Central chemosensitivity and the reaction theory

Differential effects of carbon dioxide and pH on central chemoreceptors in the rat in vitro

The brain stem, cervical cord and attached phrenic nerve were excised from neonatal rats and superfused in vitro. Respiratory activity was recorded from the phrenic nerve following transection of all the cranial nerves and dorsal roots. The frequency of spontaneous periodic activity recorded from the phrenic nerve was 6-14/min during superfusion with a saline solution equilibrated with 5% CO2 in O2 at 25 degrees C (pH 7.3). The magnitude of respiration was estimated from the peak value of phrenic activity integrated for each 0.1 s period. When the pH of the superfusion fluid was altered by changing the HCO3-concentration at constant PCO2, respiratory activity increased in low pH and decreased in high pH. These changes were maintained as long as a given pH was held. Respiratory changes observed under these conditions were characterized by alterations in both respiratory frequency and magnitude. When the CO2 level of the superfusion fluid was altered, maintaining constant pH by modified HCO3-concentrations, respiratory activity increased at high PCO2 and decreased at low PCO2. These changes were transient and lasted only for a few minutes after exposure to a new level of PCO2. Respiratory changes observed under these conditions were characterized by alterations in magnitude but not in frequency. At constant PCO2 an increase in the HCO3-concentration occasionally enhanced the magnitude of respiration before respiratory activity was depressed by the increased pH. This suggests that HCO3- may act independently as a stimulus to the central chemoreceptor. It is concluded that the mammalian central chemoreceptor for respiratory control is responsive independently to H+ and CO2 and that H+ and CO2 exert differential effects on the respiratory centre in terms of frequency and magnitude. It is suggested that frequency modulation and magnitude (tidal volume) modulation for respiratory control are triggered at different regions in the respiratory centre and/or rely on different mechanisms.

Hyperglycaemia: imitating Claude Bernard's piqûre with drugs

Hyperglycaemia lasting for hours, has been produced in unanesthetized cats, rabbits and rats by injection into the cerebral ventricles or the cisterna magna of a variety of drugs (morphine, etorphine, pethidine, beta-endorphin, enkephalin, bombesin, TRH, cholecystokinin, naloxone, propranolol, phentolamine, chloralose, magnesium chloride and GABA). These drugs probably act at the ventral surface of the brainstem and initiate a sympathetic discharge to the adrenals which results in a prolonged release of relatively small amounts of adrenaline. When adrenaline is released in this way hyperglycaemia may be the only effect. The mechanism of the piqûre hyperglycaemia of Claude Bernard may be the same, although Bernard assumed that it resulted from an effect on the floor of the fourth ventricle, i.e. on the dorsal surface of the brainstem. However, it is clear from his description that his trochar not only pricked the floor of the fourth ventricle but penetrated to the ventral surface of the brainstem. Release of adrenaline from the adrenals is usually regarded as a stress response, as in fight, flight, fear or rage when it is suddenly released in large amounts and produces its typical cardiovascular and ocular reactions. The results now obtained with drugs injected intraventricularly or intracisternally suggest an additional physiological role for adrenaline when it is released over prolonged periods and in relatively small amounts producing only hyperglycaemia. Such a release may play a role in the day-to-day control of blood glucose, and its disturbance might underlie non-insulin-dependent diabetes.

Central chemosensitivity to H+ and CO2 in the rat respiratory center in vitro

The brainstem, cervical cord and attached phrenic nerve were excised from newborn rats and superfused in vitro. Respiratory output was measured by integration of phrenic nerve discharges. Respiratory output was enhanced by an increase in pCO2 at constant pH as well as by decreased pH with constant pCO2. It is concluded that the adequate stimulus to central chemoreceptors is not restricted only to H+.

