Thromboxane A(2) modulates cyclic AMP relaxation and production in human internal mammary artery

Thromboxane receptor activation mediates isoprostane-induced increases in amyloid pathology in Tg2576 mice

Alzheimer's disease (AD) amyloid plaques are composed of amyloid-beta (Abeta) peptides produced from proteolytic cleavage of amyloid precursor protein (APP). Isoprostanes, markers of in vivo oxidative stress, are elevated in AD patients and in the Tg2576 mouse model of AD-like Abeta brain pathology. To determine whether isoprostanes increase Abeta production, we delivered isoprostane iPF(2alpha)-III into the brains of Tg2576 mice. Although treated mice showed increased brain Abeta levels and plaque-like deposits, this was blocked by a thromboxane (TP) receptor antagonist, suggesting that TP receptor activation mediates the effects of iPF(2alpha)-III on Abeta. This hypothesis was supported by cell culture studies that showed that TP receptor activation increased Abeta and secreted APP ectodomains. This increase was a result of increased APP mRNA stability leading to elevated APP mRNA and protein levels. The increased APP provides more substrate for alpha and beta secretase proteolytic cleavages, thereby increasing Abeta generation and amyloid plaque deposition. To test the effectiveness of targeting the TP receptor for AD therapy, Tg2576 mice underwent long-term treatment with S18886, an orally available TP receptor antagonist. S18886 treatment reduced amyloid plaques, insoluble Abeta, and APP levels, thereby implicating TP receptor signaling as a novel target for AD therapy.

Regulation of human internal mammary and radial artery contraction by extracellular and intracellular calcium channels and cyclic adenosine 3', 5' monophosphate

BACKGROUND

The internal mammary (IMA) and radial arteries (RA), which are routinely used in coronary artery bypass grafting, show a significant incidence of postoperative vasospasm. The present study evaluated the respective roles of calcium (Ca2+)-dependent and cyclic adenosine 3', 5' monophosphate-dependent (cAMP) signaling in mediating contraction and relaxation of the IMA and RA.

METHODS

We examined the contractile responses of the IMA and RA to potassium chloride, a depolarizing agent; phenylephrine, an alpha-adrenergic agonist; and U46619, a thromboxane analogue, in the absence and presence (0.045 to 1.500 mM) of extracellular Ca2+.

RESULTS

Potassium chloride elicited little or no contraction in the absence of extracellular Ca2+. Contractions elicited by U46619 were similar in the IMA and RA, both in the absence and presence of extracellular Ca2+. By contrast, phenylephrine elicited significantly greater extracellular Ca2+-dependent contraction of the IMA than the RA. Estimation of cyclic guanosine 3', 5' monophosphate (cGMP) and cAMP revealed levels of cAMP to be about fourfold higher than cGMP in both the RA and IMA. Whereas forskolin and milrinone elicited similar relaxation of IMA and RA precontracted with either U46619 or phenylephrine and increased adenylate cyclase-catalyzed cAMP production, isoproterenol-induced relaxation of the arteries precontracted with U46619 was significantly impaired compared with arteries precontracted with phenylephrine.

CONCLUSIONS

Our findings suggest that thromboxane A2 receptor-dependent pathways activate contraction of IMA and RA through both extracellular Ca2+-dependent and Ca2+-independent pathways. In addition, adenylate cyclase appears to play a key role in attenuating thromboxane A2 and alpha-adrenergic receptor-mediated contraction through both pathways.

pH effects on high conductance Ca2+-activated K+ channels (BKCa) in human internal mammary artery smooth muscle cells

INTRODUCTION

In vascular smooth muscle cells, different types of K+ channels participate in the regulation of membrane potential and consequently in the contractile behavior of the vessel. There is little information about the properties and role of K+ channels in human internal mammary artery (HIMA), the vessel of choice for coronary revascularization.

METHODS

Patch-clamp technique on isolated HIMA smooth muscle cells was used.

RESULTS

This work presents for the first time single-channel properties of the high conductance Ca2+-activated K+ channel (BK(Ca)) of HIMA. It presents a single-channel conductance of 228+/-4 pS (n=44, 8 cells), is sensitive to 100 nM iberiotoxin, and its open probability is Ca2+- and voltage-dependent. Inside-out results show that BK(Ca) channels in HIMA are directly activated by increasing the pH of intracellular media (NPo=0.096+/-0.032 at pH 7.4 and NPo=0.459+/-0.111 at pH 7.6, n=12 cells, p<0.05) and inhibited by lowering this pH (NPo=0.175+/-0.067 at pH 7.4 and NPo=0.051+/-0.019 at pH 6.8, n=13 cells, p<0.05).

