Vasoactive Intestinal Peptide Receptor, CRTH2, Antagonist Treatment Improves Eosinophil and Mast Cell-Mediated Esophageal Remodeling and Motility Dysfunction in Eosinophilic Esophagitis

BACKGROUND AND AIMS

Ultrasonography has shown that eosinophils accumulate in each segment of the esophageal mucosa in human EoE, ultimately promoting esophageal motility dysfunction; however, no mechanistic evidence explains how or why this accumulation occurs.

METHODS

Quantitative PCR, ELISA, flow cytometry, immunostaining, and immunofluorescence analyses were performed using antibodies specific to the related antigens and receptors.

RESULTS

In deep esophageal biopsies of EoE patients, eosinophils and mast cells accumulate adjacent to nerve cell-derived VIP in each esophageal segment. qRT-PCR analysis revealed five- to sixfold increases in expression levels of VIP, CRTH2, and VAPC2 receptors and proteins in human blood- and tissue-accumulated eosinophils and mast cells. We also observed a significant correlation between mRNA CRTH2 levels and eosinophil- and nerve cell-derived VIPs in human EoE (p < 0.05). We provide evidence that eosinophil and mast cell deficiency following CRTH2 antagonist treatment improves motility dysfunction in a chronic DOX-inducible CC10-IL-13 murine model of experimental EoE.

CONCLUSIONS

CRTH2 antagonist treatment is a novel therapeutic strategy for inflammatory cell-induced esophageal motility dysfunction in IL-13-induced chronic experimental EoE.

Steroids originating from bacterial bile acid degradation affect Caenorhabditis elegans and indicate potential risks for the fauna of manured soils

Bile acids are steroid compounds from the digestive tracts of vertebrates that enter agricultural environments in unusual high amounts with manure. Bacteria degrading bile acids can readily be isolated from soils and waters including agricultural areas. Under laboratory conditions, these bacteria transiently release steroid compounds as degradation intermediates into the environment. These compounds include androstadienediones (ADDs), which are C₁₉-steroids with potential hormonal effects. Experiments with Caenorhabditis elegans showed that ADDs derived from bacterial bile acid degradation had effects on its tactile response, reproduction rate, and developmental speed. Additional experiments with a deletion mutant as well as transcriptomic analyses indicated that these effects might be conveyed by the putative testosterone receptor NHR-69. Soil microcosms showed that the natural microflora of agricultural soil is readily induced for bile acid degradation accompanied by the transient release of steroid intermediates. Establishment of a model system with a Pseudomonas strain and C. elegans in sand microcosms indicated transient release of ADDs during the course of bile acid degradation and negative effects on the reproduction rate of the nematode. This proof-of-principle study points at bacterial degradation of manure-derived bile acids as a potential and so-far overlooked risk for invertebrates in agricultural soils.

Arterial α2-Na+ pump expression influences blood pressure: lessons from novel, genetically engineered smooth muscle-specific α2 mice

Arterial myocytes express α1-catalytic subunit isoform Na(+) pumps (75-80% of total), which are ouabain resistant in rodents, and high ouabain affinity α2-Na(+) pumps. Mice with globally reduced α2-pumps (but not α1-pumps), mice with mutant ouabain-resistant α2-pumps, and mice with a smooth muscle (SM)-specific α2-transgene (α2 (SM-Tg)) that induces overexpression all have altered blood pressure (BP) phenotypes. We generated α2 (SM-DN) mice with SM-specific α2 (not α1) reduction (>50%) using nonfunctional dominant negative (DN) α2. We compared α2 (SM-DN) and α2 (SM-Tg) mice to controls to determine how arterial SM α2-pumps affect vasoconstriction and BP. α2 (SM-DN) mice had elevated basal mean BP (mean BP by telemetry: 117 ± 4 vs. 106 ± 1 mmHg, n = 7/7, P < 0.01) and enhanced BP responses to chronic ANG II infusion (240 ng·kg(-1)·min(-1)) and high (6%) NaCl. Several arterial Ca(2+) transporters, including Na(+)/Ca(2+) exchanger 1 (NCX1) and sarcoplasmic reticulum and plasma membrane Ca(2+) pumps [sarco(endo)plasmic reticulum Ca(2+)-ATPase 2 (SERCA2) and plasma membrane Ca(2+)-ATPase 1 (PMCA1)], were also reduced (>50%). α2 (SM-DN) mouse isolated small arteries had reduced myogenic reactivity, perhaps because of reduced Ca(2+) transporter expression. In contrast, α2 (SM-Tg) mouse aortas overexpressed α2 (>2-fold), NCX1, SERCA2, and PMCA1 (43). α2 (SM-Tg) mice had reduced basal mean BP (104 ± 1 vs. 109 ± 2 mmHg, n = 15/9, P < 0.02) and attenuated BP responses to chronic ANG II (300-400 ng·kg(-1)·min(-1)) with or without 2% NaCl but normal myogenic reactivity. NCX1 expression was inversely related to basal BP in SM-α2 engineered mice but was directly related in SM-NCX1 engineered mice. NCX1, which usually mediates arterial Ca(2+) entry, and α2-Na(+) pumps colocalize at plasma membrane-sarcoplasmic reticulum junctions and functionally couple via the local Na(+) gradient to help regulate cell Ca(2+). Altered Ca(2+) transporter expression in SM-α2 engineered mice apparently compensates to minimize Ca(2+) overload (α2 (SM-DN)) or depletion (α2 (SM-Tg)) and attenuate BP changes. In contrast, Ca(2+) transporter upregulation, observed in many rodent hypertension models, should enhance Ca(2+) entry and signaling and contribute significantly to BP elevation.

Muscle ciliary neurotrophic factor receptor α promotes axonal regeneration and functional recovery following peripheral nerve lesion

Ciliary neurotrophic factor (CNTF) administration maintains, protects, and promotes the regeneration of both motor neurons (MNs) and skeletal muscle in a wide variety of models. Expression of CNTF receptor α (CNTFRα), an essential CNTF receptor component, is greatly increased in skeletal muscle following neuromuscular insult. Together the data suggest that muscle CNTFRα may contribute to neuromuscular maintenance, protection, and/or regeneration in vivo. To directly address the role of muscle CNTFRα, we selectively-depleted it in vivo by using a "floxed" CNTFRα mouse line and a gene construct (mlc1f-Cre) that drives the expression of Cre specifically in skeletal muscle. The resulting mice were challenged with sciatic nerve crush. Counting of nerve axons and retrograde tracing of MNs indicated that muscle CNTFRα contributes to MN axonal regeneration across the lesion site. Walking track analysis indicated that muscle CNTFRα is also required for normal recovery of motor function. However, the same muscle CNTFRα depletion unexpectedly had no detected effect on the maintenance or regeneration of the muscle itself, even though exogenous CNTF has been shown to affect these functions. Similarly, MN survival and lesion-induced terminal sprouting were unaffected. Therefore, muscle CNTFRα is an interesting new example of a muscle growth factor receptor that, in vivo under physiological conditions, contributes much more to neuronal regeneration than to the maintenance or regeneration of the muscle itself. This novel form of muscle-neuron interaction also has implications in the therapeutic targeting of the neuromuscular system in MN disorders and following nerve injury. J. Comp. Neurol. 521: 2947-2965, 2013. © 2013 Wiley Periodicals, Inc.

Abstract 19: Blood Pressure Correlates Directly with the Expression Level of High Ouabain Affinity a2 Na+ Pumps in Arterial Smooth Muscle

High ouabain affinity α2 Na + pumps are expressed in arterial smooth muscle (ASM). They help regulate blood pressure (BP) in several forms of hypertension, but their influence on basal BP is controversial. We determined basal BP (telemetry), and the effects of subcutaneous (sc) angiotensin II (Ang II) and dietary salt on BP in: 1) α2 SM/Tg mice, which express an α2 transgene controlled by an α-actin smooth muscle (SM) promoter; 2) α2 SM/DN mice, which express a truncated α2 gene, dominant negative for native α2 Na + pumps, controlled by the SM myosin heavy chain promoter. Expression (immunoblots) of α2 was significantly up- and down-regulated in SM from α2 SM/Tg and α2 SM/DN mice, respectively.

