Effect of age on the formation of cyclic nucleotides in guinea-pig tracheal smooth muscle in response to pharmacological agents

cAMP-independent mechanism is significantly involved in beta2-adrenoceptor-mediated tracheal relaxation

The role of cAMP in the beta2-adrenoceptor-mediated relaxation in response to salbutamol was examined in guinea pig tracheal smooth muscle. The concentration-dependent salbutamol-induced relaxation was antagonized in a competitive fashion by a beta2-selective adrenoceptor antagonist, butoxamine, with a pA2 value of 6.90. Salbutamol (10 microM) elevated the tracheal smooth muscle cAMP content by about fivefold, a response which was significantly inhibited by an adenylyl cyclase inhibitor, 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ 22,536, 100 microM). However, the salbutamol-elicited relaxation was not diminished by SQ 22,536 (100 microM). These results provide evidence for the first time that a cAMP-independent mechanism(s) is involved in beta2-adrenoceptor-mediated tracheal smooth muscle relaxation in the guinea pig.

MaxiK channel mediates beta2-adrenoceptor-activated relaxation to isoprenaline through cAMP-dependent and -independent mechanisms in guinea-pig tracheal smooth muscle

We examined the contribution of large-conductance, Ca(2+)-sensitive K+ (MaxiK) channel to beta2-adrenoceptor-activated relaxation to isoprenaline in guinea-pig tracheal smooth muscle focusing on the role for cAMP in the coupling between beta2-adrenoceptor and MaxiK channel. Isoprenaline-elicited relaxation was confirmed to be mediated through beta2-type of adrenoceptor since the response was antagonized in a competitive fashion by a beta2-selective adrenoceptor antagonist butoxamine with a pA2 value of 6.56. Isoprenaline-induced relaxation was significantly potentiated by a selective inhibitor of cyclic AMP-specific phosphodiesterase, Ro-20-1724 (0.1-1 microM). cAMP-dependent mediation of MaxiK channel in the relaxant response to isoprenaline was evidenced since the potentiated response to isoprenaline by the presence of Ro-20-1724 (1 microM) was inhibited by the channel selective blocker, iberiotoxin (IbTx, 100 nM). This concept was supported by the finding that the relaxation to a membrane permeable cAMP analogue, 8-bromo-cAMP (1 mM), was susceptible to the inhibition by IbTx. On the other hand, isoprenaline-induced relaxation was not practically diminished by an adenylyl cyclase inhibitor SQ 22,536 (100 microM). However, isoprenaline-induced relaxation in the presence of SQ 22,536 was suppressed by IbTx. Characteristics of isoprenaline-induced relaxant response, i.e., impervious to SQ 22,536 but susceptible to IbTx, were practically mimicked by cholera toxin (CTX, 5 microg/ml), an activator of adenylyl cyclase coupled-heterotrimeric guanine nucleotide-binding regulatory protein Gs. These findings indicate that in guinea-pig tracheal smooth muscle: 1) MaxiK channel substantially mediates beta2-adrenoceptor-activated relaxation; 2) both cAMP-dependent and -independent mechanisms underlie the functional coupling between beta2-adrenoceptor and MaxiK channel to induce muscle relaxation; and 3) direct regulation of MaxiK channel by Gs operates in cAMP-independent coupling between beta2-adrenoceptor and this ion channel.

Evidence for a significant role of a Gs-triggered mechanism unrelated to the activation of adenylyl cyclase in the cyclic AMP-independent relaxant response of guinea-pig tracheal smooth muscle

