Hyperosmolar solution effects in guinea pig airways. II. Epithelial bioelectric responses to relative changes in osmolarity

Osmotic challenge of airways alters the bioelectric properties of the airway epithelium and induces the release of factors that modulate smooth muscle tone. Recent studies in our laboratory suggested that methacholine-contracted airways relax in response to incremental increases in osmolarity, rather than from cell shrinkage or absolute solute concentration. In the present study, guinea pig tracheae were mounted in Ussing chambers to elucidate the bioelectric effects of challenge of the epithelium with hyperosmolar and isosmolar solutions. Transepithelial short-circuit current (Isc) across tracheae stimulated with basolateral methacholine was inhibited by apical amiloride, apical 5-nitro-2-(3-phenylpropylamino)benzoic acid, basolateral bumetanide, basolateral ouabain, and Cl(-)-free solution, but not by basolateral iberiotoxin. Apical hyperosmolar challenge with NaCl variably decreased or increased Isc, but D-mannitol (D-M) always inhibited Isc; bumetanide attenuated decreases in Isc. The effects of the transport blockers depended upon whether Isc was initially decreased or increased. Unique concentration-dependent changes in Isc and transepithelial resistance (Rt) were observed when ionic (NaCl and KCl), nonionic impermeant (D-M and sucrose), and nonionic permeant (urea) osmolytes were added to the apical and basolateral baths. At concentrations that doubled the osmolarity of the apical bath, D-M, urea, and N-methyl-D-glucamine-gluconate (NMDG-Glu) decreased Isc. Apical isosmolar NMDG-Glu solution decreased Isc, and additional NMDG-Glu caused a further decrease in Isc. Inclusion of one permeant ion, either Na+,K+, or Cl-, reversed the response to apical isosmolar and hyperosmolar solutions. Thus, bioelectric responses of the airway epithelium to hyperosmolar solution are induced by incremental increases in osmolarity.     

Hyperosmolar solution effects in guinea pig airways. IV. Lipopolysaccharide-induced alterations in airway reactivity and epithelial bioelectric responses to methacholine and hyperosmolarity

We investigated the in vivo and in vitro effects of lipopolysaccharide (LPS) treatment (4 mg/kg i.p.) on guinea pig airway smooth muscle reactivity and epithelial bioelectric responses to methacholine (MCh) and hyperosmolarity. Hyperosmolar challenge of the epithelium releases epithelium-derived relaxing factor (EpDRF). Using a two-chamber, whole body plethysmograph 18 h post-treatment, animals treated with LPS were hyporeactive to inhaled MCh aerosol. This could involve an increase in the release and/or actions of EpDRF, because LPS treatment enhanced EpDRF-induced smooth muscle relaxation in vitro in the isolated perfused trachea apparatus. In isolated perfused tracheas the basal transepithelial potential difference (Vt) was increased after LPS treatment. The increase in Vt was inhibited by amiloride and indomethacin. Concentration-response curves for changes in Vt in response to serosally and mucosally applied MCh were biphasic (hyperpolarization, <3 x 10(-7)M; depolarization, >3 x 10(-7)M); MCh was more potent when applied serosally. The hyperpolarization response to MCh, but not the depolarization response, was potentiated after LPS treatment. In both treatment groups, mucosally applied hyperosmolar solution (using added NaCl) depolarized the epithelium; this response was greater in tracheas from LPS-treated animals. The results of this study indicate that airway hyporeactivity in vivo after LPS treatment is accompanied by an increase in the release and/or actions of EpDRF in vitro. These changes may involve LPS-induced bioelectric alterations in the epithelium.     

Hyperosmolar solution effects in guinea pig airways. III. Studies on the identity of epithelium-derived relaxing factor in isolated perfused trachea using pharmacological agents

