Alveolar hypercapnia augments pulmonary C-fiber responses to chemical stimulants: role of hydrogen ion.

To determine whether the excitabilities of pulmonary C fibers to chemical and mechanical stimuli are altered by CO(2)-induced acidosis, single-unit pulmonary C-fiber activity was recorded in anesthetized, open-chest rats. Transient alveolar hypercapnia (HPC) was induced by administering CO(2)-enriched gas mixture (15% CO(2), balance air) via the respirator inlet for 30 s, which rapidly lowered the arterial blood pH from a baseline of 7.40 +/- 0.01 to 7.17 +/- 0.02. Alveolar HPC markedly increased the responses of these C-fiber afferents to several chemical stimulants. For example, the C-fiber response to right atrial injection of the same dose of capsaicin (0.25-1.0 microg/kg) was significantly increased from 3.07 +/- 0.70 impulses/s at control to 8.48 +/- 1.52 impulses/s during HPC (n = 27; P < 0.05), and this enhanced response returned to control within approximately 10 min after termination of HPC. Similarly, alveolar HPC also induced significant increases in the C-fiber responses to right atrial injections of phenylbiguanide (4-8 microg/kg) and adenosine (0.2 mg/kg). In contrast, HPC did not change the response of pulmonary C fibers to lung inflation. Furthermore, the peak response of these C fibers to capsaicin during HPC was greatly attenuated when the HPC-induced acidosis was buffered by infusion of bicarbonate (1.36-1.82 mmol. kg(-1). min(-1) for 35 s). In conclusion, alveolar HPC augments the responses of these afferents to various chemical stimulants, and this potentiating effect of CO(2) is mediated through the action of hydrogen ions on the C-fiber sensory terminals.     

Hypersensitivity of pulmonary C fibers induced by adenosine in anesthetized rats.

Compelling clinical evidence implicates the potential role of adenosine in development of airway hyperresponsiveness and suggests involvement of pulmonary sensory receptors. This study was carried out to determine the effect of a low dose of adenosine infusion on sensitivity of pulmonary C-fiber afferents in anesthetized open-chest rats. Infusion of adenosine (40 microg x kg-1x min-1 i.v. for 90 s) mildly elevated baseline activity of pulmonary C fibers. However, during adenosine infusion, pulmonary C-fiber responses to chemical stimulants and lung inflation (30 cmH2O tracheal pressure) were markedly potentiated; e.g., the response to right atrial injection of capsaicin (0.25 or 0.5 microg/kg) was increased by more than fivefold (change in fiber activity = 2.64 +/- 0.67 and 16.27 +/- 3.11 impulses/s at control and during adenosine infusion, n = 13, P < 0.05), and this enhanced response returned to control in approximately 10 min. The potentiating effect of adenosine infusion was completely blocked by pretreatment with 8-cyclopentyl-1,3-dipropylxanthine (100 microg/kg), a selective antagonist of the adenosine A1 receptor, but was not affected by 3,7-dimethyl-1-propargylxanthine (1 mg/kg), an A2-receptor antagonist, or 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (2 mg/kg), an A3-receptor antagonist. This potentiating effect was also mimicked by N6-cyclopentyladenosine (0.25 microg x kg-1 x min-1 for 90 s), a selective agonist of the adenosine A1 receptor. In conclusion, our results showed that infusion of adenosine significantly elevated the sensitivity of pulmonary C-fiber afferents in rat lungs and that this potentiating effect is likely mediated through activation of the adenosine A1 receptor.     

Sensitizing effects of chronic exposure and acute inhalation of ovalbumin aerosol on pulmonary C fibers in rats.

The effect of ovalbumin (Ova) sensitization on pulmonary C-fiber sensitivity was investigated. Brown-Norway rats were sensitized by intraperitoneal injection of Ova followed by aerosolized Ova three times per week for 3 wk. Control rats received the vehicle. At the end of the third week, single-unit fiber activities (FA) of pulmonary C fibers were recorded in anesthetized, artificially ventilated rats. Our results showed the following: 1) Ova sensitization induced airway inflammation (infiltration of eosinophils and neutrophils) and airway hyperresponsiveness in rats; 2) baseline FA in sensitized rats was significantly higher than that in control ones; 3) similarly, the pulmonary C-fiber response to right atrial injection of capsaicin was markedly higher in sensitized rats, which were significantly amplified after the acute Ova inhalation challenge; and 4) similar patterns, but smaller magnitudes of the differences in C-fiber responses to adenosine and lung inflation, were also found between sensitized and control rats. In conclusion, Ova sensitization elevated the baseline FA and excitability of pulmonary C fibers, and the hypersensitivity was further potentiated after the acute Ova inhalation challenge in sensitized rats. Chronic allergic inflammatory reactions in the airway probably contributed to the sensitizing effect on these lung afferents.     