Cyclic AMP is a key molecule in the regulation of airway smooth muscle tone. Increased cyclic AMP leads to relaxation of this smooth muscle and its inhibition results in the muscle contraction. A constitutive role for cyclic AMP in the contraction and relaxation of airway muscle is supported by the observations that direct activators of adenylyl cyclase, such as forskolin and membrane-permeable cyclic AMP analogues, relax this smooth muscle potently. This traditional view of the role for cyclic AMP is the basis for the idea that relaxation of airway smooth muscle mediated through adenylyl cyclase-linked, G(s)-coupled receptors, including the beta(2)-adrenoceptor, is achieved mainly by the elevation of cyclic AMP content [cyclic AMP-dependent mechanism(s)]. However, recent pharmacological and biochemical evidence raises a fundamental question concerning the role of cyclic AMP; can G(s)-coupled receptor-mediated relaxation of tracheal smooth muscle be attributed exclusively to cyclic AMP-dependent mechanism(s)? In the present study, we show that cholera toxin (CTX, 5 microg/ml), an activator of the heterotrimeric guanine-nucleotide-binding protein G(s), relaxes guinea-pig tracheal smooth muscle. CTX also elevates tissue cyclic AMP content by about 30-fold and this is practically abolished by an adenylyl cyclase inhibitor, SQ 22,536 (100 microM). However, unexpectedly, the relaxant response to CTX is not affected by SQ 22,536. These results firstly show that activation of G(s) is able to produce a relaxation in tracheal smooth muscle independently of the elevation of cyclic AMP. G(s)-triggered, cyclic AMP-unrelated cellular mechanism(s) seem(s) to play a substantial role in smooth muscle relaxation mediated through adenylyl cyclase-linked receptors. This mechanism may account in part for the cyclic AMP-independent relaxant response of tracheal smooth muscle.

Effect of histamine on lung contractile elements in growing cattle

OBJECTIVE

To determine the effect of histamine on the contractile elements of the respiratory tract in neonatal calves and young adult cattle.

SAMPLE POPULATION

Samples of trachealis muscle, bronchi, and intrapulmonary arteries and veins dissected from the respiratory tracts of healthy bovids (2 to 8 days and 16 to 20 months old).

PROCEDURE

Histamine cumulative concentration-effect curves (10(-6) to 10(-3) M) were constructed in duplicate smooth muscle samples mounted in organ baths. Contractile responses to histamine were compared with reference contractions elicited by methacholine (10(-5) M) for airways or KCl (127 mM) for vessels.

RESULTS

In young adult cattle, trachealis muscle had a substantial contractile response to histamine (84% of methacholine-induced contraction), whereas bronchi reacted slightly (15 and 20% for large and small bronchi, respectively). Although contractile responses to KCl were comparable in arteries and veins, histamine-induced contractions were greater for intrapulmonary veins than for arteries (202 vs 48% of KCl-induced contraction). In neonatal calves, histamine-induced contraction of veins also exceeded that of arteries (230 vs 54% of KCl-induced contraction); however, unlike in young adult cattle, histamine produced notable contraction of large and small bronchi (48 and 60% of methacholine-induced contraction, respectively).

CONCLUSIONS AND CLINICAL RELEVANCE

Compared with intrapulmonary arteries, intrapulmonary veins have greater contractile responses to histamine in neonatal and young adult cattle. Data suggest loss of histamine responsiveness in bronchial smooth muscle as neonatal calves grow to young adults. Venodilation may be useful in treatment of lung edema in cattle.

Age-related changes in airway responsiveness to phosphodiesterase inhibitors and activators of adenyl cyclase and guanylyl cyclase

The influence of animal age on the responsiveness of guinea-pig and rat isolated tracheal smooth muscle to the non-selective inhibitors of phosphodiesterase, theophylline and papaverine and to the adenylyl cyclase and soluble guanylyl cyclase activators, forskolin and sodium nitroprusside respectively, was examined. Significant age-related decreases in the potencies of theophylline and papaverine were observed across the age ranges in guinea-pig (2.8- and 3.4-fold decreases respectively) and rat (1.9- and 2.6-fold decreases respectively) trachea, suggesting age-related falls in the activity of phosphodiesterase in these tissues. However, maximum relaxant responses (E(max)) to these agents were not altered with increasing animal age. The relaxant potency of sodium nitroprusside also decreased 4.4-fold across the age range in guinea-pig isolated trachea but not in rat isolated tracheal tissue, suggesting age-related falls in soluble guanylyl cyclase activity in guinea-pig trachea. In contrast, neither forskolin potency nor E(max)changed significantly with increasing age in either guinea-pig or rat tracheal tissue. The present data indicate that ageing in both guinea-pigs and rats was associated with decreased relaxant potency of phosphodiesterase (PDE) inhibitors rather than to changes in adenylyl cyclase activity although reduced soluble guanylyl cyclase activity was also detected in the guinea-pig.