Hyperosmolar challenge of airway epithelium stimulates the release of epithelium-derived relaxing factor (EpDRF), but the identity of EpDRF is not known. We examined the effects of pharmacological agents on relaxant responses of methacholine (3 x 10(-7) M)-contracted guinea pig perfused trachea to mucosal hyperosmolar challenge using D-mannitol. Responses were inhibited by gossypol (5 x 10(-6) M), an agent with diverse actions, by the carbon monoxide (CO) scavenger hemoglobin (10(-6) M), and by the heme oxygenase (HO) inhibitor zinc (II) protoporphyrin IX (10(-4) M). The HO inhibitor chromium (III) mesoporphyrin IX (10(-4) M) was not inhibitory, and the HO activator heme-L-lysinate (3 x 10(-4) M) did not evoke relaxant responses. The CO donor tricarbonyldichlororuthenium (II) dimer (2.2 x 10(-4) M) elicited small relaxation responses. Other agents without an effect on responses included: apyrase, adenosine, 6-anilino-5,8-quinolinequinone (LY83583), proadifen, (E)-3-[[[3-[2-(7-chloro-2-quinolinyl)ethenyl]phenyl][[3-(dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid (MK 571), diphenhydramine, glibenclamide, HgCl2, tetrodotoxin, nystatin, alpha-hemolysin, 8-bromoguanosine 3',5'-cyclic monophosphothioate, Rp-isomer, 12-O-tetradecanoylphorbol-13-acetate, cholera toxin, pertussis toxin, thapsigargin, nifedipine, Ca(2+)-free mucosal solution, hydrocortisone, and epidermal growth factor. Cytoskeleton inhibitors, includingerythro-9-(2-hydroxyl-3-nonyl)adenine, colchicine, nocodazole, latrunculin B, and cytochalasins B and D, had no effect on relaxation responses. The results suggest provisionally that a portion of EpDRF activity may be due to CO and that the release of EpDRF does not involve cytoskeletal reorganization.     

The relaxant properties in guinea pig airways of S-nitrosothiols.

Several cellular constituents of the lung have the capacity to synthesize a factor capable of relaxing smooth muscle which has the physicochemical properties of nitric oxide (NO). In other systems, it has been shown that NO may be stabilized in the plasma and cellular milieu by reduced thiol in the form of an S-nitrosothiol (RS-NO). These compounds have half-lives that are significantly greater than that of NO, and also retain the vasorelaxant activity of NO, which is mediated by activating guanylate cyclase and raising cyclic GMP levels. The effects of RS-NO and their potential mechanism of action on airways, however, have not been previously investigated. In this study, we have examined the smooth muscle relaxant properties of several biological and synthetic RS-NO on guinea pig trachea. Our data reveal that RS-NO are generally potent airway smooth muscle relaxants with at least a partial effect through stimulation of cyclic GMP. Relaxations were attenuated significantly by the guanylate cyclase inhibitor methylene blue (P less than .05), and RS-NO-induced increases in cyclic GMP were demonstrated (P less than .0005). The IC50 values for S-nitroso-glutathione, S-nitroso-cysteine, S-nitroso-homocysteine, S-nitroso-N-acetylcysteine, S-nitroso-penicillamine and S-nitroso-captopril were 0.99 +/- 0.09, 3.2 +/- 0.2, 2.1 +/- 0.3, 2.1 +/- 0.8, 1.8 +/- 0.8 and 20 +/- 0.7 microM (mean +/- S.E.M.), respectively. In this system isoproterenol has an IC50 of 0.016 microM and theophylline an IC50 of 74 microM, making the relaxant properties of these NO derivatives of potential pharmacological and physiological relevance.     

Omega-conotoxin-sensitive calcium channels modulate autonomic neurotransmission in guinea pig airways.

omega-Conotoxin GVIA, a peptide derived from the marine snail Conus geographus, is an antagonist of the neuronal N type voltage-sensitive calcium channels associated with neurotransmitter release. The present study investigated effects of this peptide on neurally mediated responses in airways isolated from the guinea pig to determine whether airway nerves are modulated by omega-conotoxin-sensitive calcium channels. Electrical field stimulation was used to induce neurally mediated tachykininergic excitatory responses in guinea pig bronchus and cholinergic excitatory and nonadrenergic noncholinergic inhibitory responses in guinea pig trachea. Exogenous agonists were administered to induce contractile (acetylcholine, substance P) or relaxation (sodium nitroprusside) responses. Tissues were incubated with omega-conotoxin (1 microM) or its vehicle (10 mM acetic acid) for 30 min before establishing frequency- or concentration-response relationships to the various stimuli. Frequency-response curves for neurally mediated cholinergic, nonadrenergic nocholinergic inhibitory and tachykininergic responses were shifted to the right by omega-conotoxin to a similar extent (4- to 5-fold). omega-Conotoxin had no effect on contractile responses elicited by exogenous acetylcholine or substance P or on relaxations induced by sodium nitroprusside. These findings indicate that neurotransmission in afferent tachykininergic kininergic nerves and in efferent cholinergic excitatory and nonadrenergic noncholinergic inhibitory nerves in the airways is modulated by a prejunctional omega-conotoxin-sensitive mechanism.     