Electrical stimulation of arterial and central chemosensory afferents at different times in the respiratory cycle of the cat: I. Ventilatory responses

Ventilatory responses to stimulation of chemoreceptor afferents were studied in the anesthetized, spontaneously breathing cat. Short bursts of electrical stimuli were applied, at various times in the inspiratory or expiratory phase of consecutive breaths, to the carotid sinus (CSN) and aortic nerves (AN) and to the ventral medulla (VM), and effects on tidal volume (VT), inspiratory, expiratory and cycle durations (tI, tE, ttot) and in ventilation (VE) were measured. The responses evoked by stimulating CSN, AN and VM were qualitatively the same, although there were quantitative differences. It was found that effects of stimulation in expiration were restricted to the expiratory phase, and vice versa for inspiration. Stimulation during both inspiration and expiration resulted in increased VT, by increasing end-inspiratory or decreasing end-expiratory lung volume, respectively, and also increased ventilation, VE. These effects were most marked in response to stimulation in inspiration. During both phases there was an increasing effect with increasing delay of the stimulus, tSt, from onset of inspiration or expiration, respectively. There was a continuous increase in tI, from below control to above control values, with increasing tSt during inspiration and similarly for tE during expiration. Hence, the total respiratory cycle duration was shortened when a stimulus was applied early in either phase, and was prolonged, when it was applied late. The results show that stimulation of peripheral and of central chemoafferents exerts qualitatively similar effects on respiration. The central neuronal mechanisms generating both inspiration and expiration show the same changes in reactivity in the respiratory cycle.

An increase in metabolic acidosis induced by chloralose anaesthesia in dogs after sino-vagal denervation

Chloralose anaesthesia in dogs increased the H+ ion concentration significantly from its reference values. The findings favoured that it was most probably engendered through anaesthetic depression of neural centre regulating H+ ion concentration of blood. Such increase was largely contributed by a significant increase in its metabolic fraction. A further increase of metabolic fraction after separate and joint section of carotid sinus nerves and vagi indicated their holding effect. The section of carotid sinus nerve induced greater increase in this fraction than that of vagi. It indicated differences between the two nerves in their metabolic fraction controlling influence. Hyperpnoea after vagi section decreased the carbonic acid fraction, whereas marginally reduced ventilation after carotid sinus nerve section increased the carbonic acid fraction. Moreover, the overall changes in H+ ion concentration followed the changes in carbonic acid fraction. The present study suggested that the depressive effect of chloralose anaesthesia on H+ ion controlling neural mechanism could be largely determined by degree of increase in its metabolic fraction.

Evidence for a capsaicin-sensitive vasomotor mechanism in the ventral medullary chemosensitive area of the cat

The effects of capsaicin applied to the exposed ventral surface of the medulla were studied on the mean arterial blood pressure, heart rate, respiration and sympathetic efferent nerve activity in chloralose-urethane-anaesthetized cats. The application of capsaicin produced a marked increase in the mean arterial blood pressure and sympathetic nerve activity, but not in the heart rate. The "intermediate area" proved to be the most sensitive to capsaicin. Pressor responses could be elicited repeatedly; tachyphylaxis was not noted provided a time interval of 30 min elapsed between consecutive applications. Repeated applications of capsaicin at intervals of less than 30 min led to tachyphylaxis. However, pressor responses evoked by either topical application of glutamate or pentamethylene-tetrazole or bilateral carotid occlusion could invariably be demonstrated during this period of tachyphylaxis. Histological studies revealed the existence of a hitherto unrecognized termination of capsaicin-sensitive nerve endings within the ventral medullary chemosensitive area of the cat. The results provide both functional and morphological evidence for the presence of a capsaicin-sensitive vasomotor mechanism in the ventral medullary chemo-sensitive area of the cat. It is suggested that the pressor effects of capsaicin applied to the ventral medullary chemo-sensitive area may be mediated by an activation of capsaicin-sensitive primary sensory afferents terminating in this area. Accordingly, capsaicin-sensitive neuronal mechanisms located in the ventral medullary chemosensitive area may play an important role in the central nervous regulation of blood pressure.