Stimulation of vagal pulmonary C-fibers by a single breath of cigarette smoke in dogs

Inhalation of cigarette smoke into the lower airway via a tracheostomy evokes immediate apnea, bradycardia, and systemic hypotension in dogs. These responses can still be evoked when conduction in myelinated vagal fibers is blocked preferentially by cooling but are abolished by vagotomy, suggesting that they are mediated by afferent vagal C-fibers. To examine this possibility, we recorded impulses in pulmonary C-fibers in anesthetized, open-chest dogs and delivered 120 ml cigarette smoke to the lungs in a single ventilatory cycle. Pulmonary C-fibers were stimulated within 1 or 2 s of the delivery of smoke generated by high-nicotine cigarettes, activity increasing from 0.3 +/- 0.1 to a peak of 12.6 +/- 1.3 (SE) impulses/s, (n = 60); the evoked discharge usually lasted 3-5 s. Smoke generated by low-nicotine cigarettes evoked a milder stimulation in 33% of pulmonary C-fibers but did not significantly affect the overall firing frequency (peak activity = 2.2 +/- 1.1 impulses/s, n = 36). Hexamethonium (0.7-1.2 mg/kg iv) prevented C-fiber stimulation by high-nicotine cigarette smoke (n = 12) but not stimulation by right atrial injection of capsaicin. We conclude that pulmonary C-fibers are stimulated by a single breath of cigarette smoke and that nicotine is the constituent responsible.     

Role of bronchopulmonary C-fiber afferents in the apneic response to cigarette smoke

The role of vagal bronchopulmonary C-fiber afferents in eliciting the immediate changes in breathing pattern after acute inhalation of cigarette smoke was assessed with a selective blockade of myelinated vagal afferents (innervating both stretch and irritant receptors) utilizing the method of differential cooling. In 15 of 17 chloralose-anesthetized dogs tested, spontaneous inhalation of cigarette smoke (19.7% avg conc, 500-700 ml vol) reproducibly caused the following immediate responses: apnea, bradycardia, and hypotension. These responses occurred within 1 to 2 breaths of smoke inhalation and were followed by a delayed hyperpnea. The apneic duration reached 326 +/- 33% (SE) (n = 15) of the mean base-line expiratory duration. Differential cold block of both vagi (coolant temperature 8.4 +/- 0.3 degrees C) abolished the reflex apnea induced by a positive-pressure (7-10 cmH2O) lung inflation but did not affect the apneic response to smoke inhalation (345 +/- 35%). The smoke-induced apnea was completely abolished by lowering the coolant temperature to -1.3 +/- 0.2 degrees C (n = 10) or by bilateral vagotomy (n = 5) and returned to the control level after both vagi were rewarmed. Based on these results, we suggest that the immediate apneic response to inhaled cigarette smoke is elicited by a stimulation of vagal C-fiber afferents in the lungs and airways.     

Role of vagal C-fiber afferents in the bronchomotor response to lactic acid in the newborn dog.

We addressed the hypothesis that vagal C-fiber afferents and cyclooxygenase products are the mechanisms responsible for lactic acid (LA)-induced bronchoconstriction in the newborn dog. Perineural capsaicin and indomethacin were used to block conduction of vagal C fibers and production of cyclooxygenase products, respectively. Perineural capsaicin eliminated (85%) the increase in lung resistance (RL; 45 +/- 5.6%) due to capsaicin (25 microg/kg), whereas the increase in RL (54 +/- 6.9%) due to LA (0.4 mmol/kg) was only inhibited by 37 +/- 4.7% (P < 0.05). Atropine reduced LA-induced bronchoconstriction (42 +/- 2.1%) by an amount similar to that obtained with perineural capsaicin. However, inhibition was significantly increased when atropine was combined with indomethacin (61 +/- 2.7%; P < 0.05), implicating cyclooxygenase products in the LA-induced bronchoconstrictor response. We conclude that the mechanisms responsible for LA-induced bronchoconstriction in the newborn are 1) activation of vagal C-fibers, which, through projections to medullary respiratory centers, leads to activation of vagal cholinergic efferents; 2) production of cyclooxygenase products, which cause bronchoconstriction independent of medullary involvement; and 3) an unknown bronchoconstrictor mechanism, putatively tachykinin mediated. On the basis of our data, pharmaceutical targeting of pulmonary afferents would prevent multiple downstream mechanisms that lead to airway narrowing due to inflammatory lung disease.     