Basal 24 hr mean BP (MBP) was reduced in α2 SM/Tg mice: 103±1 ( n =15) vs 110±2 mm Hg ( n =9) in WT controls; P <0.01. Ang II, 400 ng/kg/min sc x 2 wks, increased MBP in both WT and transgenic mice ( n =5 each; P <0.01), but the increase (Δ) was smaller in α2 SM/Tg , Δ=20.3±3.3, than in WT mice, Δ=30.9±6.4 mm Hg . Also, a high (2% x 2 wks) NaCl diet, along with a sc 300 ng/kg/min (low dose) Ang II infusion, elevated systolic BP (SBP) in the WT mice (Δ=12.6±1.8 mm Hg; n =4; P <0.05), but not in the α2 SM/Tg mice (Δ=7.2±2.0 mm Hg; n =9; NS=not significant).

In contrast, in α2 SM/DN mice, basal SBP was elevated: 128±4 mm Hg ( n =11) vs 109±4 mm Hg ( n =10) in WT; P <0.01. Furthermore, both high NaCl (HS, 6% x 10 days), and low dose sc Ang II infusion (240 ng/kg/min x 8 days), individually induced significantly greater increases in MBP in α2 SM/DN than in WT mice. In α2 SM/DN mice, the HS diet raised MBP by Δ=6.8±0.7 mm Hg ( n =5; P <0.01); Ang II elevated MBP by Δ=17.1±3.7 mm Hg ( n =5; P <0.05). In WT mice, however, HS did not elevate MBP (Δ=2.6±1.4; n =5; NS), and Ang II elevated MBP by only Δ=6.9±4.0 mm Hg; n =5; P <0.05).

Conclusion: ASM α2 Na + pumps help regulate basal BP and vascular reactivity. Reduced α2 expression raises basal BP and augments arterial responses to salt and Ang II; increased ASM α2 expression has the opposite effects. The α2 Na + pumps co-localize with Na/Ca exchangers in plasma membrane (PM) microdomains at MP-sarcoplasmic reticulum junctions; they are a crucial link between circulating endogenous ouabain and the control of ASM Ca 2+ homeostasis, arterial contraction and tone.

Esophageal functional impairments in experimental eosinophilic esophagitis

Eosinophilic esophagitis (EoE) is an emerging chronic esophageal disease. Despite the increasing diagnosis of EoE globally, the causes of EoE and other esophageal eosinophilic disorders are not clearly understood. EoE pathology includes accumulation of inflammatory cells (e.g., eosinophils, mast cells), characteristic endoscopic features (e.g., furrows, the formation of fine concentric mucosal rings, exudates), and functional impairments (e.g., esophageal stricture, dysmotility). We hypothesized that the esophageal structural pathology and functional impairments of EoE develop as a consequence of the effector functions of the accumulated inflammatory cells. We analyzed eosinophils (anti-major basic protein immunostaining), esophageal stricture (X-ray barium swallowing), and esophageal motility (isometric force) in two established transgenic murine models of EoE (CD2-IL-5 and rtTA-CC10-IL-13) and a novel eosinophil-deficient model (ΔdblGATA/CD2-IL-5). Herein, we show the following: 1) CD2-IL-5 and doxycycline (DOX)-induced rtTA-CC10-IL-13 mice have chronic eosinophilic and mast cell esophageal inflammation; 2) eosinophilic esophageal inflammation promotes esophageal stricture in both transgenic murine models; 3) the eosinophil-deficient ΔdblGATA/CD-2-IL-5 mice were protected from the induction of stricture, whereas the eosinophil-competent CD2-IL-5 mice develop esophageal stricture; 4) esophageal stricture is not reversible in DOX-induced rtTA-CC10-IL-13 mice (8 wk DOX followed by 8 wk no-DOX); and 5) IL-5 transgene-induced (CD2-IL-5) EoE evidences esophageal dysmotility (relaxation and contraction) that is independent of the eosinophilic esophageal inflammation: CD2-IL-5 and ΔdblGATA/CD2-IL-5 mice have comparable esophageal dysmotility. Collectively, our present study directly implicates chronic eosinophilic inflammation in the development of the esophageal structural impairments of experimental EoE.

Na(+)-K(+)-ATPase and Ca(2+) clearance proteins in smooth muscle: a functional unit

The Na(+)-K(+)-ATPase (NKA) can affect intracellular Ca(2+) concentration regulation via coupling to the Na(+)-Ca(2+) exchanger and may be important in myogenic tone. We previously reported that in mice carrying a transgene for the NKA alpha(2)-isoform in smooth muscle (alpha(2sm+)), the alpha(2)-isoform protein as well as the alpha(1)-isoform (not contained in the transgene) increased to similar degrees (2-7-fold). Aortas from alpha(2sm+) mice relaxed faster from a KCl-induced contraction, hypothesized to be related to more rapid Ca(2+) clearance. To elucidate the mechanisms underlying this faster relaxation, we therefore measured the expression and distribution of proteins involved in Ca(2+) clearance. Na(+)-Ca(2+) exchanger, sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), and plasma membrane Ca(2+)-ATPase (PMCA) proteins were all elevated up to approximately fivefold, whereas actin, myosin light chain, and calponin proteins were not changed in smooth muscle from alpha(2sm+) mice. Interestingly, the corresponding Ca(2+) clearance mRNA levels were unchanged. Immunocytochemical data indicate that the Ca(2+) clearance proteins are distributed similarly in wild-type and alpha(2sm+) aorta cells. In studies measuring relaxation half-times from a KCl-induced contraction in the presence of pharmacological inhibitors of SERCA and PMCA, we estimated that together these proteins were responsible for approximately 60-70% of relaxation in aorta. Moreover, the percent contribution of SERCA and PMCA to relaxation rates in alpha(2sm+) aorta was not significantly different from that in wild-type aorta. The coordinate expressions of NKA and Ca(2+) clearance proteins without change in the relative contributions of each individual protein to smooth muscle function suggest that NKA may be but one component of a larger functional Ca(2+) clearance system.

Loss of the serum response factor cofactor, cysteine-rich protein 1, attenuates neointima formation in the mouse

OBJECTIVE

Cysteine-rich protein (CRP) 1 and 2 are cytoskeletal lin-11 isl-1 mec-3 (LIM)-domain proteins thought to be critical for smooth muscle differentiation. Loss of murine CRP2 does not overtly affect smooth muscle differentiation or vascular function but does exacerbate neointima formation in response to vascular injury. Because CRPs 1 and 2 are coexpressed in the vasculature, we hypothesize that CRPs 1 and 2 act redundantly in smooth muscle differentiation.

METHODS AND RESULTS

We generated Csrp1 (gene name for CRP1) null mice by genetic ablation of the Csrp1 gene and found that mice lacking CRP1 are viable and fertile. Smooth muscle-containing tissues from Csrp1-null mice are morphologically indistinguishable from wild-type mice and have normal contractile properties. Mice lacking CRPs 1 and 2 are viable and fertile, ruling out functional redundancy between these 2 highly related proteins as a cause for the lack of an overt phenotype in the Csrp1-null mice. Csrp1-null mice challenged by wire-induced arterial injury display reduced neointima formation, opposite to that seen in Csrp2-null mice, whereas Csrp1/Csrp2 double-null mice produce a wild-type response.

CONCLUSIONS

Smooth muscle CRPs are not essential for normal smooth muscle differentiation during development, but may act antagonistically to modulate the smooth muscle response to pathophysiological stress.

Ca2+ clearance and contractility in vascular smooth muscle: evidence from gene-altered murine models

The central importance of calcium clearance proteins, and their regulators, in the modulation of myocardial contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) has long been established. Key players identified include the Na(+)-Ca(2+) exchanger, the Na(+)-K(+) ATPase, the sarco(endo)plasmic reticulum Ca(2+)-ATPase and associated phospholamban. Gene-targeted and transgenic murine models have been critical in the elucidation of their function. The study of these proteins in the regulation of contractile parameters in vascular smooth muscle, on the other hand, is less well studied. More recently, gene-targeted and transgenic models have expanded our knowledge of Ca(2+) clearance proteins and their role in both tonic and phasic smooth muscle contractility. In this review, we will briefly treat the mechanisms which underlie Ca(2+) clearance in smooth muscle. These will be addressed in light of studies using gene-modified mouse models, the results of which will be compared and contrasted with those in the cardiomyocyte. The recently identified human mutations in phospholamban, which lead to dilated cardiomyopathy, are also present in vascular and other smooth muscle. Given the importance of these Ca(2+) clearance systems to modulation of smooth muscle, it is likely that mutations will also lead to smooth muscle pathology.

