1 Division of Cardiovascular and Medical Sciences and 2 Division of Developmental Medicine, University of Glasgow, Western Infirmary, Glasgow G11 6NT, UK
3 To whom correspondence should be addressed. Email: gcl203{at}clinmed.gla.ac.uk
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Abstract |
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Key words: cerebral blood flow/menopause/estrogen/progestogen
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Introduction |
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Recent large clinical trials have reported increased stroke risk in patients randomized to HRT. A post hoc analysis of the WEST study data (Viscoli et al., 2001) revealed increased risk of cerebrovascular events during the first 6 months of treatment with estradiol [relative risk of 2.3; 95% confidence interval (CI) 1.15.0]. Similarly, those patients in the active treatment arm of the WHI study (Writing Group for the Women's Health Initiative Investigators, 2002
) had increased stroke risk, with a hazard ratio of 1.41 (95% CI 1.071.85). These trials used combined HRT containing both estrogen and progestogen. These results have challenged the conclusions of older epidemiological studies which suggested significantly reduced stroke risk in HRT recipients. The mechanism through which HRT confers increased risk of stroke is elusive, although few studies have yet examined the effect of HRT on cerebral vessels directly. We performed a prospective randomized double-blind placebo-controlled study designed to examine the effect of administration of two different HRT preparations upon the cerebral vasculature.
Women over the age of 55 usually take continuous combined HRT, consisting of an estrogen combined with a progestogen. Progestogens vary in their structure according to whether they vare based on testosterone (C19 progestogens) or progesterone (C21). Until recently, it was assumed that the vascular effects of HRT were largely related to the estrogen, but it has become apparent recently from preclinical studies that the progestogenic component may be of considerable importance. In animal models of both focal (Betz and Coester, 1990) and global (Cervantes et al., 2002
) cerebral ischaemia, progesterone administration attenuates brain injury through an as yet undefined mechanism. In humans, progestogens affect a number of processes which may modify risk of vascular disease. For example, progestins may influence levels of atherogenic and atheroprotective lipids (Writing Group for the PEPI Trial. 1995
) and the expression of a number of circulating rheological factors (Nabulsi, 1993
). Progestins can also inhibit vasorelaxation (Mercuro, 1999
) and decrease proliferation of both human endothelial (Okada, 1997
) and vascular smooth muscle cells (Lee et al., 1997
). We chose to investigate the impact of the progestogen on the cerebral vasculature of post-menopausal women by keeping the estrogen constant while studying one C19 and one C21 progestogen.
Non-invasive ultrasonic assessment of vascular reactivity is possible through measurement of cerebral vasomotor reactivity (CVR) to acetazolamide. This agent is a carbonic anhydrase inhibitor which induces a mild extracellular acidosis, hence it provides a strong stimulus for intracranial vasodilatation. The vasodilatory response is readily detectable with trans-cranial Doppler (TCD) ultrasound as an increase in velocity of blood flow through the middle cerebral artery (MCA). Impaired reactivity reflects blunted ability of the cerebral vasculature to respond to a hypoxic or ischaemic insult and has been associated with increased risk of stroke (Molina et al., 1999). Impaired reactivity is reversible and has been demonstrated in the context of hypertension, hyperlipidaemia and subcortical stroke (Settakis et al., 2003
)
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Methods |
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Once informed consent had been obtained and randomization completed, patients underwent baseline assessment of intracranial haemodynamics and CVR to acetazolamide. Following baseline assessment, patients received either oral estradiol 1 mg+norethisterone 0.5 mg; oral estradiol 1 mg+dydrogesterone 5 mg; or placebo to be taken daily for 3 months. At the end of the treatment interval, they returned for repeat ultrasound assessment of intracranial haemodynamics. Demographic characteristics are shown in Table I.