CONCLUSIONS

The evidences presented about single-channel properties and intracellular pH sensitivity of BK(Ca) from HIMA smooth muscle cells provide useful information to elucidate physiological or pathological mechanisms in this vessel, as well as for future studies where drugs could have BK(Ca) channels as targets for pharmacological therapies.

Both milrinone and colforsin daropate attenuate the sustained pial arteriolar constriction seen after unclamping of an abdominal aortic cross-clamp in rabbits

We previously reported that unclamping of an abdominal aortic cross-clamp causes initial dilation of pial arteries followed by sustained constriction. Both milrinone and colforsin daropate have a vasodilator action, and both have been used in such critical conditions as abdominal aortic aneurysmectomy. We measured cerebral pial arteriolar diameters using a rabbit closed cranial window preparation before (baseline) and 15 min after the start of an IV infusion of 0.9% saline (control group), milrinone, or colforsin daropate (0.05 and 0.5 microg . /kg(-1) . min(-1)) (pre-clamp), just after aortic clamping, 20 min after clamping, and at 0 to 60 min after unclamping. In the control group, a significant decrease in diameter persisted for at least 60 min after unclamping (maximum, -15% for large and -26% for small arterioles versus baseline). These values were significantly smaller after both doses of milrinone and the larger dose of colforsin daropate (-5% and -8%, 10% and 12%, and -2% and -5%, respectively vs baseline, at 60 min). In a second experiment, changes in regional cerebral blood flow and tissue oxygen tension reflected changes in vascular variables. Thus, sustained cerebral pial arteriolar constriction induced by aortic unclamping can be attenuated by IV milrinone or colforsin daropate.

Mammary uptake and excretion of prostanoids in relation to mammary blood flow and milk yield during pregnancy-lactation and somatotropin treatment in dairy goats

Mammary arterious-venous differences (A-V) and excretion into milk of four prostanoids were related to changes in milk yield and milk vein blood velocity (MBV) in goats at different stages of pregnancy and lactation, and during somatotropin (ST) treatment in mid-lactation. Arterial concentrations and mammary A-V for the vasodilators prostacyclin (PGI(2)) and prostaglandin (PG) E(2) (measured as 6-keto-PGF(1 alpha) and bicyclic PGE(2), respectively) decreased from late pregnancy to lactation. A-V were negatively correlated to MBV (r = -0.32 to -0.34). Arterial concentrations of the vasoconstrictors PGF(2 alpha) and TXA(2) (measured as TXB(2)) changed similarly, but no A-V across the mammary gland were found. The vasodilator to vasoconstrictor ratio in plasma was around 1:1, and in skimmed milk around 0.29-0.49 due to significantly higher TXB(2) levels in milk compared to plasma. Close linear correlations were established between milk yield and excretion of TXB(2) into milk (r = 0.80, P < 0.001), and between MBV and PGE(2) excretion into milk (r = 0.69, P < 0.001). ST treatment stimulated MBV and mammary prostanoid supply, and decreased prostanoid concentration in milk vein plasma. The high arterial levels of prostaglandins during pregnancy most likely reflected uterine synthesis. Our results support a role for PGI(2) and PGE(2) in local mammary blood flow regulation during lactation. Increased mammary uptake of these two prostanoids may be involved in the mammary blood flow response to ST. TXA(2) may be synthesized by mammary epithelial as well as vascular cells, and TXA(2) may be an important factor in regulation of mammary function.

Evaluation of beta3-adrenoceptor-mediated relaxation in intact and endotoxin-treated equine digital veins

OBJECTIVE

To investigate the functional expression of beta3-adrenoceptors (beta3-ARs) in equine digital veins (EDVs) and to examine whether beta3-AR relaxation was altered in EDVs incubated with endotoxin.

SAMPLE POPULATION

Forelimbs obtained from 30 horses.

PROCEDURE

Forelimbs were obtained from horses in an abattoir. Equine digital veins were carefully removed from distal portions of the forelimbs. Rings of dissected EDVs were mounted in 5-mL organ baths to record isometric tension in the presence of various beta3-AR agonists (SR 58611A, ZD 2079, and ZM 215001).

