Immunohistochemical identification of cell types in normal and in bleomycin-induced fibrotic rat lung. Cellular origins of interstitial cells

There is substantial tissue reorganization in the interstitium in pulmonary fibrosis. In order to determine the origin of cells in the interstitium of normal or fibrotic rat lung, we stained lung tissue immunohistochemically with antiserums directed against intermediate filament proteins specific for cells of mesenchymal or muscle origin. Vimentin, specific for mesenchymal cells, was isolated from cultured 3T3 fibroblasts, and desmin, specific for muscle cells, was isolated from chicken gizzard and antiserums prepared in guinea pigs. Paraffin-embedded sections of normal or fibrotic rat lung from rats 28 days after treatment with bleomycin sulfate were reacted with each antiserum and stained by the peroxidase-antiperoxidase technique. Antivimentin antiserum stained discrete interstitial cells of both control and fibrotic lung. Smooth muscle surrounding large airways and vascular smooth muscle was not significantly stained with this antiserum but was heavily stained with antidesmin. However, antidesmin did not stain cells in the interstitium of normal or fibrotic rat lung. These results suggest that most cells in the normal as well as in the fibrotic interstitium, including contractile interstitial cells, are probably of mesenchymal (fibroblastic) rather than of smooth muscle origin.     

Actin content of normal and of bleomycin-fibrotic rat lung

A large proportion of parenchymal lung is composed of cells with an abundance of actin-containing cytoplasmic microfilaments. This may explain the substantial contractile capability of the tissue, which it appears cannot be ascribed totally to vascular or airway smooth muscle. Parenchymal contractility is increased in bleomycin-induced fibrosis, together with an increase in microfilaments and actin- and myosin-directed immunofluorescence. The present biochemical studies indicate that at least 10% of detergent-extractable protein from peripheral lung is actin. Approximately 71% of this actin is polymerized and 29% is monomeric, on the basis of differential ultracentrifugation. Isoelectric focusing and tryptic peptide analysis show that nonmuscle actin types predominate in the parenchyma, of which approximately 78% is beta-nonmuscle actin and 19% is gamma-nonmuscle actin, together with approximately 3% gamma-smooth muscle actin. Total actin as a percentage of extractable protein was not increased significantly in lungs of rats 4 or 8 wk after bleomycin instillation. Thus, actin per total lung is increased only in proportion to increased total lung weight. There is, in addition, no detectable shift in the beta/gamma actin ratio in the fibrotic lung or increase in percentage of smooth muscle actin. There is, however, a significant 16% decrease in monomeric G-actin and a commensurate significant increase in the percentage of polymerized or F-actin. Therefore, increased contractility and actin-specific immunofluorescence characteristic of fibrotic lung does not appear to be due to increases in total actin but rather to increases in its degree of polymerization, as is found in a variety of remodeling tissues. The consequence of these changes in contractile protein organization to lung function requires further investigation.     

蛋白激酶C通道对正常与致敏豚鼠气道张力的影响

探讨蛋激酶信号通道对正常和卵蛋白致敏豚鼠离体气管条和肺条张力的调节作用是否有差异。方法应用卵蛋白腹腔敏复制豚鼠哮喘模型，取气管条与肺条测定气道平滑肌的张力。     

马来酸曲美布汀对束缚应激大鼠离体结肠平滑肌张力的调节作用机制

目的 研究曲美布汀对离体结肠平滑肌收缩及舒张运动的作用机制。方法 建立束缚应激诱发排便异常大鼠动物模型，制备离体结肠平滑肌环行肌及纵行肌肌条，应用张力换能器测定其肌张力。结果 束缚应激大鼠平滑肌肌条在基础状态时，曲美布汀可增加环行肌肌条张力，且随着浓度的递增平滑肌肌条张力有增加倾向。低于10^-7g／ml曲美布汀可增加纵行肌肌条张力，但高于10^-6g／ml时则降低肌条张力。曲美布汀可降低K^ 及乙酰胆碱作用下的束缚应激大鼠环行肌及纵行肌肌条张力，随着浓度的递增曲美布汀舒张平滑肌的作用有增加倾向。在Ca^2 作用下，10^-8g／ml曲美布汀增加束缚应激大鼠纵行肌及环行肌肌条张力，但高于10^-7g／ml时降低肌条的张力。结论 曲美布汀具有抑制和兴奋平滑肌双重作用，作用特点是调节平滑肌收缩与舒张运动，纠正肠管平滑肌运动异常。     

