Effect of dichloroacetate on PaCO2 responses to hypoxia in awake goats

Lactate as a modulator of hypoxia-induced hyperventilation

In the present study, we tested the hypothesis that lactate, which is a classic companion of hypoxic stress in mammals, is a modulator of hypoxia-induced hyperventilation. To this end, pulmonary ventilation (V(E)) of male Wistar rats was measured by whole body plethysmograph, and dichloroacetate (DCA, 100 mg/kg) was used to inhibit lactate production. Plasma lactate levels, arterial pH (pHa), arterial carbon dioxide partial pressure (PaCO(2)), arterial oxygen partial pressure (PaO(2)), plasma bicarbonate (HCO3(-)) and oxygen consumption (VO(2)) were determined as well. In normoxia, intraperitoneal DCA elicited a decrease only in plasma lactate levels. Hypoxia caused an increase in V(E), pHa and plasma lactate levels and parallel to decreases in PaCO(2), PaO(2) and VO(2) in the control group. DCA administration markedly reduced the ventilatory response to hypoxia by acting on tidal volume (V(T)). This reduced ventilatory response caused by DCA was independent of VO(2). In conclusion, the present study indicates that lactate contributes to the initiation and maintenance of hypoxia-induced hyperventilation in rats, modulating the adjustments in V(T).

Hypoxia-induced anapyrexia: implications and putative mediators

Hypoxia elicits an array of compensatory responses in animals ranging from protozoa to mammals. Central among these responses is anapyrexia, the regulated decrease of body temperature. The importance of anapyrexia lies in the fact that it reduces oxygen consumption, increases the affinity of hemoglobin for oxygen, and blunts the energetically costly responses to hypoxia. The mechanisms of anapyrexia are of intense interest to physiologists. Several substances, among them lactate, adenosine, opioids, and nitric oxide, have been suggested as putative mediators of anapyrexia, and most appear to act in the central nervous system. Moreover, there is evidence that the drop in body temperature in response to hypoxia, unlike the ventilatory response to hypoxia, does not depend on the activation of peripheral chemoreceptors. The current knowledge of the mechanisms of hypoxia-induced anapyrexia are reviewed.

Ventilatory responses to specific CNS hypoxia in sleeping dogs

Our study was concerned with the effect of brain hypoxia on cardiorespiratory control in the sleeping dog. Eleven unanesthetized dogs were studied; seven were prepared for vascular isolation and extracorporeal perfusion of the carotid body to assess the effects of systemic [and, therefore, central nervous system (CNS)] hypoxia (arterial PO(2) = 52, 45, and 38 Torr) in the presence of a normocapnic, normoxic, and normohydric carotid body during non-rapid eye movement sleep. A lack of ventilatory response to systemic boluses of sodium cyanide during carotid body perfusion demonstrated isolation of the perfused carotid body and lack of other significant peripheral chemosensitivity. Four additional dogs were carotid body denervated and exposed to whole body hypoxia for comparison. In the sleeping dog with an intact and perfused carotid body exposed to specific CNS hypoxia, we found the following. 1) CNS hypoxia for 5-25 min resulted in modest but significant hyperventilation and hypocapnia (minute ventilation increased 29 +/- 7% at arterial PO(2) = 38 Torr); carotid body-denervated dogs showed no ventilatory response to hypoxia. 2) The hyperventilation was caused by increased breathing frequency. 3) The hyperventilatory response developed rapidly (<30 s). 4) Most dogs maintained hyperventilation for up to 25 min of hypoxic exposure. 5) There were no significant changes in blood pressure or heart rate. We conclude that specific CNS hypoxia, in the presence of an intact carotid body maintained normoxic and normocapnic, does not depress and usually stimulates breathing during non-rapid eye movement sleep. The rapidity of the response suggests a chemoreflex meditated by hypoxia-sensitive respiratory-related neurons in the CNS.

Human ventilatory response to acute hyperoxia during and after 8 h of both isocapnic and poikilocapnic hypoxia

During 8 h of either isocapnic or poikilocapnic hypoxia, there may be a rise in ventilation (VE) that cannot be rapidly reversed with a return to higher PO2 (L. S. G. E. Howard and P. A. Robbins. J. Appl. Physiol. 78:1098-1107, 1995). To investigate this further, three protocols were compared: 1) 8-h isocapnic hypoxia [end-tidal PCO2 (PETCO2) held at prestudy value, end-tidal PO2 (PETO2) = 55 Torr], followed by 8-h isocapnic euoxia (PETO2 = 100 Torr); 2) 8-h poikilocapnic hypoxia followed by 8-h poikilocapnic euoxia; and 3) 16-h air-breathing control. Before and at intervals throughout each protocol, the VE response to eucapnic hyperoxia (PETCO2 held 1-2 Torr above prestudy value, PETO2 = 300 Torr) was determined. There was a significant rise in hyperoxic VE over 8 h during both forms of hypoxia (P < 0.05, analysis of variance) that persisted during the subsequent 8-h euoxic period (P < 0.05, analysis of variance). These results support the notion that an 8-h period of hypoxia increases subsequent hyperoxic VE, even if acid-base changes have been minimized through maintenance of isocapnia during the hypoxic period.