Resection-induced intestinal adaptation and the role of enteric smooth muscle

BACKGROUND

Intestinal adaptation after massive small bowel resection (SBR) involves all layers of the bowel wall. Most prior work has focused on changes that occur in the intestinal mucosa. However, the contribution of the underlying intestinal smooth muscle (ISM) to the overall adaptation response remains unclear.

METHODS

Male C57BL/6 or waved-2 (diminished activity of the epidermal growth factor receptor) mice underwent a 50% proximal SBR or sham operation, and the remnant ileum was harvested 3, 7, and 28 days. Markers of adaptation (villus height, bowel length, circumference, and ISM thickness) and ISM proliferation were recorded. Contractility was measured by attaching the distal ileum to strain gauge transducers and exposed to varying doses of carbachol.

RESULTS

Intestinal smooth muscle thickness was unchanged at any given time-point after resection; however, the bowel caliber and length were increased, and augmented rates of ISM proliferation were identified. Contractility was increased at 7 days after SBR. Waved-2 mice demonstrated minimal proliferation or intestinal lengthening in response to SBR.

CONCLUSION

Compared with resection-induced thickening of the mucosa, proliferative changes in the ISM are unique and primarily affect bowel caliber, length, and contractility. Epidermal growth factor receptor signaling appears to play a significant role in adaptation of the ISM cellular compartment.

ACTH-induced hypertension is dependent on the ouabain-binding site of the alpha2-Na+-K+-ATPase subunit

ACTH-induced-hypertension is commonly employed as a model of stress-related hypertension, and despite extensive investigation, the mechanisms underlying elevated blood pressure (BP) are not well understood. We have reported that ACTH treatment increases tail-cuff systolic pressure in wild-type mice but not in mutant mice expressing ouabain-resistant alpha(2)-Na(+)-K(+)-ATPase subunits (alpha2(R/R) mice). Since tail-cuff measurements involve restraint stress, the present study used telemetry to distinguish between an effect of ACTH on resting BP vs. an ACTH-enhanced stress response. We also sought to explore the mechanisms underlying ACTH-induced BP changes in mutant alpha2(R/R) mice vs. wild-type mice (ouabain-sensitive alpha(2)-Na(+)-K(+)-ATPase, alpha2(S/S) mice). Baseline BP was not different between the two genotypes, but after 5 days of ACTH treatment, BP increased in alpha2(S/S) (104.0 +/- 2.6 to 117.7 +/- 3.0 mmHg) but not in alpha2(R/R) mice (108.2 +/- 3.2 to 111.5 +/- 4.0 mmHg). To test the hypothesis that ACTH hypertension is related to inhibition of alpha(2)-Na(+)-K(+)-ATPase on vascular smooth muscle by endogenous cardiotonic steroids, we measured BP and regional blood flow. Results suggest a differential sensitivity of renal, mesenteric, and cerebral circulations to ACTH and that the response depends on the ouabain sensitivity of the alpha(2)-Na(+)-K(+)-ATPase. Baseline cardiac performance was elevated in alpha2(S/S) but not alpha2(R/R) mice. Overall, the data establish that the alpha(2)-Na(+)-K(+)-ATPase ouabain-binding site is of central importance in the development of ACTH-induced hypertension. The mechanism appears to be related to alterations in cardiac performance, and perhaps vascular tone in specific circulations, presumably caused by elevated levels of circulating cardiotonic steroids.

Clearance of store-released Ca2+ by the Na+-Ca2+ exchanger is diminished in aortic smooth muscle from Na+-K+-ATPase alpha 2-isoform gene-ablated mice

Two alpha-isoforms of the Na+-K+-ATPase are expressed in vascular smooth muscle cells (VSMCs). The alpha 1-isoform is proposed to serve a cytosolic housekeeping role, whereas the alpha 2-isoform modulates Ca2+ storage via coupling to the Na+-Ca2+ exchanger (NCX) in a subsarcolemmal compartment. To evaluate the ramifications of this proposed interaction, Ca2+-store load and the contributions of the primary Ca2+ transporters to Ca2+ clearance were studied in aortic VSMCs from embryonic wild-type (WT) and Na+-K+-ATPase alpha 2-isoform gene-ablated, homozygous null knockout (alpha 2-KO) mice. Ca2+ stores were unloaded by inhibiting the sarco(endo)plasmic reticulum Ca2+-ATPase with cyclopiazonic acid (CPA) in Ca2+-free media to limit Ca2+ influx. Ca2+ clearance by the plasma membrane Ca2+-ATPase (PMCA), NCX, or mitochondria was selectively inhibited. In WT VSMCs, NCX accounted for 90% of the Ca2+ efflux. In alpha 2-KO VSMCs, preferential clearance of store-released Ca2+ by NCX was lost, whereas PMCA activity was increased. Selective inhibition of the alpha 2-isoform (0.5 microM ouabain for 20 min), before treatment with CPA enhanced the store load in VSMCs from WT, but not alpha 2-KO mice. A subsequent analysis of capacitative Ca2+ entry (CCE) indicated that the magnitude of Ca2+ influx was significantly greater in alpha 2-KO cells. Our findings support the concept of a subsarcolemmal space where the alpha 2-isoform coupled with NCX modulates Ca2+-store function and, thereby, CCE.

Rho kinase is an effector underlying Ca2+-desensitizing hypoxic relaxation in porcine coronary artery

Acute hypoxia dilates most systemic arteries leading to increased tissue perfusion. We have previously shown that at high-stimulus conditions, porcine coronary artery was relaxed by hypoxia without a change in intracellular [Ca2+] ( 27 ). This Ca2+-desensitizing hypoxic relaxation (CDHR) was validated in permeabilized porcine coronary artery smooth muscle (PCASM) in which hypoxia decreased force and myosin regulatory light chain phosphorylation (p-MRLC) despite fixed [Ca2+] ( 10 ). Rho kinase-dependent phosphorylation of myosin phosphatase-targeting subunit 1 (p-MYPT1) is associated with decreased MRLC phosphatase activity and increased Ca2+ sensitivity of both p-MRLC and force. We recently reported that p-MYPT1 dephosphorylation was a key effector in CDHR ( 33 ). In the current study, we tested the hypothesis that Rho kinase and not p-MYPT1 phosphatase is the regulated enzyme involved in CDHR. We used α-toxin to permeabilize deendothelialized PCASM. CDHR was attenuated in contractions attributable to myosin light chain kinase (MLCK, in the presence of the Rho kinase inhibitor Y-27632). In contrast, hypoxia relaxed contractions attributable to Rho kinase phosphorylation of MYPT1 and MRLC or MRLC alone (in the presence of the MLCK inhibitor ML7). Using an in situ assay, we showed that Rho kinase activity, measured as thiophosphorylation of MYPT1 and MRLC, was nearly abolished by hypoxia. The in vitro activity of the catalytically active fragment of Rho kinase was not affected by hypoxia. Our evidence strongly implicates that hypoxia directly inhibits Rho kinase-dependent phosphorylation of MYPT1. This underlies the decreases in both p-MYPT1 and p-MRLC and thereby leads to the Ca2+-desensitizing hypoxic relaxation.

Distinct phenotypes among plasma membrane Ca2+-ATPase knockout mice

Ca2+ gradients across the plasma membrane, required for Ca2+ homeostasis and signaling, are maintained in part by plasma membrane Ca2+-ATPase (PMCA) isoforms 1-4. Gene targeting has been used to analyze the functions of PMCA1, PMCA2, and PMCA4 in mice. PMCA1 null mutant embryos die during the preimplantation stage, and loss of a single copy of the PMCA1 gene contributes to apoptosis in vascular smooth muscle. PMCA2 deficiency in sensory hair cells of the inner ear causes deafness and balance defects, most likely by affecting both intracellular Ca2+ and extracellular Ca2+ in the endolymph. PMCA2 is required for viability of certain neurons, consistent with a major role in maintenance of intracellular Ca2+. Surprisingly, loss of PMCA2 in lactating mammary glands causes a sharp reduction in milk Ca2+, consistent with a macrocalcium secretory function. Although PMCA4 is widely expressed and is the most abundant isoform in some tissues, null mutants appear healthy. However, male PMCA4 null mutants are infertile due to a failure of hyperactivated sperm motility resulting from the absence of PMCA4 in the sperm tail, and Ca2+ signaling in B lymphocytes, involving interactions between PMCA4, CD22, and the tyrosine phosphatase SHP-1, is defective. Studies of bladder smooth muscle from PMCA4 null mutants and PMCA1 heterozygous mice suggest that PMCA1 and PMCA4 play different roles in smooth muscle contractility, with PMCA1 contributing to overall Ca2+ clearance and PMCA4 being required for carbachol-stimulated contraction. These phenotypes indicate that PMCA1 serves essential housekeeping functions, whereas PMCA4 and particularly PMCA2 serve more specialized physiological functions.

