Laser revascularization of ischemic skeletal muscle

BACKGROUND

Clinical trials have shown that transmyocardial laser revascularization is an effective secondary treatment for ischemic heart disease patients. Laser revascularization may also provide an alternative method for treating peripheral vascular disease.

METHODS

The purpose of this study was to investigate the potential for laser revascularization in ischemic skeletal muscle. Eighteen rabbits (3-4 kg) were instrumented chronically with transit time ultrasound flowprobes on both common iliac arteries. All rabbits performed graded exercise tests on a treadmill where maximal blood flow was recorded. Unilateral hindlimb ischemia was produced by ligation of one femoral artery. At week 3 postligation, 10 rabbits received laser therapy and 8 underwent a sham surgery. In each of four muscles (gracilius, medialis, sartorius, and biceps femoris) 5 to 22 laser channels were created (average = 52 channels per leg).

RESULTS

At week 3 postligation the maximal blood flow of the ischemic limb for the treated group was 64 +/- 3 ml/min (mean +/- SEM) and at 6 weeks postlaser therapy maximal blood flow increased to 75 +/- 5 ml/min. The sham surgery group had a maximal blood flow of 58 +/- 4 ml/min at week 3 postligation and 66 +/- 3 ml/min at week 6 postsham surgery.

CONCLUSION

These results indicate that laser therapy does not induce angiogenesis and vascular remodeling in the ischemic hindlimb of a rabbit which exceeds that seen with a sham surgery.

Functional anatomy of the vagal innervation of the cervical trachea of the dog

The canine cervical trachea has been used for numerous studies regarding the neural control of tracheal smooth muscle. The purpose of the present study was to determine whether there is lateral dominance by either the left or right vagal innervation of the canine cervical trachea. In anesthetized dogs, pressure in the cuff of the endotracheal tube was used as an index of smooth muscle tone in the trachea. After establishment of tracheal tone, as indicated by increased cuff pressure, either the right or left vagus nerve was sectioned followed by section of the contralateral vagus. Sectioning the right vagus first resulted in total loss of tone in the cervical trachea, whereas sectioning the left vagus first produced either a partial or no decrease in tracheal tone. After bilateral section of the vagi, cuff pressure was recorded during electrical stimulation of the rostral end of the right or left vagus. At the maximum current strength used, stimulation of the left vagus produced tracheal constriction that averaged 28.5% of the response to stimulation of the right vagus (9.0 +/- 1.8 and 31.6 +/- 2.5 mmHg, respectively). In conclusion, the musculature of cervical trachea in the dog appears to be predominantly controlled by vagal efferents in the right vagus nerve.

Direct relaxant effects of intravenous anesthetics on airway smooth muscle

Ketamine, at concentrations achieved with the usual clinical doses, has a direct relaxant effect on airway smooth muscle (ASM). This study investigates the dose-dependent direct relaxation effects of midazolam and propofol on both proximal and distal ASM compared with ketamine. The proximal and distal airways were dissected from eight mongrel dogs and cut into 2-mm rings. The rings were attached to pressure transducers and equilibrated in a Krebs-Ringer bicarbonate bath kept at 37 degrees C, pH 7.4, CO2 37 mm Hg, and PaO2 > 100 mm Hg. Optimal length was determined, a dose-response curve to acetylcholine was established, and the 50% effective dose (ED50) of acetylcholine was calculated. Ketamine, midazolam, or propofol were given in random order to each ring preconstricted with ED50 of acetylcholine in cumulative log incremental doses from 10(-6) to 10(-4) M. Relaxation response was the tension during anesthetic equilibrium, expressed as a percentage of the tension from ED50 of acetylcholine. The drug vehicles were tested for their effects on the ASM. No bronchorelaxation was seen with any of the intravenous anesthetics at 10(-6) M. Ketamine 10(-5) M produced at 17.9% +/- 2.1% relaxation in the distal ASM but had no effect on the proximal ASM. Neither propofol nor midazolam affected the ASM at 10(-5) M. The distal ASM was significantly (P < 0.005) more sensitive to 10(-4) M of all three drugs compared with the proximal ASM. In the proximal ASM, 10(-4) M of ketamine, midazolam and propofol reduced ASM tension by 14.9% +/- 4.4%, 19.0% +/-8.8%, and 14.7% +/- 5.5%, respectively, versus 36.4% +/- 3.2%, 58.6% +/- 6.1%, and 64.4% +/- 9.0% in the distal ASM. The drug vehicles had no effect on the ASM. We conclude that ketamine, midazolam, and propofol have direct relaxant effects on ASM. All three intravenous anesthetics have a greater direct relaxant effect on distal ASM than on proximal ASM. Only ketamine showed significant direct bronchorelaxing effects at concentrations that are likely to be achieved with the usual clinical dosing patterns.

Differential effects of desflurane and halothane on peripheral airway smooth muscle

Volatile anaesthetics have been shown to have direct relaxant effects on airway smooth muscle. We have examined the effects of 0.9, 1.9, and 2.8 dog MAC of desflurane and halothane on isolated proximal and distal canine airways precontracted with acetylcholine. The proximal and distal airway smooth muscle relaxed with increasing concentration of each anaesthetic in a dose-related manner. Desflurane had a greater relaxant effect than halothane on the proximal airway only at 2.8 MAC. Desflurane relaxed the distal airway to a greater extent than halothane at 1.9 and 2.8 MAC. The distal airway smooth muscle was more sensitive to volatile anaesthetics than the proximal airway smooth muscle with either halothane or desflurane at all concentrations tested. This effect may be a result of differences in cartilage content, myosin content, epithelium-dependent effects, receptor density, myofilament sensitivity to Ca2+, sarcoplasmic reticulum Ca2+ control, or ionic fluxes in the proximal airway compared with the distal airway. The increased sensitivity of airway smooth muscle to desflurane compared with halothane is not known but may be related to possible differences in the effects of Ca2+ homeostasis.

Reflex effects of stimulation of sympathetic afferents on the triangularis sterni

The purpose of this study was to determine whether contralateral inhibition of the triangularis sterni is produced by stimulation of intrathoracic sympathetic afferents. Dogs were anesthetized with sodium pentobarbital and placed on positive pressure ventilation. The chest was opened through a mid-sternal incision. Diaphragm and left and right triangularis sterni EMGs were recorded, post-vagotomy, before and during electrical stimulation of the left ventral ansa subclavia (VA), vagosympathetic trunk, ventrolateral and ventromedial cardiac nerves and, when present, the stellate cardiac nerve. Peak of the phasic diaphragm EMG and expiratory time were not significantly affected by stimulation of the VA. A significant decrease in inspiratory time was observed. Ipsilateral excitation and contralateral inhibition of the left and right triangularis sterni EMGs, respectively, were produced by stimulation of the VA. Stimulation of the other intrathoracic nerves produced a similar pattern of results. Conduction velocity determinations suggested that the afferents which produced the reflex responses are, at least in part, small A fibers.