Role of cAMP in activation of ischemically sensitive abdominal visceral afferents

A number of metabolites produced during abdominal ischemia can stimulate and/or sensitize visceral afferents. The precise mechanisms whereby these metabolites act are uncertain. Other studies have shown that the adenylate cyclase-cAMP system may be involved in the activation of sensory neurons. Therefore, we hypothesized that cAMP contributes to the activation of ischemically sensitive abdominal visceral afferents. Single-unit activity of abdominal visceral C fibers was recorded from the right thoracic sympathetic chain in anesthetized cats before and during 7 min of abdominal ischemia. Forty-six percent of ischemically sensitive C fibers responded to intra-arterial injection of 8-bromo-cAMP (0.35-1. 0 mg/kg), an analog of cAMP, with responses during ischemia increasing from 0.50 +/- 0.06 to 0.84 +/- 0.08 impulses/s (P < 0.05, n = 11 C fibers). Conversely, an inhibitor of adenylate cyclase, 2', 5'-dideoxyadenosine (DDA; 0.1 mg/kg iv), attenuated ischemia-induced increase in activity of afferents from 0.66 +/- 0.10 to 0.34 +/- 0. 09 impulses/s (P < 0.05; n = 8). Furthermore, whereas exogenous PGE(2) (3-4 microg/kg ia) augmented the ischemia-induced increase in activity of afferents (P < 0.05, n = 10), treatment with DDA (0.1 mg/kg iv) substantially reduced the increase in discharge activity of afferents during ischemia, which was augmented by PGE(2) (1.45 +/- 0.24 vs. 0.70 +/- 0.09 impulses/s, -DDA vs. +DDA; P < 0.05) in six fibers. A time control group (n = 4), however, demonstrated similar increases in the activity of afferents with repeated administration of PGE(2). These data suggest that cAMP contributes to the activation of abdominal visceral afferents during ischemia, particularly to the action of PGs on activation and/or sensitization of these endings.     

Spectrum of myelinated pulmonary afferents

Myelinated pulmonary afferents are classified as rapidly adapting receptors (RARs) or slowly adapting receptors (SARs) by their adaptation rate. Behavior of SARs varies greatly, and therefore the present study tries to further categorize SARs according to their mechanical properties. Single-fiber activity of 104 SARs was examined in anesthetized, open-chest, artificially ventilated rabbits. According to the increase or decrease in activity during removal of positive-end-expiratory pressure (PEEP), SARs were divided into two groups. In one group mean activity increased from 31 +/- 6 to 46 +/- 7 impulses per second (imp/s; n = 11); in another group mean activity decreased from 44 +/- 2 to 25 +/- 1 imp/s (n = 93). The first group of SARs has high adaptation indexes (RAR-like), which increased with inflation pressure (36 +/- 3, 44 +/- 3, and 47 +/- 3% for 10, 15, and 20 cmH(2)O, respectively; P < 0.005). Their peak activity shifted from inflation phase to deflation phase during PEEP removal. The second group of SARs has low-adaptation indexes (typical SARs), which were not affected by inflation pressure (19 +/- 1, 18 +/- 1, and 17 +/- 1% for 10, 15, and 20 cmH(2)O; P = 0. 516). Their peak activity did not shift during PEEP removal. Because there are overlaps in other characteristics, it is proposed that myelinated vagal afferents are viewed as a heterogeneous group; their behaviors are like a spectrum, where typical RARs and SARs represent two extremes of the spectrum. The receptor behavior might be determined by anatomic location and its environment.     

