cAMP inhibits IP(3)-dependent Ca(2+) release by preferential activation of cGMP-primed PKG

Calcium signaling in endothelial and vascular smooth muscle cells: sex differences and the influence of estrogens and androgens

Calcium signaling in vascular endothelial cells (ECs) and smooth muscle cells (VSMCs) is essential for the regulation of vascular tone. However, the changes to intracellular Ca2+ concentrations are often influenced by sex differences. Furthermore, a large body of evidence shows that sex hormone imbalance leads to dysregulation of Ca2+ signaling and this is a key factor in the pathogenesis of cardiovascular diseases. In this review, the effects of estrogens and androgens on vascular calcium-handling proteins are discussed, with emphasis on the associated genomic or nongenomic molecular mechanisms. The experimental models from which data were collected were also considered. The review highlights 1) in female ECs, transient receptor potential vanilloid 4 (TRPV4) and mitochondrial Ca2+ uniporter (MCU) enhance Ca2+-dependent nitric oxide (NO) generation. In males, only transient receptor potential canonical 3 (TRPC3) plays a fundamental role in this effect. 2) Female VSMCs have lower cytosolic Ca2+ levels than males due to differences in the activity and expression of stromal interaction molecule 1 (STIM1), calcium release-activated calcium modulator 1 (Orai1), calcium voltage-gated channel subunit-α1C (CaV1.2), Na+-K+-2Cl- symporter (NKCC1), and the Na+/K+-ATPase. 3) When compared with androgens, the influence of estrogens on Ca2+ homeostasis, vascular tone, and incidence of vascular disease is better documented. 4) Many studies use supraphysiological concentrations of sex hormones, which may limit the physiological relevance of outcomes. 5) Sex-dependent differences in Ca2+ signaling mean both sexes ought to be included in experimental design.

Expression and function of umami receptors T1R1/T1R3 in gastric smooth muscle

BACKGROUND

l-amino acids, such as monosodium glutamate (MSG), activate the umami receptor T1R1/T1R3. We previously showed increased peristalsis in response to activation of T1R1/T1R3 by MSG in mouse colon. However, the expression and function of these receptors in the different regions of the stomach are not clear.

METHODS

Mouse gastric smooth muscle cells (SMCs) were isolated and cultured in Dulbecco's Modified Eagle Medium. Expression of T1R1 and T1R3 was measured by RT-PCR and Western blot. The effect of MSG with and without inosine monophosphate (IMP, an allosteric activator of T1R1/T1R3) on acetylcholine (ACh)-induced contraction was measured in muscle strips and isolated SMCs by scanning micrometry. The effect of MSG with or without IMP on activation of G proteins and ACh-induced Ca²⁺ release was measured in SMCs.

KEY RESULTS

Monosodium glutamate inhibited ACh-induced contractions in muscle strips from both antrum and fundus and the effect of MSG was augmented by IMP; the effects were concentration-dependent and not affected by the nitric oxide synthase inhibitor, L-NNA, or tetrodotoxin suggesting a direct effect on SMCs. In isolated gastric SMCs, T1R1 and T1R3 transcripts and protein were identified. Addition of MSG with or without IMP inhibited ACh-induced Ca²⁺ release and muscle contraction; the effect on contraction was blocked by pertussis toxin suggesting activation of Gα_i proteins. MSG in the presence of IMP selectively activated Gα_i2 .

CONCLUSIONS AND INFERENCES

Umami receptors (T1R1/T1R3) are present on SMCs of the stomach, and activation of these receptors induces muscle relaxation by decreasing [Ca²⁺ ]_i via Gα_i2 .