The effect of sensory nerve depletion on cholinergic neurotransmission in guinea pig airways.

C-fiber primary afferent sensory nerves, containing neuropeptides such as substance P and neurokinin A, mediate excitatory nonadrenergic, noncholinergic (e-NANC) bronchoconstrictor responses in guinea pigs. We have studied the effects of sensory nerve depletion in vivo and at four different airway levels in vitro. Capsaicin pretreatment significantly reduced the cholinergic responses to electrical field stimulation at all airway levels studied when compared to similar responses obtained in tissues from both untreated animals and animals treated with the vehicle for capsaicin (at 8 Hz, 27.2 +/- 4.9% of maximal contraction to 10(-2) M acetylcholine in lower trachea from vehicle-pretreated animals compared with 11.6 +/- 2.9% in lower trachea from capsaicin-pretreated animals, P less than .01). Stimulation of lower tracheal segments at 30-min intervals revealed a cholinergic response which was followed by a small, delayed e-NANC component. In vivo cholinergic bronchoconstrictor responses to vagal stimulation were also significantly diminished (at 5 Hz from 11.6 +/- 2.2 cm H2O in vehicle-pretreated animals to 3.3 +/- 1.4 cm H2O in capsaicin-pretreated animals). Bronchoconstrictor responses to acetylcholine or substance P in normal, vehicle-pretreated or capsaicin-pretreated animals were not significantly different either in vitro or in vivo. Sensory neuropeptides released from C-fiber afferent nerves may facilitate cholinergic neurotransmission in airways. This effect appears more marked at airway levels in which e-NANC responses are demonstrable.     

Developmental changes in glucose transport of guinea pig erythrocytes.

The developmental changes in the capacity for D-glucose transport of guinea pig erythrocyte membranes were compared to alterations in the electrophoretic pattern of erythrocyte membrane components. Guinea pig erythrocytes lose their D-glucose carrier functions during development. Good correlation was observed between the loss of glucose uptake and apparent decrease of the zone 4.5 of Coomassie Blue-stained membrane proteins on electrophoresis. Reconstitution of membrane preparations in liposomes resulted in a parallel change in the D-glucose uptake and D-glucose penetration of intact erythrocytes. This suggests that the decrease of D-glucose transport capacity during development is caused by the loss of one or more protein components from the erythrocyte membranes.     

The guinea pig I region. II. Functional analysis

We have examined whether an association exists between specific Ia antigen genes and Ir genes which are encoded within the same haplotype. Functionally monospecific sera to the Ia antigens of the guinea pig MHC were selective in their ability to inhibit antigen-specific T-cell proliferation and we were thus able to demonstrate an association between individual Ia specificities and specific Ir genes. The results of these studies in inbred animals were confirmed by examining the association of Ir genes and Ia antigens in the outbred guinea pig population. Of great interest was the observation that antisera made against cross-reactive Ia antigens of strains lacking specific Ir genes would still inhibit immune responses of strains possessing the Ir gene, if the Ir gene was associated with that Ia antigen in the responder strain.     

The guinea pig I region. I. A structural and genetic analysis

The Ia antigens of the guinea pig have been shown to play a central role in the regulation of the immune response. We have previously partially characterized the chemical structure of these antigens. In this communication, we further characterize the structure of the five Ia antigens already described, as well as two new Ia antigens. Evidence is presented which shows that these seven Ia antigens can be organized into three distinct groups, each with a characteristic structure. The Ia.2 determinant of strain 2 and the Ia.3 and Ia.5 determinants of strain 13 animals are found on molecules composed of a 25,000 dalton chain and a 33,000 dalton chain in noncovalent association, or else are individually expressed on nonlinked 33,000 and 25,000 dalton molecules. The Ia.4 and Ia.5 determinants of strain 2 and the Ia.7 determinant of strain 13 are borne on 58,000 dalton molecules in which two chains are linked by disulfide bonds. The Ia.1 and Ia.6 determinants of strain 13 are found on a molecule of 26,000-27,000 daltons. Ia.6 of strain 2 has yet to be definitively assigned. Furthermore, in strain 13 animals the Ia.3 and Ia.5 determinants are borne on the same molecule, as are the Ia.1 and Ia.6 determinants. In strain 2 animals, the Ia.4 and Ia.5 determinants are found on the same molecule. On the basis of chemical structure, we have divided the guinea pig I region into three subregions. The accompanying paper presents evidence of associations between particular Ia genes and Ir genes.     

