Luc J. Teppema

Plasticity of central chemoreceptors: effect of bilateral carotid body resection on central CO2 sensitivity.

Background Human breathing is regulated by feedback and feed-forward control mechanisms, allowing a strict matching between metabolic needs and the uptake of oxygen in the lungs. The most important control mechanism, the metabolic ventilatory control system, is fine-tuned by two sets of chemoreceptors, the peripheral chemoreceptors in the carotid bodies (located in the bifurcation of the common carotid arteries) and the central CO2 chemoreceptors in the ventral medulla. Animal data indicate that resection of the carotid bodies results, apart from the loss of the peripheral chemoreceptors, in reduced activity of the central CO2 sensors. We assessed the acute and chronic effect of carotid body resection in three humans who underwent bilateral carotid body resection (bCBR) after developing carotid body tumors. Methods and Findings The three patients (two men, one woman) were suffering from a hereditary form of carotid body tumors. They were studied prior to surgery and at regular intervals for 2–4 y following bCBR. We obtained inspired minute ventilation (Vi) responses to hypoxia and CO2. The Vi-CO2 responses were separated into a peripheral (fast) response and a central (slow) response with a two-compartment model of the ventilatory control system. Following surgery the ventilatory CO2 sensitivity of the peripheral chemoreceptors and the hypoxic responses were not different from zero or below 10% of preoperative values. The ventilatory CO2 sensitivity of the central chemoreceptors decreased by about 75% after surgery, with peak reduction occurring between 3 and 6 mo postoperatively. This was followed by a slow return to values close to preoperative values within 2 y. During this slow return, the Vi-CO2 response shifted slowly to the right by about 8 mm Hg. Conclusions The reduction in central Vi-CO2 sensitivity after the loss of the carotid bodies suggests that the carotid bodies exert a tonic drive or tonic facilitation on the output of the central chemoreceptors that is lost upon their resection. The observed return of the central CO2 sensitivity is clear evidence for central plasticity within the ventilatory control system. Our data, although of limited sample size, indicate that the response mechanisms of the ventilatory control system are not static but depend on afferent input and exhibit a large degree of restoration or plasticity. In addition, the permanent absence of the breathing response to hypoxia after bCBR may aggravate the pathological consequences of sleep-disordered breathing.

The respiratory response to carbon dioxide in humans with unilateral and bilateral resections of the carotid bodies.

The acute hypercapnic ventilatory response (AHCVR) arises from both peripheral and central chemoreflexes. In humans, one technique for identifying the separate contributions of these chemoreflexes to AHCVR has been to associate the rapid component of AHCVR with the peripheral chemoreflex and the slow component with the central chemoreflex. Our first aim was to validate this technique further by determining whether a single slow component was sufficient to describe AHCVR in patients with bilateral carotid body resections (BR) for glomus cell tumours. Our second aim was to determine whether the slow component of AHCVR was diminished following carotid body resection as has been suggested by studies in experimental animals. Seven BR subjects were studied together with seven subjects with unilateral resections (UR) and seven healthy controls. A multifrequency binary sequence in end‐tidal PCO2 was employed to stimulate ventilation dynamically under conditions of both euoxia and mild hypoxia. Both two‐ and one‐compartment models of AHCVR were fitted to the data. For BR subjects, the two‐compartment model fitted significantly better on 1 out of 13 occasions compared with 22 out of 28 occasions for the other subjects. Average values for the chemoreflex sensitivity of the slow component of AHCVR differed significantly (P < 0.05) between the groups and were 0.95, 1.38 and 1.50 l min−1 Torr−1 for BR, UR and control subjects, respectively. We conclude that, without the peripheral chemoreflex, AHCVR is adequately described by a single slow component and that BR subjects have sensitivities for the slow component that are lower than those of control subjects.

Mechanism-based PK/PD Modeling of the Respiratory Depressant Effect of Buprenorphine and Fentanyl in Healthy Volunteers

The objective of this study was to characterize the pharmacokinetic/pharmacodynamic (PK/PD) relationship of buprenorphine and fentanyl for the respiratory depressant effect in healthy volunteers. Data on the time course of the ventilatory response at a fixed PETCO2 of 50u2009mm Hg and PETO2 of 110u2009mm Hg following intravenous administration of buprenorphine and fentanyl were obtained from two phase I studies (50 volunteers received buprenorphine: 0.05–0.6u2009mg/70u2009kg and 24 volunteers received fentanyl: 0.075–0.5u2009mg/70u2009kg). The PK/PD correlations were analyzed using nonlinear mixed effects modeling. A two‐ and three‐compartment pharmacokinetic model characterized the time course of fentanyl and buprenorphine concentration, respectively. Three structurally different PK/PD models were evaluated for their appropriateness to describe the time course of respiratory depression: (1) a biophase distribution model with a fractional sigmoid Emax pharmacodynamic model, (2) a receptor association/dissociation model with a linear transduction function, and (3) a combined biophase distribution‐receptor association/dissociation model with a linear transduction function. The results show that for fentanyl hysteresis is entirely determined by the biophase distribution kinetics, whereas for buprenorphine hysteresis is caused by a combination of biophase distribution kinetics and receptor association/dissociation kinetics. The half‐time values of biophase equilibration were 16.4 and 75.3u2009min for fentanyl and buprenorphine, respectively. In addition, for buprenorphine, the value of kon was 0.246u2009ml/ng/min and the value of koff was 0.0102u2009min−1. The concentration–effect relationship of buprenorphine was characterized by a ceiling effect at higher concentrations (intrinsic activity α=0.56, 95% confidence interval (CI): 0.50–0.62), whereas fentanyl displayed full respiratory depressant effect (α=0.91, 95% CI: 0.19–1.62).

