Effects of ergometrine on airway smooth muscle contractile responses

A 26-year-old asthmatic female developed severe asthma within a few hours of receiving three oral doses of 0.4 mg ergometrine maleate for the control of postpartum haemorrhaging. This experience and two previous reports of bronchospasm in asthmatic subjects following ergometrine suggested that ergometrine altered airway smooth muscle tone. In the present investigation the effect of ergometrine was studied on canine tracheal smooth muscle strips. Ergometrine (10(-9) M-10(-4) M) induced contraction of canine tracheal smooth muscle. The concentration causing 50% of maximal contraction (EC50) was 4.73 X 10(-8) M. The acetylcholine EC50 was not altered by ergometrine (10(-9) M or 10(-8) M); however, acetylcholine (10(-4) M and 10(-3) M) induced contractions were enhanced by ergometrine (10(-8) M). The data suggest that ergometrine maleate may cause broncho-constriction in some patients with asthma.     

Modulation of the calcium sensitivity of the smooth muscle contractile apparatus: molecular mechanisms, pharmacological and pathophysiological implications

Summary— Smooth muscle contraction is the basis of the physiological reactivity of several systems (vascular, respiratory, gastrointestinal, urogenital…). Hyperresponsiveness of smooth muscle may also contribute to a variety of problems such as arterial hypertension, asthma and spontaneous abortion. An increase in cytoplasmic calcium concentration ([Ca²⁺]_i) is the key event in excitation-contraction coupling in smooth muscle and the relationship linking the [Ca²⁺]_i value to the force of contraction represents the calcium sensitivity of the contractile apparatus (CaSCA). Recently, it has become evident that CaSCA can be modified upon the action of agonists or drugs as well as in some pathophysiological situations. Such modifications induce, at a fixed [Ca²⁺]_i value, either an increase (referred to as sensitization) or a decrease (desensitization) of the contraction force. The molecular mechanisms underlying this modulation are not yet fully elucidated. Nevertheless, recent studies have identified sites of regulation of the actomyosin interaction in smooth muscle. Sensitization primarily results from the inhibition of myosin light chain phosphatase (MLCP) by intracellular messengers such as arachidonic acid or protein kinase C. In addition, phosphorylation of thin filament-associated proteins, caldesmon and calponin, increases CaSCA. Activation of small (monomeric) G-proteins such as rho or ras is also involved. Desensitization occurs as a consequence of phosphorylation of myosin light chain kinase (MLCK) by the calcium-calmodulin activated protein kinase II, or stimulation of MLCP by cyclic GMP-activated protein kinase. In the present review, examples of physiological modulation of CaCSA as well as pharmacological and pathophysiological implications are illustrated for some smooth muscles.     

Mechanism by which serotonin attenuates contractile response of canine mesenteric arterial smooth muscle

We investigated the mechanism by which prior exposure to serotonin (5-HT) attenuates contractile response to norepinephrine (NE) in dog mesenteric artery. Helically cut strips from dog mesenteric artery were mounted in muscle baths for isometric force recording. Prior treatment with 5-HT (10(-8) to 2 X 10(-7) M) attenuated the response to 2.5 X 10(-7) M NE by 26 to 61%. The attenuation lasted approximately 30 min. The attenuation is not affected by adrenergic denervation, endothelial removal, propranolol, indomethacin or arachidonate. Prior treatment with 5-HT also attenuated responses to: methoxamine, clonidine, prostaglandin F2 alpha, KCl and 5-HT itself. Inhibition of sodium influx by amiloride or inhibition of the electrogenic pump by ouabain diminished the NE attenuation. Verapamil, a calcium channel blocker, diminished the attenuation by 5-HT whereas ryanodine, an intracellular calcium release blocker, had no effect. Prior treatment with 5-HT caused a reduction in the 45Ca influx stimulated by NE. Ketanserin, a 5-HT blocker, exaggerated the attenuation of the NE response caused by 5-HT. Methysergide and cyproheptadine, also 5-HT blockers, diminished the attenuation. These results are compatible with the following interpretation: 1) 5-HT attenuates both receptor- and nonreceptor-mediated contractions of vascular smooth muscle; 2) this attenuation is the result of an increased sodium influx that stimulates the electrogenic sodium pump to cause membrane hyperpolarization and decreased stimulated calcium influx; and 3) the effects of 5-HT are mediated by 5-HT receptors that are blocked by methysergide and cyproheptadine but not by ketanserin.     