The roles of medullary extracellular and cerebrospinal fluid pH in control of respiration

To determine the effective stimulus to the central chemoreceptors, we measured CSF and medullary extracellular fluid (ECF) pH and phrenic activity in 11 anesthetized, paralyzed, vagotomized and glomectomized cats. Flat-tipped pH electrodes (2 mm diam.) were used to measure ECF pH on the ventral surface of the medulla and CSF pH 2 mm above the surface. Changes in alveolar/arterial PCO2 were produced by airway occlusions of 10-20 sec durations. Changes in CSF PCO2 and pH were made by infusing 100% CO2 or an acid buffer into the CSF. Airway occlusion caused an increase of alveolar/arterial PCO2. ECF pH began to fall 6-10 sec later, with a maximum decrease of 0.032 pH unit at 21.9 sec. Phrenic activity increased as ECF pH decreased, the greatest activity occurring when ECF pH was most acid. CSF pH decreased after a longer delay. Its maximum decrease at 54.1 sec was smaller (0.026 pH unit) than ECF pH and did not correlate with the increase of phrenic activity. Addition of 100% CO2 or an acid buffer into the CSF produced an acid shift in the CSF pH but no change in ECF pH or phrenic activity. Prolonged (greater than 30 min) increase of acidity of CSF did not alter phrenic activity until ECF pH developed a delayed acid shift. Even then, the change of ECF pH was much smaller than that of CSF. We conclude that medullary chemoreceptors do not respond to changes of CSF pH or PCO2 and that change of pH of CSF minimally affects ECF pH. On the other hand, respiratory responses are closely linked to changes in ECF pH.

Respiratory responses to medullary hydrogen ion changes in cats: different effects of respiratory and metabolic acidoses

The steady-state responses of respiration, measured as integrated phrenic nerve activity, to hypercapnic acidosis of the medullary extracellular fluid (e.c.f.) and to metabolically generated acidosis were compared in paralysed, vagotomized and glomectomized cats. E.c.f. hydrogen ion concentration [( H+]) was measured directly by means of a small (2 mm diameter) pH electrode placed on the ventral medulla. The results in ten cats show that changes of medullary e.c.f. [H+] were linearly related to changes of end-tidal PCO2 both before (r = 0.999) and after (r = 0.996) development of metabolic acidosis. There was a curvilinear relation between hypercapnic e.c.f. [H+] changes and the respiratory response that reflects progressive saturation of a central neural pathway between the chemoreceptors and the respiratory controller. This relation was similar in form both before and after development of metabolic acidosis. When acidosis of metabolic origin was present, apnea occurred with only small decreases of CO2 despite a high [H+]. The respiratory responses to the same e.c.f. [H+] change were only about one-half as large when they were generated metabolically as when they were generated by raising PCO2. Both exogenously induced metabolic acidosis (HCl infusion) and endogenous acidosis yielded similar results. We conclude that the e.c.f. [H+] does not represent the unique stimulus to the central chemoreceptors. We discuss several alternate mechanisms for the action of CO2 and [H+] on central chemoreceptors but none can be considered definitive at the present time.

Cardiac inotropic responses from changes in carbon dioxide tension in the cephalic circulation of anaesthetized dogs

Experiments were performed on anaesthetized dogs to determine the effects of moderate changes in PCO2 in the cephalic circulation on the inotropic state of the heart and on the reflex inotropic responses from changes in carotid sinus pressure. The cephalic circulation was perfused, through the brachiocephalic and left subclavian arteries, with blood taken from the superior vena cava and equilibrated with various gas mixtures in a gas exchange unit. The carotid sinus regions were vascularly isolated and perfused with arterial blood at controlled pressures. Cardiac inotropic responses were assessed from the maximum rate of change of left ventricular pressure (dP/dtmax) with heart rate and mean aortic pressure held constant. An increase in cephalic blood PCO2 resulted in an increase in dP/dtmax and an increase in the unpaced heart rate. Small, graded changes in cephalic PCO2 resulted in graded responses of dP/dtmax. A change in carotid sinus pressure resulted in a significantly greater response of dP/dtmax when cephalic PCO2 was high. After interruption of the left cardiac sympathetic nerves, the responses of dP/dtmax to changes in cephalic PCO2 and carotid sinus pressure were nearly abolished. These results indicate that the tension of carbon dioxide in the cephalic circulation is likely to be of importance in the control of the inotropic state of the heart. They also imply that, in studies of cardiovascular reflex responses, it is important to control the carbon dioxide tension in the arterial blood.