Actions of second messengers synthesized by various spasmogenic agents and their relation to mechanical responses in dog tracheal smooth muscle

1. The effects of the spasmogenic agents, carbachol (CCh), histamine, 5-hydroxytryptamine (5-HT) and 9,11-epithio-11,12-methano-thromboxane A2 (STA2) were investigated on smooth muscle tissues of the dog trachea. 2. CCh (10 microM) produced a larger contraction than high K (128 mM), 10 microM histamine, 5-HT or STA2. Histamine and 5-HT produced the same amplitude of contraction as each other. In Ca-free solution containing 0.2 mM EGTA, only a phasic contraction was evoked by the above agents (except for K which induced no contraction at all). 3. In skinned muscle tissues, the maximum amplitude of contraction that could be induced by Ca (10 microM) was slightly larger than the maximum CCh-induced contraction (also at 10 microM) evoked in intact muscle tissues. Caffeine and inositol 1,4,5-trisphosphate (IP3) both produced contraction. 4. CCh, histamine and 5-HT (10 microM) produced a sustained contraction for over 30 min and also increased phosphorylation of the 20 kD protein of myosin light chain (MLC20) for over 30 min with no attenuation. Greater concentrations of the above agents caused more phosphorylation of MLC20. 5. CCh (above 1 nM), histamine (above 10 nM) and 5-HT (above 100 nM) increased the amount of IP3, in a concentration-dependent manner. Synthesis of IP3 induced by the above agents reached its peak value within 30 s and lasted for about 3 min. The potencies for the synthesis of IP3 were in the following order: CCh greater than histamine greater than 5-HT greater than STA2. 6. Isoprenaline (10 microM) markedly enhanced but CCh (10 microM) slightly reduced the amount of cyclic AMP. 5-HT (10 microM) and STA2 (10 microM) reduced, but histamine (10 microM) and CCh (1O microM) increased the amount of cyclic GMP. 7. Using fura 2, cytosolic Ca was measured by monitoring the ratio of the fluorescent signal excited at 340 and 380nm wavelengths in the presence of extracellular Ca. CCh (1O microM) increased the Ca transient from 182nm to 1.42 um. When the CCh-induced peak Ca transient (1O microM) was normalised, 10 microM histamine, 5-HT and STA2 showed smaller values such as 0.49, 0.53 and 0.04 times the control, respectively, and these values corresponded well with the amplitudes of contraction evoked by each of the stimulants. 8. The results can be summarized as follows: stronger spasmogenic responses occur on application of CCh than on application of 5-HT or histamine, and STA2 may have a minor role as a spasmogenic agent. The maximum amplitudes (peaks) of contraction evoked by the above spasmogenic agents are closely related to the maximum increase in cytosolic Ca, but sustained contraction and increased phosphorylation of myosin cannot be explained by the increased amount of Ca. In the case of 5-HT and histamine, synthesized cyclic nucleotides may interact with the action of 1P3 for the regulation of contraction in a positive or negative manner, respectively.

Aging and cholinergic responses in bovine trachealis muscle

1. The relative potencies of muscarinic agonists on bovine tracheal smooth muscle were unchanged as a consequence of aging and were carbachol greater than oxotremorine greater than muscarine greater than pilocarpine greater than McNeil A-343. 2. During aging, the potencies of carbachol, oxotremorine, McNeil A-343 and pilocarpine, but not muscarine, were reduced. 3. Maximal induced tensions to all the agents studied were reduced as a consequence of age. 4. Irreversible antagonism with benzilylcholine mustard showed that agonist efficacy was significantly reduced during aging. 5. Estimated receptor occupancy at the EC50 was significantly greater in tracheal tissues from the mature versus immature cows for every agonist studied. 6. The dissociation constants for full agonists (carbachol, oxotremorine and methacholine) were decreased with maturation while the converse was observed with partial agonists (McNeil A-343, pilocarpine). 7. We conclude that there are significant changes in the properties and coupling of muscarinic receptors during aging. These changes may contribute to the reduced airway reactivity seen in vivo.