The effect of age on -adrenoceptor activity in tracheal smooth muscle

Sensory neural responses to ozone exposure during early postnatal development in rat airways

Airway infections or irritant exposures during early postnatal periods may contribute to the onset of childhood asthma. The purpose of this study was to examine critical periods of postnatal airway development during which ozone (O(3)) exposure leads to heightened neural responses. Rats were exposed to O(3) (2 ppm) or filtered air for 1 hour on specific postnatal days (PDs) between PD1 and PD29, and killed 24 hours after exposure. In a second experiment, rats were exposed to O(3) on PD2-PD6, inside a proposed critical period of development, or on PD19-PD23, outside the critical period. Both groups were re-exposed to O(3) on PD28, and killed 24 hours later. Airways were removed, fixed, and prepared for substance P (SP) immunocytochemistry. SP nerve fiber density (NFD) in control extrapulmonary (EXP) epithelium/lamina propria (EPLP) increased threefold, from 1% to 3.3% from PD1-PD3 through PD13-PD15, and maintained through PD29. Upon O(3) exposure, SP-NFD in EXP-smooth muscle (SM) and intrapulmonary (INT)-SM increased at least twofold at PD1-PD3 through PD13-PD15 in comparison to air exposure. No change was observed at PD21-PD22 or PD28-PD29. In critical period studies, SP-NFD in the INT-SM and EXP-SM of the PD2-PD6 O(3) group re-exposed to O(3) on PD28 was significantly higher than that of the group exposed at PD19-PD23 and re-exposed at PD28. These findings suggest that O(3)-mediated changes in sensory innervation of SM are more responsive during earlier postnatal development. Enhanced responsiveness of airway sensory nerves may be a contributing mechanism of increased susceptibility to environmental exposures observed in human infants and children.

Prenatal and early, but not late, postnatal exposure of mice to sidestream tobacco smoke increases airway hyperresponsiveness later in life

BACKGROUND

Cigarette smoke exposure in utero and during early postnatal development increases the incidence of asthma and airway hyperresponsiveness (AHR) later in life, suggesting that a possible critical period of developmental sensitivity exists in the prenatal and early postnatal periods.

OBJECTIVE

We investigated mechanisms of susceptibility during critical developmental periods to sidestream smoke (SS) exposure and evaluated the possible effects of SS on neural responses.

METHODS

We exposed three different age groups of mice to either SS or filtered air (FA) for 10 consecutive days beginning on gestation day (GD) 7 by maternal exposure or beginning on postnatal day (PND) 2 or PND21 by direct inhalation. Lung function, airway substance P (SP) innervation, and nerve growth factor (NGF) levels in broncho alveolar lavage fluid were measured after a single SS exposure on PND59.

RESULTS

Methacholine (MCh) dose response for lung resistance (R(L)) was significantly elevated, and dynamic pulmonary compliance (C(dyn)) was significantly decreased, in the GD7 and PND2 SS exposure groups compared with the FA groups after SS exposure on PND59. At the same time points, the percent area of SP nerve fibers in tracheal smooth muscle and the levels of NGF were significantly elevated. MCh dose-response curves for R(L) and C(dyn), SP nerve fiber density, and the level of NGF were not significantly changed in the PND21 exposure group after SS exposure on PND59.

CONCLUSIONS

These results suggest that a critical period of susceptibility to SS exposure exists in the prenatal and early postnatal period of development in mice that results in increased SP innervation, increased NGF levels in the airway, and enhanced MCh AHR later in life.

Ontogeny of beta-adrenergic desensitization in rabbit tracheal smooth muscle

Prolonged or repeated exposure to beta-agonist medications may result in a desensitization of the agonist-mediated response. Under certain conditions, such agonist-induced desensitization may limit the efficacy of administered beta-adrenergic agonists to elicit bronchodilation. Accordingly, the present study was designed to study the mechanism of acute beta-adrenergic desensitization in maturing rabbit tracheal smooth muscle (TSM). Isometric tension was measured in tracheal ring segments isolated from newborn and mature rabbits and half-maximally contracted with Methacholine (Meth) or KCl. TSM segments were serially relaxed with repetitive single doses of isoproterenol (ISO: 0.1, 1.0, 10, or 100 microM) or prostaglandin E2 (PGE2: 0.1 or 10 microM). Serial administration of ISO-elicited dose-dependent desensitization of relaxation in mature and newborn TSM, contracted with either Meth or KCl. In contrast, the relaxant response to PGE2 was retained in the ISO-desensitized tissue. Repeated administration of PGE2 elicited no desensitization of PGE2 responsiveness, but did induce some dose-dependent desensitization of the ISO response in mature TSM. Compared to mature tissues, newborn TSM developed subtotal desensitization to 100 microM ISO and no ISO desensitization in response to PGE2. Thus, these findings demonstrate that (1) beta-adrenoceptor responsiveness undergoes dose-dependent homologous and, to a lesser extent, heterologous desensitization in rabbit TSM; and (2) both beta-adrenergic desensitization mechanisms increase with postnatal maturation.