Inositol trisphosphate receptors in smooth muscle cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of tetrameric intracellular calcium (Ca(2+)) release channels that are located on the sarcoplasmic reticulum (SR) membrane of virtually all mammalian cell types, including smooth muscle cells (SMC). Here, we have reviewed literature investigating IP(3)R expression, cellular localization, tissue distribution, activity regulation, communication with ion channels and organelles, generation of Ca(2+) signals, modulation of physiological functions, and alterations in pathologies in SMCs. Three IP(3)R isoforms have been identified, with relative expression and cellular localization of each contributing to signaling differences in diverse SMC types. Several endogenous ligands, kinases, proteins, and other modulators control SMC IP(3)R channel activity. SMC IP(3)Rs communicate with nearby ryanodine-sensitive Ca(2+) channels and mitochondria to influence SR Ca(2+) release and reactive oxygen species generation. IP(3)R-mediated Ca(2+) release can stimulate plasma membrane-localized channels, including transient receptor potential (TRP) channels and store-operated Ca(2+) channels. SMC IP(3)Rs also signal to other proteins via SR Ca(2+) release-independent mechanisms through physical coupling to TRP channels and local communication with large-conductance Ca(2+)-activated potassium channels. IP(3)R-mediated Ca(2+) release generates a wide variety of intracellular Ca(2+) signals, which vary with respect to frequency, amplitude, spatial, and temporal properties. IP(3)R signaling controls multiple SMC functions, including contraction, gene expression, migration, and proliferation. IP(3)R expression and cellular signaling are altered in several SMC diseases, notably asthma, atherosclerosis, diabetes, and hypertension. In summary, IP(3)R-mediated pathways control diverse SMC physiological functions, with pathological alterations in IP(3)R signaling contributing to disease.

Relaxant effect of prostaglandin D(2)--receptor DP agonist on liver myofibroblast contraction

Increased intrahepatic resistance causes portal hypertension in cirrhosis. Liver myofibroblasts (MFs) are now regarded as the principle cells involved in sinusoidal blood flow regulation. Many other prostaglandin-receptor agonists have been reported to regulate liver MF contraction, but the role of the prostaglandin D(2)-receptor DP is unknown. In this study, we investigated the effect of a synthetic agonist of prostanoid DP receptor, BW245C, on contractile properties of primary rat liver MFs. Collagen gel contraction assay revealed that BW245C alone (1 and 10 µM) did not induce contraction but induced cell relaxation. Pretreatment with BW245C (10 µM, 30 min) attenuated bradykinin (100 nM)-induced liver MF contraction. Elevation of [Ca(2+)](i) induced by bradykinin (100 nM) was partially suppressed by BW245C pretreatment (10 µM, 3 min). BW245C (1 and 10 µM) significantly increased intracellular cAMP level in a dose-dependent manner. Pretreatment with forskolin (30 - 300 nM, 30 min) and dibutyryl-cAMP (3 - 30 µM, 30 min) significantly reduced bradykinin-induced contraction. Furthermore, a protein kinase A (PKA) inhibitor KT5720 (10 nM to 1 µM, 30 min) blocked the relaxant effect of BW245C. These results suggest that prostanoid DP receptor agonism inhibits bradykinin-induced [Ca(2+)](i) elevation and contraction through cAMP-PKA signal activation in rat liver MFs.

InsP3R-associated cGMP kinase substrate determines inositol 1,4,5-trisphosphate receptor susceptibility to phosphoregulation by cyclic nucleotide-dependent kinases

Ca(2+) release through inositol 1,4,5-trisphosphate receptors (InsP(3)R) can be modulated by numerous factors, including input from other signal transduction cascades. These events shape the spatio-temporal characteristics of the Ca(2+) signal and provide fidelity essential for the appropriate activation of effectors. In this study, we investigate the regulation of Ca(2+) release via InsP(3)R following activation of cyclic nucleotide-dependent kinases in the presence and absence of expression of a binding partner InsP(3)R-associated cGMP kinase substrate (IRAG). cGMP-dependent kinase (PKG) phosphorylation of only the S2+ InsP(3)R-1 subtype resulted in enhanced Ca(2+) release in the absence of IRAG expression. In contrast, IRAG bound to each InsP(3)R subtype, and phosphorylation of IRAG by PKG attenuated Ca(2+) release through all InsP(3)R subtypes. Surprisingly, simply the expression of IRAG attenuated phosphorylation and inhibited the enhanced Ca(2+) release through InsP(3)R-1 following cAMP-dependent protein kinase (PKA) activation. In contrast, IRAG expression did not influence the PKA-enhanced activity of the InsP(3)R-2. Phosphorylation of IRAG resulted in reduced Ca(2+) release through all InsP(3)R subtypes during concurrent activation of PKA and PKG, indicating that IRAG modulation is dominant under these conditions. These studies yield mechanistic insight into how cells with various complements of proteins integrate and prioritize signals from ubiquitous signaling pathways.

