Vasopressin-evoked [Ca2+]i responses in neonatal rat cardiomyocytes

Effect of Sphingosine-1-Phosphate on Intracellular Free Ca²⁺ in Cat Esophageal Smooth Muscle Cells

A comprehensive collection of proteins senses local changes in intracellular Ca²⁺ concentrations ([Ca²⁺]_i) and transduces these signals into responses to agonists. In the present study, we examined the effect of sphingosine-1-phosphate (S1P) on modulation of intracellular Ca²⁺ concentrations in cat esophageal smooth muscle cells. To measure [Ca²⁺]_i levels in cat esophageal smooth muscle cells, we used a fluorescence microscopy with the Fura-2 loading method. S1P produced a concentration-dependent increase in [Ca²⁺]_i in the cells. Pretreatment with EGTA, an extracellular Ca²⁺ chelator, decreased the S1P-induced increase in [Ca²⁺]_i, and an L-type Ca²⁺-channel blocker, nimodipine, decreased the effect of S1P. This indicates that Ca²⁺ influx may be required for muscle contraction by S1P. When stimulated with thapsigargin, an intracellular calcium chelator, or 2-Aminoethoxydiphenyl borate (2-APB), an InsP₃ receptor blocker, the S1P-evoked increase in [Ca²⁺]_i was significantly decreased. Treatment with pertussis toxin (PTX), an inhibitor of G_i-protein, suppressed the increase in [Ca²⁺]_i evoked by S1P. These results suggest that the S1P-induced increase in [Ca²⁺]_i in cat esophageal smooth muscle cells occurs upon the activation of phospholipase C and subsequent release of Ca²⁺ from the InsP₃-sensitive Ca²⁺ pool in the sarcoplasmic reticulum. These results suggest that S1P utilized extracellular Ca²⁺ via the L type Ca²⁺ channel, which was dependent on activation of the S1P₄ receptor coupled to PTX-sensitive G_i protein, via phospholipase C-mediated Ca²⁺ release from the InsP₃-sensitive Ca²⁺ pool in cat esophageal smooth muscle cells.

Redox control of cardiac remodeling in atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice and is a potential cause of thromboembolic events. AF induces significant changes in the electrophysiological properties of atrial myocytes and causes alterations in the structure, metabolism, and function of the atrial tissue. The molecular basis for the development of structural atrial remodeling of fibrillating human atria is still not fully understood. However, increased production of reactive oxygen or nitrogen species (ROS/RNS) and the activation of specific redox-sensitive signaling pathways observed both in patients with and animal models of AF are supposed to contribute to development, progression and self-perpetuation of AF.

SCOPE OF REVIEW

The present review summarizes the sources and targets of ROS/RNS in the setting of AF and focuses on key redox-sensitive signaling pathways that are implicated in the pathogenesis of AF and function either to aggravate or protect from disease.

MAJOR CONCLUSIONS

NADPH oxidases and various mitochondrial monooxygenases are major sources of ROS during AF. Besides direct oxidative modification of e.g. ion channels and ion handling proteins that are crucially involved in action potential generation and duration, AF leads to the reversible activation of redox-sensitive signaling pathways mediated by activation of redox-regulated proteins including Nrf2, NF-κB, and CaMKII. Both processes are recognized to contribute to the formation of a substrate for AF and, thus, to increase AF inducibility and duration.

GENERAL SIGNIFICANCE

AF is a prevalent disease and due to the current demographic developments its socio-economic relevance will further increase. Improving our understanding of the role that ROS and redox-related (patho)-mechanisms play in the development and progression of AF may allow the development of a targeted therapy for AF that surpasses the efficacy of previous general anti-oxidative strategies. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Mechanisms of the lysophosphatidic acid-induced increase in [Ca(2+)](i) in skeletal muscle cells