Effects of halothane and isoflurane on antigen- and leukotriene-D4-induced constriction of guinea pig trachea

We investigated the mechanism of the action of volatile anesthetics on the airway smooth muscle constricted by an antigen and leukotriene-D4 (LTD4). Excised tracheal rings from ovalbumin-sensitized guinea pigs were suspended in eight tissue baths. Halothane or isoflurane was aerated into four tissue baths, while the remaining four served as time controls. To assess the antispasmogenic activity of halothane and isoflurane, concentration-response curves for antigen and LTD, were constructed exposed to anesthetics and compared to controls. The spasmolytic activity of halothane and isoflurane was measured in the tracheal rings constricted by a single antigen challenge or by EC50 of LTD. Both halothane and isoflurane produced significant rightward shifts of ovalbumin and LTD, concentration-response curve with corresponding increases in the EC50 values. Halothane increased the EC50 value for LTD, from 5.38 +/- 0.43 x 10-9 M to 1.2 +/- 0.18 x 10-8 M, and for ovalbumin from 1.2 +/- 0.06 x 10-4 mg/ml to 3.03 +/- 0.28 x 10-4 mg/ml. Isoflurane increased the EC50 value for LTD, from 5.17 +/- 0.64 x 10-9 M to 8.98 +/- 1.01 x 10-9 M, and for ovalbumin from 1.21 +/- 0.09 x 10-4 mg/ml to 2.61 +/- 0.19 x 10-4 mg/ml. Furthermore, halothane and isoflurane significantly reduced the magnitude of the antigen-and LTD4-induced constriction. In 30 min intervals, 1% and 2% halothane reduced the magnitude of the ovalbumin-induced constriction by 32% and 50%, respectively, while isoflurane (2% and 4%) caused relaxation of 16% and 35%, respectively. The magnitude of LTD4-induced constriction was reduced by 17% and 24%, with 1% and 2% halothane, respectively. Isoflurane (2% and 4%) reduced this constriction by 25% and 25% respectively. In conclusion, halothane and isoflurane attenuate and prevent the constrictive response of airway smooth muscle to allergen and LTD. A direct, nonspecific dilating effect is suggested as the mechanism responsible for the observed effects.

Pigeon-serum-induced hypersensitivity pneumonitis in the dog

Pigeon serum (PS) is one of the most common causes of hypersensitivity pneumonitis (HP). PS-induced HP was examined in a dog model. The dogs (n = 6) were immunized by i.m. injections of PS, followed by insufflation with aerosolized PS, while all control dogs (n = 3) received saline only. All animals insufflated with PS developed tachypnea 2-4 h after PS inhalation. After PS insufflation, a significant decrease in arterial oxygen tension (PaO2) was detected in sensitized dogs. No change in PaO2 was detected in sensitized dogs after saline or in the controls after PS insufflation. In intradermal skin tests with PS antigen, a positive skin reaction was found in 3/6 dogs in 30 min, and in 5/6 dogs in 6 and 48 h after the PS injections. Sensitized dogs showed a significant increase in PS-specific IgG in serum and lavage fluid (LF). In LF of sensitized dogs, an increase in the percentage of lymphocytes, eosinophils, and neutrophils was detected. Sensitized dogs developed chronic interstitial inflammation with lymphocytes, macrophages, plasma cells, and eosinophils in lungs. Granulomas with lymphocytes, histiocytes, and giant cells were detected in both the interstitium and the bronchiolar wall in the lungs of sensitized dogs. PaO2 was lowest in dogs showing the most severe interstitial inflammation in the lungs. The results indicate that dogs can be successfully used in immunologic and physiologic studies of PS-induced HP.

Inhibition of neutral endopeptidase potentiates compound 48/80-induced constriction of guinea-pig tracheal smooth muscle

Our previous studies have shown that the inhibition of neutral endopeptidase, an enzyme which degrades tachykinins, increases anaphylactic construction of guinea-pig tracheal smooth muscle. To investigate this observation further, we examined the effects of phosphoramidon, an inhibitor of a neutral endopeptidase, on constriction induced by the non-immunological mast cell degranulator-compound 48/80. Phosphoramidon produced significant leftward shift of the compound 48/80 concentration-response curve with corresponding decrease in the EC50 value from 51 (28-80) micrograms/ml to 42 (20-72) micrograms/ml. When added during the compound 48/80-induced constriction, phosphoramidon significantly increased the magnitude of this constriction by 69.7% after 30 min, and 78.9% after 45 min. Phosphoramidon was ineffective in tracheal rings from tachykinin-depleted guinea pigs. The incubation of tracheal rings with H1-histamine receptor antagonist (diphenhydramine HCl, 10 microM) and leukotriene receptor antagonist (ICI 198.615, 5 microM) significantly diminished the contractile response to compound 48/80 and prevented a phosphoramidon-dependent increase of this constriction. These results suggest that compound 48/80 induces the release of tachykinins by the stimulatory activity of histamine and leukotrienes. Anaphylactic release of tachykinins would therefore not depend directly on the antigen-antibody reaction.

Reflex effects of lung inflation on tracheomotor tone observed during apnea produced by the Hering-Breuer reflex

Reflex effects of static pressure lung inflation (SLI) on tracheomotor tone (TT) were studied during apnea produced by the Hering-Breuer expiratory-facilitatory reflex. Anesthetized dogs were placed on cardiopulmonary bypass, and diaphragm electromyogram was used as an indicator of central nervous system inspiratory output. Tracheomotor tone (TT) was reflexly produced by chemoreceptor stimulation. The volume and frequency of the ventilator [phasic lung inflation (PLI)] were adjusted to produce nearly a maximum reflex decrease in TT. The increase in TT observed while PLI was withheld (0 mmHg tracheal pressure) for 1-2 min was used as the control response. SLIs were interspersed among sets of PLIs. Although SLI produced apnea, TT returned toward the control TT recorded during 0 mmHg tracheal pressure. TT observed during apnea was reflexly sensitive to further increases in SLI and to changes in chemoreceptor stimulation, which also affected the time course of the tracheomotor responses. These results suggest that the reflex decrease in TT produced by SLI and the Hering-Breuer expiratory-facilitatory reflex are mediated by different central mechanisms but may be from the same or different pulmonary receptors.