Maturation of respiratory reflex responses in the piglet

Stimulation of chemo-, irritant, and pulmonary C-fiber receptors reflexly constricts airway smooth muscle and alters ventilation in mature animals. These reflex responses of airway smooth muscle have, however, not been clearly characterized during early development. In this study we compared the maturation of reflex pathways regulating airway smooth muscle tone and ventilation in anesthetized, paralyzed, and artificially ventilated 2- to 3- and 10-wk-old piglets. Tracheal smooth muscle tension was measured from an open tracheal segment by use of a force transducer, and phrenic nerve activity was measured from a proximal cut end of the phrenic nerve. Inhalation of 7% CO2 caused a transient increase in tracheal tension in both age groups, whereas hypoxia caused no airway smooth muscle response in either group. The phrenic responses to 7% CO2 and 12% O2 were comparable in both age groups. Lung deflation and capsaicin (20 micrograms/kg iv) administration did not alter tracheal tension in the younger piglets but caused tracheal tension to increase by 87 +/- 28 and 31 +/- 10%, respectively, in the older animals (both P less than 0.05). In contrast, phrenic response to both stimuli was comparable between ages: deflation increased phrenic activity while capsaicin induced neural apnea. Laryngeal stimulation did not increase tracheal tension but induced neural apnea in both age groups. These data demonstrate that between 2 and 10 wk of life, piglets exhibit developmental changes in the reflex responses of airway smooth muscle situated in the larger airways in response to irritant and C-fiber but not chemoreceptor stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

The involvement of hydroxyl radical and cyclooxygenase metabolites in the activation of lung vagal sensory receptors by circulatory endotoxin in rats.

Circulatory endotoxin can stimulate vagal pulmonary C fibers and rapidly adapting receptors (RARs) in rats, but the underlying mechanisms are not clear. We investigated the involvement of hydroxyl radicals and cyclooxygenase metabolites in the stimulation of C fibers and RARs by circulatory endotoxin (50 mg/kg) in 112 anesthetized, paralyzed, and artificially ventilated rats. In rats pretreated with the vehicle, endotoxin stimulated C fibers and RARs and caused a slight increase in total lung resistance (Rl) and a decrease in dynamic lung compliance. In rats pretreated with dimethylthiourea (a hydroxyl radical scavenger) alone, indomethacin (a cyclooxygenase inhibitor) alone, or a combination of the two, C-fiber and RAR responses [C fiber: change (Delta) = -62, -79, and -85%; RAR: Delta = -80, -84, and -84%, respectively] were reduced, and the Rl response was prevented. The suppressive effects of a combination of dimethylthiourea and indomethacin on the C-fiber and RAR responses were not superior to indomethacin alone. In rats pretreated with isoproterenol (a bronchodilator), the C-fiber response was not significantly affected (Delta = -26%), the RAR response was reduced (Delta = -88%), and the Rl response was prevented. None of these pretreatments affected the dynamic lung compliance response. These results suggest that 1) both hydroxyl radicals and cyclooxygenase metabolites are involved in the endotoxin-induced stimulation of C fibers and RARs, and 2) the involvement of these two metabolites in the C-fiber stimulation may be due to their chemical effects, whereas that in the RAR stimulation may be due to their bronchoconstrictive effects.     

Acute effect of cigarette smoke on laryngeal receptors

Upper airway exposure to cigarette smoke elicits reflex changes in breathing pattern. To examine whether laryngeal afferents are affected by cigarette smoke, neural activity was recorded from the peripheral cut end of superior laryngeal nerve in anesthetized dogs. A box-balloon system, connected to the breathing circuit, allowed smoke to be inhaled spontaneously through the isolated upper airway while preserving its normal respiratory flow and pressure. Our results showed the following. Inhalation of cigarette smoke (25-50% concentration, 300-400 ml) caused a marked increase in activity of laryngeal irritant receptors which were either silent or randomly discharging during control breathing [their activity increased from a control value of 1.67 +/- 0.50 (mean +/- SE; n = 21) to a peak of 5.03 +/- 0.85 impulses/s in 11-15 s]. The activity of laryngeal cold receptors was reduced to 77.3 and 63.8% of control (n = 9) during the two breaths of smoke inhalation, respectively. After returning toward the base-line activity, a more pronounced inhibition (26.3% of control) occurred at three to nine breaths after the smoke inhalation. A small but significant decrease (88.5% of control) in the inspiratory discharge of laryngeal mechanoreceptors was observed during the first test breath. These effects were independent of the CO2 content of the smoke. Furthermore, there was no difference between the responses of these laryngeal afferents to high- and low-nicotine cigarette smoke.     