Although lysophosphatidic acid (LPA) is known to increase intracellularfree calcium concentration ([Ca²⁺]_i) in different cell types, the effect of LPA on the skeletal muscle cells is not known. The present study was therefore undertaken to examine the effect of LPA on the [Ca²⁺]_i in C2C12 cells. LPA induced a concentration and time dependent increase in [Ca²⁺]_i, which was inhibited by VPC12249, VPC 32183 and dioctanoyl glycerol pyrophosphate, LPA1/3 receptor antagonists. Pertussis toxin, a Gⁱ protein inhibitor, also inhibited the LPA-induced increase in [Ca²⁺]_i. Inhibition of tyrosine kinase activities with tyrphostin A9 and genistein also prevented the increase in [Ca²⁺]_i due to LPA. Likewise, wortmannin and LY 294002, phosphatidylinositol 3-kinase (PI3-K) inhibitors, inhibited [Ca²⁺]_i response to LPA. The LPA effect was also attenuated by ethylene glycolbis(β-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA), an extracellular Ca²⁺ chelator, Ni²⁺ and KB-R7943, inhibitors of the Na⁺-Ca²⁺ exchanger; the receptor operated Ca²⁺ channel (ROC) blockers, 2-aminoethoxydiphenyl borate and SK&F 96365. However, the L-type Ca²⁺ channel blockers, verapamil and diltiazem; the store operated Ca²⁺ channel blockers, La³⁺ and Gd³⁺; a sarcoplasmic reticulum calcium pump inhibitor, thapsigargin; an inositol trisphosphate receptor antagonist, xestospongin and a phospholipase C inhibitor, U73122, did not prevent the increase [Ca²⁺]_i due to LPA. Our data suggest that the LPA-induced increase in [Ca²⁺]_i might occur through Gⁱ-protein coupled LPA_1/3 receptors that may be linked to tyrosine kinase and PI3-K, and may also involve the Na⁺-Ca²⁺ exchanger as well as the ROC. In addition, LPA stimulated C2C12 cell proliferation via PI3-K. Thus, LPA may be an important phospholipid in the regulation of [Ca²⁺]_i and growth of skeletal muscle cells.

Vasopressin-induced nitric oxide production in rat inner medullary collecting duct is dependent on V2 receptor activation of the phosphoinositide pathway

We previously reported that arginine vasopressin (AVP) stimulates the production of nitric oxide (NO) in inner medullary collecting duct (IMCD) via activation of V2 receptors (V2R) and the mobilization of intracellular Ca(2+). The aim of this study was to determine the pathway(s) through which this response is mediated. IMCDs were dissected from male Sprague-Dawley rats and intracellular Ca(2+) concentration ([Ca(2+)](i)) and NO production were measured using a fluorescence imaging system. AVP (100 nmol/l) produced a rapid increase [Ca(2+)](i) of 381 +/- 78 nmol/l that was followed by a significant increase of NO production (166 +/- 61%). The specific nonpeptide V2R antagonist OPC31260 (1 microM), but not the V1R antagonist OPC21268 (1 microM), inhibited the increase in [Ca(2+)](i) (up to 91 +/- 5%) and abolished the NO response to AVP. Both the phospholipase C inhibitor U73112 (3 microM) and the inositol (1,4,5) tri-phosphate 3 receptor blocker 2-APB (75 microM) reduced the peak [Ca(2+)](i) response to AVP (by 65 +/- 9 and 59 +/- 15%, respectively) and abolished the NO response. Although forskolin (100 microM; an activator of adenylyl cyclase) elicited a moderate increase in [Ca(2+)](i), neither preincubation with the adenylyl cyclase inhibitor 2'-5'-dideoxyadenosine (50 microM) nor the protein kinase A (PKA) inhibitor PKA(14-22) (100 microM) significantly inhibited peak [Ca(2+)](i) in response to AVP. IMCD [Ca(2+)](i) responses to AVP were reduced by 72 +/- 8% when incubated in Ca(2+)-free media and could be completely abolished by preincubation with the Ca(2+)-ATPase inhibitor thapsigargin. We conclude that AVP-induced NO production in IMCD is dependent on V2R activation of the phosphoinositide pathway and the mobilization of Ca(2+) from both intracellular and extracellular pools.

Mechanisms of lysophosphatidic acid-induced increase in intracellular calcium in vascular smooth muscle cells

Although lysophosphatidic acid (LPA) is known to cause an increase in intracellular Ca2+ concentration ([Ca2+]i) in vascular smooth muscle cells (VSMCs), the mechanisms of [Ca2+]i mobilization by LPA are not fully understood. In the present study, the effect of LPA on [Ca2+]i mobilization in cultured A10 VSMCs was examined by Fura-2 fluorescence technique. The expression of LPA receptors was studied by immunostaining. LPA was observed to increase [Ca2+]i in a concentration-dependent manner; this increase was dependent on the concentration of extracellular Ca2+. Both sarcolemmal (SL) Na(+)-Ca2+ exchange inhibitors (amiloride, Ni2+ and KB-R7943) and Na(+)-H+ exchange inhibitor (MIA) as well as SL store-operated Ca2+ channel (SOC) antagonists (SK&F 96365, tyrphostin A9 and gadolinium), unlike SL Ca2+ channel antagonists (verapamil and diltiazem), inhibited the LPA-induced increase in [Ca2+]i. In addition, sarcoplasmic reticulum (SR) Ca2+ channel blocker (ryanodine), SR Ca2+ channel opener (caffeine), SR Ca2+ pump ATPase inhibitor (thapsigargin) and inositol 1,4,5-trisphosphate (InsP3) receptor antagonists (xestospongin and 2-aminoethoxydiphenyl borate) were found to inhibit the LPA-induced Ca2+ mobilization. Furthermore, phospholipase C (PLC) inhibitor (U 73122) and protein kinase C (PKC) activator (phorbol 12-myristate 13-acetate) attenuated the LPA-induced increase in [Ca2+]i. These results indicate that Ca2+ mobilization by LPA involves extracellular Ca2+ entry through SL Na(+)-Ca2+ exchanger, Na(+)-H+ exchanger and SL SOCs. In addition, ryanodine-sensitive and InsP(3)-sensitive intracellular Ca2+ pools may be associated with the LPA-induced increase in [Ca2+]i. Furthermore, the LPA-induced [Ca2+]i mobilization in VSMCs seems to be due to the activation of both PLC and PKC.