Regulation of inositol 1,4,5-trisphosphate-induced Ca2+ release by reversible phosphorylation and dephosphorylation

The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) is a universal intracellular Ca2+-release channel. It is activated after cell stimulation and plays a crucial role in the initiation and propagation of the complex spatio-temporal Ca2+ signals that control cellular processes as different as fertilization, cell division, cell migration, differentiation, metabolism, muscle contraction, secretion, neuronal processing, and ultimately cell death. To achieve these various functions, often in a single cell, exquisite control of the Ca2+ release is needed. This review aims to highlight how protein kinases and protein phosphatases can interact with the IP3R or with associated proteins and so provide a large potential for fine tuning the Ca2+-release activity and for creating efficient Ca2+ signals in subcellular microdomains.

Contractile agonists attenuate cGMP levels by stimulating phosphorylation of cGMP-specific PDE5; an effect mediated by RhoA/PKC-dependent inhibition of protein phosphatase 1

BACKGROUND AND PURPOSE

In gastrointestinal smooth muscle cGMP levels in response to relaxant agonists are regulated by PKG-mediated phosphorylation and activation of phosphodiesterase 5 (PDE5). The aim of the present study was to determine whether contractile agonists modulate cGMP levels by cross-regulating PDE5 activity and to identify the mechanism of action.

EXPERIMENTAL APPROACH

Dispersed and cultured muscle cells from rabbit stomach were treated with the nitric oxide donor, S-nitrosoglutathione (GSNO), or with a contractile agonist, ACh and GSNO. PDE5 phosphorylation and activity, and cGMP levels were determined.

KEY RESULTS

GSNO stimulated PDE5 phosphorylation and activity and increased cGMP levels in gastric smooth muscle cells. Concurrent activation of cells with ACh augmented GSNO-stimulated PDE5 phosphorylation and activity, and attenuated cGMP levels. The effect of ACh was blocked by the m3 receptor antagonist and by inhibitors of protein kinase C (PKC) or RhoA, but not by the m2 receptor antagonist or inhibitors of PI hydrolysis. The effects of ACh on PDE5 phosphorylation and activity, and cGMP levels were mimicked by a low concentration of tautomycin (10 nM), and a high (1 microM) but not low (1 nM) concentration of okadaic acid. PDE5 was associated with protein phosphatase 1 (PP1) and dephosphorylated by the catalytic subunit of PP1 but not PP2A.

CONCLUSION AND IMPLICATIONS

In gastrointestinal smooth muscle cGMP levels are cross-regulated by contractile agonists via a mechanism that involves RhoA-dependent, PKC-mediated inhibition of PP1 activity. This leads to augmentation of PDE5 phosphorylation and activity, and inhibition of cGMP levels.