Antibody to VLA-4, but not to L-selectin, protects neuronal M2 muscarinic receptors in antigen-challenged guinea pig airways.

Antigen challenge of sensitized guinea pigs decreases the function of inhibitory M2 muscarinic autoreceptors on parasympathetic nerves in the lung, potentiating vagally induced bronchoconstriction. Loss of M2 receptor function is associated with the accumulation of eosinophils around airway nerves. To determine whether recruitment of eosinophils via expression of VLA-4 and L-selectin is critical for loss of M2 receptor function, guinea pigs were pretreated with monoclonal antibodies to VLA-4 (HP1/2) or L-selectin (LAM1-116). Guinea pigs were sensitized and challenged with ovalbumin, and M2 receptor function was tested. In controls, blockade of neuronal M2 muscarinic receptors by gallamine potentiated vagally induced bronchoconstriction, while in challenged animals this effect was markedly reduced, confirming M2 receptor dysfunction. Pretreatment with HP1/2, but not with LAM1-116, protected M2 receptor function in the antigen-challenged animals. HP1/2 also inhibited the development of hyperresponsiveness, and selectively inhibited accumulation of eosinophils in the lungs as measured by lavage and histology. Thus, inhibition of eosinophil influx into the lungs protects the function of M2 muscarinic receptors, and in so doing, prevents hyperresponsiveness in antigen-challenged guinea pigs.     

A potassium channel activator modulates both excitatory noncholinergic and cholinergic neurotransmission in guinea pig airways

The effect of a potassium channel activator, cromakalim (BRL 34915), on excitatory nonadrenergic noncholinergic (e-NANC) and cholinergic neural bronchoconstriction was studied in guinea pigs. We monitored airway opening pressure as an index of airway caliber. After atropine (1 mg/kg iv.) and propranolol (1 mg/kg iv.), bilateral vagal stimulation evoked an e-NANC response. Cromakalim did not alter basal airway caliber, but reduced the e-NANC response to vagal stimulation in a dose-dependent manner, with a maximal inhibition of 71.9 +/- 9.2% (mean +/- S.E.) at 400 micrograms/kg i.v. (P less than .01). Pretreatment with phentolamine (2.5 mg/kg i.v.) had no effect on the inhibitory response produced by cromakalim but glibenclamide (25 mg/kg iv.), an inhibitor of ATP-sensitive potassium channels, blocked its effect. Cromakalim had no inhibitory effect on exogenous substance P (5-25 micrograms/kg i.v.)-induced bronchoconstriction. In animals depleted of tachykinins by capsaicin (50 mg/kg s.c.) pretreatment, cromakalim had an inhibitory effect on both vagalcholinergic and exogenous acetylcholine (0.3-2 micrograms/kg i.v.)-induced bronchoconstriction, although the inhibitory effect was significantly greater on neural stimulation. We conclude that potassium channels modulate both e-NANC and cholinergic neurotransmission, and to a lesser extent acetylcholine-induced bronchoconstriction in guinea pig airways.     

Potentiation of nonadrenergic neural relaxation in guinea pig airways by a cyclic cAMP phosphodiesterase inhibitor

We have studied the effect of phosphodiesterase inhibitors on relaxation of guinea pig tracheal smooth muscle in an attempt to elucidate the role of cyclic nucleotides in relaxation to stimulation of inhibitory nonadrenergic noncholinergic (i-NANC) nerves. SK&F 94120 (1-10 microM) potentiated relaxation induced by isoproterenol, vasoactive intestinal peptide (VIP) and electrical field stimulation (EFS) in the presence of atropine and propranolol but had no effect on relaxation induced by sodium nitroprusside. Zaprinast (3-30 microM) potentiated relaxation induced by sodium nitroprusside but not by isoproterenol or VIP. A small potentiation of relaxation to EFS was induced by 30 microM zaprinast but not by lower concentrations. Tetrodotoxin attenuated relaxations induced by EFS suggesting that they are at least partly neurogenic in origin. SK&F 94120 and zaprinast had no effect of tetrodotoxin-resistant relaxation to EFS. The guanylate cyclase inhibitor had no effect on EFS-induced relaxation. These findings suggest that cyclic AMP may mediate relaxation of guinea pig tracheal smooth muscle in response to stimulation of i-NANC nerves, and are in agreement with the view that VIP may be the neurotransmitter released by i-NANC nerves in this tissue.     

