Dopaminergic modulation of ventilation in obese Zucker rats

Loss of ganglioglomerular nerve input to the carotid body impacts the hypoxic ventilatory response in freely-moving rats

The carotid bodies are the primary sensors of blood pH, pO2 and pCO2. The ganglioglomerular nerve (GGN) provides post-ganglionic sympathetic nerve input to the carotid bodies, however the physiological relevance of this innervation is still unclear. The main objective of this study was to determine how the absence of the GGN influences the hypoxic ventilatory response in juvenile rats. As such, we determined the ventilatory responses that occur during and following five successive episodes of hypoxic gas challenge (HXC, 10% O2, 90% N2), each separated by 15 min of room-air, in juvenile (P25) sham-operated (SHAM) male Sprague Dawley rats and in those with bilateral transection of the ganglioglomerular nerves (GGNX). The key findings were that 1) resting ventilatory parameters were similar in SHAM and GGNX rats, 2) the initial changes in frequency of breathing, tidal volume, minute ventilation, inspiratory time, peak inspiratory and expiratory flows, and inspiratory and expiratory drives were markedly different in GGNX rats, 3) the initial changes in expiratory time, relaxation time, end inspiratory or expiratory pauses, apneic pause and non-eupneic breathing index (NEBI) were similar in SHAM and GGNX rats, 4) the plateau phases obtained during each HXC were similar in SHAM and GGNX rats, and 5) the ventilatory responses that occurred upon return to room-air were similar in SHAM and GGNX rats. Overall, these changes in ventilation during and following HXC in GGNX rats raises the possibility the loss of GGN input to the carotid bodies effects how primary glomus cells respond to hypoxia and the return to room-air.

Peripheral Dopamine 2-Receptor Antagonist Reverses Hypertension in a Chronic Intermittent Hypoxia Rat Model

The sleep apnea-hypopnea syndrome (SAHS) involves periods of intermittent hypoxia, experimentally reproduced by exposing animal models to oscillatory PO₂ patterns. In both situations, chronic intermittent hypoxia (CIH) exposure produces carotid body (CB) hyperactivation generating an increased input to the brainstem which originates sympathetic hyperactivity, followed by hypertension that is abolished by CB denervation. CB has dopamine (DA) receptors in chemoreceptor cells acting as DA-2 autoreceptors. The aim was to check if blocking DA-2 receptors could decrease the CB hypersensitivity produced by CIH, minimizing CIH-related effects. Domperidone (DOM), a selective peripheral DA-2 receptor antagonist that does not cross the blood-brain barrier, was used to examine its effect on CIH (30 days) exposed rats. Arterial pressure, CB secretory activity and whole-body plethysmography were measured. DOM, acute or chronically administered during the last 15 days of CIH, reversed the hypertension produced by CIH, an analogous effect to that obtained with CB denervation. DOM marginally decreased blood pressure in control animals and did not affect hypoxic ventilatory response in control or CIH animals. No adverse effects were observed. DOM, used as gastrokinetic and antiemetic drug, could be a therapeutic opportunity for hypertension in SAHS patients’ resistant to standard treatments.

Obesity Surgery and Anesthesiology Risks: a Review of Key Concepts and Related Physiology

The obesity epidemic is swelling to epic proportions. Obese patients often suffer from a combination of hypertension, dyslipidemia, and type 2 diabetes mellitus (T2DM), also known as the "metabolic syndrome." The metabolic syndrome is an independent predictor of cardiac dysfunction and cardiovascular disease and a risk factor for perioperative morbidity and mortality. In this paper, we discuss the perioperative risk factors and the need for advanced care of obese patients needing general anesthesia for (bariatric) surgical procedures based on physiological principles.

High-fat diet increases respiratory frequency and abdominal expiratory motor activity during hypercapnia

