Vagal vasodilatory mechanisms in the pig bronchial circulation preferentially involves sensory nerves

The present study shows that in contrast to the upper trachea, where the parasympathetic vasodilatory components of both cholinergic and non-cholinergic nature are dominating, the vagal blood flow regulation in the peripheral airways of the pig supplied by the bronchial artery is entirely carried out by local release of vasodilatory mediators from capsaicin-sensitive sensory nerves. Also inhalation of the vapour phase from the major airway irritant cigarette smoke was associated with a marked increase in bronchial blood flow possibly via local axon reflexes. Capsaicin, substance P (SP) and calcitonin gene-related peptide (CGRP) caused vasodilatation in both the trachea and bronchi while vasoactive intestinal polypeptide (VIP) was most active in the trachea. These functional data were supported by immunohistochemical studies showing the presence of SP- and CGRP-containing nerves of presumably sensory origin around bronchial blood vessels while VIP-positive perivascular fibres of local parasympathetic origin were found mainly in the trachea.

Vagally mediated vasodilatation by motor and sensory nerves in the tracheal and bronchial circulation of the pig

A new in vivo model is described in which anaesthetized pigs were used to study vascular responses in the bronchial, upper tracheal and laryngeal circulation upon electrical stimulation of the vagal or superior laryngeal nerves. Vagal or superior laryngeal nerve stimulation increased blood flow in the bronchial artery and the superior laryngeal artery, respectively. After pre-treatment with atropine the vasodilatory response in the bronchial artery upon stimulation was not modified while the increase in blood flow in the superior laryngeal artery was reduced. The ganglionic blocking agent chlorisondamine further reduced the nerve stimulation evoked decrease in vascular resistance in the superior laryngeal artery, but did not influence the response of the bronchial artery. Capsaicin induced a marked increase in blood flow both in the bronchial and superior laryngeal arteries after pre-treatment with atropine, guanethidine and chlorisondamine. After capsaicin tachyphylaxis, the vasodilatation upon nerve stimulation in the bronchial artery and the smaller remaining decrease in vascular resistance in the superior laryngeal artery were strongly reduced. Thus, antidromic stimulation of afferent C fibres may increase blood flow via release of vasodilatory peptides such as tachykinins and calcitonin gene-related peptide. The present findings show that local blood flow in the larynx and upper trachea is regulated by cholinergic and non-cholinergic parasympathetic mechanisms and a small capsaicin sensitive, sensory component. On the other hand, the vagal control of the bronchial circulation seems to exclusively involve capsaicin sensitive sensory nerves.

Effect of endothelin-1 on regional vascular resistances in the pig

The actions of endothelin-1 (ET-1) on vascular resistances were investigated and characterized in anesthetized pigs in vivo. Intravenously administered ET-1 (2-400 pmol/kg) caused dose-dependent increases in mean arterial blood pressure accompanied by a reduction in heart rate. ET-1 also increased renal, coronary, femoral, and bronchial vascular resistances. The kidney was about 10 times more sensitive to ET-1 than the other regions. The ET-1-induced vasoconstriction in the kidney was attenuated by nifedipine (100 micrograms/kg). The responses to ET-1 in the femoral and bronchial vascular beds were biphasic with an initial decrease followed by an increase in vascular resistance at high doses. It is concluded that ET-1 causes dose-dependent vasoconstriction in several vascular beds in vivo with the most pronounced effects in the renal circulation.

Comparative effects of rilmenidine and clonidine on bronchial responses to histamine in asthmatic subjects

1. The effects of pretreatment with clonidine and rilmenidine, a new alpha 2-adrenoceptor agonist, on the bronchial responses to inhaled histamine were studied on 3 different days in a controlled, double-blind, randomized study in 12 asymptomatic asthmatic subjects. Clonidine and rilmenidine were orally administered as single and equipotent doses of 150 micrograms and 1 mg, respectively. All the subjects were non-smokers with normal lung function tests (forced expiratory volume in one second (FEV1) = 97 +/- 10% predicted FEV1). 2. Histamine (first dose = 543 nmol) was delivered by a breath activated dosimeter (DeVilbiss no. 646 nebulizer) every 5 min; FEV1 was measured in triplicate after each dose and the largest value was analyse. The three dose-response curves were compared by analysis of variance. 3. Both clonidine and rilmenidine decreased arterial blood pressure in all subjects. There was no difference in baseline values and pre-challenge values of FEV1 after placebo, clonidine and rilmenidine on the 3 study days. Compared with placebo, both rilmenidine and clonidine significantly increased the bronchial responses to histamine (P less than 0.05 and P less than 0.01 respectively) an effect which was significantly more marked with clonidine than rilmenidine (P less than 0.05). 4. We suggest that the enhancement of bronchial responsiveness to histamine by clonidine and rilmenidine may result from their effects on both central and peripheral alpha 2-adrenoceptors, and that the lesser aggravation of histamine-induced bronchial obstruction in asthmatic subjects on rilmenidine might be explained by its lesser central and/or greater peripheral effects than clonidine.