Transgenic mice expressing Na+-K+-ATPase in smooth muscle decreases blood pressure

The Na(+)-K(+)-ATPase (NKA) is a transmembrane protein that sets and maintains the electrochemical gradient by extruding three Na(+) in exchange for two K(+). An important physiological role proposed for vascular smooth muscle NKA is the regulation of blood pressure via modulation of vascular smooth muscle contractility (5). To investigate the relations between the level of NKA in smooth muscle and blood pressure, we developed mice carrying a transgene for either the NKA alpha(1)- or alpha(2)-isoform (alpha(1 sm+) or alpha(2 sm+) mice) driven by the smooth muscle-specific alpha-actin promoter SMP8. Interestingly, both alpha-isoforms, the one contained in the transgene and the one not contained, were increased to a similar degree at both protein and mRNA levels. The total alpha-isoform protein was increased from 1.5-fold (alpha(1 sm+) mice) to 7-fold (alpha(2 sm+) mice). The increase in total NKA alpha-isoform protein was accompanied by a 2.5-fold increase in NKA activity in alpha(2 sm+) gastric antrum. Immunocytochemistry of the alpha(1)- and alpha(2)-isoforms in alpha(2 sm+) aortic smooth muscle cells indicated that alpha-isoform distributions were similar to those shown in wild-type cells. alpha(2 sm+) Mice (high expression) were hypotensive (109.9 +/- 1.6 vs. 121.3 +/- 1.4 mmHg; n = 13 and 11, respectively), whereas alpha(1 sm+) mice (low expression) were normotensive (122.7 +/- 2.5 vs. 117.4 +/- 2.3; n = 11 or 12). alpha(2 sm+) Aorta, but not alpha(1 sm+) aorta, relaxed faster from a KCl-induced contraction than wild-type aorta. Our results show that smooth muscle displays unique coordinate expression of the alpha-isoforms. Increasing smooth muscle NKA decreases blood pressure and is dependent on the degree of increased alpha-isoform expression.

Expression and function of COOH-terminal myosin heavy chain isoforms in mouse smooth muscle

Isoforms of the smooth muscle myosin motor, SM1 and SM2, differ in length at the carboxy terminal tail region. Their proportion changes with development, hormonal status and disease, but their function is unknown. We developed mice carrying the myosin heavy chain (MyHC) transgenes SM1, cMyc-tagged SM1, SM2, and V5-tagged SM2, and all transgenes corresponded to the SMa NH(2)-terminal isoform. Transgene expression was targeted to smooth muscle by the smooth muscle alpha-actin promoter. Immunoblot analysis showed substantial expression of the cMyc-tagged SM1 and V5-tagged SM2 MyHC protein in aorta and bladder and transgene mRNA was expressed in mice carrying unlabeled SM1 or SM2 transgenes. Despite significant protein expression of tagged MyHCs we found only small changes in the SM1:SM2 protein ratio. Significant changes in functional phenotype were observed in mice carrying unlabeled SM1 or SM2 transgenes. Force in aorta and bladder was increased (72 +/- 14%, 92 +/- 11%) in SM1 and decreased to 57 +/- 1% and 80 +/- 3% in SM2 transgenic mice. SM1 transgenic bladders had faster (1.8 +/- 0.3 s) and SM2 slower (7.1 +/- 0.5 s) rates of force redevelopment following a rapid step shortening. We hypothesize that small changes in the SM1:SM2 ratio could be amplified if they are associated with changes in thick filament assembly and underlie the altered contractility. These data provide evidence indicating an in vivo function for the COOH-terminal isoforms of smooth muscle myosin and suggest that the SM1:SM2 ratio is tightly regulated in smooth muscle tissues.

Distinct roles of PMCA isoforms in Ca2+homeostasis of bladder smooth muscle: evidence from PMCA gene-ablated mice

We previously showed that plasma membrane Ca2+-ATPase (PMCA) activity accounted for 25–30% of relaxation in bladder smooth muscle ( 8 ). Among the four PMCA isoforms only PMCA1 and PMCA4 are expressed in smooth muscle. To address the role of these isoforms, we measured cytosolic Ca2+([Ca2+]i) using fura-PE3 and simultaneously measured contractility in bladder smooth muscle from wild-type (WT), Pmca1+/−, Pmca4+/−, Pmca4−/−, and Pmca1+/−Pmca4−/−mice. There were no differences in basal [Ca2+]ivalues between bladder preparations. KCl (80 mM) elicited both larger forces (150–190%) and increases in [Ca2+]i(130–180%) in smooth muscle from Pmca1+/−and Pmca1+/−Pmca4−/−bladders than those in WT or Pmca4−/−. The responses to carbachol (CCh: 10 μM) were also greater in Pmca1+/−(120–150%) than in WT bladders. In contrast, the responses in Pmca4−/−and Pmca1+/−Pmca4−/−bladders to CCh were significantly smaller (40–50%) than WT. The rise in half-times of force and [Ca2+]iincreases in response to KCl and CCh, and the concomitant half-times of their decrease upon washout of agonist were prolonged in Pmca4−/−(130–190%) and Pmca1+/−Pmca4−/−(120–250%) bladders, but not in Pmca1+/−bladders with respect to WT. Our evidence indicates distinct isoform functions with the PMCA1 isoform involved in overall Ca2+clearance, while PMCA4 is essential for the [Ca2+]iincrease and contractile response to the CCh receptor-mediated signal transduction pathway.

Vascular dysfunction in S1P2 sphingosine 1-phosphate receptor knockout mice

There is growing evidence that sphingosine 1-phosphate (S1P) plays an important role in regulating the development, morphology, and function of the cardiovascular system. There is little data, however, regarding the relative contribution of endogenous S1P and its cognate receptors (referred to as S1P(1-5)) to cardiovascular homeostasis. We used S1P(2) receptor knockout mice (S1P(2)(-/-)) to evaluate the role of S1P(2) in heart and vascular function. There were no significant differences in blood pressure between wild-type and S1P(2)(-/-) mice, measured in awake mice. Cardiac function, evaluated in situ by using a Millar catheter, was also not different in S1P(2)(-/-) mice under baseline or stimulated conditions. In vivo analysis of vascular function by flowmetry revealed decreases in mesenteric and renal resistance in S1P(2)(-/-) mice, especially during vasoconstriction with phenylephrine. In intact aortic rings, the concentration-force relations for both KCl and phenylephrine were right shifted in S1P(2)(-/-) mice, whereas the maximal isometric forces were not different. By contrast, in deendothelialized rings the concentration-force relations were not different but the maximal force was significantly greater in S1P(2)(-/-) aorta. Histologically, there were no apparent differences in vascular morphology. These data suggest that the S1P(2) receptor plays an important role in the function of the vasculature and is an important mediator of normal hemodynamics. This is mediated, at least in part, through an effect on the endothelium, but direct effects on vascular smooth muscle cannot be ruled out and require further investigation.

Effects of sex and estrogen on myosin COOH-terminal isoforms and contractility in rat aorta

We reported that estrogen treatment of ovariectomized rats increased uterine smooth muscle contractility and the ratio of the COOH-terminal myosin heavy chain isoform SM1 (204 kDa) and SM2 [200 kDa; Hewett TE, Martin AF, Paul RJ. J Physiol (Lond) 460: 351-364, 1993]. We extended this model to study sex and estrogen effects on vascular contractility. Experimental groups included 10- to 14-wk-old male (M), female (F), ovariectomized female (OF), and OF treated with estrogen (OF&E) for 7 days with a subcutaneous pellet delivery system, resulting in 17beta-estradiol of 85 (OF&E) vs. 5 (OF or M) pg/ml. The SM1-to-SM2 ratio increased from 1.8 to 2.6 in thoracic aorta, similar to uterine muscle. Isometric force was measured in 5-mm segments of intact and endothelium-denuded (-endo) aorta. With KCl, the maximum forces were in the order OF approximately M > OF&E, and ED50 OF&E > OF approximately M. Differences in ED50 with estrogen persisted after endothelial denudation. The decreased force in -endo OF aorta was not seen in OF&E, suggesting that estrogen altered an endothelium-dependent effect. No differences in maximum forces were noted with norepinephrine: ED50 OF > OF&E > M. Estrogen treatment, in contrast to KCl, increased sensitivity. Endothelial denudation increased sensitivity but reduced the differences between groups. With ACh relaxation, males were more sensitive than females, and estrogen had no effect. In the abdominal aorta, there were no changes in SM1/SM2 with 17beta-estradiol, and differences in contractility were blunted. In summary, estrogen treatment decreased responses to KCl but increased sensitivity to norepinephrine; male rats always demonstrated the highest contractility. An increase in the COOH-terminal myosin heavy chain isoform SM1-to-SM2 ratio with 17beta-estradiol treatment may underlie the changes observed in contractility.

