Cyclooxygenases 1 and 2 differentially regulate blood pressure and cerebrovascular responses to acute and chronic intermittent hypoxia: implications for sleep apnea

Abstract

Background: Obstructive sleep apnea (OSA) is associated with increased risk of cardiovascular and cerebrovascular disease resulting from intermittent hypoxia (IH)-induced inflammation. Cyclooxygenase (COX)-formed prostanoids mediate the inflammatory response, and regulate blood pressure and cerebral blood flow (CBF), but their role in blood pressure and CBF responses to IH is unknown. Therefore, this study's objective was to determine the role of prostanoids in cardiovascular and cerebrovascular responses to IH.

Methods and results: Twelve healthy, male participants underwent three, 6-hour IH exposures. For 4 days before each IH exposure, participants ingested a placebo, indomethacin (nonselective COX inhibitor), or Celebrex(®) (selective COX-2 inhibitor) in a double-blind, randomized, crossover study design. Pre- and post-IH blood pressure, CBF, and urinary prostanoids were assessed. Additionally, blood pressure and urinary prostanoids were assessed in newly diagnosed, untreated OSA patients (n=33). Nonselective COX inhibition increased pre-IH blood pressure (P ≤ 0.04) and decreased pre-IH CBF (P=0.04) while neither physiological variable was affected by COX-2 inhibition (P ≥ 0.90). Post-IH, MAP was elevated (P ≤ 0.05) and CBF was unchanged with placebo and nonselective COX inhibition. Selective COX-2 inhibition abrogated the IH-induced MAP increase (P=0.19), but resulted in lower post-IH CBF (P=0.01). Prostanoids were unaffected by IH, except prostaglandin E2 was elevated with the placebo (P=0.02). Finally, OSA patients had elevated blood pressure (P ≤ 0.4) and COX-1 formed thromboxane A2 concentrations (P=0.02).

Conclusions: COX-2 and COX-1 have divergent roles in modulating vascular responses to acute and chronic IH. Moreover, COX-1 inhibition may mitigate cardiovascular and cerebrovascular morbidity in OSA.

Clinical trial registration url: www.clinicaltrials.gov. Unique identifier: NCT01280006.

Keywords: blood pressure; cerebrovascular circulation; intermittent hypoxia; obstructive sleep apnea; prostaglandins.

Figure 1.

CONSORT diagram showing the flow of healthy participants exposed to intermittent hypoxia (IH).

Figure 2.

Healthy male participants were exposed to 6 hours of isocapnic intermittent hypoxia on 3 separate occasions (IH Exposure #1 to 3). For 4 days before each IH exposure, participants ingested either 100 mg lactose placebo (3 times/day), 50 mg (3 times/day) of the nonselective cyclooxygenase (COX) inhibitor indomethacin, or 200 mg (2 times/day) of the selective COX‐2 inhibitor Celebrex^® (Drug Ingestion #1 to 3), administered in a random order. For each IH Exposure, participants arrived at ≈0800 hours and immediately provided a urine sample () and were instrumented for Physiological Measurements of blood pressure and cerebral blood flow. Next, each participant was exposed to 6 hours of IH that consisted of cycling their end‐tidal partial pressure of oxygen () between 88 mm Hg (normal value for the altitude (≈1101 m) at which the laboratory is located) and 45 mm Hg every 60 seconds. After each IH exposure, participants provided another urine sample () and the Physiological Measurements (ie, blood pressure and cerebral blood flow) were repeated. Each IH exposure was separated by at least 4 days to allow washout of the medication from their system (Drug Washout #1 to 3).

Figure 3.

Resting mean (MAP), systolic (SBP) and diastolic (DBP) brachial blood pressures and heart rate of healthy participants before (□), and after (■) 6 hours of IH. † indicates significantly different from placebo before‐IH values with P≤0.05; * indicates significant effect of IH within each condition P≤0.05; ‡ significantly different from after‐IH placebo values with P≤0.05; and †† indicates significant difference from nonselective COX inhibition after‐IH values. IH indicates intermittent hypoxia.

Figure 4.

Using a single‐blind, placebo‐controlled, randomized, cross‐over experimental design, the acute blood pressure responses to placebo, nonselective COX inhibitor, and selective COX‐2 inhibitor medications were assessed in 10 healthy participants (9 overlap with the 12 from the IH study). Participants ingested a single oral dose of placebo (100 mg lactose), the nonselective COX inhibitor indomethacin (50 mg), and the selective COX‐2 inhibitor Celebrex^® (200 mg). Mean (MAP; A), systolic (SBP; B) and diastolic blood pressures (DBP; C) brachial blood pressures were measured before (), and either 2 hours (placebo and non‐selective COX inhibition) or 3 hours (COX‐2 inhibition) after ingestion of medications (■). The mean of 3 separate blood pressure measurements taken over 10 minutes while participants were resting in a seated position was recorded. Participants ate a standardized breakfast prior to ingesting the medications. After ingestion, the participant remained in the seated, resting position watching television until post‐drug measurements were performed. The time to post‐drug measurements was based upon the pharmacokinetics of each medication and the time required to reach peak serum levels. The 3 experimental days were separated by at least 4 days. Pre‐drug blood pressures were not different between the 3 conditions (P≥0.26). MAP and SBP were significantly elevated after ingestion of the non‐selective COX inhibitor (P≤0.01). These findings support the conclusion that prostanoids formed via COX‐1 are the primary regulator of resting blood pressure in healthy individuals. Results provided as mean±SD. * significantly different from Pre‐drug values. COX indicates cyclooxygenase; IH, intermittent hypoxia.