Calcium-independent phosphorylation of smooth muscle myosin light chain by okadaic acid isolated from black sponge (Halichondria okadai)

Previously, we have shown that okadaic acid (OA), isolated from black sponge (Halichondria okadai) causes contraction even in the absence of Ca++ in the saponin-permealized taenia isolated from guinea pig cecum. In the present study, mechanism of action of OA was examined using native actomyosin extracted from chicken gizzard smooth muscle. In the absence of Ca++, OA (0.1-1 microM) induced superprecipitation and increased the Mg++-adenosine triphosphatase activity. The OA-induced superprecipitation was enhanced by Ca++ at a concentration (greater than 0.1 microM) which did not activate the calmodulin-dependent myosin light chain (MLC) kinase. The effect of OA was not affected by the calmodulin inhibitor, trifluoperazine, at a concentration (100 microM) needed to inhibit the Ca++-induced response, but was inhibited markedly by the nonselective kinase inhibitors, amiloride (1 mM) and K-252a (5 microM). The OA-induced superprecipitation in the absence of Ca++ was accompanied by phosphorylation of the 20 K dalton MLC, which also was enhanced by low concentration of Ca++ (greater than 0.1 microM). OA did not change the phosphatase activity which dephosphorylates the phosphorylated MLC. An activator of Ca++- and phospholipid-dependent protein kinase, 12-O-tetradecanoylphorbol 13-acetate (1 microM), did not modulate superprecipitation or phosphorylation of MLC in the presence and absence of OA. Furthermore, inhibitors of Ca++ and phospholipid-dependent protein kinase, 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine dihydrochloride (400 microM) and polymyxin B (100 micrograms/ml), affected neither superprecipitation nor phosphorylation of MLC induced by OA. With a reconstituted system containing purified myosin and MLC kinase, OA induced only slight phosphorylation of MLC.(ABSTRACT TRUNCATED AT 250 WORDS)     