Effect of areca on contraction of colonic muscle strips in rats

AIM: To investigate the effects of areca on the contractileactivity of isolated colonic muscle strips in rats andmechanism involved.METHODS: Each strip (LMPC, longitudinal muscle ofproximal colon; CMPC, circular muscle of proximal colon;LMDC, longitudinal muscle of distal colon; CMlC, circularmuscle of distal colon. ) was suspended in a tissue chambercontaining 5 mL Krebs solution (37 ℃), bubbledcontinuously with 950 mL@ L-1 O2 and 50 mL@ L-1 CO2 . Themean contractile amplitude (A), the resting tension (T),and the contractile frequency (F) were simultaneouslyrecorded on recorders.RESULTS: Arsca dose dependently increased the meancontractile amplitude, the resting tension of proximal anddistal colonic smooth muscle strips in rats ( P ＜ 0.05). Italso partly increased the contractile frequency of colonicsmooth muscle strips in rats ( P ＜ 0.05). The effects werepartly inhibited by atropine (the resting tension of LMPCdecreased from 0. 44 ± 0. 12 to 0. 17 ± 0.03; the restingtension of LMDC decreased from 0.71 ± 0.14 to 0.03 ± 0.01;the mean contractile amplitude of LMPC increased from -45.8 ± 7.2 to -30.5 ± 2.9; the motility index of CMDC decreasedfrom 86.6± 17.3 to 32.8 ± 9.3; P＜ 0.05 vs areca), but theeffects were not inhibited by hexamethonium (P＞ 0.05).CONCLUSION: Areca stimulated the motility of isolatedcolonic smooth muscle strips in rats. The stimulation ofareca might be relevant with M reoeptor partly.     

Role of Crk-associated substrate in the regulation of vascular smooth muscle contraction

A pool of actin monomers is induced to polymerize into actin filaments during contractile stimulation of smooth muscle. The inhibition of actin dynamics by actin polymerization inhibitors depresses active force generation in smooth muscle. In this study, we hypothesized that Crk-associated substrate plays a role in the regulation of contraction and actin dynamics in vascular smooth muscle. Antisense or sense oligodeoxynucleotides for Crk-associated substrate were introduced into carotid smooth muscle tissues by chemical loading. The treatment of smooth muscle strips with antisense oligodeoxynucleotides inhibited the expression of Crk-associated substrates; it did not influence the expression of actin, myosin heavy chain, and paxillin. Sense oligodeoxynucleotides did not affect the expression of these proteins in smooth muscle tissues. Force generation in response to stimulation with norepinephrine or KCl was significantly lower in antisense-treated muscle strips than in sense-treated strips or in muscle strips not treated with oligodeoxynucleotides. The downregulation of Crk-associated substrate did not attenuate increases in phosphorylation of the 20-kDa regulatory light chain of myosin in response to stimulation with norepinephrine. The increase in F-actin/G-actin ratio during contractile stimulation was significantly inhibited in antisense-treated smooth muscle strips. Contractile activation of smooth muscle increased the association of profilin with actin monomers; the depletion of Crk-associated substrate inhibited the increases in the profilin-actin complex in response to contractile stimulation. These results suggest that Crk-associated substrate is a necessary molecule of signaling cascades that regulate active force generation in smooth muscle. This molecule may regulate actin dynamics in smooth muscle in response to contractile stimulation.     

COX-2 up-regulation and vascular smooth muscle contractile hyperreactivity in spontaneous diabetic db/db mice

OBJECTIVE: Abnormalities in vascular constriction and dilatation are associated with early diabetes and contribute to diabetic vascular complications. However, mechanisms underlying such vascular dysfunction remain to be fully elucidated. The current study tests the role of cyclooxygenase-2 (COX-2) in diabetic vascular smooth muscle dysfunction. METHODS: Small mesenteric artery and aorta are isolated from type 2 diabetic db/db and control mice. Isometric contractions in response to serotonin, angiotensin II, phenylephrine and high potassium are determined in small spiral mesenteric arterial or aortic strips. COX-2 mRNA and protein levels are analyzed by using DNA microarray, real-time PCR and immunoblot. RESULTS: Contractions induced by serotonin, angiotensin II, phenylephrine and high potassium are significantly higher in endothelium-denuded smooth muscle strips isolated from db/db mice than in those isolated from control mice. The contractile hyperreactivity is observed in aortic and third-order branch small mesenteric arterial smooth muscle strips. DNA microarray, real-time PCR and immunoblot analysis show that compared with control mice, COX-2 mRNA and protein are significantly increased in db/db mice aortic smooth muscle. The COX-2 up-regulation is temporally associated with the development of diabetes mellitus and vascular smooth muscle contractile hyperreactivity. Inhibition of COX-2 with NS-398 or SC-58125 partially--but significantly--alleviates agonist-induced but not potassium-induced contractile hyperreactivity. In addition, serum isolated from db/db mice induces COX-2 expression and increases thromboxane A2 production in primary cultured vascular smooth muscle cells (VSMC). SQ-29548, a TP receptor antagonist, diminished the db/db mice vascular smooth muscle contractile hyperreactivity. CONCLUSIONS: COX-2 is up-regulated and contributes at least in part to the vascular smooth muscle contractile hyperreactivity in db/db mice.     

