Ca2+ localization and sensitivity in vascular smooth muscle

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Vasorelaxation of rat thoracic aorta caused by 14-deoxyandrographolide

1. The pharmacological effects of 14-deoxyandrographolide on rat isolated thoracic aorta were examined. 2. 14-Deoxyandrographolide (2.5-120 mumol/L) inhibited contractions induced by phenylephrine (PE; 0.1 mumol/L) and high K+ (80 mmol/L) in a concentration-dependent manner in endothelium-intact aorta. The effect was attenuated in endothelium-denuded aorta without modifying the maximal response. Like verapamil, 14-deoxyandrographolide produced a much greater vasorelaxant effect in aorta precontracted by KCl than by PE. 14-Deoxyandrographolide (20-60 mumol/L) also inhibited responses of the rat aorta to PE. 3. In Ca(2+)-free medium (KCl 55 mmol/L), 14-deoxyandrographolide (20-80 mumol/L) antagonized Ca(2+)-induced vasocontraction in a concentration-dependent manner and transient contractions induced by both caffeine (10 mmol/L) and nor-adrenaline (1 mumol/L) were suppressed or almost abolished by 14-deoxyandrographolide. 4. The vasorelaxant effect of 14-deoxyandrographolide was partially antagonized by NG-nitro-L-arginine methyl ester (25 mumol/L), a specific and competitive nitric oxide synthase (NOS) inhibitor, and methylene blue (10 mumol/L), a soluble guanylate cyclase inhibitor, but was not affected by indomethacin (20 mumol/L), a cyclo-oxygenase inhibitor, or glibenclamide (10 mumol/L), an ATP-sensitive K(+)-channel blocker. 5. These results suggest that the vasorelaxant activity of 14-deoxyandrographolide may be mediated via the activation of NOS and guanylate cyclase, as well as the blockade of Ca2+ influx through both voltage- and receptor-operated Ca2+ channels.

Effects of 2,5-di-t-butyl-1,4-benzohydroquinone (BHQ) on rat aorta smooth muscle

To characterise the pharmacological activity of 2,5-di-t-butyl-1,4-benzohydroquinone (BHQ) on vascular smooth muscle, the different effects of BHQ on rat aorta were investigated under several experimental conditions. In aortic rings at rest or depolarised with 80 mM K+ in the presence of 1 microM nifedipine, BHQ evoked a slow tonic contraction which was antagonised by 1 mM Ni2+. Depolarised rings contracted in response to addition of 1 mM Ca2+, with an EC50 value of 32.4+/-1.0 mM for K+. At 20 mM K+, Ca2+-induced contraction was enhanced by BHQ. This effect was antagonised by 1 mM Ni2+, but not by 1 microM nifedipine. By contrast, at 40, 80 and 128 mM K+, BHQ antagonised Ca2+-induced contraction. This effect was partially reversed by 1 microM methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyri dine-5-carboxylate (Bay K 8644) or by increasing extracellular Ca2+ concentration. In the presence of nifedipine and Ni2+, depolarised rings (80 mM K+) contracted in response to addition of 1 microM phenylephrine; this response was fast and then slowly decreased. When the preparations were preincubated with BHQ, the phenylephrine-induced contraction was transient and antagonised in a concentration-dependent manner by BHQ. These results indicate that the myotonic effect of BHQ on rat aortic rings depends on activation of Ca2+ influx via a Ni2+-sensitive pathway, whereas its myolytic activity is due either to antagonism of Ca2+ entry via L-type Ca2+ channels or depletion of intracellular Ca2+ stores.

Downregulation of nitric oxide synthase in chronic renal insufficiency: role of excess PTH

The available data on the effect of chronic renal failure (CRF) on nitric oxide (NO) metabolism are limited and contradictory. We studied rats with CRF 6 wk after a five-sixths nephrectomy and compared the results with those in the sham-operated controls, felodipine-treated CRF, and parathyroidectomized (CRF-PTX) animals. CRF was produced by surgical resection of the upper and lower thirds of the left kidney, followed by contralateral nephrectomy. We chose this model, as opposed to that produced by renal artery branch ligation, because the latter causes exuberant hypertension (HTN), which independently affects NO metabolism. The CRF group exhibited a mild HTN coupled with elevated basal platelet cytosolic Ca2+ concentration ([Ca2+]i), blunted hypotensive response to L-arginine, decreased hypertensive response to NO synthase (NOS) inhibitor, NG-monomethyl-L-arginine, and normal hypotensive response to NO donor, sodium nitroprusside. This was associated with a significant reduction in urinary excretion of stable NO metabolites (NOX) and depressed NOS activity, as well as endothelial and inducible NO synthase (eNOS and iNOS, respectively) protein contents of thoracic aorta and the remnant kidney in the CRF animals. Calcium channel blockade and PTX lowered blood pressure, increased urinary NOX, and enhanced vascular NOS activity, as well as eNOS and iNOS protein expressions in the tested tissues. Thus CRF animals exhibited significant reductions in vascular NOS activity and eNOS and iNOS expressions. These abnormalities were reversed by calcium channel blockade and PTX, suggesting the possible causal role of CRF-induced dysregulation of [Ca2+]i.

Effects of adrenomedullin and calcitonin gene-related peptide on contractions of the rat aorta and porcine coronary artery

1. Effects of adrenomedullin and alpha-calcitonin gene-related peptide (CGRP) on the contractions and cytosolic Ca2+ concentrations ([Ca2+]i) of the rat aorta and porcine coronary artery were investigated. Characteristics of the receptors mediating the effects of adrenomedullin and alpha-CGRP were also investigated. 2. Adrenomedullin and alpha-CGRP caused a concentration-dependent relaxation in the rat aorta contracted with noradrenaline. The IC50 values for adrenomedullin and alpha-CGRP were 2.4 nM and 4.0 nM, respectively. The relaxant effects of these peptides were abolished by removal of the endothelium and significantly attenuated by an inhibitor of nitric oxide synthase, NG-monomethyl-L-arginine (L-NMMA, 100 microM), but not by a cyclo-oxygenase inhibitor, indomethacin (10 microM). 3. Adrenomedullin and alpha-CGRP increased the endothelial [Ca2+]i in the rat aorta with endothelium, whereas they did not change [Ca2+]i in the smooth muscle. 4. An antagonist of the CGRP1 receptor, CGRP (8-37), antagonized the relaxant effects of alpha-CGRP and the beta-isoform of CGRP (beta-CGRP) but not those of adrenomedullin in the rat aorta. 5. In the porcine coronary artery contracted with U46619, adrenomedullin and alpha-CGRP caused a concentration-dependent relaxation with an IC50 of 27.6 and 4.1 nM, respectively. Removal of the endothelium altered neither the IC50 values nor the maximal relaxations induced by adrenomedullin or alpha-CGRP. When the artery was contracted with high K+ solution (72.7 mM), these peptides caused a small relaxation. 6. Adrenomedullin and alpha-CGRP increased cyclic AMP content and decreased the smooth muscle [Ca2+]i in the porcine coronary artery. 7. CGRP (8-37) significantly antagonized the relaxant effects of adrenomedullin and alpha-CGRP in the porcine coronary artery. However, it had little effect on the relaxations induced by the beta-isoform of CGRP (beta-CGRP). 8. These results suggest that in the rat aorta, adrenomedullin and alpha-CGRP increase the endothelial [Ca2+]i, activate nitric oxide synthase and release nitric oxide, without a direct inhibitory action on smooth muscle. In the porcine coronary artery, in contrast, adrenomedullin and alpha-CGRP directly act on smooth muscle, increase cyclic AMP content, decrease the smooth muscle [Ca2+]i and inhibit contraction. The rat aortic endothelium seems to express the CGRP receptor which is sensitive to alpha-CGRP, beta-CGRP and CGRP (8-37) and the adrenomedullin specific receptor. The porcine coronary smooth muscle, in contrast, seems to express two types of CGRP receptor; one of which is sensitive to alpha-CGRP, CGRP (8-37) and adrenomedullin and the other is sensitive only to beta-CGRP.

Mechanisms of hydrogen peroxide-induced contraction of rat aorta

It has been suggested that reactive oxygen species may be involved in the regulation of vascular tone. However, the underlying mechanisms remain to be elucidated. The present studies were designed to investigate the contractile effects of hydrogen peroxide (H2O2), one of the reactive oxygen species, on isolated ring segments of rat aorta with and without endothelium. H2O2 induced an endothelium-independent contraction in isolated rat aorta ring segments in a concentration-dependent manner at concentrations from 5 x 10(-6) to 5 x 10(-3) M. H2O2-induced contractions of denuded rat aorta rings were stronger than those on intact rat aorta segments. The contractile effects of H2O2 were inhibited completely by 1200 u/ml catalase. The presence of 1.0 microM Fe2+ or 10 microM proadifen, a cytochrome P450 monooxygenase inhibitor, potentiated the contractile effect of H2O2 on isolated rat aorta segments. 1 mM deferoxamine (a Fe2+ chelator) or 100 microM dimethyl sulfoxide (a hydroxyl radical scavenger) significantly attenuated the vessel contractions induced by hydrogen peroxide plus Fe2+ or hydrogen peroxide itself. Removal of extracellular Ca2+ ([Ca2+]0), addition of 5 microM verapamil, administration of a protein kinase C inhibitor (staurosporine), treatment with an inhibitor of protein tyrosine phosphorylation (genistein) or employment of 5.0 microM indomethacin resulted in a significant attenuation of the contractile responses of the vessels to H2O2. Pharmacological antagonists (e.g. a muscarinic acetylcholine receptor antagonist (atropine), an antagonist of histamine H1 receptors (diphenhydramine), an antagonist of histamine H2 receptors (cimetidine), an alpha-adrenoceptor antagonist (phentolamine), a beta-adrenoceptor antagonist (propranolol) and an antagonist of serotonin receptor (methysergide)) did not inhibit or attenuate the contractions induced by H2O2. Exposure of primary aortic smooth muscle cells to H2O2 (5 x 10(-6) to 5 x 10(-3) M) produced significant rises of intracellular Ca2+ ([Ca2+]i) within 20 s. Employment of 1.0 microM Fe2+ markedly enhanced the increment in [Ca2+]i in the smooth muscle cells. 10 microM proadifen treatment failed to alter the hydrogen peroxide-induced increment in [Ca2+]i of the smooth muscle cells. However, the presence of 5 microM indomethacin significantly attenuated the rise in [Ca2+]i in smooth muscle cells. The present results suggest that H2O2 can induce contractions of rat aorta segments, at pathophysiological concentrations, which are Ca2+-dependent. Hydroxyl radicals (.OH), cyclooxygenase products, protein kinase C and products of protein tyrosine phosphorylation appear to play some role in hydrogen peroxide-induced contractions. Metabolites catalyzed by cytochrome P450-dependent enzymes (upon treatment with hydrogen peroxide) appear to exert a vasodilator effect on rat aorta segments. Lastly, some unidentified mediators, produced by a cytochrome P450 inhibitor (proadifen), during hydrogen peroxide treatment, appear to play some role in contraction of vascular smooth muscle of rat aorta segments in vitro.