Breathing disorders are commonly observed in association with obesity. Here we tested whether high-fat diet (HFD) impairs the chemoreflex ventilatory response. Male Holtzman rats (300-320 g) were fed with standard chow diet (SD) or HFD for 12 weeks. Then, tidal volume (VT), respiratory frequency (fR) and pulmonary ventilation (VE) were determined in conscious rats during basal condition, hypercapnia (7% or 10% CO2) or hypoxia (7% O2). The mean arterial pressure (MAP), heart rate (HR) and baroreflex sensitivity were also evaluated in conscious rats. A group of anesthetized rats was used for the measurements of the activity of inspiratory (diaphragm) and expiratory (abdominal) muscles under the same gas conditions. Baseline fR, VT and VE were similar between SD and HFD rats. During hypercapnia, the increase of fR was exacerbated in conscious HFD rats (60 ± 3, vs. SD: 47 ± 3 Δ breaths.min-1, P < 0.05). In anesthetized rats, hypercapnia strongly increased abdominal muscle activity in HFD group (238 ± 27, vs. basal condition: 100 ± 0.3%; P < 0.05), without significant change in SD group (129 ± 2.1, vs. basal condition: 100 ± 0.8%; P = 0.34). The ventilatory responses to hypoxia were similar between groups. In conscious HFD rats, MAP and HR were elevated and the baroreflex function was impaired (P < 0.05). These data demonstrated that 12 weeks of HFD exaggerate the ventilatory response activated by hypercapnia. The mechanisms involved in these responses need more investigation in future studies.

Adenosinergic Modulation of Ventilation in Obese Zucker Rats

OBJECTIVES

The goal of our study was to determine whether altered adenosinergic mechanisms contribute to the depressed ventilatory response observed in obese Zucker rats.

RESEARCH METHODS AND PROCEDURES

Eight lean and eight obese Zucker rats were studied at 7 to 8 weeks of age. Ventilation (V(E)) during room air, during 5-minute hypercapnic (7% CO(2), balance O(2)), and during 30-minute sustained hypoxic (10% O(2)) exposures were sequentially measured by the barometric method on three separate occasions after the randomized blinded administration of equal volumes of either saline (control), 8-(p-sulfophenyl)-theophylline (8-PST, 7 mg/kg, peripheral adenosine antagonist), or aminophylline (AMPH, 15 mg/kg, peripheral and central adenosine antagonist).

RESULTS

During room air and hypercapnic exposures, AMPH (but not 8-PST) significantly (p < 0.05) increased V(E) in both lean and obese rats. During acute (2 minute) hypoxic exposure, 8-PST (but not AMPH) significantly depressed V(E) in lean rats. In contrast, AMPH (but not 8-PST) selectively increased V(E) in obese rats. During sustained (10 to 30 minutes) hypoxic exposure, neither AMPH nor 8-PST administration altered V(E) in lean rats. In contrast, AMPH (but not 8-PST) selectively increased V(E) during the late response in obese rats.

DISCUSSION

Our findings indicate that obese rats possess altered adenosinergic modulation of ventilatory responses to acute and sustained hypoxia in two opposite ways. We conclude that the reduced hypoxic ventilatory response observed in obese Zucker rats is attributed to depressed adenosinergic peripheral excitatory mechanisms and to enhanced adenosinergic central depression mechanisms, both of which contribute to the blunted ventilatory response in obesity.

Differences in tyrosine hydroxylase expression after short-term hypoxia, hypercapnia or hypercapnic hypoxia in rat carotid body

In the carotid body (CB), it has been reported that the expressions of tyrosine hydroxylase (TH) mRNA and TH protein are enhanced by exposure to hypoxia. However, it is not known whether CO(2) affects the expression of TH in the CB. We examined the expression of TH mRNA and the immunoreactivity for TH in the CB of rats exposed to hypoxia (10% O(2)), hypercapnia (10% CO(2)) and hypercapnic hypoxia (10% O(2) and 10% CO(2)) for 2-24 h. The expression of TH mRNA in the CB was markedly enhanced in rats exposed to hypoxia for 4 h (6.6-fold), 6 h (6.0-fold) and 8 h (7.8-fold), and in rats exposed to hypercapnic hypoxia for 12 h (4.8-fold). The most intense TH immunoreactivity was observed in the CB from rats exposed to hypoxia for 12 and 24 h and to hypercapnic hypoxia for 24 h. The expressions of TH mRNA and the immunoreactivity for TH were not altered in the CB of rats exposed to hypercapnia. It is suggested that CO(2) does not affect TH expression in the CB, and that it inhibits hypoxia-enhanced TH expression.

