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1 Department of Neurology, University of Vermont College of Medicine, Burlington, Vermont, USA
* To whom correspondence should be addressed. E-mail: marilyn.cipolla{at}uvm.edu.
Eclampsia is thought to be similar to hypertensive encephalopathy in which acute elevations in intravascular pressure cause forced dilatation (FD) of intrinsic myogenic tone of cerebral arteries and arterioles, decreased cerebrovascular resistance (CVR), and hyperperfusion. In the present study, we tested the hypothesis that pregnancy and/or postpartum predispose cerebral arteries to FD by diminishing pressure-induced myogenic activity. We compared the reactivity to pressure (myogenic activity), as well as factors that modulate the level of tone, of third-order branches (<200µm) of the posterior cerebral artery (PCA) that were isolated from non-pregnant (NP, n=7), late-pregnant (d19, LP, n=10) and postpartum (d3, PP, n=8) Sprague Dawley rats under pressurized conditions. PCAs from all groups of animals developed spontaneous tone within the myogenic pressure range (50-150mmHg), constricting arteries at 100mmHg 30±3% for NP, 39±4% for LP and 42±7% for PP (ns). This level of myogenic activity was maintained in the NP arteries at all pressures, however, both LP and PP arteries dilated at considerably lower pressures compared to NP, lowering the pressure at which FD occurred from >175mmHg for NP to 146±6.5mmHg for LP (p<0.01 vs. NP) and 162±7.7mmHg for PP (p<0.01 vs. NP). The amount of myogenic tone was also significantly diminished at 175mmHg compared to NP: % tone for NP, LP and PP animals was: 35±2%, 11±3% (p<0.01 vs. NP) and 20±7% (p<0.01 vs. NP). Inhibition of nitric oxide (NO) with 0.1mM nitro- L-arginine (L-NNA) caused constriction of all vessel types that was significantly increased in the PP arteries, demonstrating significant basal NO production. Reactivity to 5-HT was assessed in the presence of L-NNA and indomethacin. There was a differential response to 5-HT: PCAs from NP animals dilated whereas LP and PP arteries constricted. These results suggest that both pregnancy and postpartum predispose the cerebral circulation to FD at lower pressures, a response that may lower CVR and promote hyperperfusion when blood pressure is elevated, as during eclampsia.
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