Protein kinase C mediates increase of Ca2+ sensitivity for contraction by cholinoceptor partial agonist in ileal longitudinal muscle of guinea pig

1. Experiments were designed to study the roles of protein kinase C in carbachol- and pilocarpine-induced contraction and the increase in cytosolic Ca2+ concentration ([Ca2+]i) in guinea pig ileal longitudinal muscle. 2. The protein kinase C inhibitors, GF 109203X (10 microM), calphostin C (10 microM) and H-7 (10 microM), reduced the maximum of the concentration response curve produced by pilocarpine more effectively than that produced by carbachol. 3. The slopes of the regression lines between [Ca2+]i and tension development for pilocarpine and carbachol in tissues treated with GF 109203X were significantly gentler than those for untreated tissues. 4. The protein kinase C alpha- and beta 1 selective inhibitor Goe 6976 (1 microM) decreased both [Ca2+]i and contraction, but did not affect the slopes of the regression lines for pilocarpine and carbachol. 5. These results suggest that protein kinase C (both n- and/or a-type) plays an important role in the increase of Ca2+ sensitivity of the contractile element, and that pilocarpine mainly activates the protein kinase C-dependent pathways for contractile mechanisms in guinea pig ileal longitudinal muscle.

Roles of K+ and Cl- channels in cAMP-induced pulmonary vasodilation

Increase in intracellular adenosine 3', 5'-cyclic monophosphate (cAMP) is a common pathway for many clinically used drugs to cause pulmonary artery (PA) relaxation. Activity of sarcolemmal K+ and Cl(-)-channels is an important determinant of membrane potential (Em), which, in turn, plays a critical role in regulating pulmonary vascular tone. Whether K+ and Cl- channels were involved in cAMP-induced PA relaxation was tested using isolated rat PA rings. Raising extracellular K+ concentration from 20 to 142.7 mM increased the K(+)-evoked contraction, but significantly decreased the relaxation induced by the adenylate cyclase activator, forskolin (FSK, 2.5 microM), suggesting that FSK-induced PA relaxation depended on transmembrane K+ gradient. Indeed, the FSK-induced relaxation was inhibited by 4-aminopyridine (4-AP, 10 mM), a voltage-gated K+ (Kv) channel blocker. Neither the Ca(2+)-activated K+ channel blocker, charybdotoxin, nor the ATP-sensitive K+ channel blocker, glibenclamide, had this effect. Furthermore, reducing extracellular Cl- concentration from 142.7 to 50 mM significantly decreased the FSK-induced relaxation in PA rings precontracted with 142.7 mM K+ (Ek approximately 0 mV), but negligibly affected the evoked contraction. This indicates that transmembrane Cl- gradient also regulates FSK-induced PA relaxation. Indeed, the Cl- channel blocker, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 10 microM), significantly inhibited the FSK-induced relaxation in PA rings preconstricted by 142.7 mM K+. In summary, the data suggest that the cAMP-induced PA relaxation is attributable, at least partly, to both activation of the 4-AP-sensitive Kv channels and stimulation of the NPPB-sensitive Cl- channels.

Effects of chronic hypoxia on Ca2+ mobilization and Ca2+ sensitivity of myofilaments in uterine arteries

The effect of chronic hypoxia on free intracellular Ca2+ concentration ([Ca2+]i) and Ca2+ sensitivity of myofilaments during agonist stimulation was examined in uterine arteries obtained from normoxic and chronically hypoxic pregnant sheep maintained at high altitude (3,820 m) for approximately 110 days. Smooth muscle [Ca2+]i was measured simultaneously with muscle contraction in the same intact tissue. Whereas both KCl and 5-HT increased [Ca2+]i and tension simultaneously in the uterine artery, 5-HT produced significantly greater contractile tension (in g) than KCl at a given amount of [Ca2+]i as indicated by the ratio of fura 2 fluorescence intensity induced by excitation at 340 nm to that induced at 380 nm (29.8 +/- 6.9 vs. 16.9 +/- 4.0, P < 0.05). Chronic hypoxia did not change KCl-induced contractions, nor did it affect KCl-mediated increases in [Ca2+]i. In contrast, chronic hypoxia significantly inhibited 5-HT-induced contractions and decreased the 5-HT-stimulated increase in [Ca2+]i (pD2 7.46 +/- 0.18-->6.86 +/- 0.11, P < 0.05, where pD2 is -log half-maximal effective concentration) in uterine arteries. In addition, the slope (g tension/nM [Ca2+]i) of the 5-HT-mediated [Ca2+]i-tension relationship was significantly decreased in chronically hypoxic arteries (0.024 +/- 0.002-->0.013 +/- 0.001, P < 0.01). The results suggest that chronic hypoxia suppresses agonist-mediated Ca2+ homeostasis in uterine arteries by inhibiting Ca2+ mobilization and the agonist-enhanced Ca2+ sensitivity of myofilaments.

Inhibition of muscarinic receptor-operated Ca2+ sensitization by short-term desensitization of alpha-toxin-permeabilized smooth muscle cells from guinea pig stomach

1. Isolated single smooth muscle cells from the guinea pig stomach were permeabilized with Staphylococcus aureus alpha-toxin. 2. The permeabilized single cells showed a shortening in response to Ca2+ in an all-or-none manner. Moreover, the addition of acetylcholine (ACh) or guanosine 5'-triphosphate (GTP) resulted in a decrease in concentration of Ca2+ required to trigger a threshold response, suggesting that Ca2+ sensitization is induced by the stimulation of muscarinic acetylcholine receptors (mAChRs) or GTP-binding protein(s). 3. Short-term desensitization was induced by incubating the permeabilized cells with 100 microM ACh for 10 min. 4. In desensitized cells, the concentration of Ca2+ required to trigger a threshold response in the presence of ACh was increased, however, the cell shortening in response to Ca2+ in the absence of ACh and GTP-induced Ca2+ sensitization was not affected by short-term desensitization. 5. These results suggest that the receptor-operated augmentation of Ca2+ sensitivity is inhibited by short-term desensitization and that the development of short-term desensitization is due to an uncoupling of mAChR/GTP-binding protein(s).

Effects of hypoxia on [Ca2+]i, pHi and myosin light chain phosphorylation in guinea-pig taenia caeci

1. Hypoxia (achieved by bubbling with N2 instead of O2) reduces the force of a KCl (40 mM)-induced contracture to approximately 10% of the control value in guinea-pig taenia caeci. The underlying mechanism of this relaxation in response to hypoxia was investigated by measuring the major cell signalling parameters, intracellular Ca2+ concentration ([Ca2+]i) and myosin regulatory light chain (LC20) phosphorylation (MLC-P1), as well as intracellular pH (pHi), a factor often suggested to mediate hypoxic relaxation of muscle. 2. [Ca2+]i, measured using the ratiometric fluorescent dye fura-2, increased when 40 mM KCl was added to physiological saline solution (PSS) (peak value assigned 100%), and the steady state after 15 min was 92.8%. There were no detectable decreases in [Ca2+]i during hypoxia. 3. MLC-Pi, measured using isoelectric focusing-polyacrylamide gel electrophoresis and identified using Western blotting, increased from 9% of the total LC20 in Ca(2+)-free PSS to a peak value of 51% in 40 mM KCl-PSS. The steady-state value in hypoxia of 43% was not significantly different from that in control oxygenated conditions at the same point in time. 4. pHi, measured using the ratiometric fluorescent dye 2',7'-bis(carboxyethyl)-5(6)-carboxy-fluorescein (BCECF), under quiescent conditions (Ca(2+)-free PSS) was 7.23 and increased to 7.36 with 40 mM KCl. After imposition of hypoxia pHi remained unchanged despite the known increase in both lactate content and production. 5. As [Ca2+]i and MLC-Pi, key factors in activation, were not decreased by hypoxia and changes in pHi were minor, hypoxic relaxation in guinea-pig taenia caeci appears to be directly related to energy limitation rather than any oxygen-sensing mechanism.