RESULTS

In intact EDVs, isoprenaline, SR 58611A, ZD 2079, and ZM 215001 induced concentration-dependent relaxation. Isoprenaline and SR 58611A-induced relaxations were reduced or unaffected by nadolol, respectively. In intact EDVs, SR 58611A-induced relaxation was significantly reduced in the presence of 2 microM ZM 215001 (used as a beta3-AR antagonist). In endothelium-denuded EDVs or intact EDVs in the presence of a nitric oxide synthase inhibitor, isoprenaline and SR 58611A-induced relaxations were significantly decreased. The endothelium-independent relaxation to SR 58611A was significantly inhibited in the presence of ZM 215001. In endotoxin-treated EDV, isoprenaline- and SR 58611A-induced relaxations were significantly reduced. In these conditions, cycloheximide (a protein synthesis inhibitor) and ibuprofen (a cyclooxygenase inhibitor) restored the relaxant response to SR 58611A.

CONCLUSIONS AND CLINICAL RELEVANCE

Beta3-adrenoceptors are functionally expressed in EDVs. Incubation in the presence of endotoxin, used as an in vitro model of laminitis, induced an alteration of beta-AR-mediated relaxations in EDVs, which could be the consequence of cyclooxygenase induction and subsequent prostanoid production.

Influence of the nature of pre-contraction on the responses to commonly employed vasodilator agents in rat-isolated aortic rings

The relaxing properties of vasodilator drugs in vitro may depend on the characteristics of the contractile state of the vessel investigated. Rat-isolated thoracic aortas were exposed to different types of pre-contraction. The following vasoconstrictor agents were used: phenylephrine (PhE), a selective alpha1-adrenoceptor agonist; St 587, a partial alpha1-adrenoceptor stimulant; U46619 (U-46). a thromboxane A2 agonist: and potassium ions causing receptor-independent depolarization of the membrane. After pre-contraction, various differential vasodilator drugs were investigated: methacholine (MCh, endothelium dependent), sodium nitroprusside (SNP, NO donor), forskolin (FSK, adenylyl cyclase stimulant) and nifedipine, a Ca2+-antagonist (selective L-type calcium antagonist). The vasodilator activity of these compounds was quantified by their vasodilator potency value (pD2) and efficacy (Emax) obtained from their concentration-response curves. PhE (0.1, 0.3, 3 microM) caused isometric responses of 4.8 +/- 0.3, 6.5 +/- 0.3 and 7.8 +/- 0.5 mN, respectively. An increase of the PhE concentration from 0.1 to 3 microM did not influence the response to FSK while it reduced the pD2 of SNP (8.6 +/- 0.1 to 7.35 +/- 0.1). Under these conditions, only the Emax of MCh was reduced (96.3 +/- 4.3% to 43.3 +/- 6.9%). U46 (0.18, 0.3, 1 microM) increased the contractile force by 7.4 +/- 0.4, 8.8 +/- 0.3 and 10.4 +/- 0.3 mN, respectively. Increasing the concentration of U-46 from 0.18 to 1 microM affected only the efficacy of SNP (84 +/- 4.4% to 17 +/- 8.8%) and MCh (64.5 +/- 12.3% to 0.0 +/- 9.2%) and reduced the potency of FSK (7.91 +/- 0.26 to 7.15 +/- 0.10). The concentration of K+-ions from 25 to 30 and 40 mM increased the contractile force by 4.0 +/- 0.4, 7.0 +/- 0.5 and 10.8 +/- 0.4 mN, respectively. The increase in [K+] caused a potency decrease of FSK (7.1 +/- 0.0 to 5.8 +/- 0.0) whereas both efficacy and potency were reduced for SNP (95.6 +/- 1.8% to 65.8 +/- 1.9% and 8.7 +/- 0.1 to 7.2 +/- 0.1) and MCh (55.4 +/- 3.5% to 24.5 +/- 0.8% and 7.4 +/- 0.3 to 6.1 +/- 0.4). Inhibiting of the endothelial NO production by L-NAME 100 microM resulted after pre-contraction with PhE and potassium in comparable differences in properties for SNP. Pre-contraction with St 587 1, 3, 10 and 30 microM shows comparable results after nifedepine relaxation. The present experiments clearly demonstrate that the characteristics of the applied pre-contraction strongly, but differentially influence both the potency and efficacy of various vasodilator drugs in vitro. Accordingly, in vitro characterization of vasodilator drugs should be performed under a carefully standardized protocol of pre-contraction.