An examination of the influence of the epithelium on contractile responses to peptidoleukotrienes and blockade by ICI 204,219 in isolated guinea pig trachea and human intralobar airways

The influence of the epithelium on antagonism by ICI 204,219 of contractile responses to peptide leukotriene (LT) agonists was examined in guinea pig tracheal and human bronchial rings. The -log molar KB values for ICI 204,219 were found to be independent of the epithelium in both tissues. Even though uninfluenced by the epithelium, the -log molar KB values for ICI 204,219 were about 10-fold smaller in human airways than in guinea pig trachea. Removal of the epithelium from guinea pig trachea resulted in small leftward shifts of the concentration-response curves to LTC4 and LTD4 and rightward shifts of the concentration-response curves to LTE4 when examined in the presence of indomethacin. The potentiation of LTC4 and LTD4 by epithelium removal was not seen in the presence of inhibitors of the transformation of LTC4 to LTD4 and LTD4 to LTE4. The influence of the epithelium on responses to LTE4 remained in the presence of these metabolic inhibitors. The lipoxygenase inhibitors nordihydroguaiaretic acid, B755C, Rev 5901 and AA861 antagonized responses to LTE4 in the presence, but not in the absence of epithelium. In human airways, epithelium removal resulted in a small leftward shift of the concentration-response curve to LTD4 whereas responses to LTC4 and LTE4 were unaltered. This effect was not observed in the presence of indomethacin, relating it to reduced release of cyclooxygenase products. These data suggest that contractile responses of guinea pig trachea to LTE4 are modulated by LTE4-induced release of 5-lipoxygenase product(s) only when the epithelium is present.(ABSTRACT TRUNCATED AT 250 WORDS)     

Phasic responses to carbachol in isolated guinea pig trachea are augmented by cooling and inhibited by nifedipine.

Steady-state and kinetic approaches were used to study the effects of cooling to 25 degrees C and nifedipine (1 microM) on the phasic and tonic responses to 0.3 and 10 microM carbachol (Carb) in isolated segments of epithelium-free guinea pig trachea. Cooling and nifedipine had no effect on the steady-state tensions of the contractile responses to Carb in Krebs-bicarbonate buffer, but did inhibit the response to 40 mM KCl. Cooling increased the tensions of the phasic responses to both concentrations of Carb in Ca(++)-depleted, ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid-containing buffer, whereas the rate constant of decay of the phasic response (kdecay) was decreased only at 0.3 microM Carb. Nifedipine inhibited the tension and increased the kdecay of the phasic response to 10 microM Carb at both 37 and 25 degrees C. The phasic response to 0.3 microM Carb was essentially completely inhibited by nifedipine. The tension of the tonic response to Carb was unaffected by cooling and was only inhibited by nifedipine at 0.3 microM Carb at 37 degrees C. Both cooling and nifedipine, at 37 degrees C, decreased the rate constant of onset of the tonic response (kon) to Carb. The predominant effect of cooling to augment the phasic component of the contractile response to Carb, whereas it does not provide the mechanism to explain the enhanced bronchoconstrictor response of airways to cold, does suggest that Ca++ mobilization is altered by cold and that this may contribute to the enhanced bronchoconstrictor response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antibody to very late activation antigen 4 prevents antigen-induced bronchial hyperreactivity and cellular infiltration in the guinea pig airways

This report examines the effect of an anti-VLA-4 monoclonal antibody (mAb) HP1/2 on antigen-induced bronchial hyperreactivity to methacholine, and on eosinophil and T lymphocyte infiltration in the airways of guinea pigs sensitized and challenged by aerosolized ovalbumin and used 24 h thereafter. The intravenous administration of 2.5 mg/kg of HP1/2, but not of its isotype-matched mAb 1E6, 1 h before and 4 h after antigen inhalation, markedly inhibited the increased bronchopulmonary responses to intravenous methacholine, as well as airway eosinophilia in bronchoalveolar lavage (BAL) fluid and in bronchial tissue. HP1/2 also suppressed the antigen-induced infiltration of the bronchial wall by CD4+ and CD8+ T lymphocytes, identified by immunohistochemical technique using specific mAbs that recognize antigenic epitopes of guinea pig T cells. Treatment with HP1/2 also resulted in a significant increase in the number of blood eosinophils, suggesting that inhibition by anti-VLA-4 mAb of eosinophil recruitment to the alveolar compartment may partially account for their accumulation in the circulation. These findings indicate that eosinophil and lymphocyte adhesion and subsequent infiltration into the guinea pig airways that follow antigen challenge are mediated by VLA-4. Furthermore, concomitant inhibition of antigen-induced bronchial hyperreactivity and of cellular infiltration by anti-VLA-4 mAb suggests a relationship between airway inflammation and modifications in the bronchopulmonary function.     