Role of endothelium and nitric oxide in histamine-induced responses in human cranial arteries and detection of mRNA encoding H1- and H2-receptors by RT-PCR

1. Histamine induces relaxation of human cranial arteries. Studies have revealed that the relaxant histamine H1-receptor predominates in human cerebral and the H2-receptor in temporal arteries, while H1- and H2-receptors are of equal importance in the middle meningeal artery. The purpose of the present study was to examine the role of the endothelium and nitric oxide in histamine-induced responses and to show the presence of mRNA encoding H1- and H2-receptors in human cranial arteries. 2. Electrophoresis of polymerase chain reaction (PCR) products from human cerebral, middle meningeal and temporal arteries, demonstrated products corresponding to mRNA encoding both H1- and H2-receptors in arteries with and without endothelium. The amplified PCR products were sequenced and showed 100% homology with the published sequences of these histamine receptors. 3. A sensitive in vitro system was used to study vasomotor responses to histamine. In precontracted cerebral, middle meningeal and temporal arteries with and without endothelium, histamine caused a concentration-dependent relaxation with Imax values between 87% and 81% and pIC50 values between 8.14 and 7.15. In arteries without endothelium the histamine-induced relaxation was significantly less potent (Imax values between 87% and 66% and pIC50 values between 7.01 and 6.67) than in cranial arteries with an intact endothelium. 4. This addition of histamine to arteries without endothelium and pretreated with the histamine H2-antagonist, cimetidine (10(-5) M), caused a concentration-dependent contraction of the cranial arteries with Emax values between 86% and 29% and pEC50 values between 7.53 and 6.77. This contraction was blocked by the histamine H1-receptor antagonist, mepyramine (10(-7) M), and even turned into a relaxation with Imax values between 84% and 14% and pIC50 values between 7.42 and 5.86. 5. The nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 3 x 10(-5) M) significantly inhibited the relaxant response to histamine in cerebral and temporal arteries (pIC50 values between 7.43 and 7.13). The combined treatment with L-NAME (3 x 10(-5) M) and cimetidine (10(-5) M) caused a further displacement of the concentration-response curve (pIC50 values between 7.14 and 6.57) and decreased the maximum relaxant responses in all three cranial arteries (Imax values between 62% and 39%). 6. In conclusion, this is the first study which show mRNA encoding histamine H1- and H2-receptors in human cranial arteries. The results indicate that histamine-induced relaxation of human cranial arteries is partially mediated via an endothelial H1-receptor coupled to the production of nitric oxide and partially via a H2-receptor associated with the smooth muscle cells. In addition, there is evidence for a contractile H1-receptor in the smooth muscle cells in these arteries.

Endothelium inhibits the palytoxin-induced depolarization and Ca2+ mobilization in porcine coronary artery through endothelium-derived hyperpolarizing factor and nitric oxide released by palytoxin

Palytoxin induced increases in cytosolic Ca2+ and tension, which were dependent on external Ca2+, and depolarized the membrane in endothelium-denuded porcine coronary arteries. When the endothelium was present, however, these effects were greatly inhibited, suggesting that some factors from endothelium inhibited the palytoxin-actions. Pretreatment with 100 microM N omega-nitro-L-arginine partially reversed the inhibitory effect of endothelium on the Ca2+ movement and the contraction but not that on the depolarization. Pretreatment with 10 microM indomethacin did not affect the inhibition. These results suggest that palytoxin released both nitric oxide and endothelium-derived hyperpolarizing factor (EDHF) from the endothelium, both of which counteracted the actions of palytoxin on smooth muscle cells. It is thought that the palytoxin-induced depolarization was attenuated by hyperpolarization due to EDHF.

Characterization of sodium-calcium exchange in rabbit renal arterioles

Experiments were performed to test the hypothesis that renal arterioles exhibit Na-Ca exchange capability and that this process is regulated by protein kinase C (PKC). Glomeruli with attached arterioles were dissected from rabbit kidney and loaded with fura-2 for measurement of intracellular calcium concentration ([Ca2+]i) using microscope-based photometry. In tissue bathed in Ringer's solution containing 150 mM Na+ and 1.5 mM Ca2+, afferent and efferent arteriolar [Ca2+]i averaged 136 +/- 6 and 154 +/- 7 nM, respectively. Removal of extracellular Na+ increased afferent arteriolar [Ca2+]i by 70 +/- 7 mM, while efferent arteriolar [Ca2+]i only increased by 39 +/- 5 nM (P < 0.01 vs. afferent arteriole). These responses were inhibited by 6 nM Ni2+ and required extracellular Ca2+, but were unaffected by 10 microM diltiazem. After incubation in 500 microM ouabain, 5 microM monensin, and 5 microM nigericin, [Ca2+]i responses to removal of extracellular Na+ were exaggerated significantly, averaging 174 +/- 50 nM in afferent arterioles and 222 +/- 82 nM in efferent arterioles (NS vs. afferent arterioles). Moreover, responses to removal of extracellular Na+ were enhanced by 100 nM phorbol 12-myristate 13-acetate, an affect which was blocked by PKC inhibition (25 nM K252b). These data indicate that both afferent and efferent arterioles express the Na-Ca exchanger, and that PKC activity impacts on exchange capacity in these vessels.

Increased inhibitory effect of phorbol ester on cytosolic Ca2+ level and contraction in rat myometrium after gestation

Activation of voltage-dependent Ca2+ channels by high K+ (40 mM) increased the cytosolic Ca2+ level ([Ca2+]i) (estimated by fura-PE3 fluorescence ratio) and force in myometrium isolated from pregnant (21 days after gestation) and non-pregnant (estrus) rats. 12-Deoxyphorbol 13-isobutyrate (DPB, 1 mM) decreased the high (K+)-stimulated [Ca2+]i and force in a concentration-dependent manner. The inhibitory effect was stronger in the pregnant myometrium than in the non-pregnant myometrium. In the pregnant myometrium, the increase in Ca2+ permeability by ionomycin (1 microM) greatly increased [Ca2+]i and force, which were only partially inhibited by verapamil (10 microM). DPB (1 microM) inhibited the verapamil-insensitive component of the increases in [Ca2+]i and muscle tension. Oxytocin (100 nM) and thapsigargin (1 microM) also induced a verapamil-insensitive increase in [Ca2+]i and force, and DPB (1 microM) inhibited these increments. Ca2+ sensitivity of contractile elements, estimated from the relationships between Ca2+ and muscle force in intact and alpha-toxin permeabilized muscle, was not significantly changed by DPB (1 microM). In summary, DPB inhibits the increase in [Ca2+]i more strongly in myometrium isolated from pregnant rats than that from non-pregnant rats without any change in the [Ca2+]i/tension relationship. Since DPB decreased [Ca2+]i-rise induced by three different mechanisms, DPB may activate Ca2+ extrusion, rather than to inhibit a specific influx pathway, to decrease [Ca2+]i.

The effects of halothane on arginine-vasopressin-induced Ca2+ mobilization from the intracellular stores and the receptor-mediated Ca2+ entry from the extracellular space in single cultured smooth muscle cells of rat aorta

Halothane has a direct action on vascular smooth muscle cells and causes relaxation of these cells, yet neither the mechanism nor the site of its action is completely understood. Using digital imaging microscopy with the Ca2+ indicator fura-2, the effects of halothane on the intracellular [Ca2+] dynamics induced by arginine vasopressin (AVP) in the perinuclear region and cytosol in single cultured smooth muscle cells of rat aorta were studied. Changes in intracellular [Ca2+] were expressed as percent increases in the ratios of fluorescence intensity at 500 nm excited by 340 nm and 380 nm. AVP (10(-7) M) elicited an initial transient increase in [Ca2+] in the perinuclear region higher than that in the cytosol in Ca(2+)-containing solution (346% +/- 21% and 213% +/- 22%, respectively). Halothane, 0.5%, attenuated the [Ca2+] increase induced by AVP in the perinuclear region and cytosol, and halothane, 1.0% and 2.0%, abolished the differential increase. Under the continuous application of AVP (10(-7) M), Ca2+ restoration in the medium after perfusion with Ca(2+)-free solution increased the perinuclear [Ca2+] more than the cytosolic [Ca2+]. Both were significantly attenuated by 2.0% halothane, but not by nicardipine (10(-5) M) or ryanodine (10(-6) M). Our results suggest that halothane may attenuate the Ca2+ release from the intracellular Ca2+ stores more than the receptor-mediated Ca2+ entry from the extracellular space in the AVP-induced response in these cells.