Science Review: Vasopressin and the cardiovascular system part 2 - clinical physiology

Vasopressin is emerging as a rational therapy for vasodilatory shock states. In part 1 of the review we discussed the structure and function of the various vasopressin receptors. In part 2 we discuss vascular smooth muscle contraction pathways with an emphasis on the effects of vasopressin on ATP-sensitive K+ channels, nitric oxide pathways, and interaction with adrenergic agents. We explore the complex and contradictory studies of vasopressin on cardiac inotropy and coronary vascular tone. Finally, we summarize the clinical studies of vasopressin in shock states, which to date have been relatively small and have focused on physiologic outcomes. Because of potential adverse effects of vasopressin, clinical use of vasopressin in vasodilatory shock should await a randomized controlled trial of the effect of vasopressin's effect on outcomes such as organ failure and mortality.

Modification of intracellular free calcium in cultured A10 vascular smooth muscle cells by exogenous phosphatidic acid

Exogenous phosphatidic acid (PA) was observed to produce a concentration-dependent increase in [Ca(2+)](i) in cultured A10 vascular smooth muscle cells. Preincubation of cells with sarcoplasmic reticulum Ca(2+)-ATPase inhibitors (cyclopiazonic acid and thapsigargin), a phospholipase C inhibitor (2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate), inositol 1,4,5-trisphosphate receptor antagonists (2-aminoethoxydiphenyl borate and xestospongin), and an activator of protein kinase C (PKC) (phorbol 12-myristate 13-acetate) depressed the PA-evoked increase in [Ca(2+)](i). Although EGTA, an extracellular Ca(2+) chelator, decreased the PA-induced increase in [Ca(2+)](i), sarcolemmal Ca(2+)-channel blockers (verapamil or diltiazem) did not alter the action of PA. On the other hand, inhibitors of PKC (bisindolylmaleimide I) and G(i)-protein (pertussis toxin) potentiated the increase in [Ca(2+)](i) evoked by PA significantly. These results suggest that the PA-induced increase in [Ca(2+)](i) in vascular smooth muscle cells may occur upon the activation of phospholipase C and the subsequent release of Ca(2+) from the inositol 1,4,5-trisphosphate-sensitive Ca(2+) pool in the sarcoplasmic reticulum. This action of PA may be mediated through the involvement of PKC.

Cysteinyl leukotriene-dependent [Ca2+]i responses to angiotensin II in cardiomyocytes

With the use of fura 2 measurements in multiple and single cells, we examined whether cysteinyl leukotrienes (CysLT) mediate angiotensin II (ANG II)-evoked increases in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in neonatal rat cardiomyocytes. ANG II-evoked CysLT release peaked at 1 min. The angiotensin type 1 (AT(1)) antagonist losartan, but not the AT(2) antagonist PD-123319, attenuated the elevations in [Ca(2+)](i) and CysLT levels evoked by ANG II. Vasopressin and endothelin-1 increased [Ca(2+)](i) but not CysLT levels. The 5-lipoxygenase (5-LO) inhibitor AA-861 and the CysLT(1)-selective antagonist MK-571 reduced the maximal [Ca(2+)](i) responses to ANG II but not to vasopressin and endothelin-1. While MK-571 reduced the responses to leukotriene D(4) (LTD(4)), the dual CysLT antagonist BAY-u9773 completely blocked the [Ca(2+)](i) elevation to both LTD(4) and LTC(4). These data confirm that ANG II-evoked increases, but not vasopressin- and endothelin-1-evoked increases, in [Ca(2+)](i) involve generation of the 5-lipoxygenase metabolite CysLT. The inositol (1,4,5)-trisphosphate [Ins(1,4,5)P(3)] antagonist 2-aminoethoxydiphenyl borate attenuated the [Ca(2+)](i) responses to ANG II and LTD(4). Thus AT(1) receptor activation by ANG II is linked to CysLT-mediated Ca(2+) release from Ins(1,4,5)P(3)-sensitive intracellular stores to augment direct ANG II-evoked Ca(2+) mobilization in rat cardiomyocytes.