Initiation of migrating myoelectric complex in sheep by duodenal acidification and hyperosmolarity: role of vagus nerves

Gastrointestinal motility was studied in conscious sheep by X-radiography and by electromyography from chronically implanted electrodes before and after total thoracic vagotomy. Duodenal infusion of 0.5-3 mmol HCl (0.035-0.1 M-HCl) induced premature duodenal regular spiking activity (r.s.a.) within 1-7 min in fifteen of seventeen sheep studied when infused at 20 min after a natural r.s.a. There was no correlation between abomasal pH and any phase of the migrating myoelectric complex (m.m.c.). Duodenal alkalinization by infusion of 0.3 M-Tris buffer (pH 10.2) or 0.1 M-NaHCO3 had no influence on the occurrence of the m.m.c. Duodenal infusion of 20-50 ml 0.5 M-NaCl induced a premature duodenal r.s.a. within 1-5 min in seven of eight sheep. Vagotomy did not prevent the initiation or migration of the m.m.c., but reduced the rate of propagation of the r.s.a. from 40.5 +/- 7.2 (mean +/- S.E. of mean) to 16.7 +/- 0.1 cm/min in the duodenum, from 27.3 +/- 4.1 to 16.6 +/- 0.8 cm/min in the jejunum, and from 21.4 +/- 1.1 to 13.7 +/- 0.7 cm/min in the proximal ileum. Initially the frequency of r.s.a. increased, especially in the duodenum where they recurred at an interval of 98.4 +/- 6.8 min before vagotomy; and at 23.4 +/- 1.8 min in the first 24 h after vagotomy; the interval had lengthened to 86.7 +/- 5.2 min 2-3 weeks after vagotomy. Premature duodenal r.s.a. was not induced by duodenal infusion of HCl in five, or by duodenal infusion of hyperosmolar NaCl in three chronically vagotomized sheep. It is concluded that the vagus nerves contribute to the regulation of the frequency and propagation of the m.m.c. in sheep; duodenal acidification is not essential nor is it the normal stimulus for initiation of r.s.a., but duodenal infusion of HCl or hyperosmolar NaCl can initiate a premature duodenal r.s.a. via the vagus nerves.

The role of spinal cord transmission in the ventilatory response to exercise in man

The ventilatory response to electrically induced exercise was studied in thirteen patients with traumatic spinal cord transection at or about the level of T6. The steady-state and on-transient responses to this exercise were compared with those obtained in eighteen normal subjects (Adams, Garlick, Guz, Murphy & Semple, 1984). Exercise was produced by surface electrode stimulation of the quadriceps and hamstring muscles so as to produce a pushing movement at 1 HZ against a spring load. At rest there was no significant difference between normals and patients, except that the patients had a lower CO2 elimination (VCO2) and end-tidal PCO2 (PET,CO2) and a higher heart rate. On exercise the mean rise in VCO2 for the patients was 172 ml min-1 (S.D. 72), and for the normals was 287 ml min-1 (S.D. 143). The corresponding mean changes in ventilation (VI) were 4.4 l min-1 (S.D. 2.2) and 7.6 l min-1 (S.D. 3.2). However, the ventilatory equivalent for CO2 (delta VI/delta VCO2) in the steady state was not significantly different between patients (26.0, S.D. 5.9) and normals (28.5, S.D. 7.4). In the steady state there was a mean rise in PET,CO2 of 0.9 mmHg (S.D. 1.4) in the normals, and 3.2 mmHg (S.D. 2.7) in the patients, but there was overlap between the two groups. In many experimental runs in both groups, PET,CO2 did not rise, and sometimes fell. Where PCO2 did rise, the ventilatory response to exercise could not be accounted for on the basis of the ventilatory sensitivity to CO2 inhalation. From arterial sampling in three of the patients it was found that when PET,CO2 rose, the corresponding change in Pa,CO2 was less. During the on transient, there was a significant rise in both VCO2 and VI by the second breath in both groups. At the end of the on transient the normal subjects had achieved 84% (S.D. 40) of the steady-state increase in VCO2 and 88% (S.D. 24) of the increase in VI. The corresponding values for the patients were 67% (S.D. 17) and 77% (S.D. 16) respectively; these differences between normals and patients are significant. The increase of VI during the on transient in the patients was achieved almost entirely by an increase in tidal volume whereas in normals, an increase in respiratory rate was a more important component. We conclude therefore that in man, spinal cord transection with a presumed loss of muscle afferents allows a ventilatory response to electrically induced exercise that cannot be explained by classical chemoreception.(ABSTRACT TRUNCATED AT 400 WORDS)