Non-genomic vascular actions of female sex hormones: physiological implications and signalling pathways

1. Epidemiological studies indicate a lower incidence of coronary heart disease in premenopausal women compared with age-matched men and post-menopausal women. Accumulating evidence suggests that this cardiovascular protection observed in premenopausal women is at least partially attributed to the direct action of oestrogens on the vascular system. 2. Research focused on vascular actions of 17beta-oestradiol indicates that this female sex hormone favourably modulates vascular reactivity at physiological concentrations. The vascular actions of 17beta-oestradiol appear independent of its genomic actions. Both endothelium-dependent and -independent signalling cascades have been implicated in the vascular effects of 17beta-oestradiol. 3. However, clinical trials on hormone-replacement therapy argue against a role of oestrogens in preventing the development of coronary heart disease. Supplementation with oestrogen is also complicated with the increased risk of breast and endometrial cancer. Hence, a better understanding of the vascular actions of 17beta-oestradiol will serve to enhance our understanding of its role in coronary heart disease.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Reciprocal regulation and integration of signaling by intracellular calcium and cyclic GMP

Calcium and guanosine-3',5'-cyclic monophosphate (cGMP) are second messenger molecules that regulate opposing physiological functions, reflected in the reciprocal regulation of their intracellular concentrations, in many systems. Indeed, cGMP and Ca2+ constitute discrete points of integration between multiple cell signaling cascades in both convergent and parallel pathways. This chapter describes the molecular mechanisms regulating intracellular Ca2+ and cGMP, and their integration in specific cellular responses.

cAMP modulates cGMP-mediated cerebral arteriolar relaxation in vivo

No studies have specifically addressed whether cAMP can influence nitric oxide (NO)/cGMP-induced cerebral vasodilation. In this study, we examined whether cAMP can enhance or reduce NO-induced cerebral vasodilation in vivo via interfering with cGMP efflux or through potentiating phosphodiesterase 5 (PDE5)-mediated cGMP breakdown, respectively, in cerebral vascular smooth muscle cells (CVSMCs). To that end, we evaluated, in male rats, the effects of knockdown [via antisense oligodeoxynucleotide (ODN) applications] of the cGMP efflux protein multidrug resistance protein 5 (MRP5) and PDE5 inhibition on pial arteriolar NO donor [S-nitroso-N-acetyl penicillamine (SNAP)]-induced dilations in the absence and presence of cAMP elevations via forskolin. Pial arteriolar diameter changes were measured using well-established protocols in anesthetized rats. In control (missense ODN treated) rats, forskolin elicited a leftward shift in the SNAP dose-response curves (approximately 50% reduction in SNAP EC50). However, in MRP5 knockdown rats, cAMP increases were associated with a substantial reduction in SNAP-induced vasodilations (reflected as a significant 35-50% lower maximal response). In the presence of the PDE5 inhibitor MY-5445, the repression of the NO donor response accompanying forskolin was prevented. These findings suggest that cAMP has opposing effects on NO-stimulated cGMP increases. On the one hand, cAMP limits CVSMC cGMP loss by restricting cGMP efflux. On the other, cAMP appears to enhance PDE5-mediated cGMP breakdown. However, because increased endogenous cAMP seems to potentiate NO/cGMP-induced arteriolar relaxation when MRP5 expression is normal, the effect of cAMP to reduce cGMP efflux appears to predominate over cAMP stimulation of cGMP hydrolysis.

G protein-mediated dysfunction of excitation-contraction coupling in ileal inflammation

Inflammation impairs the circular muscle contractile response to muscarinic (M) receptor activation. The aim of this study was to investigate whether the expression of muscarinic receptors, their binding affinity, and the expression and activation of receptor-coupled G proteins contribute to the suppression of contractility in inflammation. The studies were performed on freshly dissociated single smooth muscle cells from normal and inflamed canine ileum. Northern blotting indicated the presence of only M(2) and M(3) receptors on canine ileal circular muscle cells. Inflammation did not alter the mRNA or protein expression of M(2) and M(3) receptors. The maximal binding and K(d) values also did not differ between normal and inflamed cells. However, the contractile response to ACh in M(3) receptor-protected cells was suppressed, whereas that in M(2) receptor-protected cells was enhanced. Further experiments indicated that the expression and binding activity of G alpha(q/11) protein, which couples to M(3) receptors, were downregulated, whereas those of G alpha(i3), which couples to M(2) receptors, were upregulated in inflamed cells. We concluded that inflammation depresses M(3) receptor function, but it enhances M(2) receptor function in ileum. These effects are mediated by the differentially altered expression and binding activity of their respective coupled G alpha(q/11) and G alpha(i3) proteins.

