Journal of Applied Physiology
Vol. 81, No. 6, pp. 2349-2357, December 1996
CONTROL OF BREATHING, CIRCULATION, AND TEMPERATURE

Pulmonary chemoreflexes elicited by intravenous injection of lactic acid in anesthetized rats

Lu-Yuan Lee, Robert F. Morton, and Jan M. Lundberg

Department of Physiology, University of Kentucky Medical Center, Lexington, Kentucky 40536-0084; and Department of Pharmacology, Karolinska Institut, S-104 01 Stockholm, Sweden

ABSTRACT

INTRODUCTION

METHODS

RESULTS

DISCUSSION

ACKNOWLEDGEMENTS

FOOTNOTES

REFERENCES

ABSTRACT

Lee, Lu-Yuan, Robert F. Morton, and Jan M. Lundberg. Pulmonary chemoreflexes elicited by intravenous injection of lactic acid in anesthetized rats. J. Appl. Physiol. 81(6): 2349-2357, 1996.Experiments were carried out to characterize the cardiorespiratory reflex responses to intravenous injection of lactic acid and to determine the involvement of vagal bronchopulmonary C-fiber afferents in eliciting these responses in anesthetized rats. Bolus injection of lactic acid (0.2 mmol/kg iv) immediately elicited apnea, bradycardia, and hypotension, which were then followed by a sustained hyperpnea. The immediate apneic and bradycardiac responses to lactic acid were completely abolished by bilateral vagotomy and were absent when the same dose of lactic acid was injected into the left ventricle. The subsequent hyperpneic response was substantially attenuated by denervation of carotid body chemoreceptors. After a perineural capsaicin treatment of both vagus nerves to block the conduction of C fibers, lactic acid no longer evoked the immediate apnea and bradycardia, whereas the hyperpneic response became more pronounced and sustained, presumably because of the removal of the inhibitory effect on breathing mediated by pulmonary C-fiber activation. Single-unit electrophysiological recording showed that intravenous injection of lactic acid consistently evoked an abrupt and intense burst of discharge from the vagal C-fiber afferent endings in the lungs. In conclusion, the cardiorespiratory depressor responses induced by lactic acid are predominantly elicited by activation of vagal pulmonary C fibers.

pulmonary C fibers; vagal reflexes; apnea; hyperpnea; capsaicin; perineural capsaicin treatment

INTRODUCTION

NONMYELINATED C-fiber afferent nerve endings can be stimulated by acid solutions injected into the circulation of various tissues or organ systems, including the heart, the abdominal viscera, and the limb muscles (24, 27, 28, 30). Lactic acid, a major product of anaerobic tissue metabolism, seems to be particularly effective in stimulating these sensory endings (25, 27, 29).

The concentration of lactic acid in body fluids can be elevated substantially during various physiological and pathophysiological conditions. For example, lactic acid can be produced in large quantities by the skeletal muscles during anaerobic exercise. The production of lactic acid can be also elevated locally as the result of tissue ischemia. Lungs are perfused by the total venous return and are therefore fully exposed to the lactic acid produced by the peripheral tissues. Furthermore, lungs and airways are extensively innervated by nonmyelinated C-fiber afferents that play an important role in regulating various airway functions (3). However, although an involvement of pulmonary C fibers has been speculated in the tachypneic response to injection of lactic acid in rabbits (5), the stimulatory effect of lactic acid on these afferent endings and the consequent cardiopulmonary reflex responses have not been fully explored.

The objectives of this study were to 1) characterize the reflexogenic cardiorespiratory effects of intravenous (iv) injection of lactic acid; we postulated that if pulmonary C-fiber endings are activated by lactic acid, bolus injection of this acid should elicit the pulmonary chemoreflexes, as characterized by apnea, bradycardia, and hypotension, in spontaneously breathing animals; 2) evaluate the role of vagal C-fiber afferents in eliciting these responses by blocking the conduction of these afferents with perineural application of capsaicin (12) to both cervical vagi; and 3) determine the stimulatory effect of lactic acid on individual vagal pulmonary C-fiber afferents by using the single-fiber electrophysiological recording technique.