Stretch-induced changes in constricted lung parenchymal strips: role of extracellular matrix.

Large amplitude oscillations of contracted airway smooth muscle cause relative relaxation of the preparation. However, little is known about the effect of mechanical stretch on distal lung behaviour. Rat parenchymal strips were suspended in an organ bath and attached at one end to a force transducer and at the other end to a servo-controlled lever arm that effected length changes. Mechanical impedance of the strip was measured by applying a complex signal consisting of pseudorandom length oscillations of varying frequencies (0.5-19.75 Hz). A constant phase model was fit to changes in length and tension to calculate tissue damping (G) and elastance (H). Hysteresivity was calculated as G/H. Impedance was measured before and after sinusoidal length oscillation at different amplitudes (1, 3, 10 and 25% of resting length) at a frequency of 1 Hz under baseline conditions and after acetylcholine-induced constriction. Oscillations of 10 and 25% amplitudes significantly decreased the G and H of the lung strip. The effect of length oscillations was no different in control versus constricted strips. These data suggest that in the distal lung, large stretches affect the structural components of the extracellular matrix rather than the contractile elements.     

Effect of age on blood pressure and membrane-dependent vascular responses in the rat

Alterations in endothelium-dependent, sodium pump-mediated, and calcium-dependent responses of vascular smooth muscle were investigated in 5-7-, 24-26-, and 50-52-week-old male Sprague-Dawley rats. Age-dependent changes in systolic blood pressure were also determined. Although systolic blood pressure increased significantly with age, rats in all 3 age groups were considered normotensive. Initial studies on the passive force-response characteristics of strips of aortic and femoral arterial smooth muscle revealed that the level of passive force required for maximum active tension generation increased with increasing age. Subsequent studies were carried out using optimum passive force requirements. Endothelium-dependent relaxations of aortic smooth muscle induced by acetylcholine and the calcium ionophore A23187 decreased significantly with increasing age. An age-dependent decrease in the contractile response of aortic smooth muscle to ouabain and potassium-free physiological salt solution (PSS) was observed. Potassium relaxation of femoral smooth muscle following contraction to norepinephrine (NE) in a potassium-free PSS was also significantly attenuated with increasing age. No age-related alterations in calcium sensitivity (in the presence of 10(-7) M NE) or calcium relaxation (membrane stabilization) of femoral arterial smooth muscle was seen. These results show that endothelium-dependent and sodium pump-mediated responses are reduced in vascular smooth muscle of the rat with increasing age. However, no changes in calcium-dependent responses are apparent. These observations are discussed in relation to the vascular changes observed in hypertension.     

Effect of progesterone on canine colonic smooth muscle

This study was undertaken to ascertain the effect and mode of action of progesterone on canine colonic smooth muscle in vitro. Circularly orientated strips of smooth muscle exhibited low-amplitude contractions at the slow wave frequency. Longitudinally orientated strips exhibited larger-amplitude contractions that were associated, electrically, with a series of "spikes" superimposed on a high-frequency oscillation in membrane potential. Progesterone reduced the contractile force of both the circularly and longitudinally orientated strips and the contractile frequency of the longitudinally orientated strips in a dose-dependent manner; this inhibitory effect was observed in the presence of tetrodotoxin but not atropine. The stimulatory effects of a high-potassium superfusate were antagonized by progesterone and verapamil, the effect of progesterone being reversed by increasing the calcium concentration of the superfusate. We conclude that progesterone exerts an inhibitory effect on colonic smooth muscle and this may be mediated by changes in the cytoplasmic calcium concentration.     