Topographical differences in the direct effects of isoflurane on airway smooth muscle

Volatile anesthetics have a direct relaxant effect on airway smooth muscle, but it is not known whether this effect is similar throughout the bronchial tree. We studied the direct relaxation effect of isoflurane on isolated proximal (outer diameter [OD] 4-6 mm) and distal (OD 0.7-1.5 mm) canine airways precontracted with acetylcholine. Proximal and distal airway rings were suspended in tissue baths and stretched to their optimum length. A dose-response curve was obtained for each airway ring with log increments of acetylcholine. Maximum contraction was reached with 10(-2) mol/L of acetylcholine for the proximal airway smooth muscle (7.0 +/- 0.3 g of tension) and 10(-3) mol/L of acetylcholine for the distal airway smooth muscle (2.3 +/- 0.1 g of tension). Based on the dose-response curve, the ED50 of acetylcholine was calculated (1.26 +/- 0.37 x 10(-4) mol/L for proximal airway smooth muscle; 2.12 +/- 1.14 x 10(-5) mol/L for distal airway smooth muscle) and administered to each tissue bath, after which the stabilized response was recorded. A randomly selected dose of isoflurane (1, 2, or 2.6 dog minimum alveolar anesthetic concentration [MAC] was then administered to each bath and the relaxant responses were recorded. The proximal and distal airways relaxed with increased doses of isoflurane in a dose-related manner.

Phosphoramidon modulates effects of the 5-lipoxygenase inhibition on anaphylactic contraction of the guinea pig trachea

Our previous studies have shown that the inhibition of neutral endopeptidase, an enzyme which degrades tachykinins, increases anaphylactic contraction of guinea pig tracheal smooth muscle. It was suggested that anaphylactic release of tachykinin-like substances is likely to be responsible for the observed increases in tracheal contractions. To obtain additional information on the mechanisms responsible for anaphylactic release of tachykinins in guinea pig trachea, we examined the effects of phosphoramidon, an inhibitor of neutral endopeptidase, on contractile response to antigen after preincubation with the selective 5-lipoxygenase inhibitor AA-861. AA-861 (5 microM) significantly reduced ovalbumin-induced contraction, although the effect was not constant. A marked spontaneous increase in contraction was observed. Phosphoramidon (10 microM) produced significant increase of this contraction (27% after 30 min, and 33% after 45 min). The addition of H1-histamine receptor antagonist (diphenhydramine HCl, 10 microM) produced additional inhibition of the initial phase of antigen-induced contraction, while its later phase, apart from a spontaneous increment in magnitude, remained similar. Phosphoramidon (10 microM) increased the contraction by 26% after 30 min, and by 34% after 45 min. Since the effects of histamine and 5-lipoxygenase pathway products were prevented, we hypothesize that cyclooxygenase pathway products are responsible for the phosphoramidon-dependent increase in antigen-induced contraction. In accordance with previously reported ineffectiveness of contractile prostaglandins, we suggest that the relaxant prostaglandins are most important in mediating the release of tachykinins during the immediate hypersensitivity reaction in guinea pig trachea.

Phosphoramidon augments contraction of guinea pig tracheal smooth muscle induced by histamine and leukotriene-D4

Our previous studies have shown that the inhibition of neutral endopeptidase, an enzyme which degrades tachykinins, increases anaphylactic contraction of guinea pig tracheal smooth muscle. Anaphylactic release of tachykinin-like substances was indicated. To investigate this observation further, we examined the effects of phosphoramidon, an inhibitor of a neutral endopeptidase, on contraction induced by mediators of anaphylaxis. Phosphoramidon significantly increased histamine- and leukotriene D4-induced contractions of tracheal rings from unsensitized animals (by 14 and 48%, respectively), but failed to alter the contractile responses to prostaglandins D2 and F2 alpha. In tracheal rings preincubated with tachykinin antagonist-[D-Pro4, D-Trp7,9]-substance P(4-11), or in capsaicin-desensitized tracheal rings, phosphoramidon did not change histamine- and leukotriene D4-induced contractions. In the second part of the study, performed on tracheal rings obtained from ovalbumin-sensitized guinea pigs, we examined the effects of phosphoramidon on contractile responses to histamine and leukotrienes which are released after antigen challenge. The incubation of tracheal rings with H1-histamine receptor antagonist (diphenhydramine HCl) or leukotriene receptor antagonist (ICI 198.615) prevented a phosphoramidon-dependent increase of antigen-induced contraction. These results indicate that histamine and leukotrienes may be involved in the anaphylactic release of tachykinin-like substances or other neutral endopeptidase substratum.

Reflex bronchodilation produced by phasic ventilation of the lungs

The primary purpose of this work was to demonstrate whether oscillation of tracheal pressure in the range of normal breathing frequencies is more effective than static lung inflation in producing reflex bronchodilation. Secondarily, studies were conducted to determine a single factor that would integrate the reflex bronchodilator effects of frequency and tidal volume. Dogs were anesthetized and placed on cardiopulmonary bypass. After tracheal tone had been reflexly enhanced by chemoreceptor stimulation, oscillatory patterns of ventilation were produced around static pressure lung inflations of approximately 5.0 mmHg. The reflex bronchodilation produced by oscillation of lung volume was compared with that produced by static pressure lung inflation. Reflex bronchodilation produced by oscillation of lung volume was greater than that produced by static pressure inflation of the lungs and was strongly related to the maximum first time derivative of pressure (dP/dt, mmHg/s) observed, irrespective of whether the changes in the maximum dP/dt were produced by changes in the frequency or amplitude of the oscillations of tracheal pressure.