苦豆子对豚鼠内脏平滑肌收缩活动的干预

目的：观察苦豆子水煎剂对豚鼠离体内脏平滑肌作用的影响，分析其发挥作用途径。 方法：实验于2004—12在宁夏医学院基础学院生理实验室完成。①选用健康成年三色毛豚鼠24只，雌雄不拘。②苦豆子（由银川市中医院提供，由该院检定）被制备成1kg／L的水煎剂，实验的系列质量浓度为1&;#215;10^-4，3&;#215;10^-4，1&;#215;10^-3，3&;#215;10^-3，1&;#215;10^-2，2&;#215;10^-2kg／L。③实验方法：实验时分别取出豚鼠胆囊、胃、小肠及膀胱。每个胆囊制备3条平滑肌肌条，共72条；胃、小肠及膀胱各制备12条平滑肌肌条。累积于置胆囊、胃、小肠及膀胱平滑肌肌条各12条的肌槽中加入1&;#215;10^-4，3&;#215;10^-4，1&;#215;10^-3，3&;#215;10^-3，1&;#215;10^-2，2&;#215;10^-2kg／L苦豆子水煎剂，加药间隔2min。容量50μL。④按随机抽签法将其余60条胆囊肌条分为5组：阿托品＋苦豆子组、酚妥拉明＋苦豆子组、维拉帕米＋苦豆子组、苯海拉明＋苦豆予组、吲哚美辛＋苦豆子组，每组12条。上述5组分别加入拮抗剂阿托品（胆碱能M受体阻断剂）1&;#215;10^-6mol／L，酚妥拉明（肾上腺素能α受体阻断剂）1&;#215;10^6mol／L，维拉帕米（Ca^2＋拮抗剂）1&;#215;10^-7mol／L，苯海拉明（组织胺H1受体阻断剂）1&;#215;10^-6mol／L，吲哚美辛（前列腺隶受体阻断剂）1&;#215;10^6mol／L后2min再依次加入前述“累积加药”中各质量浓度苦豆子。⑤通过JH-2肌肉张力换能器将信号输出BL-410生物机能实验系统读出标本的张力、收缩波平均振幅及收缩频率的变化值。⑥数据间差异性比较采用均数t检验。 结果：①累积加入苦豆子水煎剂1&;#215;10^-4，3&;#215;10^-4，1&;#215;10^-3，3&;#215;10^-3，1&;#215;10^-2.2&;#215;10^-2kg／L后，胆囊肌条张力逐渐增高、收缩频率逐渐加快、收缩波平均振幅逐渐减小。（函当苦豆子水煎剂累积质量浓度为2&;#215;10^-2kg／L时，酚妥拉明＋苦豆子组离体胆囊肌条张力和收缩频率明显低于或慢于苦豆子组（t=2，3401，2．1404，P〈0．05），而收缩波平均振幅明显高于苦豆子组（t=2．8256，P〈0．05）。维拉帕米＋苦夏子组离体胆囊肌条张力明显低于苦豆子组（t=2．3464，P〈0．05）。③当苦豆子水煎剂累积质量浓度为1&;#215;10^-2kg／L时，酚妥拉明＋苦夏子组离体胆囊肌条张力明显低于苦豆子组（t=2．430，P〈0．05），而收缩波平均振幅明显高于苦豆子组（t=2．2279，P〈0．05）。维拉帕米＋苦豆子组离体胆囊肌条张力明显低于苦豆子组（t=2．6576，P〈0．05）。④累积加入不同质量浓度苦赢子水煎剂后对豚鼠胃、小肠及膀胱肌条的张力、收缩波平均振幅及收缩频率无明显影响（P〉0．05）。 结论：①苦豆子水煎刺可呈剂量依赖性增高胆囊平滑肌肌条的张力、加快收缩频率、减小收缩波平均振幅，其作用可被酚妥拉明及维拉帕米部分阻断，且该种作用可能与胆囊平滑肌细胞膜上的肾上腺素能Ⅱ受体和细胞膜上的Ca^2＋通道有关。②苦豆子水煎剂对豚鼠胃、小肠及膀胱平滑肌的收缩活动无明显影响。     