Effects of cyclopiazonic acid and ryanodine on cytosolic calcium and contraction in vascular smooth muscle

1. In smooth muscle, both Ca2+ release from the sarcoplasmic reticulum (SR) and Ca2+ influx across the plasma membrane are responsible for the increase in the cytosolic Ca2+ level ([Ca2+]i). To understand further the role of SR on smooth muscle contraction, the effects of an inhibitor of the SR Ca2+ pump, cyclopiazonic acid (CPA 10 microM), an inhibitor of the Ca(2+) -induced Ca2+ release, ryanodine, (10 microM), and an activator of the Ca(2+) -induced Ca2+ release, caffeine (20 mM), on [Ca2+]i and contractile force were examined in the ferret portal vein loaded with a photoprotein, aequorin. 2. CPA induced a small increase in the aequorin signal reaching a maximum at 7 min. Several minutes after the increase in the aequorin signal, muscle tension increased reaching a maximum at 21.5 min. In contrast, ryanodine changed neither the aequorin signal nor contraction. In the presence of ryanodine, caffeine induced a sustained increase in the aequorin signal and transient contraction. After washing ryanodine and caffeine, the aequorin signal and muscle tone returned to their respective control levels. After treatment with ryanodine and caffeine, the second addition of caffeine was almost ineffective whereas CPA still increased the aequorin signal and muscle tension. 3. In the presence of external Ca2+, noradrenaline (NA, 10 microM) induced a transient increase followed by a sustained increase in the aequorin signal and sustained contraction. In contrast, KCl (70 mM) induced sustained increases in the aequorin signal and sustained contraction. In Ca(2+) -free solution, NA induced a small transient increase in the aequorin signal and a small transient contraction. These changes were inhibited in the presence of CPA or on pretreatment of the muscle with ryanodine and caffeine. These results suggest that CPA or ryanodine and caffeine depleted Ca2+ in SR. High K+ was ineffective in the absence of external Ca2+. 4. In the presence of external Ca2+ and CPA, NA and high K+ induced larger aequorin signals than in the absence of CPA, whereas the magnitude and shape of the contractions did not change. In contrast, pretreatment with ryanodine and caffeine did not have such an effect. In the muscle pretreated with ryanodine and caffeine, CPA changed the responses to high K+ and NA in a similar manner to that in the muscle without the pretreatment with ryanodine and caffeine. 5. Dissociation of contraction from [Ca2+]i as measured with aequorin suggests that NA and high K+ increase Ca2+ in two compartments: a compartment containing contractile elements (contractile compartment) and another compartment unrelated to contractile elements (non-contractile compartment). Because CPA augmented the stimulant-induced increase in aequorin signal without changing contraction, the non-contractile compartment may be located near the SR and the CPA-sensitive SR Ca2+ pump may regulate the Ca2+ level in this compartment. However, because CPA changed neither the magnitude nor shape of the contractions in the presence of external Ca2+, the SR Ca2+ pump may have little effect on regulation of Ca2+ level in the contractile compartment. Furthermore, the release of Ca2+ from SR seems to have little effect on the increase in the contractile Ca2+ because ryanodine and caffeine changed neither the aequorin signals nor contractions induced by NA and high K+ in the presence of external Ca2+ in the ferret portal vein.

Inhibitory effect of phorbol 12,13-dibutyrate on norepinephrine-induced contraction in rabbit iris dilator muscle

The hypothesis that the increase in Ca2+ sensitivity on norepinephrine-induced contraction of smooth muscles and also the decrease of the norepinephrine-induced sustained level of intracellular Ca2+ concentration are produced by the activation of protein kinase C was tested. Phorbol 12,13-dibutyrate (PDB; 10(-6) M) relaxed the norepinephrine-induced sustained contraction in a concentration-dependent manner. On pretreatment with PDB a transient contraction was produced by the application of norepinephrine, but the sustained contraction was significantly reduced. The sustained elevations of intracellular Ca2+ concentration ([Ca2+]i) and the contraction induced by norepinephrine in fura-2-loaded preparations were decreased by the application of PDB. These inhibitory effects were antagonized by potent protein kinase inhibitors, 2-(1-(3-dimethylaminopropyl)-indol-3-yl)-3-(-indol-3-yl)-maleimide (GF 109203X) (10 (-6) M) and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) (10 (-6) M), but were not affected by a protein kinase A/G inhibitor, N-(2-cinnamylaminoethyl)-5-isoquinolinesulfonamide (H-88) (10(-6) M). The slope of the regression line for norepinephrine for [Ca2+]i and tension was significantly steeper than those obtained with high K+. Also, on pretreatment with PDB the Ca2+ sensitivity of the K(+)-induced contraction was decreased, but the Ca2+ sensitivity of norepinephrine-induced contraction tended to be increased. These observations indicate that PDB induces a decrease of [Ca2+]i on Ca2+ mobility and an increase of Ca2+ sensitivity on contraction of smooth muscle through the activation of protein kinase C.

Effects of a prostaglandin I2 analog iloprost on cytoplasmic Ca2+ levels and muscle contraction in isolated guinea pig aorta

In the isolated guinea pig aorta, the prostaglandin I2 analog iloprost (0.01-10 microM) inhibited the contractions induced by the thromboxane A2 analog U46619 (9,11-dideoxy-11 alpha,9 alpha-epoxymethanoprostaglandin F2 alpha; 30 nM) and prostaglandin F2 alpha (PGF2 alpha, 1 microM) in a concentration-dependent manner. In contrast, iloprost only partially inhibited the high K+ (65.4 mM)-induced contraction. In the muscle stimulated with high K+, verapamil (0.3 and 10 microM) inhibited [Ca2+]i and muscle tension in parallel, whereas iloprost (1 microM) inhibited muscle tension with only a small decrease in [Ca2+]i. In the muscle stimulated with U46619 (30 nM), verapamil and iloprost decreased both [Ca2+]i and muscle tension. However, as compared with the effect of verapamil, iloprost more strongly inhibited muscle tension than [Ca2+]i. The iloprost (0.1-1 microM)-induced relaxation was accompanied by a concentration-dependent increase in cAMP content. It was further demonstrated that inhibition of the U46619-contractions was augmented in the presence of cycloxygenase inhibitors, such as indomethacin (10 microM), ibuprofen (10 microM) and aspirin (10 microM). In contrast, the inhibition of PGF2 alpha-induced contraction was not affected by indomethacin. Similarly, the inhibitory effect of forskolin on U46619-induced contractions, but not on PGF2 alpha-induced contraction, was enhanced by indomethacin. These results suggest that iloprost inhibits vascular smooth muscle contraction by decreasing [Ca2+]i and the Ca2+ sensitivity of contractile elements through a cAMP-dependent mechanism. The results also suggest that in U46619-stimulated muscle, vasoactive prostaglandins that counterbalance the relaxing action of cAMP may be generated.

Dissociation of P2 purinoceptor-mediated increase in intracellular Ca2+ level from myosin light chain phosphorylation and contraction in rat aorta

1. The effects of P2 agonists, adenosine-5'-triphosphate (ATP), alpha, beta-methylene-adenosine-5'-triphosphate (alpha, beta-me-ATP) and adenosine 5-O-(3-thiotriphosphate) (ATP gamma S), on the intracellular free Ca2+ level ([Ca2+]i), myosin light chain (MLC) phosphorylation and force of contraction were examined in vascular smooth muscle of rat aorta. 2. ATP (0.1 microM-1 mM), alpha, beta-me-ATP (0.1-100 microM) and ATP gamma S (1-100 microM) induced transient increases followed by sustained increase in [Ca2+]i. The effects of these agonists were concentration-dependent. Compared with the effects of a high concentration of KCl (17.5-72.4 mM), the contractions induced by these P2 purinoceptor agonists were smaller at a given [Ca2+]i. 3. In the absence of extracellular Ca2+ (with 0.5 mM EGTA), ATP gamma S (10 microM) induced large transient increase in [Ca2+]i with only small contraction in Ca(2+)-free solution. In contrast, alpha, beta-me-ATP (10 microM) induced only a very small increase in [Ca2+]i and contraction. 4. ATP (1 mM), alpha, beta-me-ATP (10 microM) and ATP gamma S (10 microM), added during stimulation with 0.1 microM noradrenaline, induced additional and transient increases in [Ca2+]i which were also not associated with contraction. 5. High K+ (72.4 mM) increased MLC phosphorylation with a similar time course to that of the increase in [Ca2+]i (peak phosphorylation was 56% when [Ca2+]i increased to 100%). In contrast, the time course of the increase in MLC phosphorylation due to ATP (1 mM) did not coincide with that of the large increases in [Ca2+]i; MLC phosphorylation increased to only 31% when [Ca2+]i increased to 163%. The MLC phosphorylation due to alpha, beta-me-ATP (10 microM) and ATP gamma S (10 microM), measured at peak [Ca2+]i, were only 19% and 14%, respectively, irrespective of a large increase in [Ca2+]i (138% and 188%, respectively). 6. The absence of a clear relationship between P2-purinoceptor-mediated increase in [Ca2+]i (either by Ca2+ influx or Ca2+ release) and MLC phosphorylation or force generation appears to imply that elevation in [Ca2+]i does not contribute to these responses.

The mechanism of acidic pH-induced contraction in aortae from SHR and WKY rats enhanced by increasing blood pressure

1. Effect of pH on vascular smooth muscle contraction was analyzed by use of biochemical and pharmacological techniques. 2. In the aorta isolated from spontaneously hypertensive rats (SHR) decreasing extracellular pH (pH0) caused a rapid acidification of intracellular pH accompanied by a pH0-dependent increase in tension. The contraction of the SHR aorta was remarkable compared with that of the Wistar Kyoto rat (WKY) aorta. 3. Removal of NH4Cl caused a transient decrease in intracellular pH followed by a marked increase in tension. 4. Both contraction and intracellular Ca2+ mobilization induced by acidic pH0 were markedly inhibited by removal of extracellular Ca2+, verapamil and adenosine, whereas these were not affected by tetrodotoxin or Gd3+, a stretch-activated cation channel blocker. Furthermore, cromakalim (a K+ channel opener) inhibited acidic pH0-induced contraction (APIC). 5. Acidic pH0 induced a depolarization of cultured smooth muscle cells from SHR aorta. 6. Blood pressure elevated with increasing age of WKY and SHR accompanied by an increase in APIC. Feeding WKY with NG-nitro-L-arginine, an inhibitor of nitric oxide synthases caused a marked elevation of their blood pressure followed by an increase in APIC. 7. These results suggest that APIC is caused by Ca2+ influx mediated through the activation of voltage-sensitive Ca2+ channels mainly due to acidic pH0-induced depolarization of the plasma membrane of smooth muscle cells. It is also suggested that APIC is strengthened by the elevation of blood pressure.