Inhibition of neutral endopeptidase augments anaphylactic constriction of guinea pig tracheal smooth muscle

To determine whether tachykinins participate in antigen-induced constriction of tracheal smooth muscle, we examined the effects of a neutral endopeptidase inhibitor, phosphoramidon, the tachykinin antagonist (D-Pro4, D-Trp7,9,10)-substance P(4-11), and capsaicin-induced tachykinin depletion on the responses to antigen in tracheal rings from ovalbumin-sensitized guinea pigs. In these preparations, the antigen (ovalbumin, 0.1 microgram/ml) produced reproducible and durable constriction of tracheal smooth muscle. Incubation with phosphoramidon (10 min, 10 microM) prior to antigen challenge significantly augmented the magnitude of ovalbumin-induced constriction by 22% after 30 min and by 31% after 45 min. The addition of phosphoramidon at the plateau level of antigen-induced constriction produced a similar, significant increase in the magnitude of the constriction. Following incubation with tachykinin antagonist (D-Pro4,D-Trp7,9,10)-substance P(4-11) (5 microM), the contractile response of the tracheal rings to the antigen was not altered. Furthermore, the addition of phosphoramidon (10 microM) did not significantly affect this contraction. Similarly, neither tachykinin antagonist nor phosphoramidon altered the ovalbumin-induced constriction of the tracheal rings from capsaicin-treated guinea pigs. Based on these findings, we hypothesize that tachykinins or similar broncho-constricting neutral endopeptidase substrates were released from tachykinin-containing nerve endings during immediate hypersensitivity reaction in airways, manifesting a modest and delayed constrictive effect. Following alteration of endopeptidase activity, these substances could modulate the anaphylactic constriction of the airway smooth muscle.

The influence of isoflurane on the vascular reflex response to lung inflation in dogs

Positive pressure ventilation can affect hemodynamic stability by neuroreflex-mediated activity. Inhalational anesthesia is known to attenuate the arterial baroreflex function; however, little information is known about the effect of volatile anesthetics on the lung inflation reflex. The influence of isoflurane on static lung inflation reflex-induced changes in venous capacitance and systemic resistance was investigated in dogs. After controlling carotid sinus pressure at 50 mmHg and initiating total cardiopulmonary bypass, the lungs were inflated to tracheal pressures of 10 and 20 mmHg. The systemic vascular resistance index (SVRI) decreased by 0.04 +/- 0.03 and 0.13 +/- 0.03 mmHg.kg.min.ml-1 during tracheal inflation pressures of 10 and 20 mmHg, respectively. There as an accompanying change in systemic vascular capacitance index (SVCI) by 1.0 +/- 0.65 and 3.3 +/- 0.82 ml.kg-1 during tracheal inflation pressures of 10 and 20 mmHg. The addition of isoflurane decreased the reflex vascular response to lung inflation in a dose-dependent manner. A concentration of 1 MAC isoflurane administered via the cardiopulmonary bypass machine attenuated the change in SVRI to tracheal inflation pressures of 10 and 20 mmHg by 75% and 67%, respectively. Isoflurane at 1 MAC also reduced the reflex capacitance response to tracheal pressures of 10 and 20 mmHg by 36% each. Lung inflation-induced changes in SVRI and SVCI were abolished at isoflurane concentrations of 2 MAC. We conclude that under the conditions of this study, 1 MAC isoflurane was shown to attenuate lung reflex-induced changes in SVRI and SVCI and that at higher isoflurane concentrations (2 MAC) these reflex-induced changes were not seen.

Depression of baroreflex control of heart rate by halothane in growing piglets

The purpose this study was to examine the effects of halothane on baroreflex control of heart rate in developing swine. Serial tests of baroreflex function were performed over the first 2 months of life in eight piglets in the conscious state and during anesthesia with 0.45, 0.9, and 1.35% halothane. Systemic blood pressure was increased with phenylephrine (pressor test) and decreased with nitroprusside (depressor test), and stimulus-response curves relating mean blood pressure to heart rate were constructed. Baroreflex sensitivity was determined as the slope of the linear portion of the curve. Halothane markedly depressed baroreflex sensitivity at all ages in a dose-dependent manner (conscious greater than 0.45% greater than 0.9%, 1.35%). Increasing age was accompanied by decreasing baroreflex sensitivity in both the conscious and the anesthetized states. The difference in baroreflex sensitivity between conscious and anesthetized states did not change with age for the depressor test (tachycardia response), but it did change with age for the pressor test (bradycardia response). For this test, conscious values converged toward anesthetized values at higher ages; therefore, there was relatively less depression by halothane at older ages. Halothane also decreased resting heart rate and decreased the limits and narrowed the range of the baroreflex heart rate response. Increasing age was accompanied by a decreasing resting heart rate and by decreasing limits and a narrowing range of the baroreflex response. The effect of halothane on heart rate variables was similar at all ages. Halothane decreased resting blood pressure and decreased the lower limit and widened the span of the baroreflex blood pressure range.(ABSTRACT TRUNCATED AT 250 WORDS)

Reflex ventilatory effects of KCl stimulation of lung receptors with sympathetic afferents

The purpose of this study was to determine the reflex ventilatory effects produced by lung receptors with sympathetic afferent nerves. Seven dogs were anesthetized with sodium pentobarbital, placed on a ventilator, and vagotomized. The chest was opened through a mid-sternal incision. Diaphragm EMG (D-EMG), right and left triangularis sterni EMG (TS-EMG), systemic arterial blood pressure (BP), and tracheal pressure were recorded before and after the application of 2 M KCl to the right or left lung near the venous hilum. The reflex effects produced by KCl applied to the superior vena cava (SVC) and of mechanical distortion of the lungs were also studied. KCl applied to the right or left lung or right or left lung distortion produced significant increases in peak TS-EMG ipsilateral to the applied stimulus with no significant effects on the contralateral TS-EMG. BP, inspiratory time, expiratory time, or peak D-EMG were not significantly affected, except for a decrease in inspiratory time when KCl was applied to the left lung and a decrease in BP with distortion of the left lung. KCl applied to the SVC produced a significant reflex increase in the right TS-EMG. The primary reflex response to chemical stimulation of lung receptors with sympathetic afferents was an increase in expiratory muscle activity. Additionally, mechanical stimulation may also activate these receptors. Furthermore, receptors which produce similar reflex responses are located on the SVC. That the reflex responses produced by intrathoracic receptors with sympathetic afferents were unilateral suggests that spinal mechanisms are involved.

Development of baroreflex control of heart rate in swine

The purpose of this study is to describe the developmental course of arterial baroreflex control of heart rate in swine. Tests of baroreflex function were performed with eight conscious piglets serially over their first 2 mo of life. Systemic blood pressure was raised with phenylephrine (pressor test) and lowered with nitroprusside (depressor test), and stimulus-response curves relating heart rate to mean blood pressure were constructed. Baroreflex sensitivity was determined as the slope of the linear portion of the curve. Baroreflex sensitivity decreased with increasing age. Baroreflex sensitivity was not different between pressor and depressor tests except when the piglets were greater than 52 d old and sensitivity was greater with the depressor test. The heart rates at threshold and saturation, and therefore the heart rate response range, shifted to lower heart rates with increasing age. This shift was more than can be accounted for by the simultaneously decreasing resting heart rate.

