The small GTPase Rac1 is required for smooth muscle contraction.

The role of the small GTP-binding protein Rac1 in smooth muscle contraction was examined using small molecule inhibitors (EHT1864, NSC23766) and a novel smooth muscle-specific, conditional, Rac1 knockout mouse strain. EHT1864, which affects nucleotide binding and inhibits Rac1 activity, concentration-dependently inhibited the contractile responses induced by several different modes of activation (high-K+, phenylephrine, carbachol and protein kinase C activation by phorbol-12,13-dibutyrate) in several different visceral (urinary bladder, ileum) and vascular (mesenteric artery, saphenous artery, aorta) smooth muscle tissues. This contractile inhibition was associated with inhibition of the Ca2+ transient. Knockout of Rac1 (with a 50% loss of Rac1 protein) lowered active stress in the urinary bladder and the saphenous artery consistent with a role of Rac1 in facilitating smooth muscle contraction. NSC23766, which blocks interaction between Rac1 and some guanine nucleotide exchange factors, specifically inhibited the α1 receptor responses (phenylephrine) in vascular tissues and potentiated prostaglandin F2α and thromboxane (U46619) receptor responses. The latter potentiating effect occurred at lowered intracellular [Ca2+]. These results show that Rac1 activity is required for active contraction in smooth muscle, probably via enabling an adequate Ca2+ transient. At the same time, specific agonists recruit Rac1 signalling via upstream modulators, resulting in either a potentiation of contraction via Ca2+ mobilization (α1 receptor stimulation) or an attenuated contraction via inhibition of Ca2+ sensitization (prostaglandin and thromboxane receptors).

Excitation and contraction of cardiac muscle and coronary arteries of brain-dead pigs.

Excitability and contraction of cardiac muscle from brain-dead donors critically influence the success of heart transplantation. Membrane physiology, Ca2+-handling, and force production of cardiac muscle and the contractile properties of coronary arteries were studied in hearts of brain-dead pigs. Cardiac muscle and vascular function after 12 h brain death (decapitation between C2 and C3) were compared with properties of fresh tissue. In both isolated cardiomyocytes (whole-cell patch clamp) and trabecular muscle (conventional microelectrodes), action potential duration was shorter in brain dead, compared to controls. Cellular shortening and Ca2+ transients were attenuated in the brain dead, and linked to lower mRNA expression of L-type calcium channels and a slightly lower ICa,L, current, as well as to a lower expression of phospholamban. The current-voltage relationship and the current above the equilibrium potential of the inward K+ (IK1) channel were altered in the brain-dead group, associated with lower mRNA expression of the Kir2.2 channel. Delayed K+ currents were detected (IKr, IKs) and were not different between groups. The transient outward K+ current (Ito) was not observed in the pig heart. Coronary arteries exhibited increased contractility and sensitivity to the thromboxane analogue (U46619), and unaltered endothelial relaxation. In conclusion, brain death involves changes in cardiac cellular excitation which might lower contractility after transplantation. Changes in the inward rectifier K+ channel can be associated with an increased risk for arrhythmia. Increased reactivity of coronary arteries may lead to increased risk of vascular spasm, although endothelial relaxant function was well preserved.

Endotoxin induces differentiated contractile responses in porcine pulmonary arteries and veins.

BACKGROUND/AIMS: Sepsis-induced lung injury is characterized by pulmonary hypertension, edema and deteriorated gas exchange. As in vivo studies have indicated that bacterial endotoxin predominantly induces a pulmonary venous constriction, we aimed to investigate effects of endotoxin on isolated porcine pulmonary vessels. METHODS: Pulmonary arteries and veins were examined using in vitro isometric force recordings. Endothelin-receptor protein expression and distribution were analyzed by Western blot and immunohistochemistry. Freshly isolated preparations and vessels incubated (24 h) with/without endotoxin (10 µg·ml(-1)) were compared. The contractile responses to phenylephrine, UK14.304, U46619, PGF(2α), endothelin-1 (ET-1) and sarafotoxin were recorded, as well as the relaxation in response to acetylcholine, isoproterenol and nitroprusside. RESULTS: In freshly isolated vessels, phenylephrine-induced contractions had a 5-times larger amplitude in arteries than in veins. The amplitude of the contractions in response to sarafotoxin was nearly 2 times larger in veins than in arteries, but there was no difference in responses to ET-1. Endotoxin markedly reduced phenylephrine-induced contractions in both arteries and veins, whereas the responses to ET-1 and sarafotoxin were augmented in veins only. No apparent changes in ET receptor expression or distribution were detected with Western blot or immunohistochemistry. CONCLUSION: Endotoxin differentially and selectively alters the contractile responses of porcine pulmonary vessels in vitro, towards a situation where the α-1 adrenergic responses of arteries are attenuated and the ET responses of veins are augmented. In situations with high adrenergic activity and high circulating ET levels, such as sepsis, these results may provide a mechanism contributing to pulmonary hypertension and edema formation.