Effects of flavonoids on rat aortic smooth muscle contractility: structure-activity relationships

1. Flavonoids produced a concentration-dependent relaxation of the contractile responses induced by noradrenaline, KCl, or phorbol 12-myristate-13-acetate in rat aortic rings. Only the flavonoid with three contiguous hydroxyls in B rings (myricetin), at low concentrations, potentiates the contractions evoked by these agonists. 2. The relaxant effects of flavanone on the noradrenaline-induced contractions were potentiated by isoprenaline and those of morin, chrysin, flavanone, and naringenin by sodium nitroprusside. 3. Several mechanisms are implicated in the vasodilatory effects of flavonoids: inhibition of protein kinase C; inhibition of cyclic nucleotide phosphodiesterases; and/or decreased Ca2+ uptake.

Decreased vascular reactivity of portal vein in rats with portal hypertension

BACKGROUND/AIMS

Vascular hyporesponsiveness in portal hypertension is well documented in the arterial tissue. The present study aimed to investigate the possible changes in the portal vein from portal hypertensive rats.

METHODS

Portal hypertension was induced by partial portal vein ligation. Fourteen days after surgery, portal veins were removed for contractile study and measurement of cAMP, cGMP and [Ca2+]i.

RESULTS

In vitro tension preparation showed a decreased maximum response to norepinephrine in portal vein of portal vein ligated rats (38.3 +/- 4.1 vs. 23.4 +/- 1.5 mN/mm2). The pA2 values of WB4101 and yohimbine (alpha1- and alpha2-adrenoceptor antagonist, respectively) were not different between the two groups. Tissue cAMP (14.4 +/- 0.9 vs. 12.2 +/- 0.7 pmol/mg protein), but not cGMP, content was increased and intracellular calcium [Ca2+]i levels (247 +/- 9 vs. 281 +/- 13 nM) were decreased in portal vein ligated rats.

CONCLUSIONS

These results showed that in portal vein from portal vein ligated rats, vascular hyporesponsiveness and an increase in basal cAMP content and a decrease in basal [Ca2+]i were observed.

Calcium compartments in vascular smooth muscle cells as detected by aequorin signal

1. To examine whether cytosolic Ca2+ in smooth muscle cells distributes evenly, cytosolic Ca2+ levels were measured with two different Ca2+ indicators in the ferret isolated portal vein; a fluorescent indicator, fura-PE3, that shows the average Ca2+ level, and a photoprotein, aequorin, that preferentially shows a high Ca2+ compartment. 2. A noradrenaline (10 microM)-induced sustained contraction was associated with a sustained increase in the fura-PE3 signal, or a transient increase followed by small sustained increase in the aequorin signal. A high K(+)-induced contraction was associated with a sustained increase in both the fura-PE3 and aequorin signals. 3. A second application of noradrenaline or high K+ induced reproducible contractions and fura-PE3 signals. In contrast, the aequorin signal resulting from a second application of noradrenaline or high K+ was much smaller than the first signal. 4. Following a 13 h but not a 3 h resting period, the aequorin signal stimulated by noradrenaline or high K+ recovered, without any change in the contractile response. 5. In Ca(2+)-free solution, high K+ was ineffective, whereas noradrenaline induced only a small aequorin signal and contraction compared to those obtained in the presence of external Ca2+. After the addition of Ca2+, the first application of noradrenaline induced a large aequorin signal and a large contraction, although a second application induced a much smaller aequorin signal accompanied by a large contraction. 6. These results suggest that high K+ and noradrenaline increase Ca2+ in at least two cytosolic compartments; a compartment that is coupled to the contractile mechanism ('contractile' Ca2+ compartment; major portion of cytoplasm containing contractile elements) and a compartment that is not coupled to contractile mechanisms ('non-contractile' Ca2+ compartment; small sub-membrane area that does not contain contractile elements). On stimulation, the Ca2+ level in the 'contractile' compartment may increase to a level high enough to stimulate myosin light chain kinase but not so high as to consume aequorin rapidly. In contrast, the Ca2+ level in the 'non-contractile' compartment may increase so greatly that aequorin in this compartment is rapidly consumed. These two compartments may be separated by a diffusion barrier and, during a resting period, aequorin may slowly diffuse from the 'contractile' compartment to the 'non-contractile' compartment and thus restore the full aequorin signal. An increase in Ca2+ in the 'non-contractile' compartment seems to be dependent mainly on Ca2+ influx and partly on Ca2+ release.

Intraplatelet free calcium and calcium-regulating hormones in plasma are not related to the antihypertensive effect of nifedipine in hypertensive pregnancy

Intracellular free calcium regulates contraction-relaxation processes in vascular smooth muscle. We compared intraplatelet free calcium ([Ca2+]i) and pH ([pH]i) in hypertensive pregnant women to those in normotensive pregnant and non-pregnant women. Plasma parathormone and vitamin D metabolite were simultaneously assessed. In hypertensive pregnancy, [Ca2+]i tended to be lower than in normotensive pregnant (P = 0.08) and non-pregnant subjects (P = 0.06). In hypertensive pregnancy, 1,25, (OH)2 vitamin D in plasma was in the same range as in non-pregnant women and significantly lower than in normotensive pregnancy (p < 0.01). The other two vitamin D metabolites, parathormone and [pH]i were equal in the three groups. A five-day nifedipine treatment (10 mg t.i.d.) increased [Ca2+]i in hypertensive pregnant (P < 0.05) and normotensive non-pregnant subjects (P = 0.06), whereas [pH]i (P < 0.05) and 25 (OH) vitamin D (P < 0.05) decreased in the former and 24,25 (OH)2 vitamin D increased in the latter group (P < 0.05). Initial [Ca2+]i did not correlate with blood pressure in any group. The antihypertensive effect of nifedipine did not correlate with any variable measured. In conclusion, [Ca2+]i and calcium-regulating hormones seem not to be related to the antihypertensive effect of nifedipine in hypertensive pregnancy. In this type of hypertension, intraplatelet calcium may not reflect calcium balance in smooth muscle cells regulating vascular tone and blood pressure.

Focal [Ca2+]i increases detected by aequorin but not by fura-2 in histamine- and caffeine-stimulated swine carotid artery

1. We hypothesized that the homogeneity of intracellular [Ca2+] ([Ca2+]i) varies and is regulated in arterial smooth muscle. 2. We evaluated this hypothesis by exploiting the different characteristics of several [Ca2+]i indicators: (1) aequorin, which theoretically can measure focal increases in [Ca2+]i, (2) fura-2, which is predominantly a measure of mean cytoplasmic [Ca2+], and (3) myosin light chain phosphorylation and force, which reflect increases in [Ca2+] near the contractile apparatus. 3. From the differences in the observed aequorin and fura-2 signals, we developed an index of the relative degree of [Ca2+]i homogeneity as the ratio of the aequorin signal and fura-2 signal. 4. Stimulation with intermediate concentrations of histamine (1 and 10 microM) or high [K+]o (25 and 40 mM) increased [Ca2+]i and contractile stress. Relative [Ca2+]i homogeneity, estimated from the aequorin/fura-2 ratio, remained similar to levels observed in unstimulated tissues. 5. Higher concentrations of histamine (100 microM) also increased [Ca2+]i and stress, but the aequorin/fura 2 ratio declined , indicating increased [Ca2+]i homogeneity. Similarly, the aequorin/fura-2 ratio decreased when extracellular Ca2+ was removed. 6. Stimulation with histamine in low extracellular [Ca2+] transiently increased [Ca2+]i and the aequorin/fura-2 ratio. Similarly, in tissues treated with low extracellular [Ca2+], restoration of extracellular Ca2+ transiently increased both [Ca2+]i and the aequorin/fura-2 ratio. Although both of these experiments demonstrated a transient decrease in [Ca2+]i homogeneity, only histamine stimulation led to increased myosin light chain phosphorylation and force. These results indicate that the focal increases in [Ca2+]i observed with histamine stimulation and Ca2+ restoration occurred in different cellular regions. 7. Addition of caffeine (20 mM) increased [Ca2+]i and [cAMP], but this was not accompanied by sustained increased myosin light chain phosphorylation or contraction. Phosphorylation of myosin light chain kinase did not appear to underlie the lack of increase in myosin light chain phosphorylation. Rather, caffeine induced a sustained increase in the aequorin/fura-2 ratio, suggesting that caffeine inhibits smooth muscle contraction by localizing increases in [Ca2+]i to a region distant from the contractile apparatus. 8. These data suggest that there can be transient and sustained focal increases in [Ca2+]i. Aequorin detected increased [Ca2+]i in small regions of the cytoplasm during release from and refilling of the intracellular Ca2+ store and with caffeine stimulation. Dual use of aequorin and fura-2 permits determination of relative [Ca2+]i homogeneity in smooth muscle.

Comparative study of elgodipine and nisoldipine on the contractile responses of various isolated blood vessels

The effects of elgodipine, a new dihydropyridine derivative, were compared to those of nisoldipine on contractile responses in various isolated artery rings and on mechanical activity in portal vein segments. Arteries used were: rabbit aorta, mesenteric (fifth branch), femoral and basilar, and sheep coronary arteries. Elgodipine and nisoldipine (10(-16)-3 x 10(-6) M) produced a concentration-dependent inhibition of the contractile responses induced by high K+ (80 mM), 5-hydroxytryptamine (10(-5) M) or noradrenaline (10(-6) M or 10(-4) M) in all the arteries studied. The inhibitory effect of elgodipine was greater in mesenteric resistance vessels (IC50 = 8.0 +/- 2.1 x 10(-12) M and 2.0 +/- 0.5 x 10(-13) M for the depression of high K(+)- and agonist-induced contraction, respectively), and in coronary arteries (IC50 = 2.6 +/- 0.3 x 10(-10) M and 9.0 +/- 1.4 x 10(-8) M for the inhibition of high K(+)- and agonist-induced contraction, respectively). In addition, the action of elgodipine in peripheral resistance vessels and in the coronary artery was more prominent than in aorta or femoral arteries, and this tissue selectivity was more apparent for elgodipine than for nisoldipine. In rat portal vein elgodipine (IC50 = 6.5 +/- 0.9 x 10(-8) M) and nisoldipine (IC50 = 8.5 +/- 1.3 x 10(-8) M) reduced in a concentration-dependent manner the development of mechanical activity. Furthermore, contractile responses produced by the addition of Ca2+ (1-5 mM) to Ca(2+)-free high K+ solution were also concentration dependently inhibited by elgodipine. However, elgodipine did not modify noradrenaline-induced contractions attributed to intracellular Ca2+ release. The results of this study indicate that elgodipine has potent vasodilator properties and vascular selectivity. The mechanisms through which elgodipine relaxes vascular smooth fibres seem to be related to its ability to inhibit the entry of extracellular Ca2+ into the cell.

