gamma-Aminobutyric acid contributes to modulation of cardiorespiratory control after chronic ventilatory loading

GABAergic modulation of ventilation and peak oxygen consumption in obese Zucker rats

Obesity is often associated with a reduced ventilatory response and a decreased maximal exercise capacity. GABA is a major inhibitory neurotransmitter in the mammalian central nervous system. Altered GABAergic mechanisms have been detected in obese Zucker rats and implicated in their hyperphagic response. Whether altered GABAergic mechanisms also contribute to regulate ventilation and influence exercise capacity in obese Zucker rats is unknown and formed the basis of the present study. Eight lean [317 +/- 18 (SD) g] and eight obese (450 +/- 27 g) Zucker rats were studied at 12 wk of age. Ventilation at rest and ventilation during hypoxic (10% O(2)) and hypercapnic (4% CO(2)) challenges were measured by the barometric method. Peak O(2) consumption (VO(2 peak)) in response to a progressive treadmill test to exhaustion was measured in a metabolic treadmill. Ventilation and VO(2 peak) were assessed after administration of equal volumes of DMSO (vehicle) and the GABA(A) receptor antagonist bicuculline (1 mg/kg). In lean animals, bicuculline administration had no effect on ventilation and VO(2 peak). In obese rats, bicuculline administration significantly (P < 0.05) increased resting ventilation (465 +/- 53 and 542 +/- 72 ml. kg(-1). min(-1) for control and bicuculline, respectively), ventilation during exposure to hypoxia (899 +/- 148 and 1,038 +/- 83 ml. kg(-1). min(-1) for control and bicuculline, respectively), and VO(2 peak) (62 +/- 3.7 and 67 +/- 3.5 ml. kg(-0.75). min(-1) for control and bicuculline, respectively). However, in obese Zucker rats, ventilation in response to hypercapnia did not change after bicuculline administration (608 +/- 96 vs. 580 +/- 69 ml. kg(-1). min(-1)). Our findings indicate that endogenous GABA depresses ventilation and limits exercise performance in obese Zucker rats.

Chronic intermittent hypoxia increases sympathetic responsiveness to hypoxia and hypercapnia

We sought to determine whether chronic exposure to intermittent hypoxia (CIH) increases sympathetic responsiveness to subsequent chemoreflex stimulation. Sprague-Dawley rats were exposed to 30 days of CIH: exposure chamber %O2 [fractional concentration of chamber O2 (FcO2)] nadir 6.5-7% with return to 21% each minute for 8 h/day during the diurnal sleep period (Exp group). Sham controls (SC group) were similarly handled but kept at 21% FcO2 and compared with unhandled controls (UC group). Rats were then anesthetized with urethan, and preganglionic cervical sympathetic activity (CSA), diaphragm electromyogram, arterial pressure, and electrocardiogram were recorded while the rats were spontaneously breathing 100% O2, room air, 10% O2, 12% CO2, and 10% O2-12% CO2. CSA and heart rate were also recorded during phenylephrine infusion to assess baroreceptor function. Mean arterial pressure was significantly greater in Exp than in SC and UC rats during all conditions (P < 0. 05). A vasopressor response to 10% O2-12% CO2 was observed only in Exp rats. CSA was greater in Exp than in SC and UC rats during 10% O2, 12% CO2, and 10% O2-12% CO2 but not during room-air exposure. A significant increase in CSA compared with room air was noted during 10% O2, 12% CO2, and 10% O2-12% CO2 in Exp but not in SC or UC rats. No differences in baroreceptor function were observed among groups. We conclude that CIH leads to increased sympathetic responsiveness to chemoreflex stimulation.