Pharmacologic responsiveness of rat parenchymal strips, bronchi, and bronchioles

It has been inferred from previous studies that leukotrienes C4 and D4 preferentially exhibit their effects on peripheral airways. Thus we used LTC4 to examine the responsiveness of parenchymal strips, lung preparations often used as in vitro models of airway function, and to compare the responses with those observed in a preparation of isolated peripheral airways. In these studies, the effects of LTC4 on isolated bronchioles of the rat were compared to responses observed in parenchymal strips and primary intrapulmonary bronchi. Parenchymal strips contracted in response to increasing concentrations of LTC4 and to a single concentration of bethanechol. When the maximum responses were normalized to that induced by membrane depolarization, it was found that the parenchymal strip was more responsive to the leukotriene. Primary intrapulmonary bronchi similarly contracted in response to LTC4; however, the intrapulmonary bronchi were much more responsive to bethanechol than to the leukotriene. The bronchioles were not responsive to LTC4 but did contract in response to membrane depolarization and on exposure to bethanechol. When normalized, the responsiveness of the bronchiole to bethanechol was significantly greater than the responsiveness of the bronchi to this agonist. Thus contraction of the rat parenchymal strip to LTC4 cannot be attributed to the direct effects of this agonist on bronchiolar smooth muscle. We conclude that the bronchiole of the rat is not responsive to LTC4 and that the contractions observed in the parenchymal strip in response to this agonist must result from a mechanism other than direct action of LTC4 on peripheral airway smooth muscle.     

Characterization of a mammalian smooth muscle myosin heavy chain cDNA clone and its expression in various smooth muscle types.

A cDNA clone, SMHC-29, encoding the light meromyosin of smooth muscle myosin heavy chain (MHC), was isolated from a rabbit uterus cDNA library constructed in phage lambda gt11. This smooth muscle MHC cDNA demonstrates significant nucleotide and amino acid sequence homologies with known sarcomeric MHC genes from rabbit, rat skeletal, and nematode body wall myosin, and even with nonmuscle MHC gene from a slime mold (Dictyostelium discoideum), suggesting that smooth muscle, striated muscle, and nonmuscle MHC genes diverged from a common ancestor. The deduced amino acid sequences of the smooth muscle light meromyosin show very similar periodic distributions of hydrophobic and charged residues as found for the light meromyosin of striated muscle MHCs together with a high potential for alpha-helical formation, indicating an alpha-helical coiled-coil structure for the smooth muscle light meromyosin sequences. Furthermore, S1 nuclease mapping has revealed that this smooth muscle MHC gene for SMHC-29 is specifically expressed in smooth muscles of vascular and nonvascular types but not in the striated muscles or nonmuscle cells.     

Mast cell heterogeneity and hyperplasia in bleomycin-induced pulmonary fibrosis of rats

The distribution, density, and histochemical subtype of mast cells were studied in the respiratory tract of rats with bleomycin-induced pulmonary fibrosis. In normal rats, mast cell densities were highest in the trachea and lowest in the bronchus and parenchyma. Two histochemically distinct mast cell populations were identified in the mucosa adjacent to the tracheal cartilage, but elsewhere only a single population of typical connective tissuelike mast cells was found. After intratracheally administered bleomycin, lung histamine levels (micrograms/g wet weight) increased as much as 14-fold by Day 50. Pulmonary mast cell changes were present early in the fibrotic process, and by Day 14 the mast cell density in the parenchyma was 10 times normal. These parenchymal mast cells were histochemically of the connective tissue type. Thus, pronounced mast cell hyperplasia occurs during the evolution of experimental pulmonary fibrosis. This model provides a powerful tool to study pulmonary mast cells and to identify their role in fibrotic disease.     

Morphological and functional characterization of vascular muscle cells enzymatically dispersed from dog mesenteric arteries.

