Proceedings of The Physiological Society
University of Cambridge (2004) J Physiol 555P, C29
Effect of chronic intermittent hypoxia on cardiac baroreflex in rats
D. White and N.H. West
Department of Physiology, College of Medicine, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E5, Canada
The rat intermittent hypoxia (IH) model has facilitated investigation of the relation between obstructive sleep apnea and hypertension. However, the effects of IH on the baroreflex have received relatively little attention. The aim of this study was to examine the hypothesis that IH alters baroreflex function, thus contributing to increased blood pressure variability and the development of hypertension.
Wistar rats (IH-rats, n = 9) implanted with telemetry devices (halothane anaesthesia, 2 % in O2) for measurement of arterial pressure (MAP) and heart rate (HR) were exposed to IH consisting of oscillations of inspired oxygen fraction from 21 % to 1 % twice per min, 6 h per day for 7 consecutive days. Controls (Sham-IH rats, n = 6) were exposed to cycling compressed air at the same flow rates as for IH-rats. On Day 8, animals were killed by CO2 exposure and their hearts removed. All procedures were approved by the University of Saskatchewan Committee on Animal Care and Supply and are in accordance with the Guidelines of the Canadian Council on Animal Care.
IH-rats showed significant cardiac hypertrophy (0.45 ± 0.01 vs. 0.37 ± 0.02 mg/g body weight, left ventricle + septum; 0.15 ± 0.01 vs. 0.12 ± 0.01 mg/g body weight, right ventricle; IH vs. Sham). MAP was not different between or within groups over the course of the experiment. Day 1 HR did not differ between groups, however IH-rat HR tended to decrease over time (P = 0.14), becoming significantly lower than Sham-IH HR on days 5-8 (Day 1: 337 ± 8 vs. 354 ± 10 bpm, Day 8: 323 ± 6 vs. 361 ± 9 bpm; IH vs. Sham). Though not significant, there was a trend for spontaneous baroreflex sensitivity to increase with IH (4.5 ± 0.5 vs. 6.3 ± 1.0 msec/mmHg, Day 1 (n = 9) vs. Day 7 (n = 7), P = 0.10) as determined by application of the sequence method to daily recordings of MAP and HR (Bertinieri et al. 1988). Values are mean ± S.E.M. and were compared using unpaired Student's t tests and repeated measures ANOVA where appropriate. P < 0.05 was considered significant.
Contrary to previous reports (Fletcher, Bao, and Li, 1999), IH did not induce hypertension in this study. The lowering of resting HR in IH-rats without a change in MAP may reflect a change in baroreflex set point. Spontaneous baroreflex analysis was limited by the relatively few sequences meeting analysis criteria. Continued studies are required to determine whether the trends in baroreflex function inferred are in fact a significant consequence of IH exposure.
This work was supported by the Natural Sciences and Engineering Research Council of Canada.
Where applicable, experiments conform with Society ethical requirements