Influence of maturation on constrictive response to stimulation of C-fiber afferents in isolated guinea pig airways

We investigated whether the airway constrictive response to stimulation of bronchopulmonary C-fiber afferents is altered during the maturation process. Isometric tension was measured in airway rings isolated from three tracheobronchial locations (intrathoracic trachea and main and hilar bronchi) and compared in mature [M, 407 +/- 10 (SE) g body wt, n = 36] and immature (IM, 161 +/- 5 g body wt, n = 35) guinea pigs. Our results showed no difference in the ACh (10(-5) M)- or KCl (40 mM)-induced contraction between M and IM groups, regardless of the airway location. In sharp contrast, the concentration-response curves of 10(-8)-10(-6) M capsaicin were distinctly lower in IM hilar bronchi; for example, response to the same concentration of capsaicin (10(-6) M) was 89.2 +/- 15.3% of the response to 10(-5) M ACh in IM and 284.7 +/- 43.2% in M animals. Similar, but smaller, differences in the bronchoconstrictive response to capsaicin between IM and M groups were also observed in the trachea and main bronchus. Electrical field stimulation induced airway constriction in all three locations in M and IM groups. However, after administration of 10(-6) M atropine and 10(-6) M propranolol, electrical field stimulation-induced contraction was significantly smaller in the hilar bronchus of IM than M animals, and this difference was not prevented by pretreatment with 5 x 10(-5) M indomethacin. Although radioimmunoassay showed no difference in the tissue content of substance P between M and IM airways, the constrictive responses to exogenous substance P and neurokinin A were markedly greater in M airways at all three locations. In conclusion, the constriction of isolated airways evoked by C-fiber stimulation was significantly weaker in the IM guinea pigs, probably because of a less potent effect of tachykinins on the airway smooth muscle.     

Ozone enhances excitabilities of pulmonary C fibers to chemical and mechanical stimuli in anesthetized rats

Acute exposureto ozone (O₃) enhances pulmonarychemoreflex response to capsaicin, and an increased sensitivity ofbronchopulmonary C-fiber afferent endings may be involved. The presentstudy was aimed at determining the effect ofO₃ on the responses of pulmonary Cfibers to chemical and mechanical stimuli. A total of 31 C fibers werestudied in anesthetized, open-chest, and vagotomized rats. Duringcontrol, right atrial injection of a low dose of capsaicin abruptlyevoked a short and mild burst of discharge [0.77 ± 0.28 impulses (imp)/s, 2-s average]. After acute exposure toO₃ (3 parts/million for 30 min),there was no significant change in arterial blood pressure, trachealpressure, or baseline activity of C fibers. However, the stimulatoryeffect of the same dose of capsaicin on these fibers was markedlyenhanced (6.05 ± 0.88 impulses/s;P < 0.01) and prolonged immediatelyafter O₃ exposure, and returnedtoward control in 54 ± 6 min. Similarly, the pulmonary C-fiberresponse to injection of a low dose of lactic acid was also elevatedafter O₃ exposure. Furthermore,O₃ exposure significantly potentiated the C-fiber response to constant-pressure (tracheal pressure = 30 cmH₂O) lunginflation (control: 0.19 ± 0.07 imp/s; afterO₃: 1.12 ± 0.26 imp/s;P < 0.01). In summary, these results show that the excitabilities of pulmonary C-fiber afferents to lunginflation and injections of chemical stimulants are markedly potentiated after acute exposure toO₃, suggesting a possible involvement of these afferents in theO₃-induced changes in breathing pattern and chest discomfort in humans.