Increased sensitivity of rat myometrium to the contractile effect of platelet activating factor before delivery

1. The contractile effects of platelet activating factor (PAF) were compared in the myometrium isolated from non-pregnant and pregnant rats. 2. In the non-pregnant myometrium, PAF, at a concentration of 0.1 microM, did not change muscle tension and induced only a small transient contraction at 10 microM. 3. The contractile responses to PAF increased with the progress of gestation. In the late pregnant myometrium (21 day after gestation), PAF (0.1 nM-10 microM) induced large and relatively sustained contractions. The threshold concentration of PAF was decreased by approximately 10,000 times and the maximum contraction was increased 5 times by day 21 of gestation. 4. PAF (10 microM) increased the cytosolic Ca2+ concentration ([Ca2+]i) and muscle contraction to levels higher than those induced by high K+ in the pregnant rat myometrium (day 21). Verapamil (10 microM), a voltage-dependent Ca2+ channel blocker, decreased the stimulated [Ca2+]i and muscle tension to 49.6% and 22.7%, respectively, while the same concentration of verapamil completely inhibited the high K(+)-induced responses. 5. PAF (10 microM) induced a transient increase in [Ca2+]i with no contraction in the absence of external Ca2+ in the pregnant myometrium (day 21). 6. These results suggest that PAF induces contraction in rat myometrium by increasing Ca2+ influx. Although PAF released Ca2+ from stored sites, this Ca2+ does not seem to contribute to the PAF-induced contraction. Our major finding is that the sensitivity of the myometrium to PAF increased after gestation and that this may play a role in delivery.

Involvement of botulinum C3-sensitive GTP-binding proteins in alpha 1-adrenoceptor subtypes mediating Ca(2+)-sensitization

The mechanisms of alpha 1-adrenoceptor agonist-mediated sensitization of the contractile apparatus of smooth muscle to Ca2+ were studied in beta-escin-permeabilized thoracic arterial smooth muscle of rabbit. Addition of norepinephrine (10 microM) plus guanosine 5'-triphosphate (GTP, 50 microM) significantly enhanced Ca2+ sensitivity as compared with the addition of 0.3 microM Ca2+ alone (pCa6.5). In beta-escin-skinned smooth muscle of chloroethylclonidine-treated tissues, the enhancement of Ca(2+)-contraction produced by norepinephrine or clonidine was completely inhibited by guanosine 5'-O-(beta-thiodiphosphate) (GDP beta-S, 1 mM). In addition, Clostridium botulinum C3, which inactivates low molecular weight GTP-binding protein families, abolished norepinephrine- or clonidine-induced Ca(2+)-sensitization, but did not affect clonidine-induced translocation of protein kinase C to the membrane. The norepinephrine-enhanced Ca2+ sensitivity was partially reversed by a pretreatment with a selective myosin light chain kinase inhibitor (8R*, 9S*, 11S*)-(-)-9-hydroxy-9-methoxycarbonyl-8-methyl-14-n- propoxy-2,3,9,10-tetrahydro-8,11-epoxy,1H,8H,11H-2,7b,11a- triazadibenzo[a,g]cycloocta[cde]trinden-1-one (KT5926, 500 nM), but those of clonidine and in the chloroethylclonidine-treated tissues norepinephrine were not. These results suggest that Ca(2+)-sensitization produced by the activation of the alpha 1-adrenoceptor subtypes is linked via a low molecular weight GTP-binding protein (Rho), and the regulations of phosphorylation in contractile elements.

All-or-none augmentation of Ca2+ sensitivity in alpha-toxin-permeabilized single smooth muscle cells from guinea-pig taenia caecum

1. Isolated smooth muscle cells from guinea-pig taenia caecum were permeabilized by use of Staphylococcus aureus alpha-toxin, and the sarcoplasmic reticulum Ca2+ store was depleted by exposure to 0.1 microM A23187. 2. Shortening of alpha-toxin-permeabilized single smooth muscle cells was induced by increasing free Ca2+ but was not induced by 0.2 microM free Ca2+. 3. Shortening of the permeabilized cells was caused by application of acetylcholine (ACh) with free Ca2+ concentration held at 0.2 microM. Permeabilized smooth muscle cells responded to 0.3 microM or 1 microM ACh with 0.2 microM Ca2+ with maximal shortening. The concentration-response relationship to ACh had a very steep slope and the cell shortening appeared to be an all-or-none response rather than a graded response, as was the shortening of intact cells to ACh. 4. The shortening of permeabilized cells was also induced by application of guanosine 5'-triphosphate (GTP) with 0.2 microM free Ca2+, showing an all-or-none response. The threshold concentration of GTP that induced an all-or-none response was between 10 microM and 30 microM. 5. These results suggest that Ca2+ sensitivity is augmented by stimulation of the muscarinic receptor or GTP-binding protein(s) in an all-or-none manner. It seems probable that this contributes to the all-or-none response to ACh in intact smooth muscle cells.

17 beta-Estradiol inhibits Ca2+ influx and Ca2+ release induced by thromboxane A2 in porcine coronary artery

BACKGROUND

We wished to investigate the possible mechanism of the protective effect of estrogen replacement on coronary atherosclerosis observed in postmenopausal women.

METHODS AND RESULTS

Cytosolic Ca2+ concentration ([Ca2+]i) and contraction were measured simultaneously in fura 2-loaded porcine coronary arterial strips stimulated by the thromboxane A2 analogue U46619 and high-K+ depolarization in the presence and absence of 17 beta-estradiol. Pretreatment with 17 beta-estradiol (30 nmol/L to 30 mumol/L) inhibited the sustained elevation of [Ca2+]i and the sustained contraction induced by 300 nmol/L U46619. Higher concentrations of 17 beta-estradiol (1 to 100 mumol/L) also inhibited the U46619-induced transient increase in [Ca2+]i and contraction in the absence of extracellular Ca2+. In the strips precontracted by 90 mmol/L K+, 17 beta-estradiol (30 mumol/L) inhibited the increases in [Ca2+]i and contraction to resting levels. In contrast, 30 mumol/L 17 beta-estradiol only partially inhibited the U46619-induced sustained contraction, despite complete inhibition of the sustained increase in [Ca2+]i. Verapamil (10 mumol/L) also strongly inhibited the sustained increase in [Ca2+]i induced by 300 nmol/L U46619, with a partial inhibition of the U46619-induced sustained contraction. A subsequent addition of 30 mumol/L 17 beta-estradiol did not show an additional inhibitory effect on either the [Ca2+]i or the tension after the addition of verapamil. 17 beta-Estradiol (10 mumol/L) also inhibited the increase in [Ca2+]i and the contraction induced by cumulative addition of Ca2+ in the strips pretreated with 90 mmol/L K+. However, 17 beta-estradiol did not change the slope of the [Ca2+]i-tension curves. 17 beta-Estradiol (10 mumol/L) had no effect on the levels of cAMP and cGMP in the coronary strips.

CONCLUSIONS

17 beta-Estradiol inhibits the contraction of coronary vascular smooth muscle mainly inhibiting Ca2+ influx without changing Ca2+ sensitivity of contractile elements. The Ca2+ channel blocker-like action of 17 beta-estradiol may explain at least a part of the antiatherosclerotic effect of estrogen.

The inhibitory effect of Li+ on contractile elements of intestinal smooth muscle

The mechanism of the inhibitory effect of Li+ on contraction was examined in guinea pig ileal longitudinal smooth muscle. Li(+)-substitution (68.4 mM) reversed contractions induced by high K+ (45.4 mM), carbachol (1 microM) and histamine (1 microM) without changing the cytosolic Ca2+ level. Li+ also had no effect on the increase in 45Ca2+ uptake stimulated by high K+. High K+ transiently increased myosin light chain (MLC) phosphorylation, reaching a peak at 6-9 sec. Li(+)-substitution inhibited the high K(+)-induced MLC phosphorylation. In permeabilized ileal strips, contraction induced by 1 microM Ca2+ was inhibited by 10 mM Li+. The inhibitory effect was antagonized by increasing the concentration of Ca2+ or calmodulin. In the permeabilized muscle in which MLC was previously thiophosphorylated with 1 mM ATP gamma S and 3 microM Ca2+, ATP induced contraction in Ca2+ free buffer. Li+ added during this contraction did not show an inhibitory effect. In contrast, when 30 mM Li+ was added during the thiophosphorylation, the contraction induced by the subsequent addition of ATP was inhibited. Li+ (30 mM) changed neither the rate of relaxation induced by removing external Ca2+ in permeabilized muscle nor the rate of dephosphorylation of MLC induced by crude phosphatase extracted from the ileum. Li+ (15 mM), on the other hand, inhibited the rate of phosphorylation of MLC caused by crude MLC kinase extracted from the ileum. Li+ did not inhibit the calmodulin activity as measured with the (Ca2+ +Mg2+)-ATPase activity of the erythrocyte membrane. These results suggest that the inhibitory effect of Li+ on contractions is attributable to the inhibition of MLC kinase in guinea pig ileum.

