Differential effects of prostacyclin and iloprost in the isolated carotid artery of the guinea-pig

The effects on membrane potential of prostacyclin and iloprost were compared in smooth muscle cells of the guinea pig carotid artery. Both prostacyclin and iloprost induced hyperpolarization of the smooth muscle cells. In the presence of (3R)-3-(4-fluorophenyl-sulfonamido)-1,2,3,4-tetrahydro-9-carbazolepropanoic acid (Bay U3405), an antagonist of TP receptors, the response to iloprost was unaffected while that to prostacyclin was increased. Iloprost-induced hyperpolarizations were abolished by glibenclamide while those to prostacyclin were either not affected, or converted to either depolarization or to rhythmic electrical activity. The latter effects of prostacyclin were abolished by Bay U3405. After removal of the endothelium, iloprost and prostacyclin produced hyperpolarizations similar to those observed in control blood vessels. However, in the presence of glibenclamide, prostacyclin produced only depolarizations inhibited by Bay U3405. These results suggest that iloprost activates IP receptors and K(ATP) channels in smooth muscle. In contrast, prostacyclin produces additional endothelium-dependent and -independent effects via activation of TP receptors.

3-Morpholinosydnonimine (SIN-1) and K(+) channels in smooth muscle cells of the rabbit and guinea pig carotid arteries

Experiments were designed to determine the subtype of K(+) channels activated by the nitrovasodilator 3-morpholinosydnonimine (SIN-1) in smooth muscle cells of the rabbit and guinea pig carotid arteries. Membrane potential was recorded in isolated segments with intracellular microelectrode and K(+) currents in freshly dissociated smooth muscle cells, with the patch-clamp technique. In the guinea pig carotid artery, SIN-1 caused a glibenclamide-sensitive hyperpolarization. The nitrovasodilator did not affect the whole-cell K(+) current, but activated a glibenclamide-sensitive K(+) current. In the rabbit carotid artery, SIN-1 induced only an iberiotoxin-sensitive repolarization in phenylephrine-depolarized tissue and in isolated cells, enhanced the activity of an iberiotoxin-sensitive K(+) current. These findings demonstrate that the population of K(+) channels activated by nitric oxide (NO) is species-dependent and support the conclusion that, in the guinea pig carotid artery, in contrast to the rabbit carotid artery, the release of NO cannot account for the responses attributed to endothelium-derived hyperpolarizing factor (EDHF).

Endothelium-dependent hyperpolarization in isolated arteries taken from animals treated with NO-synthase inhibitors

To study the effects of chronic in vivo inhibition of NO synthase on endothelium-dependent hyperpolarization, cell-membrane potential (in individual vascular smooth-muscle cells) and changes in tension (in isolated rings) were recorded from isolated canine coronary arteries and guinea-pig carotid arteries and aortas. In coronary arteries taken from control dogs and contracted with U46619, acetylcholine- and bradykinin-induced endothelium-dependent relaxations, which were unaffected by short-term in vitro exposure to indomethacin but were inhibited partially by L-nitro-arginine (LNA). In coronary arteries taken from dogs treated over the long term in vivo with LNA (30 mg/kg on the first day and 20 mg/kg the 7 following days, i.v.), the response to acetylcholine and bradykinin was inhibited when compared with arteries from control dogs. Short-term in vitro exposure to LNA or indomethacin or both did not influence the effects of either agonist. In these arteries, the hyperpolarizing response to acetylcholine, observed in the presence of LNA and indomethacin, was enhanced, whereas that to bradykinin was partially inhibited. In the guinea pig isolated aorta, the relaxation to bradykinin was abolished by long-term in vivo treatment with L-nitro-arginine-methyl-ester (L-NAME; 1.5 mg/ml, in the drinking water for > or =4 days). In the isolated guinea pig carotid artery studied in the presence of LNA and indomethacin, acetylcholine induced a hyperpolarization that was not significantly affected by long-term in vivo treatment with L-NAME. These findings indicate that endothelium-dependent hyperpolarizations are maintained during long-term inhibition of NO synthase and probably act as a back-up mechanism to elicit endothelium-dependent relaxations.

