Age Protects From Harmful Effects Produced by Chronic Intermittent Hypoxia

Abstract

Obstructive sleep apnoea (OSA) affects an estimated 3–7% of the adult population, the frequency doubling at ages >60–65 years. As it evolves, OSA becomes frequently associated with cardiovascular, metabolic and neuropsychiatric pathologies defining OSA syndrome (OSAS). Exposing experimental animals to chronic intermittent hypoxia (CIH) can be used as a model of the recurrent hypoxic and O2 desaturation patterns observed in OSA patients. CIH is an important OSA event triggering associated pathologies; CIH induces carotid body (CB)-driven exaggerated sympathetic tone and overproduction of reactive oxygen species, related to the pathogenic mechanisms of associated pathologies observed in OSAS. Aiming to discover why OSAS is clinically less conspicuous in aged patients, the present study compares CIH effects in young (3–4 months) and aged (22–24 months) rats. To define potential distinctive patterns of these pathogenic mechanisms, mean arterial blood pressure as the final CIH outcome was measured. In young rats, CIH augmented CB sensory responses to hypoxia, decreased hypoxic ventilation and augmented sympathetic activity (plasma catecholamine levels and renal artery content and synthesis rate). An increased brainstem integration of CB sensory input as a trigger of sympathetic activity is suggested. CIH also caused an oxidative status decreasing aconitase/fumarase ratio and superoxide dismutase activity. In aged animals, CIH minimally affected CB responses, ventilation and sympathetic-related parameters leaving redox status unaltered. In young animals, CIH caused hypertension and in aged animals, whose baseline blood pressure was augmented, CIH did not augment it further. Plausible mechanisms of the differences and potential significance of these findings for the diagnosis and therapy of OSAS are discussed.

Figure 1. Effect of different times of digestion of the CB–CSN preparation on the electrical activity of the CSN evoked by hypoxia in 3M and 24M rats

Left and right panels represent, respectively, the recordings obtained in 3M and 24M animals. A, typical recordings of the chemosensory activity of the CSN evoked by hypoxia in CB–CSN preparation submitted to three different times of incubation: 5–7, 15 and 20–30 min in 3M and 24M animals. B, mean response of the CSN to hypoxia in CB–CSN preparation treated to different times of digestion, obtained in 2–3 different animals. Note that young animals submitted to lengthy digestion (15 min) exhibit a decrease in activity and in response to higher periods (30 min) of digestion it is impossible to obtain a good recording. The hypoxia used to increase the CSN response was 0% O₂ + 5% CO₂ (N₂) applied over 3 min. Data are means ± SD.

Figure 2. Effects of chronic intermittent hypoxia and age on carotid body chemoreceptor activity

A and B, time courses of the evolution of CSN activity in the four experimental groups on switching the superfusing solution from air to N₂‐equilibrated (intense hypoxia). The most salient difference is the diminution of the response in 24M animals whether control or exposed to CIH; CIH causes an increase in the response (peak response) and differences in the time course of the response. C, mean basal activity obtained in the four groups of animals: young adult control, 3‐month‐old (3M); young experimental, exposed to CIH for 15 days (3MCIH); aged control, 24 months old (24M); and aged CIH‐exposed animals (24MCIH). D, mean evoked activity elicited by mild and intense hypoxic stimuli expressed as times basal in the four experimental groups. E shows mean evoked activity elicited by a hypercapnic acidotic stimulus (superfusion with a solution equilibrated with 20% CO₂/pH 6.8). Bar charts represent mean ± SD. *P < 0.05 and ***P < 0.001 refer to significant differences due to intermittent hypoxia exposure (i.e. 3M vs. 3MCIH and 24M vs. 24MCIH). ⁺ P < 0.05, ⁺⁺ P < 0.01 and ⁺⁺⁺ P < 0.001 refer to age effects 3M vs. 24M and 3MCIH vs. 24MCIH. Statistical differences were assessed using a two‐way ANOVA followed by Bonferroni's multiple comparisons test.

Figure 3. Effects of chronic intermittent hypoxia and age on ventilation at different atmospheres

Each group of four columns corresponds to minute ventilation (MV) in the indicated atmosphere. Data are means ± SD. Comparisons for statistical significance of the differences were made with a two‐way ANOVA for repeated measures followed by Bonferroni's multiple comparisons test. *P < 0.05 and **P < 0.01 refer to significant differences due to CIH exposure (3M vs. 3MCIH). No statistical differences existed between 24M and 24MCIH. ⁺ P < 0.05, ⁺⁺ P < 0.01 and ⁺⁺⁺ P < 0.001 refer to age effects 3M vs. 24M and 3MCIH vs. 24MCIH.

Figure 4. Effects of chronic intermittent hypoxia and age on plasma catecholamine levels

A and B, respectively, absolute noradrenaline (NA) and adrenaline (A) plasma levels in the four experimental groups as drawn. C, NA/A ratios. D, mean plasma dopamine (DA) levels. Data are means ± SD. One‐way ANOVA, Tukey's multiple multicomparisons test. *P < 0.05 and **P < 0.01, statistical difference from correspondent controls (3M vs. 3MCIH); ⁺ P < 0.05, statistical difference from young adult control animals (3M vs. 24M).

Figure 5. Effects of chronic intermittent hypoxia and age on renal artery catecholamine metabolism

A, renal artery catecholamine content in the four groups of animals. B, renal artery rate of ³H‐catecholamine synthesis. Data are means ± SD. Asterisks indicate significant differences imputable to CIH exposure, and pluses age‐linked differences. One‐way ANOVA, Tukey's multiple test. *P < 0.05 and **P < 0.01, statistical difference from correspondent controls unexposed to CIH. ⁺⁺ P < 0.01, statistical difference from young adult animals (3M vs. 24M).

Figure 6. Effects of chronic intermittent hypoxia and age on plasma C‐reactive protein (CRP) and adenosine levels

A, CRP plasma levels obtained from the four group of animals. B, plasma adenosine levels. Data are means ± SD. One‐way ANOVA, Tukey's multiple multicomparisons test. ***P < 0.001 vs. 3M (CIH effect); ⁺⁺⁺ P < 0.001 vs. 3M and 3MCIH (age effect).

Figure 7. Effects of chronic intermittent hypoxia and age on and fumarase activity from liver, lung and brain mitochondria

Aconitase/fumarase ratio from liver, lung and brain tissue of the four groups of animals. Young adult control, 3‐month‐old (3M); young adult experimental, exposed to CIH for 15 days (3MCIH); aged control (24M); and aged CIH‐exposed animals (24MCIH). Data are means ± SD. One‐way ANOVA, Tukey's multiple multicomparisons test. ***P < 0.001 vs. 3M (CIH effect in young adult animals); ⁺⁺⁺ P < 0.001 vs. 3M and 3MCIH (age effect).

Figure 8. Effects of chronic intermittent hypoxia and age on mean arterial blood pressure (AP)

Mean arterial blood pressure from the four groups of animals breathing air or a 10% O₂ atmosphere. Data are means ± SD. ***P < 0.001 and *P < 0.05, statistically different from correspondent controls (3M).