Coupling of the endothelin ETA and ETB receptors to Ca2+ mobilization and Ca2+ sensitization in vascular smooth muscle

Effects of endothelins on cytosolic Ca2+ level ([Ca2+]i) and contraction were examined in the swine pulmonary artery and vein. In the artery, endothelin-1 and endothelin-3, but not sarafotoxin S6c and IRL 1620 (300 nM each), induced transient increase followed by sustained increase in [Ca2+]i and sustained contraction. These effects were inhibited by the ETA receptor antagonist, BQ-123. In the vein, endothelin-1 and endothelin-3 (300 nM each) induced sustained increase in [Ca2+]i and sustained contraction whereas sarafotoxin S6c and IRL 1620 (300 nM each) transiently increased both [Ca2+]i and contractile tension. The ETB receptor in the vein was desensitized by pretreatment with sarafotoxin S6c, abolishing the effects of sarafotoxin S6c and IRL 1620 without changing the effects of endothelin-1 and endothelin-3. In contrast, an ETB antagonist, RES-701-1, antagonized the effects of IRL 1620 without changing the effects of other stimulants. In both artery and vein, the maximum contraction induced by these stimulants was greater than that induced by KCl at a given [Ca2+]i. In the absence of external Ca2+, endothelin-1 and endothelin-3 induced transient increase in [Ca2+]i and slow sustained contraction in both artery and vein. In the vein, sarafotoxin S6c induced small sustained contraction without changing [Ca2+]i. In the permeabilized artery and vein, endothelin-1 augmented the contraction induced by Ca2+. These results suggest that the ETA receptors in the artery and vein are coupled to Ca2+ release (which does not seem to trigger contraction), Ca2+ influx and Ca2+ sensitization.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular alkalinization by NH4Cl increases cytosolic Ca2+ level and tension in the rat aortic smooth muscle

Intracellular pH (pHi) is elucidated to be an important regulator of various cell functions, but the role of pHi in smooth muscle contraction remains to be clarified. The purpose of the present study is to examine the effects of cell alkalinization by exposure to NH4Cl on cytosolic Ca2+ level ([Ca2+]i) and on muscle tone. We attempted simultaneous measurements of both [Ca2+]i and contractile force in rat isolated thoracic aorta from which the endothelium was removed. NH4Cl (10-80 mM) increased both [Ca2+]i and muscle tone in the presence of external Ca2+. These responses were reproducible. The removal of Ca2+ from the nutrient solution partially inhibited the rise in [Ca2+]i and the smooth muscle contraction induced by NH4Cl. In addition, the Ca2+ channel blocker verapamil also partially attenuated the responses to NH4Cl. The NH4Cl-induced responses were gradually reduced as NH4Cl was repeatedly added in a Ca(2+)-free solution. Norepinephrine (NE, 1 microM) induced a transient increase in [Ca2+]i and sustained contraction in the absence of external Ca2+, and the subsequent application of NE had little effect on [Ca2+]i. After internal Ca2+ stores were depleted by exposure to NE, the subsequent application of NH4Cl induced increases in [Ca2+]i and tension of the aorta in a Ca(2+)-free solution. These results suggest that NH4Cl mainly evokes Ca2+ release from the internal Ca2+ stores that are not linked with adrenergic alpha-receptor and causes Ca2+ influx through voltage-dependent Ca2+ channels in the vascular smooth muscle.

Alpha 1A-adrenoceptor subtype effectively increases Ca(2+)-sensitivity for contraction in rabbit thoracic aorta

1. Norepinephrine and phenylephrine increase cytosolic Ca2+ concentration ([Ca2+]i) and muscle tension, which shows positive correlation between [Ca2+]i and tension development. 2. The slopes of regression lines between [Ca2+]i and tension development for norepinephrine and phenylephrine in tissues treated with an irreversible alpha 1B-adrenoceptor selective blocking agent, 10(-4) M chloroethylclonidine, were significantly steeper than those with untreated tissues. 3. Myosin light chain kinase inhibitors, KT5926 (3 x 10(-6) M) and K252a (10(-6) M) more selectively reduced the contraction produced by norepinephrine (3 x 10(-7) M) than that produced by clonidine (3 x 10(-6) M). 4. In the chloroethylclonidine-treated tissues, KT5926 and K252a did not tend to affect the contraction induced by norepinephrine and clonidine. 5. These results suggest that the contractile response through the alpha 1A-adrenoceptor subtype causes a greater muscle tension than that through the alpha 1B-subtype at the same level of [Ca2+]i, and that the alpha 1A-adrenoceptor subtype mainly activates myosin light chain kinase independent pathways of contractile mechanisms in vascular smooth muscle of rabbit.

Developmental changes in response to endothelins and receptor subtypes of isolated rat duodenum

The response of isolated duodenum to endothelin-1, -3 and IRL 1620 (Suc-[Glu9,Ala11,15]endothelin-1 (8-21)), a selective endothelin ETB receptor agonist, was studied in both neonatal (1-week-old) and adult rats by recording mechanical activity isotonically. Endothelin-1, -3 and IRL 1620 (1-100 nM) elicited sustained contraction of neonatal duodenum, in a concentration-dependent manner, with a potency order of endothelin-1 = endothelin-3 > IRL 1620. The response to endothelin-1 and -3 (10-1000 nM) of adult duodenum was biphasic, i.e., transient relaxation followed by contraction, with a potency order of endothelin-1 > endothelin-3. The contractile response to endothelin-1 of adult but not neonatal duodenum was significantly antagonized by pretreatment with FR139317 (1 microM), an endothelin ETA receptor antagonist. An endothelin ETB receptor antagonist, RES-701-1 (3 microM), weakly antagonized the IRL 1620-induced contraction of neonatal duodenum. However, RES-701-1 (10 microM) did not affect the response to endothelin-1 of either adult or neonatal duodenum. These results indicate that the duodenal response to endothelins changes from a sustained contraction in neonates to a biphasic response in adults. The contractile response to endothelins of neonatal duodenum is suggested to be mediated through endothelin ETB receptors or possibly RES-701-1-resistant ETB receptor subtypes and contraction of adult duodenum through endothelin ETA receptors. The mechanism of the endothelin-induced response of duodenum was also studied.

Effect of Kil769, a novel K(+)-channel opener, on sensitivity to Ca2+ of contractile elements and inositol phosphate formation in porcine coronary artery

To determine whether Kil769, a novel K(+)-channel opener, acts intracellularly in vasorelaxation, we compared the effects of Kil769 on force of contraction, intracellular Ca2+ concentration ([Ca2+]i) and inositol phosphate (IP1) formation with those of Ca(2+)-channel blockers in isolated porcine coronary artery. Kil769 (10 microM) and verapamil (1 microM), which produced submaximal relaxation, reduced the increase in [Ca2+]i and force of contraction induced by 25 mM KCl. Verapamil reduced [Ca2+]i and the force of contraction to a similar extent but Kil769 reduced force of contraction more strongly than it did [Ca2+]i. Kil769 also inhibited U46619 (9,11-dideoxy-9 alpha,11 alpha-methano-epoxy-PGF2 alpha)-induced IP1 formation and glibenclamide blocked its inhibitory effect. These results suggest that the opening of K+ channels induced by Kil769 reduces the Ca2+ sensitivity of contractile elements and inositol phospholipid hydrolysis which is related to the Ca2+ release from intracellular storage.

Calcium channels in cerebral arteries

Electrophysiological evidence shows the existence of voltage-operated Ca2+ channels of the L- and, in some cases, T- and B-, type in the smooth muscle cells of major cerebral arteries and arterioles. Current intensity through L-type Ca2+ channels is higher in cerebral than in peripheral arteries, which points to a greater dependence on extracellular Ca2+ of contractile responses in cerebral arteries. The increase in cytosolic Ca2+ concentration is the key event leading both to maintenance of basal cerebrovascular tone and to contraction of cerebral arteries in response to depolarization and agonist-receptor interaction. Such an increase results from increased transmembrane influx of Ca2+ through L-type Ca2+ channels, as well as from the release of Ca2+ from intracellular Ca2+ stores. Ca2+ entry modulators (dihydropyridines, phenylalkylamines, benzothiazepines, and diphenylpiperazines) bind to allosterically coupled sites in the Ca2+ channel, thus inhibiting (Ca2+ entry blockers) or stimulating (Ca2+ entry activators) Ca2+ influx and, therefore, contractile responses of the cerebral arteries. In vivo, Ca2+ entry blockers increase pial vascular caliber and cerebral blood flow by their direct action on the cerebroarterial wall. However, such an action also takes place on several peripheral vascular beds, which leads to hypotension. Therefore, the brain cannot be considered a "privileged" organ when the vasodilatatory action of Ca2+ entry blockers is considered. Since increased cytosolic Ca2+ concentration (and, therefore, activation of Ca2+ channels) plays a crucial role in the pathogenesis of ischemic brain damage (e.g., acute stroke and subarachnoid hemorrhage), Ca2+ entry blockers could be useful cytoprotective drugs. However, with the exception of nimodipine in the management of subarachnoid hemorrhage, clinical trials have yielded results that are not so promising as one could expect from those obtained in experimental research.