Changes in angiotensin II receptor density and calcium handling during proliferation in SHR aortic myocytes

The aim of the present work was to characterize angiotensin II (ANG II) receptors and their effect on intracellular free Ca2+ concentration ([Ca2+]i) in proliferating aortic smooth muscle cells (VSMCs) from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Independently from the proliferating state of cultures, apparent affinities of ligands (ANG II > losartan > > CGP-42112A) were consistent with the presence of AT1 receptors in primary cells from SHR and WKY. In proliferating cultures, increases in [Ca2+]i elicited by ANG II (100 nM) were dramatically attenuated or abolished in VSMCs from both strains compared with confluent and postconfluent cultures. Ca2+ releases induced by ionomycin and by ANG II in the absence of extracellular Ca2+ were also impaired in proliferating cultures. In addition, no significant strain difference was found in proliferating cultures with respect to ANG II receptor density, basal [Ca2+]i, and ANG II-induced increases in [Ca2+]i. However, ANG II receptor density significantly increased in SHR, but not in WKY VSMCs at postconfluence. Furthermore, basal [Ca2+]i was elevated in confluent and postconfluent cultures from SHR but not WKY. In confluent cultures, ANG II- and ionomycin-induced Ca2+ releases were enhanced in SHR VSMCs compared with WKY VSMCs. These results show that ANG II-induced Ca2+ release and ionomycin-sensitive Ca2+ stores are enhanced in SHR VSMCs but dramatically decreased in proliferating VSMC cultures from both strains. Mechanisms underlying these alterations remain to be defined. However, the results suggest that alterations in ANG II AT1 receptor density and in intracellular Ca2+ handling in confluent and postconfluent cultures are not associated with the proliferative phenotype of SHR VSMCs. In addition, no evidence for any change in ANG II receptor subtype associated with proliferation of VSMCs was found in either strain.

Endothelium-derived factors and hyperpolarization of the carotid artery of the guinea-pig

1. Transmembrane potentials were recorded from isolated carotid arteries of the guinea-pig superfused with modified Krebs-Ringer bicarbonate solution. Smooth muscle cells were impaled from the adventitial side with intracellular glass microelectrodes filled with KCl (30-80 M omega). 2. Acetylcholine (1 microM) in the presence of inhibitors of nitric oxide synthase, (N omega-nitro-L-arginine (L-NOARG) 100 microM) and cyclo-oxygenase, (indomethacin 5 microM) induced an endothelium-dependent hyperpolarization (-18.9 +/- 1.6 mV, n = 15). 3. In the presence of these two inhibitors, S-nitroso-L-glutathione (10 microM), sodium nitroprusside (10 microM), 3-morpholinosydnonimine (SIN-1, 10 microM) and iloprost (0.1 microM) induced endothelium-independent hyperpolarizations of the smooth muscle cells (respectively: -16.0 +/- 2.3, -16.3 +/- 3.4, -12.8 +/- 2.0 and -14.5 +/- 1.5 mV, n = 4-6). 4. The addition of glibenclamide (1 microM) did not influence the acetylcholine-induced L-NOARG/ indomethacin-resistant hyperpolarization (-18.0 +/- 1.8 mV, n = 10). In contrast, the responses induced by S-nitroso-L-glutathione, sodium nitroprusside, SIN-1 and iloprost were abolished (changes in membrane potential: -0.8 +/- 1.1, 1.3 +/- 3.9, 4.5 +/- 4.6 and 0.3 +/- 0.8 mV respectively, n = 4-5). 5. In the presence of NO synthase and cyclo-oxygenase inhibitors, charybdotoxin (0.1 microM) or apamin (0.5 microM) did not influence the hyperpolarization produced by acetylcholine. However, in the presence of the combination of charybdotoxin and apamin, the acetylcholine-induced L-NOARG/indomethacin-resistant hyperpolarization was converted to a depolarization (4.4 +/- 1.2 mV, n = 20) while the endothelium-independent hyperpolarizations induced by S-nitroso-L-glutathione, sodium nitroprusside, SIN-1 and iloprost were not affected significantly (respectively: -20.4 +/- 3.4, -22.5 +/- 4.9, -14.5 +/- 4.7 and -14.5 +/- 0.5 mV, n = 4-5). 6. In the presence of the combination of charybdotoxin and apamin and in the absence of L-NOARG and indomethacin, acetylcholine induced a hyperpolarization (-19.5 +/- 3.7 mV, n = 4). This hyperpolarization induced by acetylcholine was not affected by the addition of indomethacin (-18.3 +/- 4.6 mV, n = 3). In the presence of the combination of charybdotoxin, apamin and L-NOARG (in the absence of indomethacin), acetylcholine, in 5 out of 7 vessels, still produced hyperpolarization which was not significantly smaller (-9.1 +/- 5.6 mV, n = 7) than the one observed in the absence of L-NOARG. 7. These findings suggest that, in the guinea-pig isolated carotid artery, the endothelium-independent hyperpolarizations induced by NO donors and iloprost involve the opening of KATP channels while the acetylcholine-induced endothelium-dependent hyperpolarization (resistant to the inhibition of NO-synthase and cyclo-oxygenase) involves the opening of Ca(2+)-activated potassium channel(s). Furthermore, in this tissue, acetylcholine induces the simultaneous release of various factors from endothelial origin: hyperpolarizing factors (NO, endothelium derived hyperpolarizing factor (EDHF) and prostaglandins) and possibly a depolarizing factor.