Influence of colforsin daropate hydrochloride on internal mammary artery grafts

The effect of colforsin daropate hydrochloride (colforsin), a water-soluble forskolin derivative, on blood flow in internal mammary artery (IMA) grafts was evaluated in a prospective randomized study of 26 patients undergoing coronary artery bypass grafting. Patients were randomized to receive either colforsin treatment (colforsin; n=14) or no colforsin treatment (control; n=14). Administration of colforsin (0.5mg x kg(-1) min(-1)) was started after induction of anesthesia and was continued for 6 h. IMA blood flow and hemodynamic measurements were assessed perioperatively. During cardiopoulmonary bypass (CPB), perfusion flow was adjusted to 2.5 L/m2 and IMA free blood flow was measured. IMA blood flow was also measured 1 h after CPB by an ultrasonic flow meter. Systemic vascular resistance was significantly lower in the colforsin group during and after CPB. IMA blood flow was significantly greater in the colforsin group than in the control group during (44 +/- 2 vs 33 +/- 3 ml min-1 x m(-2), p=0.02) and after CPB (38 +/- 6 vs 20 +/- 3ml x min(-1) m(-2), p=0.01). IMA blood flow 1 h after CPB correlated inversely with concurrent systemic vascular resistance (r=-0.61, p=0.001). Intraoperative administration of colforsin daropate hydrochloride caused potent vasodilation, resulting in an increase in IMA blood flow. The results indicate that the regimen can be used perioperatively in patients undergoing coronary artery bypass grafting.

Relaxation and modulation of cyclic AMP production in response to atrial natriuretic peptides in guinea pig tracheal smooth muscle

Relaxation and modulation of cyclic AMP production in response to atrial natriuretic peptides were investigated in epithelium-denuded guinea pig tracheal rings, treated with indomethacin (5 microM) and phosphoramidon (1 microM) and contracted with histamine (3 microM). Atrial natriuretic peptide (ANP) was a more potent relaxant than C-type natriuretic peptide whereas ANP-(4-23) was inactive suggesting the involvement of ANP(A) receptors in the relaxant effect of ANP. ODQ (1H-[1,2,4]oxadiazolo[4,3-A]quinoxalin-1-one, 10 microM), a selective inhibitor of soluble guanylyl cyclase, markedly inhibited the relaxant response to sodium nitroprusside. The relaxant response to ANP was not altered by ODQ demonstrating the involvement of particulate guanylyl cyclase. ANP-induced relaxations, as well as sodium nitroprusside-induced relaxations, were similarly potentiated by rolipram (4-(3-(cyclopentyloxy)-4-methoxyphenyl)pyrrolidin-2-one, 3 microM), a type IV phosphodiesterase inhibitor, and by zaprinast (2-(2-propyloxyphenyl)-8-azapurin-6-one, 10 microM), a type V phosphodiesterase inhibitor. ANP-mediated response was unaffected by glibenclamide (10 microM), a selective blocker of ATP-sensitive K(+) channels, and by apamin (1 microM), a selective blocker of small-conductance Ca(2+)-activated K(+) channels. Iberiotoxin (100 nM) extensively prevented the relaxant effect of ANP suggesting the activation of large-conductance Ca(2+)-activated K(+) channels. In addition, ANP (10 nM) and ANP-(4-23) (100 nM) significantly reduced forskolin (1 microM)-stimulated cAMP accumulation suggesting, for the first time, the presence of functional ANP(C) receptors in guinea pig airway smooth muscle. However, relaxations to forskolin and to isoproterenol were not altered in the presence of ANP-(4-23) or ANP demonstrating that the inhibitory effect of ANP-(4-23) and ANP on adenylyl cyclase was not sufficient to alter the functional response induced by these two activators of adenylyl cyclase.

Identification and functional characterization of thromboxane A2 receptors in Schwann cells

Previous reports have demonstrated the presence of functional thromboxane A2 (TP) receptors in astrocytes and oligodendrocytes. In these experiments, the presence and function of TP receptors in primary rat Schwann cells (rSC) and a neurofibrosarcoma-derived human Schwann cell line (T265) was investigated. Immunocytochemical and immunoblot analyses using polyclonal anti-TP receptor antibodies demonstrate that both cell types express TP receptors. Treatment with the stable thromboxane A2 mimetic U46619 (10 microM) did not stimulate intracellular calcium mobilization in rSC, whereas T265 cells demonstrated a calcium response that was inhibited by prior treatment with TP receptor antagonists. U46619 also stimulated CREB phosphorylation on Ser133 in T265 cells and, to a lesser extent, in rSC. To identify potential mechanisms of CREB phosphorylation in rSC, we monitored intracellular cAMP levels following U46619 stimulation. Elevated levels of cAMP were detected in both rSC (20-fold) and T265 (15-fold) cells. These results demonstrate that TP receptor activation specifically stimulates CREB phosphorylation in T265 cells, possibly by a calcium- and/or cAMP-dependent mechanism. In contrast, TP receptor activation in rSC stimulates increases in cAMP and CREB phosphorylation but does not elicit changes in intracellular calcium.