METHODS

Young Sprague-Dawley rats (329 ± 6 g, n = 58) of either sex were anesthetized with intraperitoneal injection of -chloralose (100 mg/kg) and urethan (500 mg/kg), and supplemental doses of the same anesthetics were administered intravenously whenever necessary to maintain abolition of the pain reflex evoked by pinching the skin of the hindlimbs. The femoral artery and vein were cannulated for recording arterial blood pressure and for iv injections, respectively; the tip of the venous catheter was positioned slightly below the entry of the right atrium. Body temperature was maintained at ~36°C throughout the experiment with a temperature servo-controller and a heating pad placed under the animal. A short tracheal cannula was inserted just below the larynx via a tracheotomy, through which rats breathed spontaneously in the supine position. Respiratory flow was measured with a heated pneumotachograph and a differential pressure transducer (Validyne MP45) and was integrated (Grass 7P10) to give tidal volume (VT). Ventilatory signals were recorded on a Grass polygraph (model 7) and also analyzed by an on-line computer (CompuAdd model 433); respiratory frequency (f), VT, and minute volume of ventilation (I) were all analyzed on a breath-by-breath basis. Results obtained from the computer analysis were routinely compared with those obtained by hand calculation for accuracy.

Lactic acid (200 mg/ml; Sigma Chemical, St. Louis, MO) was prepared in distilled water and capsaicin (400 µg/ml; Sigma Chemical) in a vehicle of 10% Tween 80, 10% ethanol, and 80% isotonic saline. Solutions of lactic acid and capsaicin at the desired concentrations were then prepared daily by dilution with distilled water and isotonic saline, respectively, on the basis of the animal's body weight. The volume of each bolus injection was 0.2 ml; the lactic acid solution injected at the dose of 0.2 mmol/kg was slightly hypertonic (osmolarity: 335-440 mosmol/l; Wescor osmometer, model 5100B). At least 20 min elapsed between two lactic acid injection challenges. Before each challenge, the rat's lungs were hyperinflated (tracheal pressure >10 cmH₂O) to establish a constant volume history (20).

Three series of experiments were carried out as described below.

Series 1: ventilatory response to lactic acid injection. This series was carried out in 10 rats to study the cardiorespiratory reflex responses elicited by iv injection of lactic acid (0.2 mmol/kg). The same dose of lactic acid was injected at least twice during control to establish the reproducibility of the response in each animal. The response to lactic acid was tested again 30 min after bilateral cervical vagotomy to assess the possible involvement of vagus nerves in eliciting these responses.

To determine the magnitude of the change in blood pH caused by lactic acid injection, arterial blood samples were drawn from four rats both before and within 3-10 s after the injection; the after-injection sampling time was chosen to coincide with the apneic response and to correct for the estimated circulation time between the lungs and the femoral artery. Because of the minimal sample volume (0.09 ml) required for the blood-gas analyzer (Instrumentation Laboratory model 1306), it took 5-7 s to collect the blood sample. The pH of mixed venous blood was also measured in five additional rats that were artificially ventilated after midline thoracotomies; blood samples were drawn from the right ventricle both before and within 6 s after the injection of lactic acid either via a catheter advanced from the right jugular vein (n = 1) or by a needle inserted directly into the right ventricle (n = 4).

To determine whether the reflex effects observed in this study were elicited from stimulation of receptors located in the lungs, the same dose of lactic acid was also injected 30 min later into a catheter that was inserted into the right carotid artery and advanced retrogradely until its tip was positioned in the left ventricle in five additional rats; the position of the catheter was monitored by the pressure trace and confirmed by postmortem examination.

After the injection of lactic acid, hyperventilation was consistently elicited after the initial depressor responses and it persisted even after bilateral vagotomy. To determine the possible involvement of carotid body chemoreceptors, we compared the ventilatory responses to injections of the same dose of lactic acid before and after sectioning both carotid sinus nerves in six vagotomized rats. The carotid sinus nerve was identified on each side of the neck with the aid of a dissecting microscope; carotid body denervation was verified by a complete abolition of the hyperpneic response to inhalation of 10 ml of 100% nitrogen.

In seven additional rats, we examined whether the responses to lactic acid injections showed any dose-related pattern. The responses to three different doses of lactic acid (0.1, 0.15, and 0.2 mmol/kg iv) were tested in random sequence in each rat. Hypertonic saline (1.4% NaCl solution) with the osmolarity (420 mosmol/l) closely matching that of the highest concentration of lactic acid was injected as control.

