We sought to determine whether chronic exposure to intermittent hypoxia (CI
H) increases sympathetic responsiveness to subsequent chemoreflex stimulati
on. Sprague-Dawley rats were exposed to 30 days of CIH: exposure chamber %O
-2 [fractional concentration of chamber O-2 (Fc(O2))] nadir 6.5-7% with ret
urn to 21% each minute for 8 h/day during the diurnal sleep period (Exp gro
up). Sham controls (SC group) were similarly handled but kept at 21% Fc(O2)
and compared with unhandled controls (UC group). Rats were then anesthetiz
ed with urethan, and preganglionic cervical sympathetic activity (CSA), dia
phragm electromyogram, arterial pressure, and electrocardiogram were record
ed while the rats were spontaneously breathing 100% O-2, room air, 10% O-2,
12% CO2, and 10% O-2-12% CO2. CSA and heart rate were also recorded during
phenylephrine infusion to assess baroreceptor function. Mean arterial pres
sure was significantly greater in Exp than in SC and UC rats during all con
ditions (P < 0.05). A vasopressor response to 10% O-2-12% CO2 was observed
only in Exp rats. CSA was greater in Exp than in SC and UC rats during 10%
O-2, 12% CO2, and 10% O-2-12% CO2 but not during room-air exposure. A signi
ficant increase in CSA compared with room air was noted during 10% O-2, 12%
CO2, and 10% O-2-12% CO2 in Exp but not in SC or UC rats. No differences i
n baroreceptor function were observed among groups. We conclude that CIH le
ads to increased sympathetic responsiveness to chemoreflex stimulation.