Relaxant effects of theophylline and clenbuterol on tracheal smooth muscle from horse and rat in vitro

A comparison between the relaxant effects of clenbuterol and theophylline on horse tracheal smooth muscle has been made in vitro. Rat tracheal smooth muscle was also investigated as a reference. The tracheal preparations were initially contracted with carbachol since the smooth muscle did not spontaneously develop tone. The response of the carbachol-contracted preparations to theophylline was the same in the two species. The response to clenbuterol varied. In only five out of eleven horses were the tracheal smooth muscles sensitive to clenbuterol (mean pD2 = 7.92 M). In the remaining six horses the tracheal smooth muscles were insensitive to clenbuterol (mean pD2 = 3.59 M), yet the preparations responded well to theophylline with complete relaxation. All rat tracheal preparations were insensitive to clenbuterol.

Beta-adrenergic function in aging. Basic mechanisms and clinical implications

Catecholamines have an important endocrine and neuroendocrine role in mediating a variety of autonomic functions. One consequence of normal aging, in particular in the cardiovascular system, is a decline in beta-adrenergic function associated with an alteration in responsiveness to beta-adrenergic therapy. The intrinsic ability for muscle contractility or relaxation is maintained with age and there appears to be an alteration in the process linking the receptor with the contractile or relaxation mechanisms. In rats, beta-adrenergic receptor density decreases with age in adipose tissues and most brain areas, is unchanged in lymphocytes, heart and lung, and increases in the liver. In humans, there are no receptor changes with age in either lymphocytes or brain. In contrast, the number of high-affinity receptors (or coupled receptors) decreases with age in most tissues. In addition, there is a decrease in membrane adenylate cyclase activity or cellular production of cyclic adenosine monophosphate (adenosine 3',5'-cyclic phosphate; cAMP). Plasma noradrenaline (norepinephrine) concentration increases with age. The reduced receptor number in some tissues (down-regulation), the reduced high-affinity receptors and the reduced hormone-stimulated adenylate cyclase activity with age suggests receptor desensitisation to increased plasma noradrenaline concentration. The inability of older animals to desensitise to beta-adrenergic agonists further supports this hypothesis. However, there is an additional post-receptor reduction in catalytic unit activity with age independent of desensitisation. Medications directed at the beta-adrenergic system are commonly used in the elderly. Many of the data on the impact of age on clinical responses are conflicting or unavailable. Concomitant disease, functional status, nutritional state and polypharmacy may play an even greater role than age. However, the available data can be used to guide the selection of therapy, anticipate side effects, and predict potential interactions with other medications and diseases.

Isoproterenol activation of prostaglandin production in guinea-pig airway muscle preparations from immature and mature animals

The aim of this study was to evaluate the capacity of isoproterenol to activate the cyclooxygenase pathway in tracheal and bronchial tissues derived from immature (198 +/- 4 g, N = 12) and mature (997 +/- 28 g, N = 12) guinea-pigs. Immunoreactive PGE2 and PGF2 alpha were measured in bath samples obtained during resting tone and when tissues had been maximally contracted or relaxed. Results from these experiments showed that histamine contractions were significantly greater in tracheal than in bronchial preparations, an effect which was independent of age. Isoproterenol and theophylline were equiactive in relaxing basal tone of tracheal and bronchial tissues when data for each tissue type was compared with results in the different age groups. This effect was also independent of age. When results were normalized for tissue wet weights, the quantities of prostaglandins produced in tissues from mature guinea-pigs were less than those generated in similar tissues derived from immature animals. These data indicate significant modifications in basal prostaglandin production in tissues from immature and mature guinea-pigs. In addition, isoproterenol stimulated prostaglandin production in airways from immature and mature animals whereas theophylline did not alter the basal production.