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Middle cerebral artery and cerebral vasomotor reactivity. As diurnal variation in CVR may occur, all examinations were performed at 11 a.m., immediately following carotid insonation. Patients underwent TCD examination in a supine position. TCD recordings (TC 2000 with 2 MHz probe, Nicolet, UK) were obtained from the MCA at a depth of 50 mm using a temporal approach. Readings were based on 36 s recordings from each MCA. Velocity readings were based on the maximal (envelope) curve. Insonation of the right MCA was performed for a period of 2 min and the mean flow velocity (MFV) recorded. Each subject then received an i.v. infusion of 13 mg/kg acetazolamide (to a maximum of 1 g) reconstituted in sterile water for injection over 10 min. The total volume of infusate was 50 ml. Twenty minutes after cessation of acetazolamide infusion, the TCD recording was repeated for a further 2 min, and the post-acetazolamide MFV recorded. Pre- and post-acetazolamide MFVs were calculated as the average MFV of all waveforms recorded in each 2 min interval. During data acquisition, end-tidal carbon dioxide concentration was measured using a standard probe to confirm reduction of systemic carbonic anhydrase activity. CVR was calculated as MFV(post)/MFV(pre)x100. The pulsatility indices of the ICA and MCA (a measure of vascular resistance distal to the insonated vessel) were obtained through off-line analysis of the velocity/time waveforms. All data were processed without access to treatment group information. A more detailed account of the Doppler methodology employed in our laboratory can be found elsewhere (Dyker et al., 1997
).
Statistical measures
Statistical analysis was performed using Student's t- test and two-way analysis of variance (StatsDirect Ltd, Cheshire, UK). The power calculation was based upon variability data acquired during earlier TCD studies. A sample size of 12 patients per group would allow a 10% difference in CVR between groups to be detected with 80% power.
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Results |
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Cerebral vasomotor reactivity
There were no significant between-group differences in baseline CVR. HRT administration did not affect CVR [% (SE) change from baseline, estradiol+norethisterone +4.2% (11); estradiol+dydrogesterone +3.8% (5.5); placebo +4.0% (3.8)] (Figure 1). Absolute values of CVR were consistent with those from healthy volunteers.
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Discussion |
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Although no change in CVR was seen following HRT administration, our results suggest a modest but consistent effect of HRT on intracranial vessels, suggestive of HRT-induced intracranial vasodilatation (increased MFV and reduced carotid PI). Interestingly, significant differences between the effects of different progestins on the cerebral vasculature were seen, a phenomenon not reported before. Although this was a small study and differences observed were modest, the results support the hypothesis that not all HRT preparations exert the same effects on the cerebral vasculature, and that choice of HRT preparation may influence cerebrovascular risk.
Acetazolamide is a potent vasodilatory stimulus, acting through both endothelium-dependent and endothelium-independent mechanisms. As such, a subtle effect of HRT administration upon cerebrovascular endothelial function has not been excluded by the lack of a significant difference in reactivity to acetazolamide between groups. One plausible explanation for the changes consistent with intracranial vasodilatation observed following HRT administration is improved cerebrovascular endothelial function, similar to the estrogen-mediated improvement in endothelial function reported in other vascular beds (Gilligan et al., 1994; Liebermanhard et al., 1994
).
Systemic infusion of a more specific vasoactive agent such as the nitric oxide synthase inhibitor L-NMMA would allow the effect of HRT upon cerebrovascular nitric oxide bioavailability to be examined in more detail, and further work in this area is planned.
Conclusion
In this study, HRT induced subtle but consistent and readily demonstrable changes in intracranial haemodynamics, consistent with relaxation of intracranial resistance vessels. Our results suggest a differential effect of progestins on patterns of cerebral blood flow; these may in part explain the disparity between the apparent beneficial effects of HRT upon vascular surrogate end points and the disappointing results of large clinical trials. The results of this study suggest that differences exist between the effects of different HRT preparations on the cerebral vasculature.
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Acknowledgements |
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References |
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Bousser MG (1999) Stroke in women: the 1997 Paul Dudley White International Lecture. Circulation 99, 463467.