Sdhd and Sdhd/H19 Knockout Mice Do Not Develop Paraganglioma or Pheochromocytoma

Background Mitochondrial succinate dehydrogenase (SDH) is a component of both the tricarboxylic acid cycle and the electron transport chain. Mutations of SDHD, the first protein of intermediary metabolism shown to be involved in tumorigenesis, lead to the human tumors paraganglioma (PGL) and pheochromocytoma (PC). SDHD is remarkable in showing an ‘imprinted’ tumor suppressor phenotype. Mutations of SDHD show a very high penetrance in man and we postulated that knockout of Sdhd would lead to the development of PGL/PC, probably in aged mice. Methodology/Principal Findings We generated a conventional knockout of Sdhd in the mouse, removing the entire third exon. We also crossed this mouse with a knockout of H19, a postulated imprinted modifier gene of Sdhd tumorigenesis, to evaluate if loss of these genes together would lead to the initiation or enhancement of tumor development. Homozygous knockout of Sdhd results in embryonic lethality. No paraganglioma or other tumor development was seen in Sdhd KO mice followed for their entire lifespan, in sharp contrast to the highly penetrant phenotype in humans. Heterozygous Sdhd KO mice did not show hyperplasia of paraganglioma-related tissues such as the carotid body or of the adrenal medulla, or any genotype-related pathology, with similar body and organ weights to wildtype mice. A cohort of Sdhd/H19 KO mice developed several cases of profound cardiac hypertrophy, but showed no evidence of PGL/PC. Conclusions Knockout of Sdhd in the mouse does not result in a disease phenotype. H19 may not be an initiator of PGL/PC tumorigenesis.

Antioxidants reverse depression of the hypoxic ventilatory response by acetazolamide in man

The carbonic anhydrase inhibitor acetazolamide may have both inhibitory and stimulatory effects on breathing. In this placebo‐controlled double‐blind study we measured the effect of an intravenous dose (4 mg kg−1) of this agent on the acute isocapnic hypoxic ventilatory response in 16 healthy volunteers (haemoglobin oxygen saturation 83–85%) and examined whether its inhibitory effects on this response could be reversed by antioxidants (1 g ascorbic acid i.v. and 200 mg α‐tocopherol p.o.). The subjects were randomly divided into an antioxidant (Aox) and placebo group. In the Aox group, acetazolamide reduced the mean normocapnic and hypercapnic hypoxic responses by 37% (P < 0.01) and 55% (P < 0.01), respectively, and abolished the O2–CO2 interaction, i.e. the increase in O2 sensitivity with rising PCO2. Antioxidants completely reversed this inhibiting effect on the normocapnic hypoxic response, while in hypercapnia the reversal was partial. In the placebo group, acetazolamide reduced the normo‐ and hypercapnic hypoxic responses by 33 and 47%, respectively (P < 0.01 versus control in both cases), and also abolished the O2–CO2 interaction. Placebo failed to reverse these inhibitory effects of acetazolamide in this group. We hypothesize that either an isoform of carbonic anhydrase may be involved in the regulation of the redox state in the carotid bodies or that acetazolamide and antioxidants exert independent effects on oxygen‐sensing cells, in which both carbonic anhydrase and potassium channels may be involved. The novel findings of this study may have clinical implications, for example with regard to a combined use of acetazolamide and antioxidants at high altitude.

CrossTalk opposing view: peripheral and central chemoreceptors have hyperadditive effects on respiratory motor control.

Since the discovery of the O2 and CO2 respiratory chemoreceptors there has been a long debate as to their relative contributions to eupnoea and the ventilatory responses to hypoxia and hypercapnia. Recent evidence suggests that attempting to assign relative contributions to the central and peripheral chemoreceptors may not be a useful approach (e.g. Teppema & Dahan, 2010; Smith et al. 2010) if the two sets of chemoreceptors interact in other than a simply additive way and are thus capable of modulating the responsiveness of one another. This means that neural signals arising from stimuli at both sets of chemoreceptors have the potential to interact; indeed, such interdependence is a pre-condition for hypoor hyperadditive interaction (Adams & Severns, 1982). In this ‘pro–con’ debate three potential interaction modes are discussed: hypoadditive, additive and hyperadditive. The literature reports a broad spectrum of results and opinions between the extremes of hypoand hyperaddition (reviewed in Blain et al. 2010; Smith et al. 2010; Teppema & Dahan, 2010). Here we focus on recent evidence that supports a hyperadditive or multiplicative