Differential aspects in ratio measurements of [Ca(2+)](i) relaxation in cardiomyocyte contraction following various drug treatments

This study is concerned with the analysis of the time dependency of [Ca(2+)](i), monitored by indo-1-AM, via the ratiometric time response curve R(t) as measured during contractions of spontaneous or electrical stimulated cardiomyocytes (in culture). A mathematical formulation which describes the relaxation phase of R(t) was developed. By fitting formulation to the measured data of R(t), the extraction of characteristic parameters is feasible, which may reflect the factors regulating intracellular Ca concentration. The usefulness of the suggested formulation was examined by monitoring changes induced in those parameters following the exposure of the myocytes to different drugs, among which are: caffeine, ryanodine, thapsigargin db, cyclic AMP, isoprenaline, doxorubicin, and Cl-IB-MECA.

Regulated expression of GRP78 during vasopressin-induced hypertrophy of heart-derived myocytes

Although the development of cellular hypertrophy is widely believed to involve Ca(2+) signaling, potential supporting roles for sequestered Ca(2+) in this process have not been explored. H9c2 cardiomyocytes respond to arginine vasopressin with an initial mobilization of Ca(2+) stores and reduced rates of mRNA translation followed by repletion of Ca(2+) stores, up-regulation of translation beyond initial rates, and the development of hypertrophy. Rates of synthesis of the endoplasmic reticulum (ER) chaperones, GRP78 and GRP94, were found to increase preferentially at early times of vasopressin treatment. Total GRP78 content increased 2- to 3-fold within 8 h after which the chaperone was subject to post-translational modification. Preferential synthesis of GRP78 and the increase in chaperone content both occurred at pM vasopressin concentrations and were abolished at supraphysiologic Ca(2+) concentrations. Co-treatment with phorbol myristate acetate decreased vasopressin-dependent Ca(2+) mobilization and slowed appearance of new GRP78 molecules in response to the hormone, whereas 24 h pretreatment with phorbol ester prolonged vasopressin-dependent Ca(2+) mobilization and further increased rates of GRP78 synthesis in response to the hormone. Findings did not support a role for newly synthesized GRP78 in translational up-regulation by vasopressin. However up-regulation, which does not depend on Ca(2+) sequestration, appeared to expedite chaperone expression. This report provides the first evidence that a Ca(2+)-mobilizing hormone at physiologic concentrations signals increased expression of GRP78. Translational tolerance to depletion of ER Ca(2+) stores, typifying a robust ER stress response, did not accompany vasopressin-induced hypertrophy.

Vasopressin-induced hypertrophy in H9c2 heart-derived myocytes

Protein synthesis in H9c2 heart-derived myocytes responds biphasically to arginine vasopressin (1 microM). An initial 50% inhibition attributable to Ca(2+) mobilization from the sarcoplasmic/endoplasmic reticulum is followed by a recovery that subsequently converts to a 1.5-fold stimulation. This study was undertaken to ascertain whether vasopressin programs H9c2 cells to undergo hypertrophy or to proliferate and whether early translational inhibition is required for programming. Translational suppression was observed only at vasopressin concentrations (>1 nM) causing extensive (>50%) depletion of Ca(2+) stores and was diminished at supraphysiologic extracellular Ca(2+) concentrations. Stimulation of protein synthesis, by contrast, was unaffected by changes in extracellular Ca(2+), depended on gene transcription, was suppressed by a protein kinase C pseudosubstrate sequence (peptide 19-27), and was observed at pM vasopressin concentrations. Activation of MAP kinases, phosphoinositide 3-kinase, calcineurin, S6 kinase, or eIF4 could not be implicated in the stimulation, which persisted for 24 h. Vasopressin-treated H9c2 cells underwent hypertrophy by standard criteria. Cellular protein accumulation occurred at pM hormone concentrations, was blocked by peptide 19-27, was observed regardless of retinoic acid pretreatment to prevent myogenic transdifferentiation, and preceded full repletion of Ca(2+) stores. It is proposed that H9c2 cells, which possess all basic features of V1-vasopressin receptor signaling, provide a convenient model for investigating vasopressin-induced myocyte hypertrophy. Early translational suppression is not needed for vasopressin-induced H9c2 myocyte hypertrophy whereas activation of protein kinase C appears essential.