     

Different roles of two subgroups of lung vagal C-fiber afferents in the tachypneic response to pulmonary air embolism in dogs

It is known that lung vagal C-fiber afferents play an important role in eliciting the tachypneic response to pulmonary air embolism (PAE), and can be subgrouped as those with low resistance (LRC) and those with high resistance (HRC) to perivagal capsaicin. In this study, we investigated the relative contributions of vagal LRC and HRC C-fiber afferents to the PAE-induced tachypneic response. Phrenic activity was recorded from 10 anesthetized, paralyzed, and artificially ventilated dogs. PAE was induced by infusion of air into the vein (2 ml/min, 1 ml/kg). During control conditions, induction of PAE produced a shortening in expiratory duration with no significant change in inspiratory duration, resulting in tachypnea. The PAE-induced tachypneic response was totally abolished by perivagal capsaicin treatment with a method (capsaicin concentration, 6 mg/ml; treatment duration, 25-30 min) that blocks the conduction of LRC C-fiber afferents, but not that of HRC C-fiber afferents. This tachypneic response was not affected by cooling of both vagi to a temperature (4.5 degrees C) that blocks the conduction of HRC C-fiber afferents, but not that of LRC C-fiber afferents. A bilateral cervical vagotomy virtually eliminated this tachypneic response. These results suggest that LRC C-fiber afferents are responsible for eliciting the reflex tachypneic response to PAE, whereas HRC C-fiber afferents play no vital role.     

CO2 relaxes parenchyma in the liquid-filled rat lung.

CO(2) regulation of lung compliance is currently explained by pH- and CO(2)-dependent changes in alveolar surface forces and bronchomotor tone. We hypothesized that in addition to, but independently of, those mechanisms, the parenchyma tissue responds to hypercapnia and hypocapnia by relaxing and contracting, respectively, thereby improving local matching of ventilation (Va) to perfusion (Q). Twenty adult rats were slowly ventilated with modified Krebs solution (rate = 3 min(-1), 37 degrees C, open chest) to produce unperfused living lung preparations free of intra-airway surface forces. The solution was gassed with 21% O(2), balance N(2), and CO(2) varied to produce alveolar hypocapnia (Pco(2) = 26.1 +/- 2.4 mmHg, pH = 7.56 +/- 0.04) or hypercapnia (Pco(2) = 55.0 +/- 2.3 mmHg, pH = 7.23 +/- 0.02). The results show that lung recoil, as indicated from airway pressure measured during a breathhold following a large volume inspiration, is reduced approximately 30% when exposed to hypercapnia vs. hypocapnia (P < 0.0001, paired t-test), but stress relaxation and flow-dependent airway resistance were unaltered. Increasing CO(2) from hypo- to hypercapnic levels caused a substantial, significant decrease in the quasi-static pressure-volume relationship, as measured after inspiration and expiration of several tidal volumes, but hysteresis was unaltered. Furthermore, addition of the glycolytic inhibitor NaF abolished CO(2) effects on lung recoil. The results suggest that lung parenchyma tissue relaxation, arising from active elements in response to increasing alveolar CO(2), is independent of (and apparently in parallel with) passive tissue elements and may actively contribute to Va/Q matching.     

Prostaglandin involvement in lung C-fiber activation by substance P in guinea pigs.

Airway hyperresponsiveness is a cardinal feature of asthma. Lung C-fiber activation induces central and local defense reflexes that may contribute to airway hyperresponsiveness. Initial studies show that substance P (SP) activates C fibers even though it is produced and released by these same C fibers. SP may induce release of other endogenous mediators. Bradykinin (BK) is an endogenous mediator that activates C fibers. The hypothesis was tested that SP activates C fibers via BK release. Guinea pigs were anesthetized, and C-fiber activity (FA), pulmonary insufflation pressure (PIP), heart rate, and arterial blood pressure were monitored before and after intravenous injection of capsaicin (Cap), SP, and BK. Identical agonist challenges were repeated after infusion of an antagonist cocktail of des-Arg9-[Leu8]-BK (10(-3) M, B1 antagonist), and HOE-140 (10(-4) M, B2 antagonist). After antagonist administration, BK increased neither PIP nor FA. Increases in neither PIP nor FA were attenuated after Cap or SP challenge. In a second series of experiments, Cap and SP were injected before and after infusion of indomethacin (1 mg/kg iv) to determine whether either agent activates C fibers through release of arachidonic acid metabolites. Indomethacin administration decreased the effect of SP challenge on FA but not PIP. The effect of Cap on FA or PIP was not altered by indomethacin. In subsequent experiments, C fibers were activated by prostaglandin E2 and F2alpha. Therefore, exogenously applied SP stimulates an indomethacin-sensitive pathway leading to C-fiber activation.     