Role of plasma membrane Ca2+-ATPase in contraction-relaxation processes of the bladder: evidence from PMCA gene-ablated mice

We investigated the roles and relationships of plasma membrane Ca2+-ATPase (PMCA), sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)2, and Na+/Ca2+ exchanger (NCX) in bladder smooth muscle contractility in Pmca-ablated mice: Pmca4-null mutant ( Pmca4 −/−) and heterozygous Pmca1 and homozygous Pmca4 double gene-targeted ( Pmca1 +/− Pmca4 −/−) mice. Gene manipulation did not alter the amounts of PMCA1, SERCA2, and NCX. To study the role of each Ca2+ transport system, contraction of circular ring preparations was elicited with KCl (80 mM) plus atropine, and then the muscle was relaxed with Ca2+-free physiological salt solution containing EGTA. We measured the contributions of Ca2+ clearance components by inhibiting SERCA2 (with 10 μM cyclopiazonic acid) and/or NCX (by replacing NaCl with N-methyl-d-glucamine/HCl plus 10 μM KB-R7943). Contraction half-time (time to 50% of maximum tension) was prolonged in the gene-targeted muscles but marginally shortened when SERCA2 or NCX was inhibited. The inhibition of NCX significantly inhibited this prolongation, suggesting that NCX activity might be augmented to compensate for PMCA4 function in the gene-targeted muscles under nonstimulated conditions. Inhibition of SERCA2 and NCX as well as gene targeting all prolonged the relaxation half-time. The contribution of PMCA to relaxation was calculated to be ∼25–30%, with that of SERCA2 being 20% and that of NCX being 70%. PMCA and SERCA2 appeared to function additively, but the function of NCX might overlap with those of other components. In summary, gene manipulation of PMCA indicates that PMCA, in addition to SERCA2 and NCX, plays a significant role in both excitation-contraction coupling and the Ca2+ extrusion-relaxation relationship, i.e., Ca2+ homeostasis, of bladder smooth muscle.

Ca2+-desensitizing hypoxic vasorelaxation: pivotal role for the myosin binding subunit of myosin phosphatase (MYPT1) in porcine coronary artery

Acute hypoxia dilates most systemic arteries leading to increased tissue perfusion. We showed that at high stimulus conditions, porcine coronary artery was relaxed by hypoxia without a change in [Ca(2+)](i). This 'Ca(2+)-desensitizing hypoxic relaxation' was validated in permeabilized porcine coronary artery smooth muscle (PCASM) in which hypoxia decreased force and myosin regulatory light chain phosphorylation (p-MRLC) despite fixed [Ca(2+)]. Rho kinase-dependent phosphorylation of MYPT1 (p-MYPT1) is associated with decreased MRLC phosphatase (MLCP) activity, and increased Ca(2+) sensitivity of both p-MRLC and force. We tested the hypothesis that hypoxia induces Ca(2+)-desensitizing hypoxic relaxation via dephosphorylation of p-MYPT1, consequently increasing MLCP activity and thus decreasing p-MRLC. alpha-Toxin-permeabilized PCASM pretreated with ATPgammaS did not relax in response to hypoxia. Moreover, when MRLC but not MYPT1 was protected from ATPgammaS thiophosphorylation by the MRLC kinase inhibitor ML7 (300 mum), hypoxia remained ineffective. In contrast, hypoxic relaxation was preserved with further addition of the Rho kinase inhibitor Y27632 (1 mum), to attenuate thiophosphorylation of MYPT1. Importantly, measurements of p-MRLC, and p-MYPT1 at T696 and T853 (human sequence) paralleled that of force. We conclude that Ca(2+)-desensitizing hypoxic relaxation requires dephosphorylation of p-MYPT1. Moreover, no kinases, other then those inhibited by ML7 and Y27632, nor their associated phosphoproteins can be involved in Ca(2+)-desensitizing hypoxic relaxation.

Effects of hypoxia acclimation on morpho-physiological traits over three generations of Daphnia magna

The mechanisms, dynamics and effects of hypoxia acclimation were studied in the water flea Daphnia magna over three successive generations (parental, first and second filial generation: P, F1 and F2). The P generation was raised under normoxic conditions at 20 degrees C and became exposed to environmental hypoxia (10-19% air saturation) at maturity. Their progenies (F1 and F2) experienced hypoxia from birth onwards. Controls were kept under normoxic conditions. Individuals were successively sampled in a 3-day interval from each acclimation group to determine morpho-physiological parameters relevant in oxygen transport and regulation. Hypoxia acclimation induced adjustments at the haemoglobin (Hb) and metabolic level (within 3 days) but none at the systemic level. The convective performance and oxygen-sensitive control of the ventilatory and circulatory systems were the same in both acclimation groups. The Hb concentration and oxygen affinity increased by 266% and 32%, respectively. The 22% decrease in mass-specific oxygen consumption rate reduced the energy allocation to somatic growth without greatly affecting reproduction. The onset and duration of hypoxic exposure during ontogenesis have had a significant influence on Hb oxygen affinity and body size. Transgenerational effects of hypoxia acclimation could not be observed. The adjustments at the Hb and metabolic levels in combination with the smaller body size, which is advantageous to diffusive oxygen transport, reduced the critical ambient oxygen tension by approximately 50%.

Regulation of the spontaneous contractile activity of the portal vein by the sarcoplasmic reticulum: evidence from the phospholamban gene-ablated mouse

The rapid contraction/relaxation cycles of phasic smooth muscles necessitates intracellular calcium cycling at a more rapid rate than that of tonic smooth muscles. Recent studies suggest that sarcoplasmic reticulum calcium handling is an important determinant of portal vein phasic contractions. We evaluated the importance and role of phospholamban, a protein which inhibits the sarcoplasmic reticulum (SR) calcium ATPase (SERCA), in regulating the contractility of the phasic mouse portal vein. PLB gene ablation significantly reduced the basal frequency of spontaneous mechanical activity and increased force development of the portal vein. Cyclopiazonic acid (CPA), an inhibitor of SERCA, did not significantly affect the spontaneous activity of the wild-type (WT) portal vein. CPA (1 microM) eliminated the differences in frequency and force between the PLB-KO and WT, localizing the effects to the SR. The PLB-KO portal vein had a lower resting membrane potential than WT controls. There were no significant differences between WT and KO responses to charybdotoxin (250 nM), indicating that calcium-activated potassium channels do not contribute to altered KO portal vein contractility. While contractile sensitivity to acetylcholine was not different between WT and PLB-KO portal veins, force generated in response to a given concentration of acetylcholine was significantly greater in the PLB-KO portal vein, both in the absence and presence of CPA. Our results confirm that SR activity can play a major role in modulating the frequency of the spontaneous mechanical activity of portal veins and removal of PLB inhibition of the SR calcium ATPase has significant effects on the spontaneous activity of the portal vein.

Crater landscape: two-dimensional oxygen gradients in the circulatory system of the microcrustacean Daphnia magna

Oxygen transport processes in millimetre-sized animals can be very complex, because oxygen molecules do not exclusively follow the pathway predetermined by the circulating fluid but may also simultaneously move from the respiratory surfaces to the tissues along different paths by diffusion. The present study made use of the oxygen-sensitive phosphorescence probe Oxyphor R2 to analyze the internal oxygen pathway in the transparent microcrustacean Daphnia magna. Oxyphor R2 was injected into the circulatory system and the distribution of oxygen partial pressure (P(O(2))) in the haemolymph was measured by phosphorescence lifetime imaging in the P(O(2)) range 0-6 kPa (0-30% air saturation). There were substantial differences in the shape of the two-dimensional P(O(2)) profiles depending on the concentration of haemoglobin (Hb) in the haemolymph. A steep global gradient, from posterior to anterior, occurred in animals with low concentrations of Hb (90-167 micromol l(-1) haem). In contrast, animals with a five- to sixfold higher concentration of Hb showed flat internal P(O(2)) gradients which, however, were only present under reduced ambient oxygen tensions (P(O(2)amb)=3-1 kPa), when Hb was maximally involved in oxygen transport. Under these conditions, the presence of Hb at high concentrations stabilized the unloading P(O(2)) in the central body to 0.9-0.4 kPa. Independent of Hb concentration and body size, the loading P(O(2)) was always 0.5 kPa below the P(O(2)amb). From these P(O(2)) profiles, it was possible (i) to follow the track of oxygen within the animal, and (ii) to visualize the shift from a diffusion-dominated to a convection-dominated transport as a result of increased Hb concentration.