Big breathing: the complex interaction of obesity, hypoventilation, weight loss, and respiratory function

Obesity places a significant load on the respiratory system, affecting lung volumes, respiratory muscle function, work of breathing, and ventilatory control. Despite this, most morbidly obese individuals maintain eucapnia. However, a subgroup of morbidly obese individuals will develop chronic daytime hypercapnia, described as the obesity hypoventilation syndrome (OHS). While obesity is obviously a crucial component of this syndrome, the relationship between excess fat accumulation and the development of awake hypercapnia is complex and extends beyond simply impairments of pulmonary mechanics and lung volumes as a consequence of obesity. Various compensatory mechanisms operate to maintain eucapnia even in the presence of extreme obesity. However, if compensation is impaired, hypoventilation will ensue. While obesity alone does not account for the development of hypoventilation, weight loss will produce significant improvements in lung function and awake gas exchange. Such improvements have the potential to substantially reduce morbidity and mortality in these individuals. Nevertheless, many individuals remain overweight despite substantial weight loss, with persistence of upper airway obstruction. Attention to this residual abnormality is important given the high incidence of cardiovascular abnormalities, including pulmonary hypertension, in individuals with OHS.

Long-term ventilatory adaptation and ventilatory response to hypoxia in plateau pika (Ochotona curzoniae): role of nNOS and dopamine

We assessed ventilatory patterns and ventilatory responses to hypoxia (HVR) in high-altitude (HA) plateau pikas, repetitively exposed to hypoxic burrows, and control rats. We evaluated the role of neuronal nitric oxide synthase (nNOS) and dopamine by using S-methyl-l-thiocitrulline (SMTC) inhibitor and haloperidol antagonist, respectively. Ventilation (Vi) was measured using a whole body plethysmograph in conscious pikas (n = 9) and low-altitude (LA) rats (n = 7) at different Pi(O(2)) (56, 80, 111, 150, and 186 mmHg) and in HA acclimatized rats (n = 9, 8 days at 4,600 m) at two different Pi(O(2)) (56 and 80 mmHg). The effects of NaCl, SMTC, and haloperidol on ventilatory patterns were assessed in pikas at Pi(O(2)) = 56 and 80 mmHg. We observed a main species effect with larger Vi, tidal volume (VT), inspiratory time/total time (T(i)/T(tot)), and a lower expiratory time in pikas than in LA rats. Pikas had also a larger VT and lower respiratory frequency compared with HA rats in hypoxia. HVR of pikas and rats were not statistically different. In pikas, SMTC induced a significant increase in Vi and VT for a Pi(O(2)) of 56 mmHg, but had no effect for a PiO(2) of 80 mmHg, i.e., the living altitude of pikas. In pikas, haloperidol injection had no effect on any ventilatory parameter. Long-term ventilatory adaptation in pikas is mainly due to an improvement in respiratory pattern (VT and T(i)/T(tot)) with no significant improvement in HVR. The sensitivity to severe acute hypoxia in pikas seems to be regulated by a peripheral nNOS mechanism.

In hamsters the D1 receptor antagonist SCH23390 depresses ventilation during hypoxia

During exposure of animals to hypoxia, brain and blood dopamine levels increase stimulating dopaminergic receptors which influence the integrated ventilatory response to low oxygen. The purpose of the present study is to test the hypothesis that in conscious hamsters, systemic antagonism of D(1) receptors would depress their breathing in air and in response to hypoxic and hypercapnic challenges. Nine male hamsters were treated with saline or 0.25 mg/kg SCH-23390 (SCH), a D(1) receptor antagonist that crosses the blood-brain barrier. Ventilation was determined using the barometric method, and oxygen consumption and CO(2) production were evaluated utilizing the flow-through method. During exposure to air, SCH decreased frequency of breathing. During exposure to hypoxia (10% oxygen in nitrogen), relative to saline, SCH-treated hamsters decreased minute ventilation by decreasing tidal volume and oxygen consumption but not CO(2) production. During exposure to hypercapnia (5% CO(2) in 95% O(2)), frequency of breathing was decreased with SCH, but there was no significant effect on minute ventilation. Relative to saline treatment body temperature was lower in SCH-treated hamsters by 0.6 degrees C. These results demonstrate that in hamsters D(1) receptors can modulate control of ventilation in air and during hypoxia and hypercapnic exposures. Whether D(1) receptors located centrally or on carotid bodies modulate these effects is not clear from this study.