苦豆子对豚鼠内脏平滑肌收缩活动的干预

目的:观察苦豆子水煎剂对豚鼠离体内脏平滑肌作用的影响,分析其发挥作用途径。方法:实验于2004-12在宁夏医学院基础学院生理实验室完成。①选用健康成年三色毛豚鼠24只,雌雄不拘。②苦豆子(由银川市中医院提供,由该院检定)被制备成1kg/L的水煎剂,实验的系列质量浓度为1×10-4,3×10-4,1×10-3,3×10-3,1×10-2,2×10-2kg/L。③实验方法:实验时分别取出豚鼠胆囊、胃、小肠及膀胱。每个胆囊制备3条平滑肌肌条,共72条;胃、小肠及膀胱各制备12条平滑肌肌条。累积于置胆囊、胃、小肠及膀胱平滑肌肌条各12条的肌槽中加入1×10-4,3×10-4,1×10-3,3×10-3,1×10-2,2×10-2kg/L苦豆子水煎剂,加药间隔2min,容量50μL。④按随机抽签法将其余60条胆囊肌条分为5组:阿托品 苦豆子组、酚妥拉明 苦豆子组、维拉帕米 苦豆子组、苯海拉明 苦豆子组、吲哚美辛 苦豆子组,每组12条。上述5组分别加入拮抗剂阿托品(胆碱能M受体阻断剂)1×10-6mol/L,酚妥拉明(肾上腺素能α受体阻断剂)1×10-6mol/L,维拉帕米(Ca2 拮抗剂)1×10-7mol/L,苯海拉明(组织胺H1受体阻断剂)1×10-6mol/L,吲哚美辛(前列腺素受体阻断剂)1×10-6mol/L后2min再依次加入前述“累积加药”中各质量浓度苦豆子。⑤通过JH-2肌肉张力换能器将信号输出BL-410生物机能实验系统读出标本的张力、收缩波平均振幅及收缩频率的变化值。⑥数据间差异性比较采用均数t检验。结果:①累积加入苦豆子水煎剂1×10-4,3×10-4,1×10-3,3×10-3,1×10-2,2×10-2kg/L后,胆囊肌条张力逐渐增高、收缩频率逐渐加快、收缩波平均振幅逐渐减小。②当苦豆子水煎剂累积质量浓度为2×10-2kg/L时,酚妥拉明 苦豆子组离体胆囊肌条张力和收缩频率明显低于或慢于苦豆子组(t=2.3401,2.1404,P<0.05),而收缩波平均振幅明显高于苦豆子组(t=2.8256,P<0.05)。维拉帕米 苦豆子组离体胆囊肌条张力明显低于苦豆子组(t=2.3464,P<0.05)。③当苦豆子水煎剂累积质量浓度为1×10-2kg/L时,酚妥拉明 苦豆子组离体胆囊肌条张力明显低于苦豆子组(t=2.430,P<0.05),而收缩波平均振幅明显高于苦豆子组(t=2.2279,P<0.05)。维拉帕米 苦豆子组离体胆囊肌条张力明显低于苦豆子组(t=2.6576,P<0.05)。④累积加入不同质量浓度苦豆子水煎剂后对豚鼠胃、小肠及膀胱肌条的张力、收缩波平均振幅及收缩频率无明显影响(P>0.05)。结论:①苦豆子水煎剂可呈剂量依赖性增高胆囊平滑肌肌条的张力、加快收缩频率、减小收缩波平均振幅,其作用可被酚妥拉明及维拉帕米部分阻断,且该种作用可能与胆囊平滑肌细胞膜上的肾上腺素能α受体和细胞膜上的Ca2 通道有关。②苦豆子水煎剂对豚鼠胃、小肠及膀胱平滑肌的收缩活动无明显影响。     

The effect of lysolecithin on contractile force of isolated gastric smooth muscle

Mechanical activity of smooth muscle preparations was recorded in circular strips of antrum, corpus, and fundus from guinea-pig stomach. Whereas acetylcholine stimulated both rhythmic and tonic (shift of the base line) activity of the tested muscles, lysolecithin induced tonic contractions, but had no influence on the rhythmic activity. Lysolecithin was ineffective in the absence of external calcium. The tonic activations of corpus and fundus preparations induced by lysolecithin were suppressed by sodium nitroprusside. D 600 (methoxy-verapamil) did not counteract the lysolecithin effect.     

Inhibition by amiloride of contractile elements in smooth muscle of guinea pig taenia cecum and chicken gizzard