Series 2: effect of perineural capsaicin treatment of vagi on responses to lactic aid injection. To assess the role of C-fiber afferents, we used the method of perineural application of capsaicin to both cervical vagus nerves to block the neural conduction in these fibers; this method was modified from that reported first by Jancso and Such (12) and used successfully in our previous study (14). Briefly, cotton strips soaked in capsaicin solution (0.25 mg/ml) were wrapped around a 2- to 3-mm segment of the isolated cervical vagus nerves for 15-20 min and then removed. Our criterion for a successful treatment was a complete abolition of the reflex responses to capsaicin injection (1.0 µg/kg iv); capsaicin was chosen because of its specificity and potency in stimulating the nonmyelinated C-fiber afferent endings. The response to lactic acid injection was tested twice before and then again within 30 min after completion of the treatment; in a previous study, we showed that the blocking effect of this treatment lasted for >60 min (14, 16). To determine whether the perineural capsaicin treatment affected neural conduction in myelinated afferent fibers, we also compared the reflex apneic responses to lung inflation (tracheal pressure 6 cmH₂O) during the control phase and after the treatment in each rat. A total of 10 rats were studied in this series.

Previous investigators have shown that slow continuous iv infusion of hydrochloric acid causes the endogenous release from platelets of thromboxane A₂, which, in turn, can stimulate pulmonary C fibers (13, 26). To test the possible involvement of this effect in the ventilatory response to lactic acid, we compared the responses to lactic acid injections during the control phase and 30 min after administration of indomethacin (15 mg/kg iv), a cyclooxygenase inhibitor, in six additional rats. Indomethacin (Sigma Chemical) was first dissolved in polyethylene glycol and then diluted at a 1:1 ratio in saline to a final concentration of 15 mg/ml for slow infusion. We have previously shown that one-third of this dose was sufficient to prevent the bronchoconstrictive effect mediated by the endogenous release of thromboxane A₂ in guinea pig lungs (11).

Series 3: effect of lactic acid injection on pulmonary C-fiber afferents. To further examine whether lactic acid can exert a stimulatory effect on vagal pulmonary C-fiber endings, we recorded directly the afferent activity of vagal C fibers in the lungs of nine rats using a "single-fiber" recording technique, similar to that described previously (15, 16). Briefly, rats were paralyzed with pancuronium bromide (Pavulon, 0.05 mg/kg iv); its effect was allowed to wear off periodically so that the depth of anesthesia could be monitored. After a midsternal thoracotomy, the lungs were ventilated with a respirator, and the expiratory line was placed 3 cm under water to prevent the lungs from collapsing. The right or left vagus nerve was isolated from the adjacent carotid artery and sectioned as far rostrally as possible. The distal end of the cut vagus nerve was immersed in a pool of mineral oil and placed on a small dissecting platform. A thin nerve filament was then teased away from the desheathed nerve trunk, and its afferent activity was recorded with a monopolar platinum-iridium hook electrode. The nerve activity was amplified (Grass P511K), monitored on an audio monitor (Grass AM-8), and displayed on an oscilloscope (Tektronix 502A). The thin filament was further split until the afferent nerve activity from a single unit was electrically isolated. The vagus nerve was also sectioned or ligated just above the diaphragm to eliminate the nerve activity arising from the lower visceral organs.

Pulmonary C fibers usually have a sparse and irregular discharge but can be activated by hyperinflation of the lungs. Once the afferent activity of the single unit(s) had been identified by lung inflation (to 3 or 4 VT), capsaicin (1.0 µg/kg) was injected as a bolus via the venous catheter; only fibers that were activated within 2 s (usually <1 s) after the injection were included in this study. The response to bolus injection of lactic acid (0.2 mmol/kg iv) was tested 15 min later. Finally, the general location of each receptor was identified by gentle palpation of the lungs with a small glass rod after the experiment; those receptors having a location that could not be identified in the lung structures were not included.

Statistical analysis. Results were analyzed statistically by either a one-way or a two-way repeated-measures analysis of variance, unless otherwise mentioned. When a positive interaction was indicated by the analysis of variance test, the responses were further compared by using a post hoc analysis (Fisher's least significant difference). P < 0.05 was considered significant. All data are presented as means ± SE.