Effect of aging on the response of guinea pig trachea to isoprenaline

To study the effect of aging on the beta-adrenergic receptor potency of isoprenaline, tracheas from 3-, 6-, 10-, 40- and 100-week-old guinea pigs were used as test tissues. The pD2 values (potency) of isoprenaline increased with age in tracheas treated with corticosterone, but decreased in untreated tracheas. The pA2 value of propranolol against isoprenaline estimated in the treated tracheas did not change with age. Specific binding of [3H]-dihydroalprenolol to the microsomal fractions from the tracheal muscles from 6-, 10- and 40-week-old guinea pigs was analyzed with Scatchard plots. The capacity of maximum binding sites of [3H]-dihydroalprenolol increased with increasing age, while its dissociation constant did not change. Age-related increase in the potency of isoprenaline in the tracheas treated with corticosterone is considered to be due to the increase in the total amount (density) of beta-adrenoceptors. However, the potency of the drug in untreated tracheas decreased with age. These results suggest that extraneuronal uptake plays an important role in the beta-adrenergic action of isoprenaline in older (40 and 100 weeks) guinea pigs.

Adrenoceptors in airway smooth muscle

This review examines the roles and functional significance of alpha and beta-adrenoceptor subtypes in airway smooth muscle, with emphasis on human airway function and the influence of asthma. Specifically, we have examined the distribution of beta-adrenoceptors in lung and the influence of age, the epithelium, respiratory viruses and inflammation associated with asthma on airway smooth muscle beta-adrenoceptor function. Sites of action, beta 2-selectivity, efficacy and tolerance are also examined in relation to the use of beta 2-agonists in man. In addition, alpha-adrenoceptor function in airway smooth muscle has been reviewed, with some emphasis on comparing observations made in airway smooth muscle with those in animal models.

Effects of dystrophy and age on hamster tracheal smooth muscle function

Pharmacological and morphometric evaluations of airway smooth muscle were performed on tracheal segments from control and dystrophic hamsters at 50, 145 and 315 days of age. An age-related change in muscle volume index (a measure of the total smooth muscle per tracheal segment) was not observed in the control group but decreased with age in the dystrophic strain. In the control strain, correlations were not observed between age and maximum pharmacological response to the drugs tested, but a change in sensitivity to carbachol and KCl was noted at 145 days. In contrast, tracheal segments of dystrophic hamsters exhibited decreased maximum responses to carbachol, KCl, histamine, and isoproterenol at 315 days of age. When maximal contractile responses were normalized by muscle volume index, only the KCl maximum tension at 315 days was less than at 50 days of age. Changes in sensitivity to carbachol and KCl were also seen in tracheal smooth muscle of the dystrophic hamster at 145 days of age. At 315 days, histamine and isoproterenol-induced relaxation was less in dystrophic than in control tracheal smooth muscle. It is not possible to ascertain whether the decreased responses of the tracheal segments from the oldest dystrophic hamsters are due to dystrophy alone, or to a combination of dystrophy and aging.

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

The effects of isoproterenol, carbachol and other drugs on the cyclic AMP and cyclic GMP levels in tracheal smooth muscles of guinea-pigs of four different ages were investigated. Isoproterenol increased the cyclic AMP level several-fold in tracheal muscles from newborn (1 week) and young (4-7 weeks) guinea-pigs, but it caused less increase in the level in muscles from middle-aged (12 weeks) and old (20-24 weeks) guinea-pigs, although the basal cyclic AMP level at these ages was not significantly lower. The effects of prostaglandin E1 and cholera toxin in increasing the cyclic AMP level were also markedly less in muscle preparations from old guinea-pigs than in those from young ones. The increase in cyclic AMP levels caused by forskolin, an activator of adenylate cyclase did not decrease with age. Carbachol caused a 3- to 4-fold increase in the cyclic GMP level in muscle preparations from newborn and young guinea-pigs and more increase in the cyclic GMP level in preparations from middle-aged and old guinea-pigs. The increases in cyclic GMP level induced by high K+, histamine and sodium nitroprusside also increased with the age of the animals. These results suggest that the changes in the formation of cyclic AMP and cyclic GMP induced by various agents are due to changes at the post-receptor level.

Beta-adrenergic inhibition of bovine mesenteric lymphatics

The beta-action of catecholamines on lymphatic smooth muscle was studied by observing the effect of isoprenaline on electrical and mechanical activity in the double sucrose-gap. Action potentials and phasic contractions evoked by depolarizing pulses were abolished within 2 min of drug addition. Isoprenaline hyperpolarized the membrane and increased membrane conductance. Tetraethylammonium (10 mM) did not itself affect membrane resistance but reduced the hyperpolarization and the increase in conductance caused by isoprenaline. Removal of K+ from the external solution reduced membrane conductance and increased the hyperpolarization due to isoprenaline. When the NaCl content of Krebs solution was replaced with LiCl or choline chloride, isoprenaline no longer blocked action potential firing and its effects on phasic contractions and membrane conductance were reduced. In contrast, ouabain (10(-5) M) did not block the effect of isoprenaline on membrane potential and membrane conductance. These results suggest that beta-adrenergic inhibition of lymphatic smooth muscle involves an increase in an outward K+ current, though an additional metabolic effect cannot be ruled out.

