Angiogenesis and improved cerebral blood flow in the ischemic boundary area detected by MRI after administration of sildenafil to rats with embolic stroke

Abstract

To dynamically investigate the long-term response of an ischemic lesion in rat brain to the administration of sildenafil, male Wistar rats subjected to embolic stroke were treated with sildenafil (n = 11) or saline (n = 10) at a dose of 10 mg/kg administered subcutaneously 24-h after stroke and daily for an additional 6 days. Magnetic resonance images were acquired and functional performance was measured in all animals at 1 day, 2 days and weekly for 6 weeks post-stroke. All rats were sacrificed 6 weeks after stroke and endothelial barrier antigen immunostaining was employed for morphological analysis and quantification of cerebral vessels. Map-ISODATA was computed from T₁, T₂ and T_1sat maps. ISODATA derived tissue signatures characterize the degree of ischemic injury. Based on the map-ISODATA calculated at 6 weeks, the ischemic lesion for each animal was divided into two specific regions, the ischemic boundary and ischemic core. The temporal profiles of cerebral blood flow (CBF) and tissue signature were retrospectively tracked in these two regions and were compared with histological evaluation and functional outcome. After 1 week of sildenafil treatment, the ischemic lesion exhibited two significantly different regions, with higher CBF level and correspondingly, lower tissue signature value in the boundary region than in the core region. Sildenafil treatment did not significantly reduce the lesion size, but did enhance angiogenesis. Functional performance was significantly increased after sildenafil treatment compared with the control group. Administration of sildenafil to rats with embolic stroke enhances angiogenesis and selectively increases the CBF level in the ischemic boundary, and improves neurological functional recovery compared to saline-treated rats.

Introduction

As a vasodilator with good hemodynamic effects, sildenafil has been successfully used in the treatment of patients with pulmonary arterial hypertension (Sheth et al., 2005, Lopez-Guarch et al., 2004, Kataoka et al., 2004, Rosenkranz et al., 2004, Michelakis et al., 2003, Watanabe et al., 2002) and cardiovascular disease (Jackson et al., 2005, Cheitlin et al., 1999). By selectively inhibiting phosphodiesterase type-5 (PDE-5) and thus effectively reducing the breakdown of cGMP, sildenafil administration can markedly improve pulmonary and cardiac functional capacity, and hemodynamics (Du Toit et al., 2005, Traverse et al., 2000).

Sildenafil also significantly increases cortical levels of cGMP in ischemic rat brain (Zhang et al., 2002, Zhang et al., 2005) and transiently elevates localized cerebral blood flow (CBF) in non-ischemic rat brain (Zhang et al., 2002). However, the long-term, especially, dynamic evolution of CBF in the ischemic brain after sildenafil intervention remains largely unexplored. In the present study, we therefore dynamically and non-invasively investigated the long-term (up to 6 weeks) response of ischemic rat brain to sildenafil treatment using magnetic resonance imaging (MRI). Our data indicate that treatment with sildenafil leads to a significantly increased CBF level in the ischemic boundary area at the late stage after stroke (6-week) compared to the control group.