration.

Integrin stimulation-induced hypertrophy in neonatal rat cardiomyocytes is NO-dependent

Prolonged myocardial stretch typically leads to hypertrophy of cardiomyocytes. As integrins are cellular receptors of stretch, we hypothesize that integrin stimulation induces cardiomyocyte hypertrophy. Integrins of neonatal rat cardiomyocytes (NRCMs) were stimulated with a peptide containing the Arg-Gly-Asp (RGD) sequence for 24 h. For comparison, alpha(1)-adrenergic stimulation by phenylephrine (PE) for 24 h was applied. Saline-treated NRCMs were used as control. The hypertrophic response was quantified by measuring cell surface area (CSA). Phosphorylation of NO-synthase-1 (NOS1) was assessed by immunocytochemistry. CSA was increased by 38% (IQR 31-44%) with RGD and by 68% (IQR 64-84%) with PE versus control (both P < 0.001). NOS-1 phosphorylation was increased by 61% with RGD and by 21% with PE versus control (both P < 0.01). A general NOS-inhibitor (L-NAME) inhibited RGD-induced hypertrophy completely, but had no significant effect on PE-induced hypertrophy. Administration of NO-donor to NRCMs co-incubated with RGD + L-NAME partly restored hypertrophy (to 62% of the hypertrophic effect of RGD alone), but had no effect if incubated with PE + L-NAME. Ryanodine and BAPTA-AM inhibited RGD-induced hypertrophy completely but not that induced by PE. Integrin stimulation of NRCMs by RGD leads to hypertrophy, likely by activation of NOS-1. Abrogation of RGD-induced hypertrophic response upon NOS-inhibition and rescue of this hypertrophic effect by NO-donor suggest that integrin stimulation-induced hypertrophy of NRCMs is NO-dependent.

Nitric oxide mediates stretch-induced Ca2+ release via activation of phosphatidylinositol 3-kinase-Akt pathway in smooth muscle

BACKGROUND

Hollow smooth muscle organs such as the bladder undergo significant changes in wall tension associated with filling and distension, with attendant changes in muscle tone. Our previous study indicated that stretch induces Ca(2+) release occurs in the form of Ca(2+) sparks and Ca(2+) waves in urinary bladder myocytes. While, the mechanism underlying stretch-induced Ca2+ release in smooth muscle is unknown.

METHODOLOGY/PRINCIPAL FINDINGS

We examined the transduction mechanism linking cell stretch to Ca(2+) release. The probability and frequency of Ca(2+) sparks induced by stretch were closely related to the extent of cell extension and the time that the stretch was maintained. Experiments in tissues and single myocytes indicated that mechanical stretch significantly increases the production of nitric oxide (NO) and the amplitude and duration of muscle contraction. Stretch-induced Ca(2+) sparks and contractility increases were abrogated by the NO inhibitor L-NAME and were also absent in eNOS knockout mice. Furthermore, exposure of eNOS null mice to exogenously generated NO induced Ca(2+) sparks. The soluble guanylyl cyclase inhibitor ODQ did not inhibit SICR, but this process was effectively blocked by the PI3 kinase inhibitors LY494002 and wortmannin; the phosphorylation of Akt and eNOS were up-regulated by 204+/-28.6% and 258+/-36.8% by stretch, respectively. Moreover, stretch significantly increased the eNOS protein expression level.

CONCLUSIONS/SIGNIFICANCE

Taking together, these results suggest that stretch-induced Ca2+ release is NO dependent, resulting from the activation of PI3K/Akt pathway in smooth muscle.