Effect of radiation on cAMP, cGMP and Ca(2+)(i) pathways and their interactions in rat distal colon

The secretory response implicated in the intestinal response to luminal attack is altered by radiation. The cAMP, cGMP and Ca(2+)(i) pathways leading to secretion as well as the interactions between the cAMP pathway and the cGMP or Ca(2+)(i) pathway were studied in the rat distal colon 4 days after a 9-Gy abdominal X irradiation, when modifications mainly occurred. The secretory response in Ussing chambers and cAMP and cGMP accumulation in single isolated crypts were measured. The muscarinic receptor characteristics were determined in mucosal membrane preparations. The secretory response by the cAMP pathway (stimulated by vasoactive intestinal peptide or forskolin) and the cAMP accumulation in crypts were decreased (P < 0.05) after irradiation. The weak secretory response induced by the cGMP pathway (stimulated by nitric oxide or guanylin) was unaltered by radiation, and the small amount of cGMP determined in isolated crypts from the control group became undetectable in the irradiated group. Inducible NOS was not involved in the hyporesponsiveness to VIP after irradiation (there was no effect of an iNOS inhibitor). The secretory response by the Ca(2+)(i) pathway (stimulated by carbachol) was unaffected despite a decreased number and increased affinity of muscarinic receptors. The non-additivity of VIP and carbachol co-stimulated responses was unmodified. In contrast, VIP and SNP co-stimulation showed that NO enhanced the radiation-induced hyporesponsiveness to VIP through a reduced accumulation of cAMP in crypts. This study provides further understanding of the effect of ionizing radiation on the intracellular signaling pathways.

Selective phosphorylation of the IP3R-I in vivo by cGMP-dependent protein kinase in smooth muscle

This study examined the expression of inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R) types and PKG isoforms in isolated gastric smooth muscle cells and determined the ability of PKG and PKA to phosphorylate IP(3)Rs and inhibit IP(3)-dependent Ca(2+) release, which mediates the initial phase of agonist-induced contraction. PKG-Ialpha and PKG-Ibeta were expressed in gastric smooth muscle cells, together with IP(3)-R-associated cG-kinase substrate, a protein that couples PKG-Ibeta to IP(3)R-I. IP(3)R-I and IP(3)R-III were also expressed, but only IP(3)R-I was phosphorylated by PKA and PKG in vitro and exclusively by PKG in vivo. Sequential phosphorylation by PKA and by PKG-Ialpha in vitro showed that PKA phosphorylated the same site as PKG (presumably S(1755)) and an additional PKA-specific site (S(1589)). In intact muscle cells, agents that activated PKG or both PKG and PKA induced IP(3)R-I phosphorylation that was reversed by the PKG inhibitor (8R,9S,11s)-(-)-9-methoxy-carbamyl-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1H,8H,1H,-2,7b,11a-trizadizo-benzo9(a,g)cycloocta(c,d,e)-trinden-1-one. Agents that activated PKA induced IP(3)R-I phosphorylation in permeabilized but not intact muscle cells, implying that PKA does not gain access to IP(3)R-I in intact muscle cells. The pattern of IP(3)R-I phosphorylation in vivo and in vitro was more consistent with phosphorylation by PKG-Ialpha. Phosphorylation of IP(3)R-I in microsomes by PKG, PKA, or a combination of PKG and PKA inhibited IP(3)-induced Ca(2+) release to the same extent, implying that inhibition was mediated by phosphorylation of the PKG-specific site. We conclude that IP(3)R-I is selectively phosphorylated by PKG-I in intact smooth muscle resulting in inhibition of IP(3)-dependent Ca(2+) release.