The relaxant action of amiloride was investigated in the smooth muscles of guinea pig taenia ceci and chicken gizzard. Amiloride inhibited the contractions induced by high K+ (45.4 mM) and carbachol (10 microM) in the taenia with the concentrations needed to induce 50% inhibition (IC50) of approximately 41 microM. A prolonged incubation period (greater than 1 hr) was necessary to obtain the full inhibition of these contractions. The taenia gradually accumulated amiloride and the tissue/medium ratio exceeded 2.0 after a 120-min incubation period. Amiloride had no effect on the high K+-stimulated 45Ca++ uptake or the ATP content of the taenia. Amiloride inhibited the Ca++-induced contraction of the saponin-treated taenia with an IC50 of 186 microM. Amiloride (10-1000 microM) also inhibited superprecipitation and Mg++-adenosine triphosphatase activity of the gizzard native actomyosin as well as the phosphorylation of myosin light chain. The inhibition of the phosphorylation was antagonized competitively by ATP. Amiloride (1 mM) had no effect on the dephosphorylation of myosin light chain upon removal of Ca++ from reaction medium. Amiloride, at concentrations up to 1 mM, had not effect on calmodulin activity as monitored by the Ca++-calmodulin-activated erythrocyte membrane (Ca++ + Mg++)-adenosine triphosphatase and phosphodiesterase activities. In contrast to this, trifluoperazine inhibited the calmodulin activity at the concentration needed to inhibit the Ca++-induced contraction of the permeabilized taenia and the superprecipitation and the phosphorylation of myosin light chain of gizzard. We conclude that amiloride, unlike trifluoperazine, may inhibit directly the myosin light chain kinase activity to induce muscle relaxation.     

Interaction of hydralazine and hydrazone derivatives with contractile mechanisms in rabbit aortic smooth muscle

The mechanism of action and relative potency of hydralazine (H) and tow hydrazone derivatives were investigated using isolated rabbit aortic strips. H, hydralazine acetone hydrazone (HA) and hydralazine butanone hydrazone (HBH) relaxed established K+ and norepinephrine (NE) contractures, and inhibited the development of contractures to these two agents on preincubation. H, HA and HBH increased the threshold to Ca++ and decreased the maximum tension responses during K+-Ca++-contractures (HA greater than H, P less than .05; HBH greater than H P less than .01). The Ca++-dependent and Ca++-independent components of NE contractures were both inhibited by H, HA and HBH. NE contractures were more sensitive to the effects of H than K+ contractures. These results are consistent with the conclusion that H and hydrazone derivatives produce effects on vascular muscle both by interactions with the fluxes of Ca++ from the extracellular space and effects on release from cell stores. However, other possibilities need to be assessed experimentally.     

Alteration of contractile function and calcium ion movements in vascular smooth muscle by gentamicin and other aminoglycoside antibiotics

Experiments were conducted to examine the effects of certain aminoglycoside antibiotics on contractile responses and related calcium ion (Ca(2+)) movements in isolated vascular smooth muscle. Gentamicin, kanamycin, and streptomycin decreased contractile responses produced by norepinephrine, histamine, and high K(+) in rabbit aortic strips. The inhibitory action of these antibiotics on mechanical function was more pronounced when the Ca(2+) concentration of the bathing solution was decreased from 1.5 mM (normal Ca(2+) solution) to 0.05 mM (low Ca(2+) solution). The uptake of radiocalcium ((45)Ca) into the isolated media-intimal layer of rabbit aortae was decreased in a maintained manner by each antibiotic. With gentamicin, the inhibitory effect on (45)Ca uptake was shown to be dependent upon the concentration of gentamicin employed and to be more evident in a 0.1 mM Ca(2+) solution than in a normal Ca(2+) solution. In addition, the rate of (45)Ca efflux from the rabbit aortic media-intimal layer was increased in a sustained manner by gentamicin, streptomycin, and kanamycin. Furthermore, contractile responses induced by high K(+) and norepinephrine in canine carotid arterial strips were inhibited by gentamicin. Present findings indicate that aminoglycoside antibiotics interfere with Ca(2+)-linked events leading to activation of the contractile mechanism of vascular smooth muscle. These in vitro findings may partially explain the occurrence of in vivo cardiovascular depression that has occasionally been observed after the administration of chemically related antimicrobial agents.     