Chronic vascular catheters in growing piglets

Chronic vascular catheterization of growing piglets is problematic because the animals grow rapidly and disrupt each others catheters when housed together. We successfully maintained chronic arterial and venous catheters in growing piglets for the first two months of life using the Vascular-Access-Port, a totally implantable catheter system. Two Vascular-Access-Ports (one venous and one arterial) were surgically placed in each of ten, 3-7 days-old piglets. Nine piglets survived the perioperative period, and for eight piglets the ports were successfully used for experimental purposes to infuse drugs, monitor arterial blood pressure and obtain blood samples for approximately two months. During this period the piglets averaged an eight-fold increase in body weight. This technique of chronic vascular catheterization is useful for experiments employing conscious, growing animals.

Influence of lung inflation reflex on vascular capacitance in the systemic circulation

The effects of sustained lung inflation on systemic vascular capacitance (SVC), systemic vascular resistance (SVR), and cardiac sympathetic efferent nerve activity (SENA) were investigated in anesthetized dogs. By use of a total cardiopulmonary bypass, the lungs were inflated to tracheal pressures of 10, 15, and 20 mmHg. Tracheal pressures of 10, 15, and 20 mmHg increased system vascular capacitance by 1.4, 3.1, and 4.3 ml/kg and decreased systemic vascular resistance by 0.11, 0.15, and 0.16 mmHg.kg.min-ml-1, respectively, at low carotid sinus pressure (CSP) of 41 mmHg. SENA showed a concomitant decrease. Bilateral vagotomy attenuated the change in SVR by 69%, SVC by 62%, and SENA by 97% when lungs were inflated to a tracheal pressure of 20 mmHg at a low CSP. These results indicate that lung inflation causes a reflex induced increase in SVC as well as a decrease in both SVR and SENA. The lung inflation reflex is mediated primarily through vagal afferent nerve fibers with a small contribution from other afferent nerve pathways.

Left ventricular reflex control of venous return and systemic vascular capacitance in dogs

The reflex effects of left ventricular distension on venous return, vascular capacitance, vascular resistance, and sympathetic efferent nerve activity were examined in dogs anesthetized with sodium pentobarbital. In addition, the interaction of left ventricular distension and the carotid sinus baroreflex was examined. Vascular capacitance was assessed by measuring changes in systemic blood volume, using extracorporeal circulation with constant cardiac output and constant central venous pressure. Left ventricular distension produced by balloon inflation caused a transient biphasic change in venous return; an initial small increase was followed by a late relatively large decrease. Left ventricular distension increased systemic blood volume by 3.8 +/- 0.6 mL/kg and decreased systemic blood pressure by 27 +/- 2 mmHg (1 mmHg = 133.3 Pa) at an isolated carotid sinus pressure of 50 mmHg. These changes were accompanied by a simultaneous decrease in sympathetic efferent nerve activity. When the carotid sinus pressure was increased to 125 and 200 mmHg, these responses were attenuated. It is suggested that left ventricular mechanoreceptors and carotid baroreceptors contribute importantly to the control of venous return and vascular capacitance.

Reinnervation of pulmonary stretch receptors

The Breuer-Hering reflex (BHR) reappears 12-14 wk after surgical lung denervation in beagle dogs (J. Appl. Physiol. 54: 1451-1456, 1983). To demonstrate that this is due to reinnervation of pulmonary stretch receptors, we recorded nerve activity from regenerated branches of the left vagus nerve in five beagle dogs. Ten days postdenervation the BHR was absent, whereas by 19 mo it was clearly present. Multifiber pulmonary afferent activity was observed in all five dogs with single-fiber activity observed in three. Sectioning the right vagus nerve did not alter the BHR, but sectioning all the regenerated branches of the left vagus abolished the reflex. In two additional dogs studied 17 mo postsurgery, recordings were made from few fiber nerve bundles of the left cervical vagus. Nerve activity was increased during gentle stroking of the surface of the left upper and lower lobes, indicating receptive fields in both lobes. These data demonstrate that reinnervation of pulmonary stretch receptors does occur and provides evidence that reinnervation of these receptors is responsible for return of the BHR after pulmonary denervation.

Pulmonary depressor reflex elicited by capsaicin in conscious intact and lung-denervated dogs

A pulmonary depressor reflex has been shown to be elicited in anesthetized cats, dogs, and rats by intravenous injection of capsaicin. The effects observed include apnea, hypotension, and bradycardia with some investigators reporting tachypnea following the apneic period. We investigated the response to a bolus injection of capsaicin (20 micrograms/kg) into the cephalic vein in 11 conscious beagle dogs. Five control dogs underwent sham thoracotomies, and six dogs underwent selective denervation of the lungs. A low dead-space latex rubber mask was used to monitor ventilation, and arterial blood pressure was obtained by catheterizing an exteriorized carotid artery. In four of the five control dogs the observed response was apnea concomitant with hypotension and bradycardia, followed by tachypnea. In five of the six lung-denervated dogs there was a slight tachypnea along with hypertension. It is concluded that the pulmonary depressor reflex can be elicited in conscious dogs by intravenous injection of capsaicin but is absent in lung-denervated dogs.

Arterial hypocapnia during exercise in beagle dogs

Previous investigators have assumed that during exercise there is a tight coupling of ventilation with CO2 delivery to the lungs such that arterial blood remains isocapnic. We measured arterial blood gases in a group of 10 beagle dogs in which arterial blood sampling could be accomplished via exteriorized carotid artery loops and in six of the same dogs following chronic pulmonary denervation. Samples were taken at rest, at 15-s intervals during the first minute of unrestrained treadmill exercise at 5.0 km/h, 0% grade, and then at 2 and 3 min at the same work load. Mean resting arterial PCO2 for the control dogs was 37.1 Torr. At the onset of exercise arterial PCO2 fell progressively to a nadir of 34.6 Torr during the 30- to 45-s sampling period. Samples at 2 and 3 min remained significantly hypocapnic (PCO2 = 34.8 Torr). The arterial PCO2 and pH responses to exercise in the lung-denervated dogs were not significantly different from those of the control dogs, although arterial PO2 was lower at rest and during exercise following denervation of the lungs. The arterial hypocapnia exhibited in intact beagle dogs at the onset of exercise persists into the steady state and suggests that there is not a tight coupling of ventilation with pulmonary CO2 delivery. The similarity of the response in lung-denervated dogs suggests that intrapulmonary receptors with afferents in the vagi are not the primary mediators of the ventilatory response to exercise.