Hypoxic vasorelaxation: Ca2+-dependent and Ca2+-independent mechanisms

The mechanisms of oxygen sensing in vascular smooth muscle have been studied extensively in a variety of tissue types and the results of these studies indicate that the mechanism of hypoxia-induced vasodilation probably involves several mechanisms that combined to assure the appropriate response. After a short discussion of the regulatory mechanisms for smooth muscle contractility, we present the evidence indicating that hypoxic vasorelaxation involves both Ca2+-dependent and Ca2+-independent mechanisms. More recent experiments using proteomic approaches in organ cultures of porcine coronary artery reveal important changes evoked by hypoxia in both Ca2+-dependent and Ca2+-independent pathways.

Ca2+ clearance in smooth muscle: lessons from gene-altered mice

The regulation of intracellular [Ca(2+)](i) is important for all cells, but in particular for smooth muscle, as [Ca(2+)](i) is a key second messenger leading to contraction. Mechanisms for the cellular clearance of [Ca(2+)](i) form one side of Ca(2+) homeostasis and include: Plasma Membrane Ca(2+) ATPases (PMCA), Sarcoplasmic/Endoplasmic Reticulum Ca(2+) ATPases (SERCA), Na(+)-Ca(2+)-exchangers (NCX) when coupled to the Na(+)-K(+) ATPases (NKA) and in some cases mitochondria. The nature and relative contribution of these various components of cytosolic Ca(2+) clearance have long been an important topic for study in smooth muscle, particularly as related to regulation of contractility. These studies have largely depended on inhibition of the various components. Recently advances in gene-targeting and transgenesis have made it possible to add or delete individual components, and importantly specific isoforms from the cell. In this brief review, we will focus on new information on Ca(2+) clearance in smooth muscle gained from studies on gene-altered mice models. These provide a deeper understanding of distinct functional roles for individual isoforms and the interactions between various components.

Thrombin-Induced Force Development in Vascular Endothelial Cells: Contribution to Alteration of Permeability Mediated by Calcium-Dependent and -Independent Pathways

Endothelial cell (EC) barrier dysfunction is associated with many types of vascular diseases. Investigators have hypothesized that altered EC contraction in conjunction with morphological changes may lead to EC dysfunction. However, the nature of EC contraction and its regulatory mechanisms are not fully understood. In this study we measured thrombin-induced force in bovine arterial EC force using EC fibers that were grown in a collagen matrix. Contraction, which occurred in time- and dose-dependent fashion, was elicited by thrombin. The thrombin-enhanced EC layer permeability was correlated with EC fiber contraction. These results suggest that EC contractile response is involved in alteration of EC barrier function. During the initial period of thrombin stimulation, cadherin complexes were disrupted and cell-to-cell connections were reduced. This was dependent on the transient increase in intracellular calcium concentration and myosin phosphorylation. Rho kinase activation led to rearrangement of actin stress fibers (ASF). Paracellular holes were created in the EC layer in parallel to EC morphological change. Our findings suggest that EC layer permeability is regulated by two distinguishable steps. In the initial period, the cell-to-cell connection was reduced in a calcium-dependent fashion. Subsequently, Rho kinase and ASF-mediated force development increased EC layer permeability via morphological change of EC.

Ca2+-independent hypoxic vasorelaxation in porcine coronary artery

To demonstrate a Ca(2+)-independent component of hypoxic vasorelaxation and to investigate its mechanism, we utilized permeabilized porcine coronary arteries, in which [Ca(2+)] could be clamped. Arteries permeabilized with beta-escin developed maximum force in response to free Ca(2+) (6.6 microm), concomitant with a parallel increase in myosin regulatory light chain phosphorylation (MRLC-P(i)), from 0.183 +/- 0.023 to 0.353 +/- 0.019 MRLC-P(i) (total light chain)(-1). Hypoxia resulted in a significant decrease in both force (-31.9 +/- 4.1% prior developed force) and MRLC-P(i) (from 0.353 to 0.280 +/- 0.023), despite constant [Ca(2+)] buffered by EGTA (4 mm). Forces developed in response to Ca(2+) (6.6 microm), Ca(2+) (0.2 microm) + GTPgammaS (1 mM), or in the absence of Ca(2+) after treatment with ATPgammaS (1 mM), were of similar magnitude. Hypoxia also relaxed GTPgammaS contractures but importantly, arteries could not be relaxed after treatment with ATPgammaS. Permeabilization with Triton X-100 for 60 min also abolished hypoxic relaxation. The blocking of hypoxic relaxation after ATPgammaS suggests that this Ca(2+)-independent mechanism(s) may operate through alteration of MRLC-P(i) or of phosphorylation of the myosin binding subunit of myosin light chain phosphatase. Treatment with the Rho kinase inhibitor Y27632 (1 microm) relaxed GTPgammaS and Ca(2+) contractures; but the latter required a higher concentration (10 microm) for consistent relaxation. Relaxations to N(2) and/or Y27632 averaged 35% and were not additive or dependent on order. Our data suggest that the GTP-mediated, Rho kinase-coupled pathway merits further investigation as a potential site of this novel, Ca(2+)-independent O(2)-sensing mechanism. Importantly, these results unambiguously show that hypoxia-induced vasorelaxation can occur in permeabilized arteries where the Ca(2+) is clamped at a constant value.

Control of oxygen transport in the microcrustacean Daphnia: regulation of haemoglobin expression as central mechanism of adaptation to different oxygen and temperature conditions

The pathway for oxygen, the control of oxygen transport and the role of haemoglobin expression for the physiological adaptation to different oxygen and temperature conditions were studied in the ecological model organism Daphnia magna. Ventilation of the inner walls of the carapace as the main gas exchange area as well as of the embryos in the brood pouch are controlled, oxygen-dependent processes. The P(O2)-dependent increase of heart rate as well as perfusion rate during short-term, progressive hypoxia improves the circulatory oxygen transport within the body. The regulation of haemoglobin (Hb) expression is the central mechanism for a medium-term adaptation to hypoxia. Genetic control elements and oxygen conditions near the two Hb synthesis sites (fat cells, epipodite epithelial cells) determine, which types of Hb subunits and, accordingly, hetero-multimeric Hb macromolecules are produced. One synthesis site may respond mainly to internal, the other one to external oxygen conditions. Depending on environmental condition, either higher quantities of macromolecules of unchanged functionality (P50) or increasing amounts of macromolecules with higher oxygen affinity are synthesized. The Hb subunit DmHbA is probably of considerable importance for this functional change. The physiological benefits of haemoglobin in Daphnia are discussed. Physiological adaptation of Daphnia to different temperatures is also related to the control of oxygen transport processes with the regulation of haemoglobin expression again as a central mechanism.

The alpha2-isoform of Na-K-ATPase mediates ouabain-induced hypertension in mice and increased vascular contractility in vitro

Although ouabain is known to induce hypertension, the mechanism of how this cardiac glycoside affects blood pressure is uncertain. The present study demonstrates that the alpha2-isoform of the Na-K-ATPase mediates the pressor effects of ouabain in mice. To accomplish this, we analyzed the effect of ouabain on blood pressure in wild-type mice, where the alpha2-isoform is sensitive to ouabain, and genetically engineered mice expressing a ouabain-insensitive alpha2-isoform of the Na-K-ATPase. Thus differences in the response to ouabain between these two genotypes can only be attributed to the alpha2-isoform of Na-K-ATPase. As the alpha1-isoform is naturally resistant to ouabain in rodents, it will not be inhibited by ouabain in either genotype. Whereas prolonged administration of ouabain increased levels of ouabain in serum from both wild-type and targeted animals, hypertension developed only in wild-type mice. In addition, bolus intravenous infusion of ouabain increased the systolic, mean arterial, and left ventricular blood pressure in only wild-type anesthetized mice. In vitro, ouabain increased vascular tone and thereby phenylephrine-induced contraction of the aorta in intact and endothelium-denuded wild-type mice but in alpha2-resistant mice. Ouabain also increased the magnitude of the spontaneous contractions of portal vein and the basal tone of the intact aorta from only wild-type mice. The increase in aortic basal tone was dependent on the presence of endothelium. Our studies also demonstrate that the alpha2-isoform of Na-K-ATPase mediates the ouabain-induced increase in vascular contractility. This could play a role in the development and maintenance of ouabain-induced hypertension.

