Exp Physiol. 2014 Dec 16. doi: 10.1113/expphysiol.2014.082990
Volumes of research indicate that males are more susceptible to developing hypertension than premenopausal females. There is also data that shows that obstructive sleep apnea, a form of chronic intermittent hypoxia (CIH), leads to hypertension and that this apnea is more common in men than women. However, data linking this apnea to differences in men and women is scarce and a bit controversial. In this study they looked at female rats exposed to CIH and examined changes in breathing patterns and blood pressure.
In this paper, they used a different CIH protocol than what was in some of the other papers I recently blogged. Here, they did 9 minutes of normoxia, and used nitrogen displacement to bring oxygen levels down to 6% for 40seconds, before being brought back to normoxia for another 9 minutes. This continued for 8hrs a day for 35 days. Rats were anesthetized and fitted with a femoral catheter to record arterial pressure. Respiratory activity was recorded 30 minutes after placing the rat inside of the plethysmographic chamber.
What they noticed was that CIH really messed up female rats’ cardiovascular system - they weighed less than controls (273 vs 347g), they had higher systolic (136 vs 129mmHg), diastolic (92 vs 86mmHg), and mean arterial pressures (111 vs 104mmHg), as well as increased heart rate (400 vs 376bpm). The systolic pressure was also more variable in CIH females. However, the dramatic changes seen in the cardiovascular system was not reflected in the respiratory system. Compared to controls, CIH rats didn’t have higher minute volume, tidal volume, or respiration frequency. However, short and long term variability was indeed higher in CIH rats.
Because respiratory and cardiovascular systems are coupled, they looked at how this relation changed between rat groups. CIH rats had a stronger fall in MAP during deep breaths, and were more likely to have an apneic event after a deep breath than control rats, and the apnea lasted longer as well. They noted that their results were very comparable to the results of a different study on male rats that used the same protocol, but conflicted with another study in female rats that used a different CIH protocol which said that females DO NOT experience the strong changes seen in this paper. This suggests that females may have some protection against the changes, presumably due to differences in hormone levels, but that protection can be overcome by more severe CIH. -DH
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