Decreased vascular sensitivity to histamine during aging

The sensitivity of thoracic aorta preparations to histamine decreased with age. The responsiveness to histamine was unchanged. Indomethacin (17 microM) did not affect the basal tone of preparations from either young or old guinea pigs nor did the drug alter aortic sensitivity or responsiveness to histamine. The data show that (a) changes in vascular sensitivity to histamine during aging are not due to prostaglandins and that (b) prostaglandins do not act locally to modulate vascular tissue response to histamine.

Adrenergic responses in different sections of rat airways

Effects of adrenergic agonists on 4 different preparations (trachea, bronchus, bronchiole and parenchymal strip) of rat airway smooth muscle were studied. Phenylephrine, an alpha-agonist, contracted the trachea, bronchus and parenchymal strip but had no effect on the bronchiole. Similarly adrenaline, both a beta- and an alpha-agonist, contracted the trachea, bronchus and parenchymal strip, but neither contracted nor relaxed the bronchiole. These results suggest that the rat trachea and bronchus contain alpha receptors but the bronchiole does not. Alpha response of the parenchymal strip preparation seems to originate from the vascular smooth muscle. Isoprenaline, a nonselective beta-agonist, could not elicit any response on resting airways, but caused a relaxation of all four preparations after being contracted by carbacholine. Expressed in % of maximal carbacholine contractions, the relaxations were: 21% in the trachea, 45% in the bronchus, 92% in the bronchiole and 141% in the parenchymal strip. This shows that beta response is weak in the large central airways but becomes stronger towards the small peripheral airways, when measured against cholinergic contraction.

Beta-adrenoceptor stimulation and cyclic AMP levels in bovine tracheal muscle of old and young animals

The relationship between cyclic AMP levels and mechanical activity after exposure to isoprenaline was studied in tracheal smooth muscle. It was found that the basal cyclic AMP content decreased with age. The relaxing effects of isoprenaline, theophylline and papaverine were tested on muscles contracted by histamine, acetylcholine or carbacholine. Isoprenaline completely relaxed histamine contracted tracheas, but not those contracted by acetylcholine or carbacholine. Theophylline and papaverine completely relaxed the tracheas irrespective of whether the contracting agent was histamine, acetylcholine or carbacholine. Isoprenaline increased the cyclic AMP content of bovine trachea; this effect was stronger in muscles with spontaneous tension than in histamine contracted muscles. In muscles contracted by carbachol, isoprenaline increased the cyclic AMP level after 5 min. The correlations between the changes in the cyclic AMP levels and the tension in tracheal smooth muscle support the hypothesis that this nucleotide plays a role in the relaxation process.

The relationship between age and relaxation of vascular smooth muscle in the rabbit and rat

Relaxation of rabbit and rat aorta by isoproterenol decreases with increasing age, whereas such responses caused by nitroglycerin or sodium nitrite are not age-dependent. In the present study, we sought to determine whether this relationship also exists in pulmonary arteries and portal veins. As was the case with the aorta, isoproterenol-induced relaxation of pulmonary arteries decreased as the animal aged; relaxation by nitroglycerin was minimally altered. Aging did not influence responses of rabbit and rat portal veins to isoproterenol or nitroglycerin. If the responses of these blood vessels are characteristic of the responses of other vascular smooth muscles to vasodilators, then this study suggests a difference in the manner by which arteries and veins age. We also confirmed that isoproterenol-induced relaxation of rabbit and rat aorta markedly decreases with increasing age and that the responses of rat aorta to nitroglycerin are independent of age. Because of the agonist used to contract the tissues prior to drug-induced relaxation, the results of the first series of experiments with nitroglycerin on rabbit aorta were at variance with our earlier findings. When KCl was used to contract the aortas, the mean effective dose (ED50) obtained for nitroglycerin for tissues from 2-year-old rabbits was 8 to 19 times larger than that obtained from 2-month-old rabbits. This ratio dropped to 4 when the tissues were contracted with histamine. Since KCl and histamine contract rabbit aorta by different mechanisms, this finding suggests that, in addition to a specific loss in beta-receptor activity, increasing age results in an alteration in the contraction-relaxation process of rabbit aortic tissue.