Effect of chronic pulmonary denervation on ventilatory responses to exercise

To assess the role of intrapulmonary receptors on the ventilatory responses to exercise we studied six beagle dogs before and after chronic pulmonary denervation and five dogs before and after sham thoracotomies. Each exercise challenge consisted of 6 min of treadmill exercise with measurements taken during the third minute at 3.2 km/h, 0% grade, and during the third minute at 5.0 km/h, 0% grade. Inspiratory and expiratory airflows were monitored with a low-dead-space latex mask and pneumotachographs coupled to differential pressure transducers. Both pre- and postsurgery, all dogs exhibited a significant arterial hypocapnia and alkalosis during exercise. Denervation of the lungs had no significant effect on minute ventilation at rest or during exercise, although there was a lower frequency and higher tidal volume in the lung-denervated dogs at all measurement periods. Breathing frequency increased significantly during exercise in lung-denervated dogs but to a lesser magnitude than in the control dogs. The changes that occurred in breathing frequency in all animals were due predominantly to the shortening of expiratory time. Inspiratory time did not shorten significantly during exercise following lung denervation. We conclude from these data that intrapulmonary receptors which are deafferented by sectioning the vagi at the hilum are not responsible for setting the level of ventilation during rest or exercise but are involved in determining the pattern of breathing.

Respiratory arrhythmias and airway CO2, lung receptors, and central inspiratory activity

The purpose of this study was to determine whether hypocapnia affects heart rate secondary to an effect on pulmonary receptors. Dogs were anesthetized and placed on cardiopulmonary bypass. Interrelationships among airway CO2, central inspiratory activity, and lung receptor effects on respiratory-related heart rate changes (respiratory arrhythmias) were studied after vagal efferent activity was increased secondary to baroreceptor stimulation. Hypocapnia, isolated to the lungs, produced an increase in the magnitude of the respiratory arrhythmias observed. Two mechanisms may produce these results. Hypocapnia affects pulmonary receptors, which 1) reflexly alter heart rate and 2) modulate breathing frequency, thus altering the dynamics of the respiratory arrhythmias that were produced. The results also suggested that the reflex increase in heart rate in response to lung inflation and the Hering-Breuer expiratory-facilitatory reflex are either produced by different pulmonary receptors or by the same pulmonary receptors but may be mediated by different central mechanisms.

Effects of chronic right-to-left cardiac shunt on hypoxic sensitivity of mongrel dogs

Resting ventilation (VI), blood gases, hypoxic sensitivity, and the ventilatory responses to intravenous sodium cyanide (NaCN, 100 micrograms/kg), doxapram (DOX, 500 micrograms/kg), and dopamine (DOPA, 20 micrograms/kg) were analyzed in four normal mongrel dogs (group I-N) and seven mongrel dogs with chronic (5-11 yr) right-to-left cardiac shunt (group II). The group I-N animals were also studied during steady-state isocapnic hypoxia (group I-H). The shunt procedure used for these studies produced a model for ventilatory studies during chronic shunt hypoxemia. The increases in VI per percent decrease in O2 saturation, which occurred during a four-breath N2 test, were 30, 43, and 13 ml X kg-1 X min-1 in groups I-N, I-H, and II, respectively. The decrease in hypoxic sensitivity of the group II animals, compared with groups I-N and I-H, occurred in the presence of an increase in PaCO2 from 21.9 to 26.0 Torr during the four-breath N2 test. A decrease in PaCO2 from 34.7 to 30.0 and from 33.6 to 30.4 Torr was observed in groups I-N and I-H. The response to DOX, a general analeptic agent, was greatest in group II and least in group I-N. However, the ventilatory responses to NaCN and DOPA were not sufficiently different among the three groups to suggest a difference in carotid body function as assessed by these drugs.

Systemic arterial pH servocontrolled ventilator simulation of the respiratory control system

The terminology 'isocapnic hyperpnea' has been used to describe the ability of the respiratory control system to increase ventilation in response to inhalation of low levels of CO2 without an apparent change in the error signal (arterial pH or PCO2). Recently a control system for the systemic arterial pH (pHa) servocontrol of mechanical ventilation has been developed. The combination of proportional and integral control used produced a system by which the desired set point was maintained with virtually a zero steady-state error. The purpose of these experiments was to use this system to produce isocapnic hyperpnea in response to low levels of inspired CO2 and thus to demonstrate how, through integral control, a biological system could produce a particular response without an apparent change in the controlled variable. Adding 1.0 to 3.5% CO2 to the inspired gas of dogs connected to the pHa servocontrolled ventilator produced increases in minute ventilation with little or no change in pHa or PaCO2. Whether such a control system has any relevance to the physiological control system is questionable. It does however allow a unique way of investigating the possibilities by which the physiological system may work.

Pulmonary denervation in the dog

To produce a chronically lung-denervated animal model, a single-stage surgical procedure was performed on five beagle dogs. A left thoracotomy allowed hilar stripping of the pulmonary contributions to the left vagus nerve and transection of the right vagal trunk. The criterion for denervation was defined as the absence of the Hering-Breuer reflex (HBR). The five denervated dogs (DD) as well as five control dogs (CD) were evaluated for presence of the HBR under pentobarbital sodium (Nembutal, 30 mg/kg) anesthesia. Between the 3rd and 8th wk postsurgery, the HBR was clearly abolished in the DD but present in the CD. By the 12th-14th wk postsurgery, the HBR was again present in the DD. This relatively uncomplicated surgical procedure effectively produced a lung-denervated animal model. However, reinnervation occurred relatively soon, thus demonstrating the importance of regular frequent evaluation of the HBR when using this and any other surgically produced pulmonary denervation model.

Interrelationships among airway CO2, airway pressure, and breathing frequency

The breathing frequency response to changes in airway CO2 of a vascularly isolated lobe of the canine lung has previously been shown to be primarily dependent on CO2-mediated changes in airway pressure. This study was carried out to determine what contribution changes in airway pressure make in the whole lung airway CO2-mediated breathing frequency response. Mongrel dogs were anesthetized and placed on cardiopulmonary bypass. Diaphragm electromyogram (EMG) was used to monitor respiratory center output and to trigger ventilation of the lungs. Isoproterenol administered to the lungs prevented hypocapnic airway constriction but only partially blocked the decrease in breathing frequency, suggesting that in the whole lung preparation, airway CO2 in part alters breathing frequency through a direct effect on pulmonary receptors. At constant positive end-expired pressures (1-6 Torr), 0% airway CO2 produced greater increases in expiratory time than 10% CO2. Thus airway CO2 can affect breathing frequency in the absence of CO2-related changes in airway pressure at pressures that would produce lung volumes similar to those observed at end expiration in the intact animal. An argument is presented that the receptors directly affected by CO2 are probably not located in the airways constricted by hypocapnia.