Respiratory effects of carbon dioxide-induced changes of medullary extracellular fluid pH in cats

Acute and steady-state responses to hypercapnia of respiratory output, measured as integrated phrenic nerve activity, and medullary extracellular fluid (e.c.f.) pH, measured directly, were determined in paralysed, vagotomized and glomectomized cats. Medullary e.c.f. pH responds within seconds to an acute change of alveolar and arterial PCO2. The respiratory response closely and inversely matches the e.c.f. pH change, but not the cerebrospinal fluid pH change. The medullary e.c.f. pH change following a rapid step-change in end-tidal PCO2 requires at least 5 min for a new steady state to be achieved. Steady-state studies in twenty-six cats show: (a) that the respiratory response to progressive hypercapnic stimulation of the central chemoreceptors is curvilinear (Eldridge, Gill-Kumar & Millhorn, 1981), (b) that the relationship between increasing end-tidal PCO2 and medullary hydrogen ion concentration [(H+]) or changes of pH is linear (r = 0.995); a doubling of PCO2 causes 0.260 units pH change, (c) there is a curvilinear relationship between e.c.f. [H+] and the respiratory response that is the same as that found with CO2. We conclude that medullary e.c.f. pH measured by means of a surface electrode accurately reflects the CO2-induced [H+] stimulus to respiration. The decreasing respiratory responses to identical changes of central chemoreceptor input are due to progressive neuronal saturation of a central pathway between the chemoreceptors and the respiratory controller.

Cholinergic synaptic activation due to HCO-3 in the superior cervical ganglion of the rat

Influence of acid-base change on synaptic transmission was studied in the isolated superior cervical ganglion of the rat. Effects of changes in PCO2, [HCO-3], or pH of the superfusing solution were studied, using as an index of synaptic excitation the amplitude of the initial negative deflection of surface potential induced by preganglionic stimulation. An increase or decrease in the extracellular fluid (ECF) pH by changing [HCO-3] at a normal PCO2 elicited respectively augmentation or suppression of the negative deflection. Similar shifts in the ECF pH with varying PCO2 at a normal [HCO-3] had small or almost negligible effects on the negative deflection. Simultaneous increase in both the PCO2 and [HCO-3], which compensated for the pH change in the ECF, induced a consistent increase in the amplitude of the negative deflection. The amplitude of negative deflection in various acid-base conditions was positively correlated with the ECF [HCO-3] but not with the ECF pH or PCO2. These results suggest that an increase in the ECF [HCO-3] activates cholinergic (nicotinic) synaptic transmission in the ganglion.