RESULTS

Series 1: ventilatory response to lactic acid injection. Bolus injection of lactic acid (0.2 mmol/kg) immediately elicited apnea, bradycardia, and hypotension, which were followed by a more sustained hyperpnea (e.g., Fig. 1). On the first breath after the injection, I decreased from a baseline of 124.3 ± 11.8 to 38.2 ± 5.4 ml/min (n = 10, P < 0.001) and then quickly started to increase, reaching a peak of 189.6 ± 37.1 ml/min (P < 0.01) at the third breath (Fig. 2); the baseline ventilatory parameters were calculated by averaging 10 consecutive breaths immediately before injection in each rat. Both the immediate apneic and the subsequent hyperpneic responses to the lactic acid injection were highly reproducible in the same animals (Fig. 2). Although the apnea and bradycardia occurred only transiently, the hypotensive response usually lasted for >10 s after the injection (e.g., Fig. 1). Arterial blood pH decreased substantially from a baseline of 7.44 ± 0.02 to 7.16 ± 0.04 (n = 4, P < 0.01; paired t-test) after the injection of lactic acid, and it returned to the baseline within 1 min in the two rats tested. In five additional rats, mixed venous blood pH was measured in the blood samples drawn directly from the right ventricle, and it decreased from a baseline of 7.39 ± 0.06 to 7.01 ± 0.05 after the injection (n = 5, P < 0.01; paired t-test).

After bilateral vagotomy, the immediate apnea and bradycardia were completely abolished, but a small reduction in VT was frequently found on the first breath after the acid injection (e.g., Figs. 1 and 2). In contrast, the hyperpneic response became more pronounced after vagotomy; both the intensity and the duration of the hyperpnea markedly increased (Figs. 1 and 2).

When the same dose of lactic acid was injected directly into the left ventricle via the carotid arterial catheter in five additional rats with intact vagi, the apnea and bradycardia that were elicited immediately by the iv injection 30 min earlier were absent in four of these animals (e.g., Fig. 3) and markedly reduced in the remaining one. In contrast, the hyperpneic response still persisted and occurred sooner in all animals, occasionally accompanied by an augmented breath 2-5 breaths later (e.g., Fig. 3). In addition, left ventricular injection of lactic acid also induced a systemic hypotension but to a lesser degree than did the iv injection, and the accompanying bradycardia was absent (e.g., Fig. 3).

Sectioning both carotid sinus nerves completely abolished the hyperpneic response to inhalation of 10 ml nitrogen in the six vagotomized rats. In the same animals, denervation of carotid bodies also markedly diminished the increase in I induced by the injection of lactic acid (0.2 mmol/kg iv); the peak increases in I (averaged over five consecutive breaths) were 265.2 ± 43.8 and 174.3 ± 31.2 ml/min (n = 6, P < 0.01) before and after the carotid body denervation, respectively (Fig. 4).

The cardiorespiratory responses to lactic acid injections showed a dose-related pattern (Fig. 5). Injection of lactic acid at a lower dose (0.1 mmol/kg iv) did not elicit significant change in any of the ventilatory parameters, although a bradypnea was found immediately after the injection in two of the seven rats tested. A higher dose (0.15 mmol/kg) evoked a mild but significant reduction in f (P < 0.05) and I (P < 0.05) on the first breath after the injection but did not induce any delayed hyperpnea. Injection of the dose of 0.2 mmol/kg elicited both the immediate apneic and the subsequent hyperpneic responses, similar to those found in other series (e.g., Fig. 2); these responses were markedly greater than those induced by the lower doses of lactic acid in the same rats. Control injections of hypertonic saline did not elicit any detectable responses (Fig. 5).

Series 2: effect of perineural capsaicin treatment of vagi on responses to lactic acid injection. Perineural capsaicin treatment of both vagi did not cause any significant change in the baseline VT, f, I, arterial blood pressure, or heart rate (n = 10; e.g., Figs. 6 and 7). The reflex apnea induced by lung inflation was not significantly different during the control phase (12.3 ± 2.4 s, n = 10) and after the treatment (9.7 ± 0.7 s, P > 0.05; paired t-test). However, the cardiorespiratory depressor responses elicited by capsaicin injection were completely eliminated by the treatment (Figs. 6 and 7). Similarly, in the same animals, the apnea and bradycardia elicited immediately by the injection of lactic acid were also abolished after the treatment, whereas the more sustained hypotensive response was attenuated but not completely blocked (Figs. 6 and 7). In contrast, the subsequent hyperpneic response to lactic acid injection was clearly enhanced, primarily because of a marked increase in f (Figs. 6 and 7).

Pretreatment with indomethacin altered neither the immediate apneic response nor the subsequent hyperpneic response to lactic acid injection in six additional rats (Fig. 8). Cardiovascular depressor responses to lactic acid were also not altered by the indomethacin pretreatment.