Comparative distribution of smooth muscle postsynaptic contractile responses in canine trachea and bronchus in vivo

The response of canine tracheal and bronchial smooth muscle to i.a. administered agonists causing smooth muscle contraction was compared in 22 mongrel dogs in vivo. Tracheal and bronchial contractile responses were measured isometrically in situ in the same dogs. In nine dogs, dose-response curves were generated with i.a. acetylcholine and histamine (10(-10) to 10(-6) mol) in a 4-cm tracheal segment and a 1-cm segment of third order bronchus. The tracheal response to i.a. histamine was 36.5 +/- 4.48% of the response to equivalent doses of acetylcholine. In bronchus, the contraction caused by histamine was 81.0 +/- 2.83% of the cholinergic contractile response (P less than .001). In five dogs having beta adrenergic blockade with propranolol, tracheal contraction to 10(-8) to 10(-6) mol i.a. norepinephrine was 27.3 +/- 4.2% of the response to acetylcholine. However, in bronchus, contraction to norepinephrine was 218 +/- 16.6% of the response to equivalent doses of acetylcholine (P less than .001). Phentolamine (200-400 micrograms/kg i.a.) caused 79 to 100% blockade of the tracheal and bronchial response to i.a. norepinephrine. Cholinergic contraction was blocked specifically with 5 micrograms/kg i.a. of atropine. It is concluded that there is substantial heterogeneity in the contractile responses of canine trachea and bronchus in situ. Relative to cholinergic contraction, both histamine and alpha adrenergic stimulation cause substantially greater contraction of airway smooth muscle in the third order bronchus than in trachea.     

Effect of isoproterenol on contractile force of isolated rabbit renal pelvic smooth muscle

Isoproterenol produced a concentration-dependent increase in contractile force of rabbit upper renal pelvic smooth muscle. Conversely, isoproterenol produced a concentration-dependent decrease in contractile force of rabbit lower renal pelvic smooth muscle. These data show that in response to isoproterenol upper renal pelvic smooth muscle, which is thought to act as a pacemaker region for pelviureteral peristalsis, resembles cardic muscles, whereas lower pelvic smooth muscle is similar to ureter and other smooth muscles.     

延胡索对未孕大鼠离体子宫平滑肌运动的抑制作用

目的:观察延胡索乙素对未孕大鼠离体子宫平滑肌条运动的抑制作用。方法:实验于2003-11/2004-03在兰州大学医学院生理学教研室进行。①成年未孕Wistar大鼠90只,于实验前72h皮下注射乙烯雌酚0.5mg/kg,造成人工动情期以提高子宫对药物的敏感性。②猛击大鼠头部致昏,取卵巢及子宫分叉处之间的中段子宫,制成8mm×2mm的肌条,将肌条安置在恒温平滑肌肌槽中,每个肌槽中分别盛有5mL37℃Krebs液。③肌条在1g的前负荷下温育,每20min更换一次5mL新鲜的Krebs液(37℃),待肌条的自发活动平稳后加入不同剂量的延胡索乙素(2×10-5,2×10-4,4×10-4,8×10-4,1.6×10-3kg/L),观察延胡索乙素对大鼠离体子宫平滑肌条收缩活动的影响。④分别加入不同的受体拮抗剂5min后(2×10-6mol/L酚妥拉明,2×10-7mol/L异搏定,2×10-5mol/吲哚美辛和2×10-6mol/L苯海拉明),再加入8×10-4延胡索乙素,观察使用拮抗剂后延胡索乙素对大鼠离体子宫平滑肌条作用的影响。⑤计算给药前5min和给药后5min子宫平滑肌条的收缩波的频率、平均振幅和收缩波间隔平均持续时间。结果:①不同浓度的延胡索乙素均能抑制子宫平滑肌收缩运动,可使子宫平滑肌收缩波的频率减慢,在延胡索乙素浓度8×10-4kg/L,1.6×10-3kg/L时为其抑制作用差异具有显著性(P<0.01);延胡索乙素各浓度虽可以降低子宫平滑肌收缩波的振幅但差异无显著性(P>0.05);而当延胡索乙素浓度为4×10-4,8×10-4,1.6×10-3kg/L时,可显著延长收缩波的间歇时间(P<0.01)。②在加入H1受体阻断剂苯海拉明后延胡索乙素使子宫平滑肌的收缩频率降低(P<0.05),收缩波间隔的持续时间延长(P<0.05),而收缩振幅增高(P<0.05);在加入α受体阻断剂酚妥拉明后,延胡索乙素抑制子宫平滑肌的作用与单独使用延胡索乙素组相比差异无显著性(P>0.05);在加入吲哚美辛抑制前列腺素合成酶合成和释放后,延胡索乙素抑制子宫平滑肌的作用与延胡索乙素组相比仍然可以使收缩波的频率降低(P<0.05),振幅减小(P<0.05),收缩波间隔持续时间延长(P<0.05)。加入L型Ca2 通道阻断剂异搏定后延胡索乙素对未孕大鼠离体子宫平滑肌的运动有抑制作用,收缩间隔时间延长(P<0.05),但振幅不变(P<0.05)。结论:延胡索乙素抑制大鼠子宫平滑肌收缩运动的部分作用可能与L型电压依从性Ca2 通道使平滑肌细胞内Ca2 浓度降低有关,也可能与前列腺素的合成与释放有关。     