Isoform switching from SM-B to SM-A myosin results in decreased contractility and altered expression of thin filament regulatory proteins

We previously generated an isoform-specific gene knockout mouse in which SM-B myosin is permanently replaced by SM-A myosin. In this study, we examined the effects of SM-B myosin loss on the contractile properties of vascular smooth muscle, specifically peripheral mesenteric vessels and aorta. The absence of SM-B myosin leads to decreased velocity of shortening and increased isometric force generation in mesenteric vessels. Surprisingly, the same changes occur in aorta, which contains little or no SM-B myosin in wild-type animals. Calponin and activated mitogen-activated protein kinase expression is increased and caldesmon expression is decreased in aorta, as well as in bladder. Light chain-17b isoform (LC(17b)) expression is increased in aorta. These results suggest that the presence or absence of SM-B myosin is a critical determinant of smooth muscle contraction and that its loss leads to additional changes in thin filament regulatory proteins.

Targeted Ablation of Plasma Membrane Ca2+-ATPase (PMCA) 1 and 4 Indicates a Major Housekeeping Function for PMCA1 and a Critical Role in Hyperactivated Sperm Motility and Male Fertility for PMCA4

The relative importance of plasma membrane Ca2+-ATPase (PMCA) 1 and PMCA4 was assessed in mice carrying null mutations in their genes (Atp2b1 and Atp2b4). Loss of both copies of the gene encoding PMCA1 caused embryolethality, whereas heterozygous mutants had no overt disease phenotype. Despite widespread and abundant expression of PMCA4, PMCA4 null (Pmca4-/-) mutants exhibited no embryolethality and appeared outwardly normal. Loss of PMCA4 impaired phasic contractions and caused apoptosis in portal vein smooth muscle in vitro; however, this phenotype was dependent on the mouse strain being employed. Pmca4-/- mice on a Black Swiss background did not exhibit the phenotype unless they also carried a null mutation in one copy of the Pmca1 gene. Pmca4-/- male mice were infertile but had normal spermatogenesis and mating behavior. Pmca4-/- sperm that had not undergone capacitation exhibited normal motility but could not achieve hyperactivated motility needed to traverse the female genital tract. Ultrastructure of the motility apparatus in Pmca4-/- sperm tails was normal, but an increased incidence of mitochondrial condensation indicated Ca2+ overload. Immunoblotting and immunohistochemistry showed that PMCA4 is the most abundant isoform in testis and sperm and that it is localized to the principle piece of the sperm tail, which is also the location of the major Ca2+ channel (CatSper) required for sperm motility. These results are consistent with an essential housekeeping or developmental function for PMCA1, but not PMCA4, and show that PMCA4 expression in the principle piece of the sperm tail is essential for hyperactivated motility and male fertility.

The Na(+)-K(+)-ATPase alpha2-subunit isoform modulates contractility in the perinatal mouse diaphragm

This study uses genetically altered mice to examine the contribution of the Na(+)-K(+)-ATPase alpha2 catalytic subunit to resting potential, excitability, and contractility of the perinatal diaphragm. The alpha2 protein is reduced by 38% in alpha2-heterozygous and absent in alpha2-knockout mice, and alpha1-isoform is upregulated 1.9-fold in alpha2-knockout. Resting potentials are depolarized by 0.8-4.0 mV in heterozygous and knockout mice. Action potential threshold, overshoot, and duration are normal. Spontaneous firing, a developmental function, is impaired in knockout diaphragm, but this does not compromise its ability to fire evoked action potential trains, the dominant mode of activation near birth. Maximum tetanic force, rate of activation, force-frequency and force-voltage relationships, and onset and magnitude of fatigue are not changed. The major phenotypic consequence of reduced alpha2 content is that relaxation from contraction is 1.7-fold faster. This finding reveals a distinct cellular role of the alpha2-isoform at a step after membrane excitation, which cannot be restored simply by increasing alpha1 content. Na+/Ca2+ exchanger expression decreases in parallel with alpha2-isoform, suggesting that Ca2+ extrusion is affected by the altered alpha2 genotype. There are no major compensatory changes in expression of sarcoplasmic reticulum Ca(2+)-ATPase, phospholamban, or plasma membrane Ca(2+)-ATPase. These results demonstrate that the Na(+)-K(+)-ATPase alpha1-isoform alone is able to maintain equilibrium K+ and Na+ gradients and to substitute for alpha2-isoform in most cellular functions related to excitability and force. They further indicate that the alpha2-isoform contributes significantly less at rest than expected from its proportional content but can modulate contractility during muscle contraction.

Glucose-Dependent Enhancement of Spontaneous Phasic Contraction Is Suppressed in Diabetic Mouse Portal Vein: Association with Diacylglycerol-Protein Kinase C Pathway

We investigated portal vein (PV) contractility in diabetes using a mouse model (ob/ob mouse) of spontaneous noninsulin-dependent diabetic mellitus. Spontaneous phasic contraction in control mice (C57Bl) was increased in the presence of the thromboxane A(2) analog 9,11-dideoxy-11alpha, 9alpha-epoxymethanoprostaglandin F(2)alpha (U46619) in a time- and concentration-dependent manner. This response was enhanced under high glucose conditions (22.2 mM). Diacylglycerol (DG) was synthesized from glucose and was not affected by phospholipase C (PLC) inhibition under resting conditions in normal glucose. Inhibition of DG-induced PKC activation with 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo-(2,3-alpha)pyrrolo(3,4-c)-carbazole (Gö6976), a calcium-dependent protein kinase C (PKC) inhibitor, was only observed under normal glucose conditions. High glucose levels enhanced PLC-independent DG formation followed by an induction of total phosphatidylinositol turnover via calcium-independent PKC activation in C57Bl mice. In ob/ob mice, the high glucose-induced enhancement of PV contraction in response to U46619 was suppressed. These findings suggest that these differences are associated with long-term exposure of tissue to a hyperglycemic state. Under high glucose conditions, DG derived from glucose fell below 50% in C57Bl mice. Moreover, the DG-related calcium-independent PKC was desensitized in ob/ob mice. These results suggest that suppression of the glucose-induced enhancement of PV contraction involves both a decrease in glucose-derived DG formation and reduction of the glucose sensitivity of DG-related PKC.

Vascular oxygen sensing: detection of novel candidates by proteomics and organ culture

We have shown that the specific inhibition of hypoxia-induced relaxation by organ culture in porcine coronary arteries can be mimicked by treatment of control vessels with the protein synthesis inhibitor, cycloheximide. We hypothesize that organ culture of vascular smooth muscle results in the decreased expression of proteins that are critical for vascular oxygen sensing. Using two-dimensional gel electrophoresis and mass spectroscopy, we identified such candidate proteins. The expressions of the smooth muscle-specific protein, SM22, and tropomyosin are decreased after 24 h in organ culture. These results were confirmed by Western blot analysis. Other smooth muscle proteins (actin and calponin) exhibited little change. We also demonstrate a 50% downregulation in the small G protein, Rho, a potent modulator of Ca2+-independent force. These results indicate that organ culture preferentially inhibits the expression of certain smooth muscle proteins. This change in protein expression after organ culture correlates with the specific inhibition of hypoxic vasorelaxation. These results provide novel target pathways for investigation that are potentially important for vascular oxygen sensing.

Fibroblast fiber contraction: role of C and Rho kinase in activation by thromboxane A2

We investigated the mechanisms underlying regulation of contraction with measurements of isometric force and intracellular Ca2+ concentration ([Ca2+]i) in NIH 3T3 fibroblast reconstituted into fibers with the use of a collagen matrix. Treatment with the major phospholipids, neurotransmitters, and growth factors had little effect on baseline isometric force. However, U-46619, a thromboxane A2 (TxA2) analog, increased force and [Ca2+]i; EC50 values were 11.0 and 10.0 nM, respectively. The time courses were similar to those induced by calf serum (CS), and the maximal force was 65% of a CS-mediated contraction. The selective TxA2 receptor antagonist SQ-29548 abolished the U-46619-induced responses. CS-induced contractions are dependent on an intracellular Ca2+ store function; however, the U-46619 response depended not only on intracellular Ca2+ stores, but also on Ca2+ influx from the extracellular medium. Inhibition of Rho kinase suppressed U-46619- and CS-induced responses; in contrast, inhibition of C kinase (PKC) reduced only the U-46619 response. Moreover, addition of U-46619 to a CS contracture enhanced force and [Ca2+]i responses. These results indicate that U-46619-induced responses involve PKC and Rho kinase pathways, in contrast to activation by CS. Thus TxA2 may have a role in not only the initial step of wound repair as an activator of blood coagulation, but also in fibroblast contractility in later stages.

