Antenatal betamethasone alters vascular reactivity in adult female ovine cerebral arteries.

Although the use of antenatal glucocorticoids has resulted in decreased neonatal morbidity/mortality, recent animal studies have raised concerns regarding adverse effects of these medications on postnatal cardiovascular function. We hypothesized that antenatal betamethasone (Beta) exposure alters cerebral vascular reactivity in adult female sheep. We observed that K-induced constriction was comparable in middle cerebral artery (MCA) from Beta-exposed animals and age-matched controls. Pressure-induced constriction was significantly attenuated in MCA from Beta-exposed compared with control sheep. Inhibition of NOS significantly augmented pressure-induced constriction in MCA from both Beta-exposed and control sheep, whereas cyclooxygenase (COX) inhibition augmented pressure-induced constriction only in MCA from Beta-exposed sheep. Furthermore, NOS and COX inhibition significantly attenuated bradykinin (BK)-induced dilation in MCA from both Beta-exposed and control sheep. However, there seemed to be a greater contribution of both NOS and COX to BK-induced dilation in Beta-exposed compared with control MCA. Our findings demonstrate that fetal exposure to a clinically relevant course of Beta alters cerebral vascular tone and reactivity in adult female sheep.

Piglet pial arteries respond to N-methyl-D-aspartate in vivo but not in vitro.

Controversy exists concerning whether activation of N-methyl-D-aspartate (NMDA) receptors exerts direct dilator effects on cerebral arteries. The purpose of this study was to examine the responses of isolated piglet arteries to NMDA to determine whether isolated arteries, apart from surrounding neuronal tissue, are capable of responding to NMDA. Piglet arteries (100-200 microm) were isolated from branches of the middle cerebral artery and carefully dissected free of adherent tissue. Arteries were then mounted in an arteriograph system and pressurized to either 30 mm Hg (n=8), 60 mm Hg (n=10), 80 mm Hg (n=6), or 100 mm Hg (n=5). After development of spontaneous tone, NMDA (10(-5) to 10(-3) M) was administered abluminally to the vessels, and no appreciable response was noted (for example; 10(-4) M, 30 mm Hg: 3+/-3% change in active diameter; 60 mm Hg: -4+/-3% change in active diameter). Following a thorough washout, vessels were treated with bradykinin (10(-9) to 10(-7) M), and the arteries did respond (10(-7) M, 30 mm Hg: 26+/-3% change in active diameter; 60 mm Hg: 65+/-10% change in active diameter). In contrast, 10(-5) M and 10(-4) M NMDA dilated arteries in vivo by 9+/-2% and 29+/-6% change in active diameter, respectively (n=6). These results demonstrate that isolated cerebral arteries do not respond directly to NMDA receptor activation. This work confirms our previous in vivo data and is consistent with the hypothesis that cerebral arteries respond to NMDA through a secondary interaction mediated by neuronal release of NO and not to NMDA directly.

Insulin resistance does not impair contractile responses of cerebral arteries.

Insulin resistance (IR) impairs endothelium-mediated vasodilation in cerebral arteries as well as K+ channel function in vascular smooth muscle. Peripheral arteries also show an impaired endothelium-dependent vasodilation in IR and concomitantly show an enhanced contractile response to endothelin-1 (ET-1). However, the contractile responses of the cerebral arteries in IR have not been examined systematically. This study examined the contractile responses of pressurized isolated middle cerebral arteries (MCAs) in fructose-fed IR and control rats. IR MCAs showed no difference in pressure-mediated (80 mmHg) vasoconstriction compared to controls, either in time to develop spontaneous tone (control: 61+/-3 min, n=30; IR: 63+/-2 min, n=26) or in the degree of that tone (control: 60 min: 33+/-2%, n=22 vs. IR 60 min: 34+/-3%, n=17). MCAs treated with ET-1 (10(-8.5) M) constrict similarly in control (53+/-3%, n=14) and IR (53+/-3%, n=14) arteries. Constrictor responses to U46619 (10(-6) M) are also similar in control (48+/-9%, n=8) and IR (42+/-5%, n=6) MCAs as are responses to extraluminal uridine 5'-triphosphate (UTP; 10(-4.5) M) (control: 35+/-7%, n=11 vs. IR: 38+/-3%, n=10). These findings demonstrate that constrictor responses remain intact in IR despite a selective impairment of dilator responses and endothelial and vascular smooth muscle K+ channel function in cerebral arteries. Thus, it appears that the increased susceptibility to cerebrovascular abnormalities associated with IR and diabetes (including cerebral ischemia, stroke, vertebrobasilar transient ischemic attacks) is not due to an enhanced vasoreactivity to constrictor agents.