The objective of this study was to compare the properties of single smooth muscle cells enzymatically dispersed from the dog mesenteric arteries to the properties of similar cells functioning in tissue strips. The isolated cells remained relaxed in nominally Ca(2+)-free medium for about 1-2 h after exposure to 1 mM Ca2+ and like intact mesenteric artery rings did not contract spontaneously. Enzymatically dispersed cells maintained all the characteristic morphological features observed in strips of muscle prior to isolation except that the amorphous materials covering the smooth muscle cell surfaces (basal lamina) were absent after enzymatic dispersion. Addition of 100 mM KCl to these vascular muscle cells elicited maximal shortening in the presence but not in the absence of extracellular Ca2+ and KCl-induced cell shortening was prevented by 10(-7) M nifedipine indicating the presence of functional voltage-operated Ca2+ channels. However, in contrast to the vascular muscle strips, in which graded contractile responses were observed with increasing KCl concentrations, isolated vascular muscle cells underwent nearly maximal contraction at concentrations as low as 15 mM KCl. Both intact tissue and isolated cell preparations responded similarly to phenylephrine in a concentration-dependent manner and the responses were blocked by prazosin. In contrast to muscle strips, the isolated cells did not shorten in response to phenylephrine in Ca(2+)-free medium. Isolated muscle shortened in the presence of sarcoplasmic reticulum selective Ca2+ transport ATPase inhibitors, cyclopiazonic acid or thapsigargin. Ryanodine also caused contraction. We conclude that enzymatically dispersed smooth muscle cells from dog mesenteric arteries are potentially useful for studies of the regulation of smooth muscle contractility, but have significantly increased sensitivity to external K+, implying an altered membrane potential or voltage dependence of ion channels. Their impaired ability to contract to phenylephrine in Ca(2+)-free medium implies some alteration in intracellular Ca2+ stores of their coupling to cellular activation. These differences will affect how the data obtained from freshly isolated enzymatically dispersed vascular muscle cells may be extrapolated to cell studies in intact tissues.     

Mechanisms of angiotensin II- and arginine vasopressin-induced increases in protein synthesis and content in cultured rat aortic smooth muscle cells. Evidence for selective increases in smooth muscle isoactin expression

Previous studies from this laboratory have demonstrated that angiotensin II (Ang II) and arginine vasopressin (AVP) are potent hypertrophic agents in cultured rat aortic smooth muscle cells. The present study identified major proteins that accumulate in Ang II-induced and AVP-induced hypertrophic cells and initiated studies of the mechanisms that contribute to their accumulation. Smooth muscle cell hypertrophy induced by Ang II and/or AVP (1 microM each) was associated with widespread increases in the content of many cellular proteins that were resolved by one- and two-dimensional gel electrophoresis. However, increases were also selective in nature, with increases in certain individual proteins, including actin (twofold to threefold), vimentin (2.5-fold to sevenfold), tropomyosin (threefold to sixfold), and myosin heavy chain, far exceeding overall increases in cellular protein content (20-40%). Increases in actin content were due largely to increased expression of smooth muscle alpha-actin (3.6- to 7.5-fold), as opposed to nonmuscle beta-actin (1.7- to 2.5-fold). Increases in smooth muscle alpha-actin were accompanied by a fivefold to eightfold increases in smooth muscle alpha-actin mRNA, indicating that these changes were not due exclusively to translational controls. Results demonstrate that contractile agonist-induced hypertrophy in cultured smooth muscle cells is due, in part, to increased expression of smooth muscle contractile proteins. Furthermore, the fact that Ang II and AVP induced selective increases in smooth muscle alpha-actin suggests that these agonists may not only regulate growth of vascular smooth muscle but may also promote expression of smooth muscle-specific contractile proteins during differentiation of vascular smooth muscle.     

Mechanics and crossbridge kinetics of tracheal smooth muscle in two inbred rat strains.

The aim of the study was to determine whether the nonspecific in vivo airway hyperresponsiveness of the inbred Fisher F-344 rat strain was associated with differences in the intrinsic contractile properties of tracheal smooth muscle (TSM) when compared with Lewis rats. Isotonic and isometric contractile properties of isolated TSM from Fisher and Lewis rats (each n=10) were investigated, and myosin crossbridge (CB) number, force and kinetics in both strains were calculated using Huxley's equations adapted to nonsarcomeric muscles. Maximum unloaded shortening velocity and maximum extent of muscle shortening were higher in Fisher than in Lewis rats (approximately 46% and approximately 42%, respectively), whereas peak isometric tension was similar. The curvature of the hyperbolic force/velocity relationship did not differ between strains. Myosin CB number and unitary force were similar in both strains. The duration of CB detachment and attachment was shorter in Fisher than in Lewis rats (approximately -46% and -34%, respectively). In Fisher rats, these results show that inherited, genetically determined factors of airway hyperresponsiveness are associated with changes in crossbridge kinetics, contributing to an increased tracheal smooth muscle shortening capacity and velocity.     

Evidence implicating nonmuscle myosin in restenosis. Use of in situ hybridization to analyze human vascular lesions obtained by directional atherectomy.