Interaction between fenoterol, ipratropium, and acetylcholine on human isolated bronchus

Functional antagonism between fenoterol (F) and acetylcholine (ACh) and interaction between F and ipratropium (Ipr) on ACh-induced contraction were evaluated on isolated human bronchi. In the presence of increasing concentrations of ACh (2 X 10(-4) and 2 X 10(-3) mol/L), dose-response curves of the relaxant effect of F were shifted to the right (0.45 and 0.92 log units), and the maximal effect of F, expressed as a percentage of the effect of theophylline, 3 X 10(-3) mol/L, was reduced from control values of 92.1 +/- 4% to 70.8 +/- 7.0%, and to 67.2 +/- 7.1%, according to the functional antagonism. In the presence of Ipr, 10(-9) and 10(-8) mol/L, the functional antagonism between ACh and F was partially reversed. Concentration response-curves to F versus ACh, 2 X 10(-3) mol/L, were shifted to the left, and although the -log molar concentration producing 50% of maximal effect was not significantly modified, the maximal effect of F was significantly increased in the presence of Ipr, 10(-8) mol/L. It is concluded that the effect of combined F and Ipr on the isolated human bronchus contracted with ACh appears to be of the additive type.

Effects of noradrenaline on the isolated human bronchus. Comparison with the isolated guinea pig trachea

The pharmacodynamic activity of noradrenaline was evaluated comparatively in vitro on isolated human bronchi and on guinea pig tracheal spirals. Noradrenaline exerted a contractile effect on both preparations under resting tone and in the presence of propranolol 10(-6) M; maximal noradrenaline-induced contraction was 15-20% of maximal acetylcholine (ACh)-induced contraction. Without propranolol, the contractile effect of noradrenaline was negligible when the preparations were under resting tone and absent when they were precontracted with ACh. In contrast, noradrenaline exerted a strongly relaxant effect on both human bronchi (-log ED50 5.24 +/- 0.17; N = 5) and guinea pig tracheae (-log ED50 6.15 +/- 0.29; N = 8). With maximal contraction induced by ACh 3.10(-3) M the -log ED50 of both preparations were shifted to the right by functional antagonism and became 4.72 +/- 0.17 and 5.31 +/- 0.11, respectively. The pKD values of noradrenaline, calculated according to Furchgott and Bursztyn (1967), were 4.79 +/- 0.04 in human bronchi (N = 5) and 4.77 +/- 0.16 in guinea-pig tracheae (N = 8). In the presence of cocaine plus phenoxybenzamine these values were not significantly modified in human bronchi and only slightly modified in guinea pig tracheae. It is concluded that noradrenaline induces a strong beta-adrenergic response and a negligible alpha-adrenergic response from both human bronchi and guinea pig tracheae in vitro.

Effects of clonidine on bronchial responses to histamine in normal and asthmatic subjects

Our aim was to examine the effects of clonidine (C), an agonist of central and peripheral alpha-2 adrenoceptors, on bronchomotor responsiveness to histamine (H). In a double-blind study, we compared on two different days the effects of pretreatment with placebo (P) and with 200 micrograms or 150 micrograms of C given orally, in ten normal (NS) and eight asymptomatic asthmatic subjects (AS) respectively, the response to inhalation of serially increasing doses of H. On each day, five doubling doses of H (first dose = 3.5 and 1.1 mumol in NS and AS, respectively) were administered every 5 min; forced expiratory volume in one second (FEV1) was measured after each dose. The dose-response curves were compared by an analysis of variance. Clonidine caused hypotension with bradycardia in all subjects. Baseline values and pre-challenge values of FEV1 after P and C were identical on the two study days. Compared to P, C did not modify the response to H in NS but significantly increased it in AS (p less than 0.01). Our results suggest that the neural control of the airways differs in AS compared to NS and could be explained either by a decrease in sympathetic inhibitory activity or a greater responsiveness of the airways to parasympathetic stimulation and/or a higher parasympathetic tone in AS.

Effects of clonidine on bronchial responses to histamine in normal and asthmatic subjects

Our aim was to examine the effects of clonidine (C), an agonist of central and peripheral alpha-2 adrenoceptors, on bronchomotor responsiveness to histamine (H). In a double-blind study, we compared on two different days the effects of pretreatment with placebo (P) and with 200 micrograms or 150 micrograms of C given orally, in ten normal (NS) and eight asymptomatic asthmatic subjects (AS) respectively, the response to inhalation of serially increasing doses of H. On each day, five doubling doses of H (first dose = 3.5 and 1.1 mumol in NS and AS, respectively) were administered every 5 min; forced expiratory volume in one second (FEV1) was measured after each dose. The dose-response curves were compared by an analysis of variance. Clonidine caused hypotension with bradycardia in all subjects. Baseline values and pre-challenge values of FEV1 after P and C were identical on the two study days. Compared to P, C did not modify the response to H in NS but significantly increased it in AS (p less than 0.01). Our results suggest that the neural control of the airways differs in AS compared to NS and could be explained either by a decrease in sympathetic inhibitory activity or a greater responsiveness of the airways to parasympathetic stimulation and/or a higher parasympathetic tone in AS.