Cardiovascular responses evoked from the nicotine-sensitive area on the ventral surface of the medulla oblongata in the cat

Experiments were carried out in cats anaesthetized with chloralose, (a) to examine the effect on blood pressure, heart rate and respiratory frequency produced by topical application of leptazol, nicotine and sodium pentobarbitone to the ventral surface of the medulla at an area around the rootlets of the XII cranial nerve, and (b) to study the role of this area in some cardiovascular reflexes. Leptazol applied uni- or bilaterally to this area produced hypotension, bradycardia and bradypnoea. The area from which leptazol produced these effects was localized 3-6 mm lateral to the mid line and 5-9 mm caudal to the lower border of the trapezoid bodies. When comparing the effects of leptazol and nicotine applied to this area it was found that in concentrations that produced similar falls in arterial blood pressure and heart rate leptazol produced a much stronger bradypnoea than nicotine. The hypotension produced by leptazol was mainly due to inhibition of sympathetic vasomotor tone since it was little affected by section of the vagi and by atropine given intravenously. Bilateral application of sodium pentobarbitone produced a small hypertension, tachycardia and pronounced tachypnoea. Unilateral application of sodium pentobarbitone had no effect by itself but inhibited the effects of leptazol applied to the same site. Cardiovascular reflexes produced by sinus nerve stimulation, by increased sinus pressure or by injections of veratridine into a vein or into the left ventricle of the heart were potentiated by topical application of leptazol to the ventral surface and depressed by the topical application of sodium pentobarbitone. The chemoreceptor reflex, produced by retrograde injections of lobeline into the lingual artery, was partially affected by topical application of sodium pentobarbitone: the evoked bradycardia was attenuated but the tachypnoea and hypertension were not affected. These results suggest that this medullary area on the ventral surface of the medulla plays an important role in normal cardiovascular regulation.

Ventilation during acute HCl infusion in intact and chemodenervated conscious rabbits

The ventilatory response to acute (0-90 min) intravenous infusion of HCl was studied in awake, conscious rabbits with intact (CB+) and denervated (CB-) carotid bodies. The HCl dose was delivered such as to produce an increasing degree of acidosis over 90 min reaching blood pH values of 7.032-7.115 at 60-90 min and plasma [HCO-3] values of 8.5-10.6 mmol . L-1 . CB- rabbits exhibit an increase in VT by 7 min and a decrease in PaCO2 by 15 min of infusion, changes that increased over 90 min. However, at all times the CB- response was significantly less than that in CB+ rabbits. The relationships, percent decrease in PaCO2 vs delta [H+] and percent increase VT vs delta [H+], were well approximated by linear regression analysis in both CB+ and CB- groups and the slope, as an index of the response sensitivity, was, in CB- rabbits, 0.33-0.37 of that in CB+ rabbits. In the conscious, awake rabbit, peripheral chemoreceptors seem to account for up to 2/3 of the ventilatory response to metabolic acidosis.

Difference between actions of high PCO2 and low [HCO-3] on neurons in the rat medullary chemosensitive areas in vitro

To evaluate the contribution of extracellular fluid (ECF) pH in stimulating the ventral medullary chemosensors, effects on neuronal activities of changing ECF PCO2 and/or [HCO-3] were studied in tissue slices taken from the medulla oblongata of the rat. In many cases changes in neuronal discharges produced by a high PCO2-normal [HCO-3] solution differed from those produced by a low [HCO-3] normal PCO2 solution although the ECF pH was reduced to the same degree (from 7.40 to about 7.15). Only 9 of a total of 76 neurons showed an increase in discharge in response to both acid solutions. Neuronal activation due to high PCO2 was augmented when the ECF pH was returned to a normal value (7.40) by simultaneous increase in [HCO-3]. High PCO2-high [HCO-3] solution (pH 7.40) increased the activity of many neurons which were either inhibited or uninfluenced by high PCO2-normal [HCO-3] (pH 7.15). Neuronal activation due to low [HCO-3] was partially suppressed by compensating for the pH reduction with a concomitant decrease in PCO2. The results suggest that CO2 and HCO-3 independently influence the activity of neurons. Possible roles of ECF H+, CO2 and HCO-3 in activating the ventral medullary chemosensitive structures are discussed.