Relationship between smooth muscle volume and contractile response in airway tissue. Isometric versus isotonic measurement

Two different methods of recording contractile response (isometric and isotonic) were used in both spirally cut and intact ring preparations of rabbit airways. The volume of smooth muscle in each airway preparation was compared with the amount of contraction induced by histamine and carbachol. There were significant relationships between smooth muscle volume and carbachol-induced contraction measured isometrically in spiral and ring preparations. There also was a significant relationship between the proportion of airway smooth muscle and the histamine-induced contraction measured isometrically on ring preparations. There were no significant relationships between smooth muscle volume or proportion of airway smooth muscle and the carbachol or histamine contraction measured isotonically. It is concluded that isometric measurements may provide a more accurate representation of smooth muscle changes in response to agonists in vitro. This may indicate that hyper-responsiveness of airway smooth muscle in vitro is more likely to be detected by isometric rather than isotonic measurements.     

Metabolic activation of sodium nitroprusside to nitric oxide in vascular smooth muscle.

Sodium nitroprusside (SNP) is thought to exert its vasodilating activity, at least in part, by vascular activation to nitric oxide (NO), but the activation mechanism has not been delineated. This study has examined the potential for vascular metabolism of SNP to NO in bovine coronary arterial smooth muscle subcellular fractions using a sensitive and specific redox-chemiluminescence assay for NO. SNP was readily metabolized to NO in subcellular fractions, and the dominant site of metabolism appeared to be located in the membrane fractions. NO-generating activity was significantly enhanced by, but did not absolutely require, the addition of a NADPH-regenerating system, NADPH per se, NADH or cysteine. A correlation analysis of NO-generating activity (in the presence of a NADPH-regenerating system) with marker enzyme activities indicated that the SNP-directed NO-generating activity was primarily membrane-associated. Radiation inactivation target-size analysis revealed that the microsomal SNP-directed NO-generating activity was relatively insensitive to inactivation by radiation exposure, suggesting that the functioning catalytic unit might be quite small. A molecular weight of 5 to 11 kDa was estimated. NO-generating activity could be solubilized from the crude microsomes with 3-[(3-cholamidopropyl)- dimethylammonio]-1-propane sulfonate, and the solubilized extract was subjected to gel filtration chromatography. NO-generating activity was eluted in two peaks: one peak corresponding to an approximate molecular weight of 4 kDa, thus confirming the existence of a small molecular weight NO-generating activity, and a second activity peak corresponding to a molecular weight of 112 to 169 kDa, the functional significance of which is unclear at present.(ABSTRACT TRUNCATED AT 250 WORDS)