Series 3: effect of lactic acid injection on pulmonary C-fiber afferents. Vagal pulmonary C fibers discharged only sparsely and irregularly during control; their average baseline activity (over a 10-s interval) was 0.14 ± 0.08 impulses/s (n = 12) . All these C-fiber afferents were activated by hyperinflation of the lungs (e.g., Fig. 9), and their locations were identified in the lung structures. Each of these afferent fibers was excited abruptly after a bolus injection of capsaicin (1.0 µg/kg iv). A bolus injection of lactic acid (0.2 mmol/kg iv) evoked a short burst of discharge (e.g., Figs. 9 and 10) within 1 s after the injection in 11 of these 12 receptors, and the activity usually returned to the baseline level within 5 s (Figs. 9 and 10). The peak fiber activity after the injection was 6.84 ± 1.12 impulses/s (averaged over a 3-s interval), and the responses were reproducible in the same fibers.

DISCUSSION

This study clearly describes a biphasic ventilatory response to iv injection of a bolus of lactic acid in anesthetized spontaneously breathing rats: an immediate apneic response followed by hyperpnea. The initial apnea was accompanied by bradycardia and hypotension; these responses resembled the classic "pulmonary chemoreflex," which has been repeatedly demonstrated by previous investigators with injections of various chemical agents (e.g., capsaicin) or endogenous mediators (e.g., bradykinin) into the pulmonary circulation (3, 4). Because the chemical substances that elicit the pulmonary chemoreflex invariably stimulate the vagal pulmonary C-fiber afferents, it is believed that activation of these afferents is responsible for triggering the cardiorespiratory depressor responses (3, 22). The results of this study lend additional support to this notion, because the initial apnea and bradycardia that followed the lactic acid injection were completely abolished by perineural capsaicin treatment of both cervical vagi, which has been shown to selectively block the conduction of C-fiber afferents (12, 14). Indeed, our study of the individual vagal pulmonary C-fiber afferents has further demonstrated the stimulatory effect of lactic acid on these sensory endings.

The excitatory effect of lactic acid on nociceptors has been described in various other organ systems. For example, it has been reported that lactic acid is a very effective stimulus of C-fiber afferent endings in the limb skeletal muscles (24, 28) and in the gastrointestinal tract (27). However, the action of lactic acid on the C-fiber afferent endings in the lungs has not been previously characterized. Continuous perfusion of isolated guinea pig lungs with acidic buffer at a pH of 5.0 has been shown to evoke the release of various sensory neuropeptides, such as tachykinins and calcitonin gene-related peptides, presumably from C-fiber afferents (18). Sustained application of acidic buffer of the same low pH has also been demonstrated to stimulate C fibers in guinea pig trachea in vitro (6). In the present study, the change in blood pH caused by lactic acid injection was considerably smaller and lasted for only a very short duration, but the injection nevertheless caused a distinct excitatory effect on C-fiber endings in the lungs. Although C fibers in the abdominal viscera and skeletal muscles seem to be particularly sensitive to hydrogen ions carried by the lactic acid, as compared with the responses to other acidic solutions (25, 27, 29), whether lactic acid is more potent than other forms of acid in stimulating pulmonary C fibers remains to be determined.

After injection into the venous blood, lactic acid can react with bicarbonate ions and produce CO₂, which is then eliminated from the lungs (19). This reaction has been shown in our preliminary trials in anesthetized rats; the end-tidal CO₂ concentration increased immediately after injection of lactic acid (0.2 mmol/kg iv) from ~5 to 7-8% and lasted for three to four breaths (L.-Y. Lee and R. F. Morton, unpublished observations). This change may explain the marked difference in pH between mixed venous blood and systemic arterial blood found in this study. We believe that these changes in blood pH caused by the lactic acid injection led to a transient reduction of pH in the pulmonary interstitial fluid, which, in turn, activated these C-fiber sensory endings in the lungs.

The transduction mechanism of the acid-induced excitation of C-fiber endings is not fully understood. It has been suggested that low-pH buffer may trigger the release of an endogenous ligand for the "capsaicin receptor" and thereby activate the C-fiber afferents and evoke the release of tachykinins from these endings, because these effects of acid could be completely blocked in the isolated guinea pig trachea and lungs by capsazepine (6, 18), a specific antagonist to the capsaicin receptor. The possible involvement of cyclooxygenase metabolites in the acid-induced activation of C fibers has also been reported in other visceral organs (7, 17). Indeed, it has been shown that a slow, continuous infusion of hydrochloric acid triggers platelet to release thromboxane A₂, which, in turn, can stimulate pulmonary C fibers (13, 26). However, we can rule out the possibility that the stimulatory effect on pulmonary C fibers by the bolus injection of lactic acid in the present study is mediated by the action of endogenous cyclooxygenase metabolites because the pulmonary chemoreflex elicited by lactic acid was not affected by pretreatment with indomethacin (Fig. 8). Alternatively, it has been shown that hydrogen ions can directly excite capsaicin-sensitive C-fiber neurons of the rat dorsal root ganglia by evoking a sustained inward current as the result of an increase in monovalent cation (Na⁺, K⁺, Cs⁺) conductance in the neuronal membrane (2, 23). Whether this proton-activated current is responsible for generating action potentials in the pulmonary C-fiber endings by lactic acid remains to be investigated.