Activation of signaling molecules and matrix metalloproteinases in right ventricular myocardium of rats with pulmonary hypertension

Pulmonary hypertension induces right ventricular (RV) overload, which is transmitted to cardiomyocytes via integrins that activate intracellular messengers, including focal adhesion kinase (FAK) and neuronal nitric oxide synthase (NOS1). We investigated whether RV hypertrophy (RVH) and RV failure (RVF) were associated with activation of FAK, NOS1, and matrix metalloproteinases (MMPs). Rats were treated without (RVC) or with a low dose of monocrotaline (30mg/kg) to induce RVH, and with a high dose (80mg/kg) to induce RVF. After approximately 30 days, RV function was determined using a combined pressure-conductance catheter. After sacrifice, FAK, NOS1, their phosphorylated forms (FAK-P and NOS1-P), MMP-2, and MMP-9 were quantified in RV myocardium by immunohistochemistry. In RVH and RVF, RV weight/ body weight increased by 36% and 109%, whereas RV ejection fraction decreased by 23% and 57% compared to RVC, respectively. FAK-P and FAK-P/FAK were highest in RVH (2.87+/-0.12 and 2.52+/-0.23 fold compared to RVC, respectively) and slightly elevated in RVF (1.76+/-0.17 and 1.15+/-0.13 fold compared to RVC, respectively). NOS1-P and NOS1-P/NOS1 were increased in RVH (1.63+/-0.12 and 3.06+/-0.80 fold compared to RVC, respectively) and RVF (2.16+/-0.03 and 3.30+/-0.38 fold compared to RVC, respectively). MMP-2 was highest in RVH and intermediate in RVF (3.50+/-0.12 and 1.84+/-0.22 fold compared to RVC, respectively). MMP-9 was elevated in RVH and RVF (2.39+/-0.35 and 2.92+/-0.68 fold compared to RVC, respectively). Activation of FAK in RVH points to an integrin-dependent hypertrophic response of the myocardium. Activation of NOS1 in failing RV suggests a role of excessive NO in the development of failure and activation of MMPs leading to ventricular remodeling.

Release of cardiac troponin I from viable cardiomyocytes is mediated by integrin stimulation

Elevated cardiac troponin-I (cTnI) levels have been demonstrated in serum of patients without acute coronary syndromes, potentially via a stretch-related process. We hypothesize that this cTnI release from viable cardiomyocytes is mediated by stimulation of stretch-responsive integrins. Cultured cardiomyocytes were treated with (1) Gly-Arg-Gly-Asp-Ser (GRGDS, n = 22) to stimulate integrins, (2) Ser-Asp-Gly-Arg-Gly (SDGRG, n = 8) that does not stimulate integrins, or (3) phosphate-buffered saline (control, n = 38). Cells and media were analyzed for intact cTnI, cTnI degradation products, and matrix metalloproteinase (MMP)-2. Cell viability was examined by assay of lactate dehydrogenase (LDH) activity and by nuclear staining with propidium iodide. GRGDS-induced integrin stimulation caused increased release of intact cTnI (9.6 +/- 3.0%) as compared to SDGRG-treated cardiomyocytes (4.5 +/- 0.8%, p < 0.001) and control (3.0 +/- 3.4%, p < 0.001). LDH release from GRGDS-treated cardiomyocytes (15.9 +/- 3.8%) equalled that from controls (15.2 +/- 2.3%, p = n.s.), indicating that the GRGDS-induced release of cTnI is not due to cell necrosis. This result was confirmed by nuclear staining with propidium iodide. Integrin stimulation increased the intracellular and extracellular MMP2 activity as compared to controls (both p < 0.05). However, despite the ability of active MMP2 to degrade cTnI in vitro, integrin stimulation in cardiomyocytes was not associated with cTnI degradation. The present study demonstrates that intact cTnI can be released from viable cardiomyocytes by stimulation of stretch-responsive integrins.

