ALTERED RESPIRATORY RESPONSES TO HYPOXIA IN MUTANT MICE DEFICIENT IN NEURONAL NITRIC-OXIDE SYNTHASE

1. The role of endogenous nitric oxide (NO) generated by neuronal nitr ic oxide synthase (NOS-l) in the control of respiration during hypoxia and hypercapnia was assessed using mutant mice deficient in NOS-1. 2. Experiments were performed on awake and anaesthetized mutant and ir w ild-type control mice. Respiratory responses to varying levels of insp ired oxygen (100, 21 and 12% O-2) and carbon dioxide (3 and 5% CO2 bal anced oxygen) were analysed. In a awake animals, respiration was monit ored by body plethysmograph along with oxygen consumption ((V) over do t(O2)), CO2 production ((V) over dot(CO2)) and body temperature. In an aesthetized, spontaneously breathing mice, integrated efferent phrenic nerve activity was monitored as an index of neural respiration along with arterial blood pressure and blood gases. Cyclic 3',5'-guanosine m onophosphate (cGMP) levels in the brainstem were analysed by radioimmu noassay as an index of nitric oxide generation. 3. Unanaesthetized mut ant mice exhibited greater respiratory responses during 21 and 12% O-2 than the wild-type controls. Respiratory responses were associated wi th significant decreases in oxygen consumption in both groups of mice, and the magnitude of change was greater in mutant than wild-type mice . Changes in CO2 production and body temperature, however, were compar able between both groups of mice. 4. Similar augmentation of respirato ry responses during hypoxia was also observed in anaesthetized mutant mice. In addition, five of the fourteen mutant mice displayed periodic oscillations in respiration (brief episodes of increases in respirato ry rate and tidal phrenic nerve activity) while breathing 21 and 12% O -2, but not during 100% O-2. The time interval between the episodes de creased by reducing inspired oxygen from 21 to 12% O-2. 5. Changes in arterial blood pressure and arterial blood gases were comparable at an y given level of inspired oxygen between both groups of mice, indicati ng that changes in these variables do not account for the differences in the response to hypoxia. 6. Respiratory responses to brief hyperoxi a (Dejours test) and to cyanide, a potent chemoreceptor stimulant, wer e more pronounced in mutant mice, suggesting augmented peripheral chem oreceptor sensitivity. 7. cGMP levels were elevated in the brainstem d uring 21 and 12% O-2 in wild-type but not in mutant mice, indicating d ecreased formation of nitric oxide in mutant mice. 8. The magnitude of respiratory responses to hypercapnia (3 and 5% CO2 balanced oxygen) w as comparable in both groups of mice in the awake and anaesthetized co nditions. 9. These observations suggest that the hypoxic responses wer e selectively augmented in mutant mice deficient in NOS-1. Peripheral as well as central mechanisms contributed to the altered responses to hypoxia. These results support the idea that nitric oxide generated by NOS-1 is an important physiological modulator of respiration during h ypoxia.