In hamsters dopamine D2 receptors affect ventilation during and following intermittent hypoxia

We tested the hypothesis that in golden Syrian hamsters (Mesocricetus auratus) carotid body dopaminergic D2 receptors modulate ventilation in air, during exposure to intermittent hypoxia (IH) and reoxygenation. Ventilation was evaluated using the barometric method and CO2 production was determined using the flow through method. Hamsters (n=8) received either subcutaneous injections of vehicle, haloperidol (0.5 mg/kg) or domperidone (0.5 mg/kg). Ventilatory and metabolic variables were determined 30 min following injections, after each of 5 bouts of 5 min of 10% oxygen interspersed by normoxia (IH), and 15, 30, 45 and 60 min following IH when hamsters were exposed to air. Haloperidol, but not domperidone decreased body temperature in hamsters. Neither treatment affected CO2 production. Vehicle-treated hamsters exhibited ventilatory long-term facilitation (VLTF) following IH. Haloperidol or domperidone decreased ventilation in air, during IH and eliminated VLTF due to changes in tidal volume and not frequency of breathing. Thus, in hamsters D2 receptors are involved in control of body temperature and ventilation during and following IH.

GABAA and GABAB receptors differentially modulate volume and frequency in ventilatory compensation in obese Zucker rats

The aim of this study was to investigate whether GABAA and/or GABAB receptor-mediated mechanisms contribute to the impaired ventilatory response and reduced maximal aerobic exercise capacity in obese Zucker rats. Ten lean and 10 obese Zucker rats were studied at 12 wk of age. Minute ventilation (V̇e), tidal volume (Vt), and breathing frequency (f) during room air breathing and in response to 10 min of hypercapnia (8% CO2) and 30 min of hypoxia (10% O2) were measured by the barometric method, and peak oxygen consumption (V̇o2 peak) was measured by an enclosed metabolic treadmill following the randomized blinded subcutaneous administration of equal volumes of DMSO (vehicle), bicuculline (selective GABAA receptor antagonist, 1 mg/kg), and phaclofen (selective GABAB receptor antagonist, 1 mg/kg). Administration of bicuculline and phaclofen to lean animals had no effect on V̇e and V̇o2 peak. Similarly, phaclofen failed to alter V̇e and V̇o2 peak in obese rats, although it did significantly increase f after 5–20 min of hypoxia. In contrast, bicuculline increased V̇e and Vt relative to DMSO during room air breathing and after 10–30 min of hypoxic exposure in obese rats, but it did not increase V̇e at 5 min of hypoxemia. Bicuculline increased V̇o2 peak relative to DMSO in obese Zucker rats. We conclude that endogenous GABA acting on GABAA receptors can modulate V̇e and V̇o2 peak in obese but not in lean Zucker rats, whereas endogenous GABA acting on GABAB receptors modulates f during hypoxia (5–20 min) in obese rats in a very different manner from that when acting on GABAA receptors.

Domperidone and ventilatory behavior: Sprague–Dawley versus Brown Norway rats

Domperidone, a dopamine D(2) receptor antagonist, is a tool for uncovering the tonic and dynamic effects of the peripheral dopaminergic system in unanesthestized animals. The hypothesis was that domperidone effects would vary between strains of the same species. Ventilatory behavior -- frequency and minute ventilation -- was measured by the plethysmographic method in unrestrained adult male Sprague-Dawley (SD: n=8) and Brown Norway (BN: n=8) rats before, during and after rapid transition to 100% O(2) after 5 min of 13% O(2)/3% CO(2). Tests were done 60 min after intraperitoneal injection of either vehicle (0.1% lactic acid in saline) or a dose of domperidone (0.1, 0.5, 1.0, or 5.0mg/kg) dissolved in vehicle, each on a separate day. Resting frequency and minute ventilation (mean+/-standard deviation) decreased after domperidone in the BN strain (e.g. 94.63/min+/-4.99 versus 87.37/min+/-9.59, p=0.42; 77.3 ml/min+/-9.25 versus 62.13 ml/min+/-11.5, p=0.019, respectively), but did not change in the SD. With increasing doses of domperidone the ventilatory response to hypoxia and reoxygenation became similar owing to a decrease in frequency and minute ventilation in the SD. At a dose altering SD hypoxic responses, the hypercapnic ventilatory response was not significantly affected. In conclusion, breathing frequency and minute ventilation over a challenge with hypoxia and reoxygenation differ with domperidone depending upon genetic background. We speculate that hypoxic ventilatory responses may be differently configured even among strains of the same species.