Possible involvement of a small G-protein sensitive to exoenzyme C3 of Clostridium botulinum in the regulation of myofilament Ca2+ sensitivity in beta-escin skinned smooth muscle of guinea pig ileum

The effects of exoenzyme C3 of Clostridium botulinum on Ca(2+)- and drug-induced tension developments were investigated in beta-escin skinned smooth muscle of guinea pig ileum to test the involvement of a small G-protein in the regulation of myofilament Ca2+ sensitivity. C3 is known to ADP-ribosylate the rho p21 family of small G-proteins. Treatment with C3 (0.35 microgram/ml, for 30 min) shifted the pCa-tension curve rightward along the Ca2+ concentration axis, indicating a decrease in Ca2+ sensitivity of the contractile elements. The inhibitory effect of C3 was not preserved after treatment with GDP beta S (1 mM), an antagonist of GTP for the binding to G-proteins. Stimulation of muscarinic receptors with carbachol (CCh, 100 microM) shifted the pCa-tension curve leftward, indicating Ca2+ sensitization of tension development. The Ca(2+)-sensitizing effect of CCh was not observed after C3 treatment. When GTP gamma S (10 microM), an activator of G-proteins, was applied at a plateau of tension development produced by a moderate concentration of Ca2+, further increase in tension was elicited and the effect of GTP gamma S was inhibited by C3 treatment. The results suggest the possible involvement of a rho p21-like small G-protein in the regulation of Ca2+ sensitivity of smooth muscle myofilaments.

Effects of calcitonin gene-related peptide on intracellular calcium concentration in longitudinal muscle of guinea pig ileum

The effect of rat calcitonin gene-related peptide (rCGRP) on intracellular free calcium concentration ([Ca2+]i) and its relationship with muscle relaxation were examined in plexus-free longitudinal muscle (LM) of the guinea pig ileum using a [Ca2+]i-tension simultaneous recording technique. Tissue was stimulated with either histamine (0.5 microM) or KCl (30 mM). rCGRP at a concentration of 263 nM (which displayed maximal relaxation of the LM) caused a small and brief but significant decrease in the [Ca2+]i in histamine-treated tissue. On the other hand, the same concentration of rCGRP relaxed the muscle without affecting the [Ca2+]i in KCl-treated tissue. rCGRP caused a dissociation between the changes in [Ca2+]i and tension. The effects of forskolin 0.5 microM) and nifedipine (10 nM) on both [Ca2+]i and tension were examined in comparison with rCGRP. Forskolin lowered the tension to greater extent than the [Ca2+]i whereas nifedipine (10 nM) diminished both the [Ca2+]i and tension in a parallel manner in histamine-treated tissues. These results suggest that rCGRP may not reduce [Ca2+]i as its primary mechanism of relaxation and it may change the calcium sensitivity of the contractile elements of the smooth muscle.

Effects of pHi on isometric force and [Ca2+]i in porcine coronary artery smooth muscle

During ischemia or hypoxia, alterations in pHi may play a significant role in alteration of vessel wall function. We studied the effects of altering pHi on isometric force and [Ca2+]i in porcine coronary artery. pHi was altered at constant pHo by use of NH4Cl and measured with the fluorescent dye BCECF. [Ca2+]i was monitored with fura 2 and ratiometric fluorescence measurements. Addition of NH4Cl elicited a concentration-dependent (2 to 30 mmol/L) sustained increase in isometric force in unstimulated tissues. In tissues stimulated with KCl (29 mmol/L) or U46619 (1 mumol/L), addition of NH4Cl elicited a rapid but transient decrease followed by a sustained increase in force above the initial stimulated levels. Removal of NH4Cl was associated with a transient decrease and increase followed by a prolonged depression of force and slow recovery to initial levels. Addition of NH4Cl elicited a rapid monotonic increase in pHi and then a slow recovery toward initial levels; washout of NH4Cl led to a rapid acidification followed by recovery. In contrast to the steady state effects of NH4Cl on force, its effects on [Ca2+i were in the opposite direction. During the sustained increase in force elicited by NH4Cl alkalinization, [Ca2+]i was substantially decreased, whereas when force was depressed during the acidification elicited by NH4Cl washout, [Ca2+i was increased to values observed before addition of NH4Cl. The initial transients in force elicited by NH4Cl addition or washout were also associated with opposite changes in [Ca2+]i. Thus, the effects on force of the NH4Cl-induced changes in pHi are associated with changes in the Ca2+ sensitivity of the contractile apparatus rather than mediated through changes in [Ca2+]i.

Sources of calcium utilized in cholinergic responses in canine colonic smooth muscle

Ratiometric fura 2 fluorescence techniques were used to investigate the sources of Ca2+ that lead to an increase in cytosolic Ca2+ concentration ([Ca2+]i) and the generation of force during cholinergic stimulation of canine colonic circular smooth muscle tissues. Acetylcholine (ACh; 1 microM) caused a biphasic increase in [Ca2+]i and force. The initial upstroke phase was characterized by an increase in [Ca2+]i and a pronounced increase in force. The sustained phase was characterized by concurrent oscillations in [Ca2+]i and force (2-3/min) that persisted as long as ACh was present. The increase in [Ca2+]i in response to ACh was reduced to approximately 30% in the presence of nicardipine (1 microM), suggesting that L-type Ca2+ channels contribute to the rise in [Ca2+]i but that other sources also contribute. Preincubation in caffeine (10 mM) and ryanodine (10 microM) reduced the upstroke phase of the increase in [Ca2+]i and contractile responses to ACh, indicating that release of Ca2+ from intracellular stores contributes only to the initial cholinergic response. Responses to ACh persisted when nicardipine (1 microM) was included after emptying of caffeine-ryanodine-sensitive stores, suggesting the presence of additional sources of Ca2+. Data suggest that cholinergic regulation of [Ca2+]i in colonic smooth muscle occurs by a number of parallel pathways. Influx through voltage-dependent Ca2+ channels, release of Ca2+ from intracellular stores, and possibly Ca2+ entry through additional conductances activated by ACh all contribute to the regulation of [Ca2+]i.

Alpha 1-adrenoceptor subtypes mediating the regulation and modulation of Ca2+ sensitization in rabbit thoracic aorta

Norepinephrine (10 microM), methoxamine (100 microM) and clonidine (100 microM) with guanosine 5'-triphosphate (GTP, 50 microM) or guanosine 5'-O-(3-thiotriphosphate) (GTP gamma-S, 10 microM) all significantly enhanced the contraction induced by 0.3 microM Ca2+ (pCa6.5) in beta-escin-skinned smooth muscle of rabbit thoracic aorta. The enhancement of Ca2+ contraction produced by norepinephrine was greater than that produced by methoxamine or clonidine. In beta-escin-skinned strips of chloroethylclonidine-pretreated smooth muscle, the enhancement of Ca2+ contraction produced by norepinephrine was significantly decreased, whereas the amplitude was the same as that produced by methoxamine or clonidine; this enhancement was inhibited by the selective alpha 1A-adrenoceptor antagonist WB 4101 (100 nM). The enhancement of Ca2+ contraction produced by methoxamine and clonidine was not affected by chloroethylclonidine pretreatment. The effects of methoxamine, clonidine and norepinephrine in the chloroethylclonidine-pretreated tissue were all inhibited by guanosine 5'-O-(2-thiodiphosphate) (GDP beta-S, 1 mM) and 1-(5-isoquinolinylsulfonyl)-methylpiperazine (H-7, 20 microM). Furthermore, the phosphorylation of myosin light chain produced by norepinephrine was greater than that produced by clonidine. These results suggest that both alpha 1-adrenoceptor subtypes (alpha 1A and alpha 1B) increase the Ca2+ sensitivity of contractile elements, and that the Ca2+ sensitization produced by alpha 1A-subtype receptors is mediated through G-protein and protein kinase C, and plays an important role in contraction of smooth muscle of rabbit thoracic aorta.

Parathyroid hormone-related protein reduces cytosolic free Ca2+ level and tension in rat aortic smooth muscle

The effect of parathyroid hormone-related protein (PTHrP) on cytosolic free Ca2+ level ([Ca2+]i) and tension in rat aortic smooth muscle was investigated with special reference to the role of production and action of cyclic AMP. Rat aortic spiral strip preparations without endothelium were treated with the acetoxymethyl ester of fura 2, and the ratio of fluorescences (R340/380), an index of [Ca2+]i, emitted from smooth muscle, was measured. The tension of the preparations was simultaneously measured. PTHrP-(1-34) produced concentration-dependent decreases both in the tension and in R340/380 increased by phenylephrine (10(-7) M). These effects were significantly inhibited by pretreatment with either PTHrP-(7-34) (10(-6) M), a PTHrP receptor antagonist, or with Rp diastereomer of adenosine cyclic 3',5'-phosphorothioate (RpcAMPS; 10(-4) M), a cyclic AMP-dependent protein kinase inhibitor. Dibutyryl cyclic AMP (10(-5)-10(-3.5 M) elicited effects similar to those of PTHrP-(1-34). PTHrP-(1-34) was found to significantly elevate aortic cAMP level, measured by specific radioimmunoassay, after 5 min incubation with PTHrP-(1-34). These results suggest that the decrease in [Ca2+]i is involved in the vasodilator action of PTHrP, and that the decreases both in tension and in [Ca2+]i might be attributed to cyclic AMP production stimulated by PTHrP.

Calcium antagonists and vasodilatation

Calcium antagonists comprise a diverse group of chemically unrelated agents that interact with voltage-operated calcium channels (L-type) and thereby inhibit smooth muscle contractility. They are used to treat several major cardiovascular disorders, including hypertension and angina pectoris; they are also studied in congestive heart failure and in atherosclerosis. The current view is that their therapeutic action is related to vasodilatation. This view is an oversimplification, as will be shown in this review. It will also be illustrated that all calcium antagonists are not identical pharmacological agents.