After either vagotomy or perineural capsaicin treatment of both vagi, the same dose of lactic acid no longer triggered the initial apneic response. However, neither vagotomy nor perineural treatment prevented the subsequent hyperpnea. On the contrary, the hyperpneic response to lactic acid was markedly enhanced and prolonged, presumably because of the elimination of the inhibitory effect caused by the pulmonary C-fiber activation. Bolus injection of lactic acid (0.2 mmol/kg iv) lowered pH in the arterial blood abruptly and transiently from 7.44 to 7.16 in this study; this acidotic load presumably stimulated peripheral chemoreceptors and thereby elicited a reflex hyperpneic response (1). This hypothesis is supported by our observation that sectioning both carotid sinus nerves eliminated ~75% of the hyperpnea induced by lactic acid injection in vagotomized rats. The remaining hyperpneic response is probably mediated through the excitation of other sensory receptors (e.g., aortic body chemoreceptors). Furthermore, injection of lactic acid is known to lead to a transient increase of arterial CO₂ tension (19), which may stimulate the central chemoreceptors and induce hyperventilation.

In vagotomized rats, injection of lactic acid still triggered a small reduction in VT of the first breath immediately after the injection (e.g., Figs. 1, 2, and 6). This response is presumably elicited by activation of nonvagal afferents, and the receptor type(s) and location(s) are not identified in this study. Because of the rapid onset of the response, the receptors are probably located in or near the pulmonary circulation. We suspect that sympathetic afferents arising from the lungs and/or heart may be involved (3, 30).

Although stimulation of pulmonary C-fiber afferents is known to induce the transient systemic hypotension that accompanies the reflex bradycardia (3, 4, 16), the more sustained hypotensive response to a bolus injection of lactic acid observed in this study probably involved, to some extent, a direct vasodilatory effect of hydrogen ions on the peripheral vascular smooth muscles (9). This speculation is supported by our observations that the hypotensive response was also induced by the left ventricular injection of lactic acid (e.g., Fig. 3) and could not be completely abolished by either bilateral vagotomy or perineural capsaicin treatment of both vagi (e.g., Figs. 1 and 6).

It is well recognized that pulmonary C-fiber afferent endings are very sensitive to various inhaled irritants and to certain blood-borne autocoids and that they, therefore, play an important role in regulating the respiratory defense functions in both physiological and pathological conditions (3). Activation of these afferent endings by chemical substances such as lactic acid is known to elicit the pulmonary chemoreflex, as described above, accompanied by reflex bronchoconstriction and hypersecretion of mucus (3). In addition, activation of these afferents is believed to be involved in evoking the dyspneic sensation in certain pathophysiological conditions (e.g., pulmonary edema or lung inflammation) (8, 21). It has been postulated that the dyspneic sensation during strenuous exercise could result from an excitation of pulmonary C-fiber afferents caused by pulmonary congestion or a mild increase in interstitial pressure in the lungs under those conditions (21, 22). However, on the basis of our results in this study, it seems conceivable that lactic acid may also play a part in the genesis of the dyspneic sensation during severe exercise. Lactic acid is an important product of anaerobic metabolism, and the production of lactic acid can increase drastically during heavy exercise; blood lactate concentrations in the range of 10-20 mmol/l have been reported during heavy exercise in healthy human volunteers (10). Several other pathological conditions that could lead to excessive production of lactic acid in the tissue include local tissue ischemia and inflammation. Whether pulmonary C-fiber endings are activated by lactic acid endogenously produced under those conditions remains to be determined.

ACKNOWLEDGEMENTS

The authors thank Dr. Mary K. Rayens for statistical analysis of the data and Margareta Stensdotter, Kevin Kwong, and Ju-Lun Hong for technical assistance.

FOOTNOTES

Address for reprint requests: L.-Y. Lee, Dept. of Physiology, Univ. of Kentucky, Lexington, KY 40536-0084.

Received 18 April 1996; accepted in final form 22 July 1996.

REFERENCES