Chronic intermittent hypoxia increases sympathetic responsiveness to hypoxia and hypercapnia

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.