Electrophysiological effects of sodium channel blockers on guinea pig left atrium.

The electrophysiological effects of five sodium channel blockers (mexiletine, lidocaine, disopyramide, aprindine and flecainide) on the guinea pig left atrium were investigated by recording the action potential and its maximum rate of rise (Vmax). The onset and offset kinetics of use-dependent block of Vmax were analyzed. Lidocaine, aprindine and flecainide were classified clearly as fast, intermediate and slow, respectively. Mexiletine and disopyramide had two components in onset and offset of use-dependent block. Mexiletine showed fast and intermediate kinetics, whereas disopyramide showed intermediate and slow kinetics. Action potential duration at 90% repolarization (APD) was prolonged by disopyramide and mexiletine. The other drugs did not change the action potential duration. Effective refractory period was prolonged by all drugs with relative potency in the following order: disopyramide greater than mexiletine greater than lidocaine greater than aprindine = flecainide. In conclusion, the modes of actions of sodium channel blockers on the atrium were disclosed to be different from those on the ventricle. The pharmacological therapy for atrial arrhythmias should be based on the electrophysiological effects of the drugs on the atrium, not on the ventricle.     

Antigen-induced enhancement of noncholinergic contractile responses to vagus nerve and electrical field stimulation in guinea pig isolated trachea.

Nonadrenergic, noncholinergic contractions were elicited by electrical field stimulation (EFS) (2 Hz, 1 msec, 12 V for 15 sec) of the distal aspect of guinea pig trachea pretreated with atropine (1 microM), propranolol (1 microM) and indomethacin (3 microM). The contractions were abolished by pretreatment with the sensory C-fiber toxin capsaicin or by a combination of the neurokinin (NK)1 receptor antagonist, CP 96,345 (0.1 microM), and the NK2 receptor antagonist, MEN 10376 (3 microM), and were markedly attenuated by tetrodotoxin. In animals actively sensitized to ovalbumin, the addition of threshold concentrations of antigen markedly increased the noncholinergic contractile responses to EFS (approximately 3- to 6-fold). This potentiation was long lasting, persisting virtually unchanged for 60 min, whereas the antigen-induced contractions were shorter lived, usually lasting less than 30 min. The ovalbumin-induced potentiation of the neuronal response was not observed in tissues pretreated with capsaicin or treated with tetrodotoxin. This antigen-induced potentiation of capsaicin-sensitive, EFS-induced contractions was not mimicked by serotonin or prostaglandin D2. However, it was mimicked by histamine. Moreover, the histamine H1 receptor antagonist pyrilamine (0.3 microM) reversed the potentiation elicited by ovalbumin. The effect of ovalbumin challenge was also examined on the distal trachea with the right vagus nerve intact. Noncholinergic contractions to EFS and vagus nerve stimulation were enhanced equally by threshold concentrations of antigen. The results support the hypothesis that antigen challenge releases histamine which acts via H1 receptors to enhance noncholinergic contractions due to the release of tachykinins from capsaicin-sensitive fibers in the guinea pig trachea.     

Negative and positive inotropic responses to muscarinic agonists in guinea pig and rat atria in vitro

In the guinea pig atria, carbachol, acetylcholine and bethanechol elicited negative inotropic and positive inotropic effects. In the rat atria, a negative inotropic response occurred, but the positive inotropic response was small. The positive and negative inotropic responses to carbachol and bethanechol (but not acetylcholine) were unaffected by pretreating the animals with reserpine and were antagonised by pirenzepine with pKB values of 6.7. Pretreatment with pertussis toxin abolished the negative inotropic responses, but was without effect on the positive inotropic responses in the guinea pig. Pretreatment of rats with pertussis toxin abolished the negative inotropic response and enhanced the positive inotropic response. The positive inotropic response was attenuated by pretreatment with dietary lithium for 2 weeks, whereas no effect was observed on the negative inotropic response. Negative and positive inotropic responses to muscarinic agonists in these species are mediated directly through an M2 muscarinic receptor. The ability of dietary lithium to selectively inhibit the positive inotropic response may provide evidence for the involvement of inositol phospholipid hydrolysis in this effect.