Signalling mechanisms in contraction-mediated stimulation of intracellular NO production in cat ventricular myocytes

In this study we sought to determine whether contractile activity has a role as a signalling mechanism in the activation of intracellular nitric oxide (NO(i)) production induced by electrical stimulation of cat ventricular myocytes. Field stimulation (FS) of single ventricular myocytes elicited frequency-dependent increases in NO(i) that were blocked by the calmodulin (CaM) inhibitor 10 microM W-7 and partially inhibited by the phosphatidylinositol 3'-kinase (PI-(3)K) inhibitor 10 microMm LY294002. Increasing extracellular [Ca(2+)] caused a concentration-dependent increase in FS-induced NO(i) that was partially inhibited by LY294002. The negative inotropic agents BDM (5 mm) or blebbistatin (10 microM) decreased cell shortening and NO(i) production without concomitant changes in L-type Ca(2+) current (I(Ca,L)) or [Ca(2+)](i) transients. The positive inotropic agents EMD 57033 or CGP 48506 (1 microM) increased cell shortening and NO(i) production without concomitant changes in I(Ca,L) or [Ca(2+)](i) transients. FS-induced NO(i) production was decreased in myocytes infected (100 multiplicity of viral infection (MOI); 24 h) with a replication-deficient adenovirus expressing a dominant-negative mutant of protein kinase B (Akt) compared with cells infected with a control adenovirus expressing beta-galactosidase. FS-induced NO(i) was partially inhibited by either endothelial (eNOS) or neuronal nitric oxide synthase (nNOS) inhibitors and completely blocked by simultaneous exposure to both. FS-induced [Ca(2+)](i) transients were increased by the nNOS inhibitor nNOS-I (0.24 microM), decreased by the eNOS inhibitor L-NIO (1 microM) and unchanged by exposure to both inhibitors. We conclude that in cat ventricular myocytes, FS-induced NO(i) production requires both Ca(2+)-dependent CaM signalling and Ca(2+)-independent PI-(3)K-Akt signalling activated by contractile activity. FS activates NO(i) production from both eNOS and nNOS, and each source of NO(i) exerts opposing effects on [Ca(2+)](i) transient amplitude. These findings are important for understanding the regulation of NO(i) signalling in the normal and mechanically failing heart.

Integrin ligands mobilize Ca2+ from ryanodine receptor-gated stores and lysosome-related acidic organelles in pulmonary arterial smooth muscle cells

Extracellular matrix (ECM) protein receptors, or integrins, participate in vascular remodeling and the systemic myogenic response. Synthetic ligands and ECM fragments regulate the vascular smooth muscle cell contractile state by altering intracellular Ca2+ levels ([Ca2+]i). Information on the Ca2+ effect of integrins in vascular smooth muscle cells is limited, but nonexistent in pulmonary arterial smooth muscle cells (PASMCs). We therefore characterized integrin expression in endothelium-denuded pulmonary arteries, and explored [Ca2+]i mobilization pathways induced by soluble ligands in rat PASMCs. Reverse transcriptase-PCR showed mRNA expression of integrins alpha1, alpha2, alpha3, alpha4, alpha5, alpha7, alpha8, alpha(v), beta1, beta3, and beta4, and immunoblots of alpha5, alpha(v), beta1, and beta3 confirmed protein expression. Exposure of PASMCs to integrin-binding peptides (0.5 mM) containing the arginine-glycine-aspartate (RGD) motif elicited [Ca2+]i responses with an order of potency of GRGDNP > GRGDSP > GRGDTP = cyclo-RGD. Pharmacological analysis revealed that the GRGDSP-induced Ca2+ response was unrelated to Ca2+ influx and the inositol triphosphate receptor-gated Ca2+ store, but partially blocked by ryanodine or inhibition of lysosome-related acidic organelles with bafilomycin A1. Simultaneous inhibition of both pathways was necessary to abolish the response. GRGDSP treatment increased cyclic ADP-ribose, the endogenous activator of ryanodine receptors, by 70%. GRGDSP also rapidly reduced Lysotracker Red accumulation, confirming direct modulation of acidic organelles. These data are the first demonstration of integrin-mediated Ca2+ regulation in PASMCs. The presence of an array of integrins, and activation of ryanodine-sensitive Ca2+ stores and lysosome-like organelles by GRGDSP suggest important roles for integrin-dependent Ca2+ signaling in regulating PASMC function.