Location of lung receptors mediating the breathing frequency response to pulmonary CO2

Pulmonary stretch receptors are thought to mediate the breathing frequency (bf) response to changes in pulmonary CO2. However, the location and distribution of these receptors is disputed. The purpose of this study was to determine what contribution the extrapulmonary receptors make in the pulmonary CO2 bf response. Mongrel dogs were anesthetized and placed on cardiopulmonary bypass. The diaphragm electromyogram was used to monitor respiratory center output and to trigger a ventilator. Exposure of an upper airway segment to CO2 or positive end-expired pressure failed to produce changes in the bf. Denervation of the upper airway down to but not including the hilum caused similar insignificant changes in the CO2 bf response. Lungs collapsed by suction showed minimal Hering-Breuer inhibition when compared with inflated lungs. Bronchial arterial perfusion with hypocapnic followed by hypercapnic blood failed to produce changes in the bf while similar perfusion of the pulmonary arterial system resulted in significant increases in bf. It appears that the receptors mainly responsible for the pulmonary CO2 response are located in the more peripheral regions of the lung.

Measurement of dead space ventilation using a pHa servo-controlled ventilator

A control system for the systemic arterial pH (pHa) servo control of mechanical ventilation has recently been developed. If pHa is maintained constant by the change, separation of minute volume into alveolar ventilation and physiological dead space ventilation (VE = fVA VDp) can be manipulated to show that VDp = (VE1 - VE 2)/(f1 - fe) where f1 and f2 are different ventilator frequencies and VE1 and VE2 are expired minute volumes at these frequencies. Also, added dead space can be measured. VDadded = (VE2 - VE1)/f where VE1 and VE2 are the minute volumes before and after the dead space was added. The validity of these equations was tested in the anesthetized dog. The measured added dead space was in close agreement with the volume of dead space which was added and with that measured by another independent method. The measurement of VDp, probably as a result of tidal volume-related changes in VDp, did not agree as well with VDp measured by an independent method.

Accessory muscle activity and respiration

The relationship between the accessory muscle activity (sternohyoid and sternothyroid) and respiration was studied in canines. These animals do not have an omohyoid muscle such as found in primates. Therefore, chair-trained monkeys that have all three accessory muscles were used in a portion of the study. Findings in canines supported those previously reported. The sternothyroid muscle fired spontaneously with the onset of inspiration, but there was no similar activity in the sternohyoid. The activity of both muscles was observed after sectioning the recurrent laryngeal nerves and inducing hypoxia and hypercarbia. The severely hypoxic animals produced some negligible activity in the sternohyoid muscle. The data obtained from the chair-trained monkeys showed no consistent accessory muscle activity during normal respiration with recordings taken immediately after electrode placement, at 24 hours, and one week later. Respiratory activity was consistent in the omohyoid and sternothyroid but not in the sternohyoid muscle when partial airway obstruction and hypoxia were induced.

Segmental ventricular adjustments to brief periods of ischemia in the dog

The changes in left ventricular segmented contractile force induced by brief periods of ischemia (15-90 s) and subsequent reperfusions were analyzed in anesthetized dogs. Segmental coronary artery occlusion (left anterior descending or left circumflex) produced a decrease in segmental ventricular function in the occluded area and an increase in contractile force in the myocardial segment away from the occluded area. With reperfusion, a transient overshoot in contractile force above preischemic control levels was observed in the occluded segment. This overshoot was shown not to be dependent on adrenergic mechanisms but appears to indicate changes in calcium permeability.

Effect of verapamil on pulmonary reflexes in the vascularly isolated canine lung: physiology

Breathing frequency (BF) may be affected by changes in the percent inspired CO2 administered to vascularly isolated lungs. Pulmonary CO2 probably affects BF, in part, through a secondary effect of CO2 on airway smooth muscle. To further determine the role of pulmonary mechanics in the pulmonary CO2-mediated BF response, Verapamil, a Ca++ blocking agent which blocks hypocapnic airway constriction, was administered to the vascularly isolated lungs of the dog. Verapamil blocked the hypocapnic airway constriction which occurred when pulmonary CO2 was reduced; however, the decrease in BF was not only blocked but in some animals there was an increase in BF. Also, the decrease in BF produced by hyperinflation of the lungs (Hering-Breuer reflex) was either blocked or an increase in BF occurred after administration of Verapamil.

Effect of anodal blockade of myelinated fibers on vagal C-fiber afferents

The effects of monopolar cathodal, monopolar anodal, and bipolar anodal polarizing currents on vagal A- and C-fiber activity were studied in anesthetized dogs. Monopolar cathodal polarization consistently produced excitation of spontaneous and evoked A- and C-fibers. Monopolar anodal and bipolar anodal polarizations differentially blocked A-fibers as a function of fiber diameter. The specific purpose of this study was to compare the effects of monopolar anodal and bipolar anodal currents on C-fiber excitability. Small fiber preparations were dissected from the vagal trunk, cut centrally, and placed on recording electrodes. A constant-current stimulus pulse was applied to the nerve at various distances from the blocking electrodes. The stimulus current strength was increased until an isolated C-fiber spike was observed. This value was defined as 100% of threshold. The stimulus current was then reduced to zero, the blocking current was increased slowly to 100 microA, and the procedure repeated. Threshold data obtained in this manner for each set of stimulation electrodes was plotted as a function of distance from the blocking electrode(s) for both modes of anodal blockade. No significant change in C-fiber excitability was observed with bipolar anodal blockade, whereas excitability was significantly (P less than or equal to 0.05) decreased using the monopolar technique. Thus, monopolar anodal block may reduce the possibility of asynchronous C-fiber discharge, which has been associated with a bipolar block of A-fibers.