Anesthetics and control of breathing

An important side effect of general anesthetics is respiratory depression. Anesthetics have multiple membrane targets of which ionotropic receptors such as gamma-aminobutyric acid-A (GABA(A)), glycine, N-methyl-D-aspartate and nicotinic acetylcholinergic (nACh) receptors are important members. GABA, glutamate and ACh are crucial neurotransmitters in the respiratory neuronal network, and the ability of anesthetics to modulate their release and interact with their receptors implies complex effects on respiration. Metabotropic receptors and intracellular proteins are other important targets for anesthetics suggesting complex effects on intracellular signaling pathways. Here we briefly overview the effects of general anesthetics on protein targets as far as these are relevant for respiratory control. Subsequently, we describe some methods with which the overall effect of anesthetics on the control of breathing can be measured, as well as some promising in vivo approaches to study their synaptic effects. Finally, we summarize the most important respiratory effects of volatile anesthetics in humans and animals and those of some intravenous anesthetics in animals.

Low-dose acetazolamide reduces the hypoxic ventilatory response in the anesthetized cat.

Low intravenous dose acetazolamide causes a decrease in steady-state CO(2) sensitivity of both the peripheral and central chemoreflex loops. The effect, however, on the steady-state hypoxic response is unknown. In the present study, we measured the effect of 4 mg x kg(-1) acetazolamide (i.v.) on the isocapnic steady-state hypoxic response in anesthetized cats. Before and after acetazolamide administration, the eucapnic steady-state hypoxic response in these animals was measured by varying inspiratory P(O2) levels to achieve steady-state Pa(O2) levels between hyperoxia Pa(O2) approximately 55 kPa, approximately 412 mmHg) and hypoxia (Pa(O2) approximately 7 kPa, approximately 53 mmHg). The hypoxic ventilatory response was described by the exponential function V(I) = G exp (-DP(o2) + A with an overall hypoxic sensitivity G, a shape parameter D and ventilation during hyperoxia A. Acetazolamide significantly reduced G from 3.057 +/- 1.616 to 1.573 +/- 0.8361 min(-1) (mean +/- S D). Parameter A increased from 0.903 +/- 0.257 to 1.193 +/- 0.321 min(-1), while D remained unchanged. The decrease in overall hypoxic sensitivity by acetazolamide is probably mediated by an inhibitory effect on the carotid bodies and may have clinical significance in the treatment of sleep apneas, particularly those cases that are associated with an increased ventilatory sensitivity to oxygen and/or carbon dioxide.

Naloxone reversal of opioid-induced respiratory depression with special emphasis on the partial agonist/antagonist buprenorphine

Buprenorphine is relatively resistant to reversal by naloxone. We tested the effect of various doses and infusion schemes of naloxone on buprenorphine-induced respiratory depression and compared the data with naloxone-reversal of morphine and alfentanil-induced respiratory depression. Both morphine and alfentanil were easily reversed by low doses of naloxone (0.4 mg). Increasing doses of naloxone caused a bell-shaped reversal curve of buprenorphine with maximal reversal at naloxone doses between 2 and 4 mg. However, reversal was short-lived. The bell-shaped reversal curve may be related to the existence of two mu-opioid receptor subtypes, one mediating the agonist effects of opioids at low dose, the other mediating antagonistic effects at high dose.

The carbonic anhydrase inhibitors methazolamide and acetazolamide have different effects on the hypoxic ventilatory response in the anaesthetized cat

We compared the effects of the carbonic anhydrase inhibitors methazolamide and acetazolamide (3 mg kg−1, i.v.) on the steady‐state hypoxic ventilatory response in 10 anaesthetized cats. In five additional animals, we studied the effect of 3 and 33 mg kg−1 methazolamide. The steady‐state hypoxic ventilatory response was described by the exponential function: where is the inspired ventilation,G is hypoxic sensitivity, D is the shape factor and A is hyperoxic ventilation. In the first group of 10 animals, methazolamide did not change parameters G and D, while A increased from 0.86 ± 0.33 to 1.30 ± 0.40 l min−1 (mean ±s.d., P= 0.003). However, the subsequent administration of acetazolamide reduced G by 44% (control, 1.93 ± 1.32; acetazolamide, 1.09 ± 0.92 l min−1, P= 0.003), while A did not show a further change. Acetazolamide tended to reduce D (control, 0.20 ± 0.07; acetazolamide, 0.14 ± 0.06 kPa−1, P= 0.023). In the second group of five animals, neither low‐ nor high‐dose methazolamide changed parameters G, D and A. The observation that even high‐dose methazolamide, causing full inhibition of carbonic anhydrase in all body tissues, did not reduce the hypoxic ventilatory response is reminiscent of previous findings by others showing no change in magnitude of the hypoxic response of the in vitro carotid body by this agent. This suggests that normal carbonic anhydrase activity is not necessary for a normal hypoxic ventilatory response to occur. The mechanism by which acetazolamide reduces the hypoxic ventilatory response needs further study.