Figure 5.

Estimated 24‐hour sodium excretion before (□), and after (■), intermittent hypoxia exposure.

Figure 6.

Cerebral blood velocity through the middle cerebral artery () and cerebrovascular resistance (CVR) of healthy participants before (□), and after (■) 6 hours of IH. † indicates significantly different from placebo before‐IH values with P≤0.05; * indicates significant effect of IH within each condition P≤0.05. IH inidcates intermittent hypoxia.

Figure 7.

Acute cerebral blood velocity through the middle cerebral artery () and cerebrovascular resistance (CVR) responses to a single dose of of placebo (100 mg lactose), nonselective COX inhibitor (50 mg indomethacin), and selective COX‐2 inhbitor (200 mg Celebrex^®) medications. and CVR were assessed assessed before (), and either 2 hours (placebo (100 mg lactose) and nonselective COX inhibition) or 3 hours (COX‐2 inhibition) after ingesting medications (■). Pre‐drug and CVR were not different between the 3 drug conditions (P≥0.24). At the post‐drug time point, was significantly lower within all 3 drug conditions (P≤0.05) while CVR was significantly increased after ingestion of only the non‐selective COX inhibitor (P<0.01). The magnitude of the decrease in from the pre‐drug to post‐drug condition was significantly greater with nonselective COX inhibition compared to placebo (P<0.01) and selective COX‐2 inhibition (P<0.01). These findings support the conclusion that prostanoids formed via COX‐1 are the primary regulators of resting cerebral blood flow in healthy individuals. Results provided as mean±SD. * indicates significant difference from pre‐drug values with P≤0.05; ‡ significantly different from post‐drug placebo values with P≤0.05; and †† indicates significant difference from nonselective COX inhibition post‐drug values with P≤0.05. COX indicates cyclooxygenase.

Figure 8.

Urinary concentrations (normalized to creatinine) of prostacyclin (PGI₂), prostaglandin E₂ (PGE₂), thromboxane A₂ (TXA₂), and prostaglandin F_2α (PGF_2α) and the PGI₂:TXA₂ ratio in healthy participants before (□), and after (■), IH exposures within the placebo (A), nonselective COX inhibition (B) and the selective COX‐2 inhibition (C) conditions. † indicates significantly different from placebo before‐IH values with P≤0.05; * indicates significant effect of IH within each condition P≤0.05; and ‡ indicates significant difference from after‐IH placebo values with P≤0.05. COX indicates cyclooxygenase; IH, intermittent hypoxia.

Figure 9.

Urinary prostanoid concentrations (normalized to creatinine) in newly diagnosed OSA patients. † indicates significantly different from placebo pre‐IH values with P≤0.05 and ‡ significantly different from post‐IH placebo values with P≤0.05. IH indcates intermittent hypoxia; OSA, obstructive sleep apnea; PGE₂, prostaglandin E₂; PGF₂α, prostaglandin F₂α; PGI₂, prostacyclin; TXA₂, thromboxane A₂.

Figure 10.

Putative pathways through which selective COX‐2 inhibition may have prevented the IH‐induced blood pressure elevation and resulted in a decreased cerebral blood flow. With IH exposure, there is an up regulation of the renin‐angiotensin system (RAS) via increased sympathetic nervous system activation resulting in increased renin activity and angiotensin‐II formation^– (Right; A). Subsequently, angiotensin‐II binds to angiotensin type 1 receptors (AT1r) on vascular smooth muscle cells causing vasoconstriction and an increase in blood pressure (Right; B). Via binding to the AT1r, angiotensin‐II increases COX‐2 expression, which enhances the vascular effects of angiotensin‐II on the vascular smooth muscle cells^– (Right; C). Additionally, since renin release is COX‐2 dependent,^, IH induced increases in COX‐2 may also enhances renin activity and formation of angiotensin‐II which, in turn, will further enhance COX‐2 expression (Right; D). Thus, it is proposed that the required magnitude of RAS up regulation to produce an increase in blood pressure with IH is dependent upon the augmenting effects of COX‐2. In addition, COX‐2 expression is enhanced via IH induced inflammation (eg, IL‐1β, TNFα, and NFκβ—Right; E). Therefore, selectively inhibiting COX‐2 may have prevented the augmentation of the vascular effects of angiotensin‐II as well as minimizing renin activity. As a result, the RAS system was not sufficiently up regulated by IH resulting in maintenance of blood pressure. In contrast, within the cerebral vasculature, an elevation of NF‐κβ, IL‐1β, and TNFα may lead to augmented expression and activity of endothelial COX‐2^– and enhanced release of vasodilatory prostanoids involved in regulating resting CBF leading to the maintenance of CBF after IH^– (Left; F). Selective inhibition of COX‐2 may have blocked this increase in vasodilatory prostanoids and caused CBF to decrease with IH exposure. CBF inidcates cerebral blood flow; COX, cyclooxygenase; IH, intermittent hypoxia; RAS, renin‐angiotensin system.