Cardiac, aortic, pericardial, and pulmonary receptors in the dog

Afferent nerve activity from left and right atrial, left and right ventricular, interventricular septal, papillary muscle, pericardial, aortic and pulmonary vascular receptors was recorded from the left T3 white ramus communicans and the innominate, dorsal, recurrent, ventromedial, craniovagal and caudovagal cardiac nerves in alpha-chloralose-anesthetized dogs. The receptors were localized and the nature of the stimuli required to excite these receptors was also determined. Some ventricular receptors were excited during maximal contraction of the myocardium. Other ventricular and aortic receptors were stimulated by elevation of intracardiac or aortic pressure. The pericardial, atrial, and papillary muscle receptors were excited by stretching the surrounding tissue. The discharge patterns of these receptors were not always synchronous with the events of the cardiac cycle. The stimuli required to excite each type of receptor (ventricular, atrial, etc.) and their resultant discharge patterns were not identical for all of the receptors. Excitation of cardiac receptors with sympathetic afferents resulted in 1--2 spikes per cardiac cycle, whereas receptors with vagal afferents resulted in bursts of spikes per cardiac cycle.

Breathing frequency responses to pulmonary CO2 in an isolated lobe of the canine lung

Recent studies have indicated that the breathing frequency responses to inspired CO2 in part result from changes in pulmonary stretch receptor activity. Pulmonary CO2 may alter frequency by direct inhibition of stretch receptor discharge, or secondarily, by changes in airway mechanics. The vascularly isolated left lower lobe (LLL) of the canine lung was used to determine the effect of hypocapnic airway constriction on the pulmonary CO2 reflex. The upper and middle lobes of the left lung were removed and the right vagus nerve sectioned. Blood was recirculated through the LLL. Diaphragm electromyogram was used as an index of respiratory center activity and to trigger ventilation of the left lower lobe. Lobar hypocapnia increased peak airway pressure and reduced respiratory rate. However, infusion of isoproterenol or the use of a mechanical overflow system to block the airway pressure response prevented the frequency changes associated with CO2. Although both the direct and mechanical effects of CO2 on stretch receptors may contribute to the reflex, in the LLL preparation the mechanical effects predominate.

Halothane, tracheal compliance and upper-airway mechanoreceptors

This study was designed to define the effects of halothane on the compliance of the trachea. An isolated in-situ tracheal preparation was studied in 14 mongrel dogs anesthetized with pentobarbital. Compliance of the closed tracheal segment was measured with continuous intraluminal pressure recordings during repeated injections of known volumes of air: Slow-adapting neural activity observed in paratracheal branches of the recurrent laryngeal nerve accurately reflected pressure in the tracheal segment. Halothane at 0.5--4.0 per cent concentrations caused a significant (P less than 0.001) average 10 per cent increase in the compliance of the trachea. Stimulation of the efferent vagus caused a significant (P less than 0.001) average 8 per cent decrease in compliance of the trachea. After exposure to halothane, vagal stimulation still caused a significant decrease in compliance of the trachea.

Systemic arterial blood pH servocontrol of mechanical ventilation

Servocontrol of mechanical ventilation using systemic arterial blood pH, measured by a dual-function pH/PCO2 intra-arterial sensor, as the controlled variable uas carried out in 30 dogs anesthetized with pentobarbital, 30 mg/kg. The control loop consisted of the animal, an intra-arterial dual-function pH/PCO2 sensor and sensor amplifier, a controller, and a Siemans-Elema 900 servoventilator. The system responded appropriately to changes in set-point pH from 7.30 to 7.50, as well as to infusions of lactic acid, which, with the control loop open, decreased systemic arterial blood pH 0.1 TO 0.2 PH units. Long-term (16 hr) ventilation of one dog with the systemic arterial blood pH servocontrol ventilator was shown to be feasible.

Respiratory inhibition with sympathetic afferent stimulation in the canine and primate

Inhibition of phrenic efferent nerve activity, diaphragm electromyogram (EMG), and external intercostal EMG was observed in halothane- and pentobarbital-anesthetized mongrel dogs and pentobarbital-anesthetized monkeys with stimulation of sympathetic afferents. The central end of the transected ventral limb of the left ansa subclavia, the sympathetic chain, or individual white rami were stimulated while simultaneously recording phrenic efferent nerve activity, diaphragm EMG, or the external intercostal EMG. Averaged phrenic efferent bursts or MEG were used to trigger an electronic respirator. In all of the dogs and monkeys, electrical stimulation of sympathetic afferent pathways resulted in inhibition of phrenic efferent nerve activity, diaphragm EMG, or external intercostal EMG. Although the exact origin of these fibers was not determined, the conduction velocities of these afferents were 4-7 m/s, which places them in the Adelta fiber-type range. The importance of these afferents in the regulation of respiration in the awake animal remains unknown.

Cardiac responses to stimulation of thoracic afferents in the primate and canine

Excitatory cardiovascular responses to electrically stimulated upper thoracic sympathetic afferent nerves were observed in halothane-anesthetized mongrel dogs and monkeys. The central end of the transected ventral limb of the left ansa subclavia was stimulated before and after several types of denervation. Significant increases in right and left ventricular maximum systolic pressures, systolic and diastolic systemic blood pressures, and aortic flow were observed. The carotid sinuses were denervated bilaterally and stimulation of the ansa was repeated. The cardiovascular responses to stimulation of the ventral ansa after carotid sinus denervation were greater in magnitude than those observed prior to denervation. This carotid sinus modulation of cardiovascular responses was observed in dogs and monkeys. Cardiovascular responses to stimulation of the ventral ansa after bilateral vagotomy were significantly less than the responses observed after carotid sinus denervation prior to vagotomy. However, the responses after vagotomy were statistically identical to responses obtained while stimulating the ventral ansa when the carotid sinuses and vagi remained intact.

Tracheal afferent nerves

The individual fibers of the inferior laryngeal nerve and anastomosing branch of the superior laryngeal nerve arising from the upper five to six tracheal rings were studied. In 30 mongrel dogs using simple and multifiber nerve preparations, afferent nerve activity was studied both in the intact trachea and in an isolated segment of trachea with artificial stimuli of graded pressure and flow. The adaptation rate, threshold, spontaneous firing frequency, maximum firing frequency, and rate of change in afferent discharge were recorded. Subsequently, these fibers were electrically stimulated to determine what reflex effects could be produced. Afferent nerve activity recorded from the upper trachea was found to be sensitive to tracheal pressure changes of 0.5 to 12 mm Hg. Nerve activity paralleled the frequency of pressure changes. Electrical stimulation of these fibers demonstrated reflex bradycardia, bradypnea and alteration of blood pressure. These data and that of other investigators indicate that upper airway mechano-receptors may be significant, not only in the initiation of upper airway protective reflexes, but also in the regulation of normal respiration.