Inhibitors of the cytochrome P450-mono-oxygenase and endothelium-dependent hyperpolarizations in the guinea-pig isolated carotid artery

1. Transmembrane potentials were recorded from isolated carotid arteries of the guinea-pig superfused with modified Krebs-Ringer bicarbonate solution. Smooth muscle cells were impaled with sharp intracellular microelectrodes. 2. Acetylcholine (1 microM) induced an endothelium-dependent hyperpolarization (14.3 +/- 2.8 mV, n = 6) which was not affected (15.1 +/- 1.1 mV, n = 35) by inhibitors of cyclo-oxygenase (indomethacin, 5 microM) and nitric oxide synthase (N omega nitro-L-arginine: L-NOARG, 100 microM). 3. The hyperpolarization produced by acetylcholine was abolished in the presence of elevated potassium (35 mM) in the superfusing physiological saline solution. 4. The acetylcholine-induced hyperpolarization was not affected by the inhibitors of cytochrome P450 mono-oxygenases, SKF525a (10 and 100 microM, 13.9 +/ 2.2 and 15.3 +/- 4.6 mV), metyrapone (100 microM, 13.1 +/- 1.9 mV), clotrimazole (100 microM, 13.5 +/- 2.7 mV), 17-octadecynoic acid (5 microM, 16.5 +/- 1.9 mV), methoxsalen (10 microM, 15.3 +/- 1.6 mV), the inhibitor of phospholipase A2 quinacrine (10 microM 12.8 +/- 2.5 mV) and the non specific lipoxygenases/cyclo-oxygenases/cytochrome P450 inhibitor, eicosatetraynoic acid (50 microM, 15.0 +/- 2.2 mV). However, the muscarinic antagonist, atropine (100 nM), abolished the hyperpolarization. 5. These results suggest that in guinea-pig carotid artery, the metabolism of arachidonic acid, either through cyclo-oxygenase, lipoxygenase or cytochrome p450 mono-oxygenase, is not involved in acetylcholine-induced endothelium-dependent hyperpolarizations.

Effects of losartan on contractile responses of conductance and resistance arteries from rats

We investigated the selectivity of losartan as an angiotensin II (ANG II) antagonist in contractile experiments using segments of small mesenteric arteries and rings of aorta from rat. The concentration-effect curve of ANG II was not different in mesenteric arteries with an without endothelium. In both resistance and conductance vessels, it was shifted toward larger concentrations by losartan (3 nM) with similar apparent inhibition constant (KB) values: 4.1 +/- 1.8 nM (n = 6) in small mesenteric arteries and 1.9 nM (n = 6) in aorta. These values agree with the known affinity of losartan for AT1 receptors. At 1 microM, the AT2-selective ligand CGP 42112A had no effect on contractile responses induced by norepinephrine (NE), serotonin, or neuropeptide Y (NPY). However, it inhibited vasoconstriction elicited by prostaglandin F2 alpha (PGF2 alpha). This latter effect was also noted in the aorta. Similarly, losartan also competitively antagonized aortic contractile responses elicited by U 46619, a thromboxane A2 analogue (TXA2), with a pA2 value of 5.7. Two losartan analogues, DuP 532 and EXP 3174 (a metabolite of losartan), < or = 30 microM, did not antagonize U 46619, showing structural requirements for this antagonistic action of losartan. We conclude that in both rat resistance and conductance vessels, ANG II induces vasoconstriction through activation of AT1 receptors which are selectively blocked by losartan at nanomolar concentrations and that at micromolar concentrations, losartan may also block the vascular TXA2/PGF2 alpha (TP) receptor.