Cardiac and pulmonary vagal neurons receive excitatory chemoreceptor input

The effects of hypercapnia and hypocapnia on the activities of the cardiac and pulmonary vagal single fibers were examined in the decerebrated, unanesthetized, paralyzed, and vagotomized cats. The animals breathed 100% O2. Fractional end tidal CO2 concentration was raised to 9% by adding CO2 into the O2 inlet. Average discharge rate of efferent cardiac vagal units (n=10) increased from 1.0+/-0.3 to 2.2+/-0.3 Hz. Hypocapnia apnea was produced by hyperventilation. Activities of cardiac vagal units tested (n = 4) showed dramatic decrease (0.1+/-0.0 Hz). Mean arterial blood pressure did not change significantly under these conditions. In contrast, only instantaneous firing rate during inspiration was significantly increased for efferent pulmonary vagal units (n = 11) during hypercapnia. The activities of the 3 pulmonary vagal units tested with hypocapnia decreased significantly. We concluded that cardiac and pulmonary vagal neurons were excited by chemoreceptor input.     

Depression of force production and ATPase activity in different types of human skeletal muscle fibers from patients with chronic heart failure.

Isometric force production and ATPase activity were determined simultaneously in single human skeletal muscle fibers (n = 97) from five healthy volunteers and nine patients with chronic heart failure (CHF) at 20 degrees C. The fibers were permeabilized by means of Triton X-100 (1% vol/vol). ATPase activity was determined by enzymatic coupling of ATP resynthesis to the oxidation of NADH. Calcium-activated actomyosin (AM) ATPase activity was obtained by subtracting the activity measured in relaxing (pCa = 9) solutions from that obtained in maximally activating (pCa = 4.4) solutions. Fiber type was determined on the basis of myosin heavy chain isoform composition by polyacrylamide SDS gel electrophoresis. AM ATPase activity per liter cell volume (+/-SE) in the control and patient group, respectively, amounted to 134 +/- 24 and 77 +/- 9 microM/s in type I fibers (n = 11 and 16), 248 +/- 17 and 188 +/- 13 microM/s in type IIA fibers (n = 14 and 32), 291 +/- 29 and 126 +/- 21 microM/s in type IIA/X fibers (n = 3 and 5), and 325 +/- 32 and 205 +/- 21 microM/s in type IIX fibers (n = 7 and 9). The maximal isometric force per cross-sectional area amounted to 64 +/- 7 and 43 +/- 5 kN/m(2) in type I fibers, 86 +/- 11 and 58 +/- 4 kN/m(2) in type IIA fibers, 85 +/- 6 and 42 +/- 9 kN/m(2) in type IIA/X fibers, and 90 +/- 5 and 59 +/- 5 kN/m(2) in type IIX fibers in the control and patient group, respectively. These results indicate that, in CHF patients, significant reductions occur in isometric force and AM ATPase activity but that tension cost for each fiber type remains the same. This suggests that, in skeletal muscle from CHF patients, a decline in density of contractile proteins takes place and/or a reduction in the rate of cross-bridge attachment of approximately 30%, which exacerbates skeletal muscle weakness due to muscle atrophy.     

Pulmonary venous pressure increases during alveolar hypoxia in isolated lungs of newborn pigs.

The purpose of this study was to determine whether pulmonary venous pressure increases during alveolar hypoxia in lungs of newborn pigs. We isolated and perfused with blood the lungs from seven newborn pigs, 6-7 days old. We maintained blood flow constant at 50 ml.min-1.kg-1 and continuously monitored pulmonary arterial and left atrial pressures. Using the micropuncture technique, we measured pressures in 10 to 60-microns-diam venules during inflation with normoxic (21% O2-69-74% N2-5-10% CO2) and hypoxic (90-95% N2-5-10% CO2) gas mixtures. PO2 was 142 +/- 21 Torr during normoxia and 20 +/- 4 Torr during hypoxia. During micropuncture we inflated the lungs to a constant airway pressure of 5 cmH2O and kept left atrial pressure greater than airway pressure (zone 3). During hypoxia, pulmonary arterial pressure increased by 69 +/- 24% and pressure in small venules increased by 40 +/- 23%. These results are similar to those obtained with newborn lambs and ferrets but differ from results with newborn rabbits. The site of hypoxic vasoconstriction in newborn lungs is species dependent.