Inflammation-induced airway smooth muscle responsiveness is strain dependent in mice.

Different mouse strains display different degrees of inflammation-induced airway hyperresponsiveness in vivo. It is not known whether these variations are attributable to distinct properties of the airway smooth muscle. Therefore, tracheal ring segments from C57BL/6 and BALB/c mice were exposed to three different pro-inflammatory stimuli for 4 days while maintained under tissue-culture conditions: tumour necrosis factor α (100 ng/ml), the Toll-like receptor (TLR) 3 agonist polyI:C (10 µg/ml), and the TLR4 agonist LPS (10 µg/ml). The contractile responses to carbachol, 5-hydroxytryptamine (5-HT) and bradykinin were assessed after culture. In addition, gene expression of TLR1-TLR9, pivotal inflammatory signal transduction proteins (jun-kinase, p38 and p65) and critical negative regulators of inflammation (A20, Itch, Tax1bp1 and RNF11) were studied in tracheal smooth muscle strips, fresh and following treatment for 4 days with LPS, from both strains. No differences between the strains were detected regarding the response of freshly isolated preparations to carbachol, 5-HT and bradykinin. After stimulation with pro-inflammatory mediators, contractions in response to 5-HT and bradykinin, but not to carbachol, were up-regulated. This up-regulation was markedly larger in BALB/c than in C57BL/6 segments and depended on the type of inflammatory stimulus. Expression of the genes investigated did not differ between the two strains. These findings indicate that strain differences in airway hyperresponsiveness can be linked to differences in the responsiveness of airway smooth muscle to pro-inflammatory mediators per se. The differences do not appear to be due to differential expression of TLR or common inflammatory transduction and repressor proteins.

Toxic effects of cobalt in primary cultures of mouse astrocytes. Similarities with hypoxia and role of HIF-1alpha.

Cobalt is suspected to cause memory deficit in humans and was reported to induce neurotoxicity in animal models. We have studied the effects of cobalt in primary cultures of mouse astrocytes. CoCl(2) (0.2-0.8mM) caused dose-dependent ATP depletion, apoptosis (cell shrinkage, phosphatidylserine externalization and chromatin rearrangements) and secondary necrosis. The mitochondria appeared to be a main target of cobalt toxicity, as shown by the loss of mitochondrial membrane potential (DeltaPsi(m)) and release from the mitochondria of apoptogenic factors, e.g. apoptosis inducing factor (AIF). Pre-treatment with bongkrekic acid reduced ATP depletion, implicating the involvement of the mitochondrial permeability transition (MPT) pore. Cobalt increased the generation of oxygen radicals, but antioxidants did not prevent toxicity. There was also an impaired response to ATP stimulation, evaluated as a lower raise in intracellular calcium. Similarly to hypoxia and dymethyloxallyl glycine (DMOG), cobalt triggered stabilization of the alpha-subunit of hypoxia-inducible factor HIF-1 (HIF-1alpha). This early event was followed by an increased expression of HIF-1 regulated genes, e.g. stress protein HO-1, pro-apoptotic factor Nip3 and iNOS. Although all of the three stimuli activated the HIF-1alpha pathway and decreased ATP levels, the downstream effects were different. DMOG only inhibited cell proliferation, whereas the other two conditions caused cell death by apoptosis and necrosis. This points to cobalt and hypoxia not only inducing HIF-1alpha regulated genes but also affecting similarly other cellular functions, including metabolism.