ANG II receptor expression and function during phenotypic modulation of rat aortic smooth muscle cells

Angiotensin II (ANG II) receptors were investigated in primary cultured rat aortic smooth muscle cells (SMC) that expressed either a proliferative phenotype (during the growth phase) or a contractile phenotype (at postconfluence). For each phenotype, alpha-smooth muscle actin expression, 125I-labeled ANG II specific binding, D-myo-inositol 1,4,5-triphosphate [Ins(1,4,5)P3] production, and ANG II-mediated increases in intracellular calcium (Cai2+) were studied. In both phenotypes, 1) ANG II-specific high-affinity binding (KD 0.5 +/- 0.1 nM and Bmax 196 +/- 106 pmol/mg protein in proliferative state, KD 1.5 +/- 0.3 nM and Bmax 560 +/- 299 pmol/mg protein in postconfluent state) was entirely inhibited by the selective AT1-antagonist losartan as well as by [Sar1,Ala8]ANG II and ANG III; 2) the AT2-antagonist CGP 42112A was ineffective, except at very high concentrations (> or = 10 microM); 3) the specific binding of ANG II was inhibited by guanosine 5'-[gamma-thio]triphosphate; and 4) ANG II induced a losartan-sensitive increase in Ins(1,4,5)P3. In postconfluent cultures, ANG II elicited a rapid biphasic elevation in Cai2+, which was abolished by losartan, whereas in growing cultures, this response was either absent or greatly attenuated. It is concluded that AT1-receptors coupled to phospholipase C via a G protein are expressed in the proliferative as well as in the contractile SMC phenotype and that their coupling to Cai2+ release is impaired in the proliferative phenotype. No evidence for AT2-receptor expression during phenotypic modulation of SMC was found.

Prejunctional opioid ?-receptors and adenosine A1-receptors on the sympathetic nerve endings of the rat tail artery interact with the ?2-adrenoceptors

Experiments were designed to study the interaction between prejunctional alpha 2-adrenoceptors and both adenosine and opioid receptors at the postganglionic sympathetic nerve endings innervating the tail artery of the rat. Segments of this vessel were preincubated with [3H]-noradrenaline and then perfused/superfused with [3H]-noradrenaline-free medium. Their perivascular nerves were field stimulated with standard stimulation parameters: 24 pulses at 0.4 Hz, 0.3 ms, 200 mA. In some experiments, the stimulation parameters were adjusted in order to obtain similar reference release values despite the presence of a first release-modulating drug. The adenosine agonist 5'-N-ethylcarboxamidoadenosine (NECA; 0.3-10 mumol/l) and [D-Ala2,MePhe4,Glyol5]enkephalin (DAGO; 0.3-10 mumol/l) depressed the stimulation-evoked overflow of tritium in a concentration dependent manner. The release-inhibiting effect of both NECA and DAGO was enhanced in the presence of the alpha 2-adrenoceptor antagonist rauwolscine (3 mumol/1) while it was attenuated in the presence of the alpha 2-adrenoceptor agonist 5-bromo-6-[2-imidazolin-2yl-amino]-quinoxaline (UK-14,304; 0.1 mumol/l). These changes occurred both at standard and adjusted stimulation parameters. These results demonstrate that the prejunctional adenosine A1- and opioid mu-receptors interact with the prejunctional alpha 2-adrenoceptors. The level at which these interactions take place (receptors themselves or transduction mechanisms) as well as the physiological significance of the phenomenon remain to be determined.