Cacciatore B, Paakkari I, Toivonen J et al. (1998) Randomized comparison of oral and transdermal hormone replacement on carotid and uterine artery resistance to blood flow. Obstet Gynecol 92, t-8.[CrossRef]
Cervantes M, Gonzales Vidal M and Ruelas R (2002) Neuroprotective effects of progesterone on damage elicited by acute global cerebral ischemia in neurons of the caudate nucleus. Arch Med Res, 614.
Dyker AGB, Grosset DGB and Lees KBMF (1997) Perindopril reduces blood pressure but not cerebral blood flow in patients with recent cerebral ischemic stroke. Stroke 28, 580583.
Gangar KF, Vyas S, Whitehead M et al. (1991) Pulsatility index in internal carotid artery in relation to transdermal oestradiol and time since menopause. Lancet 338, 839842.[ISI][Medline]
Gilligan DM, Badar DM, Panza JA et al. (1994) Acute vascular effects of estrogen in postmenopausal women. Circulation 90, 786791.[Abstract]
Hankey GJ (1999) Stroke: how large a public health problem, and how can the neurologist help? Arch Neurol 56, 748754.
Lee W, Harder J and Yoshizumi M (1997) Progesterone inhibits arterial smooth muscle cell proliferation. Nature Med 3, 10051008.[ISI][Medline]
Lieberman EH, Gerhard MD, Uehata A et al. (1994) Estrogen improves endothelium-dependent, flow-mediated vasodilation in postmenopausal women. Ann Intern Med 121, 936941.
Mercuro G (1999) Effects of acute administration of natural progesterone on peripheral vascular responsiveness in healthy postmenopausal women. Am J Cardiol 84, 214218.[CrossRef][ISI][Medline]
Molina C, Sabin JA, Montaner J et al. (1999) Impaired cerebrovascular reactivity as a risk marker for first-ever lacunar infarction: a case-control study. Stroke 30, 22962301.
Murray CJ and Lopez AD (1997) Mortality by cause for eight regions of the world: Global Burden of Disease Study. Lancet 349, 12691276.[CrossRef][ISI][Medline]
Nabulsi A (1993) Association of hormone-replacement therapy with various cardiovascular risk factors in postmenopausal women. N Engl J Med 328, 10691075.
Okada M (1997) Effects of 17[beta]-estradiol and progesterone on migration of human monocytic THP-1 cells stimulated by minimally oxidized low-density lipoprotein in vitro. Cardiovasc Res 34, 529535.[CrossRef][ISI][Medline]
Penotti M, Nencioni T, Gabrielli L et al. (1993) Blood flow variations in internal carotid and middle cerebral arteries induced by postmenopausal hormone replacement therapy. Am J Obstet Gynecol 169, 12261232.[ISI][Medline]
Penotti M, Sironi L, Castiglioni E et al. (1999) Blood flow in the internal carotid and middle cerebral arteries: effects of continuous oral conjugated equine estrogens administration with monthly progestogen supplementation on postmenopausal women. Menopause 6, 225229.[ISI][Medline]
Settakis G, Molnar C, Kerenyi L et al. (2003) Acetazolamide as a vasodilatory stimulus in cerebrovascular diseases and in conditions affecting the cerebral vasculature. Eur J Neurol 10, 609620.[CrossRef][ISI][Medline]
Viscoli CM, Brass LM, Kernan WN et al. (2001) A clinical trial of estrogen-replacement therapy after ischemic stroke. N Engl J Med 345, 12431249.
Writing Group for the PEPI Trial (1995) Effects of estrogen or estrogen/progestin regimens on heart disease risk factors in postmenopausal women: the Postmenopausal Estrogen/Progestin Interventions (PEPI) Trial. J Am Med Assoc 273, 199208.[Abstract]
Writing Group for the Women's Health Initiative Investigators (2002) Risks and benefits of estrogen plus progestin in healthy postmenopausal women: principal results from the Women's Health Initiative randomized controlled trial. J Am Med Assoc 288, 321333.
Submitted on February 24, 2004; accepted on June 7, 2004.
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