Influence of epithelium on the responsiveness of guinea-pig isolated trachea to adenosine

1. The influence of epithelium removal on the effects of adenosine on airway contractility was investigated on the guinea-pig isolated trachea. 2. In preparations under resting tone or precontracted with histamine 10(-5) M, removal of the tracheal epithelium resulted in similar shifts to the left of the adenosine concentration-response curves (0.61 +/- 0.18 (P less than 0.05) and 0.80 +/- 0.09 (P less than 0.001) log units; n = 5), corresponding to 4.07 and 6.31 fold potentiations of the relaxant effect of adenosine. 3. In the presence of dipyridamole 10(-5) M the relaxant effects of adenosine were potentiated 85.1 fold on tracheae with epithelium; removal of the epithelium did not produce a significant additional shift to the left of the adenosine concentration-response curves (0.07 +/- 0.03 log units; n = 5; NS). 4. In the absence of dipyridamole, the theophylline-adenosine antagonism was not of the competitive type, irrespective of whether the tracheae were with or without epithelium. 5. In the presence of dipyridamole, this antagonism was likely to be of the competitive type and its characteristics were the same when the epithelium was present or absent. Regression slope and pA2 values were 0.84 and 5.07, respectively, in the presence of epithelium and 0.76 and 4.89, respectively, in its absence. 6. It is suggested that, at least in the guinea-pig isolated trachea model, the airway epithelium seems to be involved only in the uptake and metabolism of adenosine.

Role of extracellular calcium in the effects of substance P and neurokinin A on guinea pig trachea and human bronchus

Whether the influx of calcium through voltage-operated channels is involved in the stimulatory effects of substance P and neurokinin A in airways smooth muscle is not yet firmly established. This question was addressed in the present study using guinea pig trachea and human bronchi suspended in normal or calcium-free Krebs solution and tested with inhibitors of calcium channels. In calcium-free Krebs solution, the myotropic effects of substance P (10(-7) M), neurokinin A (3.10(-8) M), acetylcholine (2.10(-5) M), and histamine (2.10(-5) M) were reduced by 27-57%, while those of potassium chloride and tetraethylammonium were practically abolished. Calcium antagonists such as verapamil or nicardipine, when applied at concentrations of 10(-8)-10(-6) M, inhibited the contractions produced by potassium chloride and tetraethylammonium, whereas higher concentrations (10(-5)-10(-4) M) of both inhibitors were needed to reduce the effects of substance P, neurokinin A, acetylcholine, and histamine. In neither preparation did the calcium agonist Bay K 8644 (10(-6) M) modify the effects of neurokinin A, substance P, acetylcholine, or histamine, but it potentiated potassium chloride's effect on human bronchi. We conclude that transmembrane calcium influx through voltage-operated channels plays a minor role in the stimulatory effects of neurokinins in airways smooth muscle.

Bronchial response to hyperventilation of dry air at room temperature in normals and asthmatics

The bronchial effects of three levels (25, 40 and 60 1 X min-1) of voluntary isocapnic hyperventilation of dry air at room temperature (20-22 degrees C) have been studied in 18 normal, non-atopic subjects and in 25 nonperennial asthmatics who were asymptomatic and whose airway obstruction at the time of the study was mild, with a peak expiratory flow rate of 6.1 +/- 1.5 (SD) 1 X s-1 vs a predicted 8.4 +/- 1.3 1 X s-1. The bronchial response was assessed by use of maximal expiratory flow-volume curves obtained before and 1, 5, 10 and 15 min after the 5 min hyperventilation challenge. In normal subjects, there was a minimal though significant (p less than 0.001; two-way analysis of variance) fall in maximal expiratory flows which did not increase with the level of hyperventilation and was not accompanied by a fall in forced vital capacity. The bronchial response of asthmatics differed from that in normal: the fall in maximal expiratory flows was significantly greater, associated with a significant fall in forced vital capacity and increased with the level of hyperventilation. Results in 10 asthmatics studied on two different study days were highly reproducible. Sensitivity and specificity are excellent (approximately equal to 1) for the 40 1 X min-1 hyperventilation challenge. Our results suggest that isocapnic voluntary hyperventilation of dry air at room temperature (20-22 degrees C) is a highly satisfactory screening test to detect bronchial hyperreactivity.