Blood pressure regulation by CD4+ lymphocytes expressing choline acetyltransferase.

Blood pressure regulation is known to be maintained by a neuro-endocrine circuit, but whether immune cells contribute to blood pressure homeostasis has not been determined. We previously showed that CD4+ T lymphocytes that express choline acetyltransferase (ChAT), which catalyzes the synthesis of the vasorelaxant acetylcholine, relay neural signals. Here we show that these CD4+CD44hiCD62Llo T helper cells by gene expression are a distinct T-cell population defined by ChAT (CD4 TChAT). Mice lacking ChAT expression in CD4+ cells have elevated arterial blood pressure, compared to littermate controls. Jurkat T cells overexpressing ChAT (JTChAT) decreased blood pressure when infused into mice. Co-incubation of JTChAT and endothelial cells increased endothelial cell levels of phosphorylated endothelial nitric oxide synthase, and of nitrates and nitrites in conditioned media, indicating increased release of the potent vasorelaxant nitric oxide. The isolation and characterization of CD4 TChAT cells will enable analysis of the role of these cells in hypotension and hypertension, and may suggest novel therapeutic strategies by targeting cell-mediated vasorelaxation.

Increased nociceptive response in mice lacking the adenosine A1 receptor.

The role of the adenosine A1 receptor in nociception was assessed using mice lacking the A1 receptor (A1R-/-) and in rats. Under normal conditions, the A1R-/- mice exhibited moderate heat hyperalgesia in comparison to the wild-type mice (A1R+/+). The mechanical and cold sensitivity were unchanged. The antinociceptive effect of morphine given intrathecally (i.t.), but not systemically, was reduced in A1R-/- mice and this reduction in the spinal effect of morphine was not associated with a decrease in binding of the mu-opioid ligand DAMGO in the spinal cord. A1R-/- mice also exhibited hypersensitivity to heat, but not mechanical stimuli, after localized inflammation induced by carrageenan. In mice with photochemically induced partial sciatic nerve injury, the neuropathic pain-like behavioral response to heat or cold stimulation were significantly increased in the A1R-/-mice. Peripheral nerve injury did not change the level of adenosine A1 receptor in the dorsal spinal cord in rats and i.t. administration of R-PIA effectively alleviated pain-like behaviors after partial nerve injury in rats and in C57/BL/6 mice. Taken together, these data suggest that the adenosine A1 receptor plays a physiological role in inhibiting nociceptive input at the spinal level in mice. The C-fiber input mediating noxious heat is inhibited more than other inputs. A1 receptors also contribute to the antinociceptive effect of spinal morphine. Selective A1 receptor agonists may be tested clinically as analgesics, particularly under conditions of neuropathic pain.

Luminal adenosine stimulates chloride secretion through A1 receptor in mouse jejunum.

Adenosine is known to stimulate chloride secretion by mouse jejunum. Whereas the receptor on the basolateral side is believed to be A2B, the receptor involved in the luminal effect of adenosine has not been identified. We found that jejuna expressed mRNA for all adenosine receptor subtypes. In this study, we investigated the stimulation of chloride secretion by adenosine in jejuna derived from mice lacking the adenosine receptors of A1 (A1R) and A2A (A(2A)R) or control littermates. The jejunal epithelium was mounted in a Ussing chamber, and a new method on the basis of impedance analysis was used to calculate the short-circuit current (I(sc)) values. Chloride secretion was assessed by the I(sc) after inhibition of the sodium-glucose cotransporter by adding phloridzin to the apical bathing solution. The effect of apical adenosine on chloride secretion was lost in jejuna from mice lacking the A1R. There was no difference in the response to basolaterally applied adenosine or to apical forskolin. Furthermore, in jejuna from control mice, the effect of apical adenosine was also abolished in the presence of 8-cyclopentyl-1,3-dipropylxanthine, a specific A1R antagonist. Responses to adenosine were identical in jejuna from control and A(2A)R knockout mice. This study demonstrates that A1R (and not A(2A)R) mediates the enhancement of chloride secretion induced by luminal adenosine in mice jejunum.