Na(+) pump alpha 2-isoform specifically couples to contractility in vascular smooth muscle: evidence from gene-targeted neonatal mice

The relative expression of alpha(1)- and alpha(2)-Na(+)/K(+)-ATPase isoforms found in vascular smooth muscle is developmentally regulated and under hormonal and neurogenic control. The physiological roles of these isoforms in vascular function are not known. It has been postulated that the alpha(1)-isoform serves a "housekeeping" role, whereas the alpha(2)-isoform localizes to a subsarcolemmal compartment and modulates contractility. To test this hypothesis, isoform-specific gene-targeted mice in which the mRNA for either the alpha(1)- or the alpha(2)-Na(+)/K(+)-ATPase isoform was ablated were utilized. Both of these knockouts, alpha(1)(-/-) and alpha(2)(-/-), are lethal; the latter dies at birth, which allows this neonatal aorta to be studied. Isometric force in alpha(2)(-/-)-aorta was more sensitive to contractile agonists and less sensitive to the vasodilators forskolin and sodium nitroprusside (SNP) than wild-type (WT) aorta; alpha(2)(+/-)-aortas had intermediate values. In contrast, neonatal alpha(1)(+/-)-aorta was similar to WT. Western blot analysis indicated a population of 70% alpha(1)- and 30% alpha(2)-isoforms in the WT. Thus in terms of the total Na(+)/K(+)-ATPase protein, the alpha(2)(-/-)-aorta (at 70%) would be similar to the alpha(1)(+/-)-aorta (at 65%) but with a dramatically different phenotype. These data suggest that individual alpha-isoforms of the Na(+)/K(+)-ATPase differ functionally and that the alpha(2)-isoform couples more strongly to activation-relaxation pathways. Three-dimensional image-acquisition and deconvolution analyses suggest that the alpha(2)-isoform is distributed differently than the alpha(1)-isoform. Importantly, these isoforms do not localize to the same regions.

Antithetic regulation by beta-adrenergic receptors of Gq receptor signaling via phospholipase C underlies the airway beta-agonist paradox

beta-adrenergic receptors (betaARs) relax airway smooth muscle and bronchodilate, but chronic beta-agonist treatment in asthma causes increased sensitivity to airway constriction (hyperreactivity) and is associated with exacerbations. This paradox was explored using mice with ablated betaAR genes (betaAR-/-) and transgenic mice overexpressing airway smooth muscle beta2AR (beta2AR-OE) representing two extremes: absence and persistent activity of airway betaAR. Unexpectedly, betaAR-/- mice, lacking these bronchodilating receptors, had markedly decreased bronchoconstrictive responses to methacholine and other Gq-coupled receptor agonists. In contrast, beta2AR-OE mice had enhanced constrictive responses. Contraction to permeabilization with beta-escin was unaltered by gene ablation or overexpression. Inositol phosphate accumulation by Gq-coupled M3-muscarinic, thromboxane-A2, and 5-HT2 receptors was desensitized in airway smooth muscle cells from betaAR-/- mice and sensitized in cells from beta2AR-OE mice. Thus, betaAR antithetically regulates constrictive signals, affecting bronchomotor tone/reactivity by additional means other than direct dilatation. Studies of signaling elements in these pathways revealed the nodal point of this cross talk as phospholipase C-beta1, whose expression was altered by betaAR in a direction and magnitude consistent with the physiologic and cellular responses. These results establish a mechanism of the beta-agonist paradox and identify a potential asthma modifier gene (phospholipase C-beta1), which may also be a therapeutic target in asthma when chronic beta-agonists are required.

Energy cost of contraction in rat urinary bladder smooth muscle during anoxia

1. The aim of the present study was to investigate the effects of hypoxia on energy metabolism and contraction of rat urinary bladder smooth muscle, thereby gaining insight into the capacity of this smooth muscle to maintain contractile function when rendered hypoxic. 2. Isometric force, oxygen consumption, lactate production, heat production and unloaded shortening velocity were measured in isolated muscle strips under both aerobic and anaerobic conditions. Muscle strips were bathed in physiological saline solution with the anaerobic condition being created by replacing the oxygen bubbling the solution with nitrogen. 3. During contraction under anaerobic conditions, the rate of lactate production was increased 2.5-fold above that observed under aerobic conditions. This, however, only provided for a rate of ATP production of approximately 30% of that measured under aerobic conditions. Despite this, force maintenance was only slightly depressed, indicating that the metabolic cost of contraction was reduced in hypoxia. In support of this, the rate of heat production during contractions in anoxia was only approximately half of that under aerobic conditions, whereas, again, force was only slightly lower. Unloaded shortening velocity was significantly lower in anoxia, suggesting a slower cross-bridge turnover rate. 4. The results indicate that the economy of force maintenance is increased in bladder smooth muscle under hypoxic conditions and that this is due, at least in part, to a reduced rate of cross-bridge cycling. This may help to preserve bladder contractile function during periods of ischaemia that may be associated with bladder filling and emptying.

Effects of organ culture on arterial gene expression and hypoxic relaxation: role of the ryanodine receptor

Organ culture specifically inhibits vasorelaxation to acute hypoxia and preferentially decreases specific voltage-dependent K(+) channel expression over other K(+) and Ca(2+) channel subtypes. To isolate further potential oxygen-sensing mechanisms correlated with altered gene expression, we performed differential display analysis on RNA isolated from control and cultured coronary arterial rings. We hypothesize that organ culture results in altered gene expression important for vascular smooth muscle contractility important to the mechanism of hypoxia-induced relaxation. Our results indicate a milieu of changes suggesting both up- and downregulation of several genes. The altered expression pattern of two positive clones was verified by Northern analysis. Subsequent screening of a porcine cDNA library indicated homology to the ryanodine receptor (RyR). RT-PCR using specific primers to the three subtypes of RyR shows an upregulation of RyR2 and RyR3 after organ culture. Additionally, the caffeine- and/or ryanodine-sensitive intracellular Ca(2+) store was significantly more responsive to caffeine activation after organ culture. Our data indicate that organ culture increases expression of specific RyR subtypes and inhibits hypoxic vasorelaxation. Importantly, ryanodine blunted hypoxic relaxation in control coronary arteries, suggesting that upregulated RyR might play a novel role in altered intracellular Ca(2+) handling during hypoxia.

Enhancement effect under high-glucose conditions on U46619-induced spontaneous phasic contraction in mouse portal vein

The effect of the thromboxane A(2) analog 9,11-dideoxy-11alpha, 9alpha-epoxymethanoprostaglandin F(2alpha) (U46619) on spontaneous phasic contractions in the mouse portal vein was studied. U46619 induced concentration-dependent (1-100 nM) increases in amplitude, frequency, and contractile period (ON-time) of the contraction. Both amplitude and ON-time were enhanced significantly under high-glucose (HG; 4-fold greater than normal) conditions. This hyperactivation may be associated with portal vein dysfunction in diabetes. However, the mechanisms remain unclear. HG enhanced the U46619-induced accumulation of endogenous diacylglycerol (DG). Phospholipase C inhibition suppressed accumulation under normal conditions; however, this suppression was not observed under HG conditions. The HG-induced enhancement of U46619-induced contraction was inhibited by protein kinase C (PKC) inhibition. This finding indicated that accumulated DG might increase PKC activity. Activated PKC stimulated DG kinase activation as a feedback mechanism. DG kinase inhibition also suppressed the HG-induced enhancement of contraction. Increased myo-inositol incorporation was detected under HG conditions, indicating an acceleration of phosphatidylinositol (PI) turnover. This acceleration was inhibited by PKC and DG kinase inhibitors. These findings indicated that HG treatments increased DG synthesis derived from incorporated glucose, PKC, and DG kinase activation. These responses induce hyperactivation of the amplitude and contractile period of contraction mediated by acceleration of PI turnover. This series of responses may be involved in the dysfunction of the portal vein under the HG conditions occurring with diabetes.