BACKGROUND. Identification of genes that are specifically activated in restenosis lesions after percutaneous transluminal angioplasty represents a necessary step toward molecular manipulation designed to inhibit cellular proliferation responsible for such lesions. Whereas quiescent smooth muscle cells (contractile phenotype) preferentially express smooth muscle myosin, proliferating smooth muscle cells (synthetic phenotype) have been shown to preferentially express nonmuscle myosin in vitro. Accordingly, we analyzed the expression of a recently cloned isoform of human nonmuscle myosin heavy chain (MHC-B) in fresh human restenotic lesions. METHODS AND RESULTS. A total of 10 lesions, including four restenosis (three superficial femoral arterial lesions and one saphenous vein bypass lesion) and six primary (four superficial femoral arterial lesions and two coronary arterial lesions) obtained percutaneously by directional atherectomy, were processed for examination by in situ hybridization. In total, 150 tissue sections of restenotic lesions (66 sections), primary lesions (78 sections), and normal internal mammary artery (six sections) were hybridized with the nonmuscle MHC-B probe. Restenotic lesions showed intense hybridization to the nonmuscle MHC-B cRNA probe, as demonstrated by a clustering of more than 20 grains per cell nucleus in 80% of the cells examined within a high-power field (x250); in contrast, an equivalent degree of hybridization was observed in only 7% of cells within primary lesions (p less than 0.001). Results of immunocytochemistry using monoclonal antibody to smooth muscle actin indicated that cells demonstrating strong hybridization were smooth muscle in origin. CONCLUSIONS. These findings demonstrate that 1) human vascular tissue obtained by percutaneous directional atherectomy constitutes appropriate biopsy material for gene expression studies at the mRNA level, and 2) nonmuscle MHC-B mRNA is present in greater abundance among restenotic versus primary vascular stenoses. These observations thus provide a rational basis to explore restenotic lesions on a larger scale to identify genes that are activated in these lesions and establish potential targets for future gene therapy.     

Effects of almitrine on diaphragm contractile properties in young and old rats

BACKGROUND: Diaphragm muscle force and fatigue are key factors in the development of respiratory failure. Almitrine is used to improve ventilatory drive and ventilation-perfusion matching in respiratory failure. Recently, it has also been shown to improve diaphragm muscle force and endurance in young rats, but it is not known if this effect persists with ageing. OBJECTIVES: To determine the effects of almitrine on diaphragm contractile properties in young and old rats. METHODS: In young and old rats, isometric contractile properties were measured in strips of isolated diaphragm muscle in physiological saline solution at 30 degrees C with or without almitrine. RESULTS: In young animals, almitrine increased twitch tension, reduced half-relaxation time and increased endurance, but had no effect on tetanic tension, contraction time or tension-frequency relationship. Ageing had no effect on endurance, but did reduce twitch and tetanic tension and contraction and half-relaxation time. Almitrine had no effect on contractile tension and kinetics, tension-frequency relationship or on endurance in the old animals. CONCLUSIONS: Ageing negates the beneficial effects of almitrine on diaphragm muscle force and endurance.     

Bronchodilator action of an agonist for histamine H3-receptors in guinea pig perfused bronchioles and lung parenchymal strips

Isolated guinea pig perfused bronchioles and lung parenchymal strips were examined as an in vitro model for assessment of the direct effect of pharmacologic agents on the airway smooth muscle. The experiments were performed with a perfusion technique in bronchioles, the input pressure being measured as an index of the state of dilation, while changes in tension of the lung parenchymal strips were measured with an isometric force transducer. In both preparations, histamine and acetylcholine elicited dose-related contractile responses whereas fenoterol induced a concentration-dependent relaxation. After the 3 agonists' activities were compared in these 2 preparations, we tested the intrinsic effects of a specific H₃ agonist, (R)-methylhistamine ([R]-MeHA). Statistical analysis was by Student's t test on the E_max (expressed as a percentage of 10^–4 M papaverine relaxation), EC₅₀, and slopes of regression lines calculated from the concentration-response curves plotted for (R)-MeHA alone or in presence of antagonists. Our results showed that (R)-MeHA induced a concentration-dependent relaxation of perfused bronchioles and lung parenchymal strips competitively inhibited by 10^–7 M thioperamide (H₃-antagonist), whereas 10^–5 M cimetidine (H₂-antagonist) failed to prevent this effect. These results suggest the presence of H₃-histaminergic dilatory receptors in the guinea pig airway. Offprint requests to: J.-L. Burgaud