Breathing pattern and hypoxic sensitivity during ageing in a new model of obesity-resistant rat

The Lou/C rat, an inbred strain of Wistar origin, is presented both as an obesity-resistant rat and as a relevant model of successful ageing. In the present study, we examined throughout ageing the breathing pattern and the ventilatory response to hypoxia in Lou/C and age-matched Wistar rats. Ventilation was assessed by whole body plethysmography in resting condition and during a hypoxic challenge (10% O(2)) at 2, 6, 12 and 24 months of age. Lou/C rats maintained constant their resting ventilation rate throughout ageing while Wistar rats tended to decrease their ventilation with advancing age. Elderly Lou/C as well as Wistar rats increased their frequency rather than their tidal volume in response to hypoxia. As Lou/C rats did not develop obesity, ageing per se surpasses the effect of fat accumulation and prevents old rats to mobilise their tidal volume in response to hypoxia.

GABAergic modulation of ventilatory response to acute and sustained hypoxia in obese Zucker rats

OBJECTIVE

To determine whether altered central and/or peripheral gamma-aminobutyric acid (GABA)ergic mechanisms acting in GABA(A) receptors contribute to the abnormal ventilatory response to acute and sustained hypoxia in obese Zucker rats.

METHODS

In all, 10 lean and 10 obese Zucker rats were studied at 12 weeks of age. Ventilation (V(.-)(E)), tidal volume (V(T)), and breathing frequency (f) during room air breathing and in response to sustained (30 min) hypoxic (10% O(2)) challenges were measured on three separate occasions by the barometric method following the randomized blinded administration of equal volumes of DMSO (vehicle), bicuculline methiodide (B(M), 1 mg/kg, peripheral GABA(A) receptor antagonist), or bicuculline hydrochloride (B(HCl), 1 mg/kg, peripheral and central GABA(A) receptor antagonist).

RESULTS

Administration of B(M) and B(HCl) in lean animals had no effect on ventilation either during room air breathing or 30 min of sustained exposure to hypoxia. Similarly, B(M) failed to alter ventilation in obese rats. In contrast, B(HCl) significantly (P<0.05) increased V(.-)(E) and V(T) during room air breathing and 10-30 min of hypoxic exposure in obese rats. During 5 min of acute hypoxic exposure, V(T) remained elevated with B(HCl) in obese rats, but the V(.-)(E) appeared not to be increased with B(HCl) due to a decrease in f.

CONCLUSION

Thus, endogenous GABA modulates both ventilation during room air breathing and ventilatory response to sustained hypoxia in obese, not in lean, Zucker rats by acting specifically on GABA(A) receptors located within the central, not peripheral, nervous system. However, endogenous GABA does not modulate ventilation but the pattern of breathing during acute hypoxia in obesity in a different manner from that during sustained hypoxia.

The fa leptin receptor mutation and the heritability of respiratory frequency in a Brown Norway and Zucker intercross

We sought to determine whether the fa (leptin receptor) mutation was a major determinant of the putative obesity effects on respiratory frequency in an intercross between the Brown Norway (low breathing frequency, nonobese strain) and the Zucker (moderately high breathing frequency, with the fa mutation) strains. The hypothesis was that rats bearing one (heterozygote) or two (homozygote) alleles of the Glu296Pro point mutation (fa) would have a uniformly high respiratory frequency in the second filial (F2) generation, compared with wild-type animals. In addition to breathing frequency, tidal volume and minute ventilation were assessed during baseline, acute hypoxic (10% O2-0% CO2-balance nitrogen), hypercapnic (93% O2-7% CO2), hyperoxic (100% O2-0% CO2), and combined (10% O2-3% CO2-balance nitrogen) challenges in fa homozygote (fa/fa; n = 24), fa heterozygote (fa/wt; n = 33), and wild-type (wt/wt; n = 19) animals. Phenotypes were adjusted with stepwise regression analyses for the effects of age, sex, length, and litter size. Broad-sense heritability was estimated by examining the variance of the traits in first filial and F2 generations. ANOVAs were used to determine the mode of inheritance of the fa allele in the F2 generation. As anticipated, weight demonstrated the greatest overall broad-sense heritability (77%) and was the result of the recessive mutation. Breathing parameters during the hypoxic, hypercapnic, and combined challenges demonstrated a wide range of heritability from 5 to 96%, with a very nonuniform proportion of heritability explained by the leptin receptor. At best, for frequency 4.5 min into the hypercapnic hypoxic challenge, approximately 20% of the total heritability (approximately 67%) could be attributed to an effect of the leptin receptor mutation. We conclude that, unlike its major effect on weight, the effect of the fa allele is not a major gene involved in the regulation of breathing frequency.