Acetaminophen-sensitive prostaglandin production in rat cerebral endothelial cells.

Acetaminophen is a widely used antipyretic and analgesic drug whose mechanism of action has recently been suggested to involve inhibitory effects on prostaglandin synthesis via a newly discovered cyclooxygenase variant (COX-3). Because COX-3 expression is high in cerebral endothelium, we investigated the effect of acetaminophen on the prostaglandin production of cultured rat cerebral endothelial cells (CECs). Acetaminophen dose-dependently inhibited both basal and LPS-induced PGE(2) production in CECs with IC(50) values of 15.5 and 6.9 microM, respectively. Acetaminophen also similarly inhibited the synthesis of 6-keto-PGF(1alpha) and thromboxane B(2). LPS stimulation increased the expression of COX-2 but not COX-1 or COX-3. In addition, the selective COX-2 inhibitor NS398 (1 microM) was equally as effective as acetaminophen in blocking LPS-induced PGE(2) production. Acetaminophen did not influence the expression of the three COX isoforms and the inducible nitric oxide synthase. In LPS-stimulated isolated cerebral microvessels, acetaminophen also significantly inhibited PGE(2) production. Our results show that prostaglandin production in CECs during basal and stimulated conditions is very sensitive to inhibition by acetaminophen and suggest that acetaminophen acts against COX-2 and not COX-1 or COX-3. Furthermore, our findings support a critical role for cerebral endothelium in the therapeutic actions of acetaminophen in the central nervous system.

Mechanisms of vascular dysfunctionin insulin resistance.

Insulin resistance (IR) has profound, negative effects on the function of arteries and arterioles throughout the body. In addition to the obvious link between IR and the development of type 2 diabetes, IR-associated dysfunction of resistance vessels is associated with arterial hypertension and vascular occlusive diseases, such as heart attacks and strokes. IR affects arteries and arterioles at both the endothelium and smooth muscle levels. For example, IR causes reduced responsiveness of vascular smooth muscle to dilator agents; predominantly due to impaired potassium channel function. The common, underlying mechanism of vascular dysfunction, at both endothelium and smooth muscle levels, appears to involve the augmented availability and subsequent actions of reactive oxygen species (ROS). However, in some circulations, other factors, such as increased production of, and actions by, constrictor agents also appear to restrict normal dilator responses. The underlying cause of augmented ROS availability is not completely understood, but vascular inflammatory processes appear to be involved. Furthermore, application of superoxide dismutase, a specific scavenger of superoxide anion, is able to immediately restore normal vascular responsiveness in IR arteries. Additional treatments involving behavioral and pharmacological approaches, such as dietary adjustments, weight loss, exercise and the use of statins or insulin-sensitizing agents also appear to offer some benefit against the detrimental effects of IR.

Potassium channel dysfunction in cerebral arteries of insulin-resistant rats is mediated by reactive oxygen species.

BACKGROUND AND PURPOSE: Insulin resistance (IR) increases the risk of stroke in humans. One possible underlying factor is cerebrovascular dysfunction resulting from altered K(+) channel function. Thus, the goal of this study was to examine K+ channel-mediated relaxation in IR cerebral arteries. METHODS: Experiments were performed on pressurized isolated middle cerebral arteries (MCAs) from fructose-fed IR and control rats. RESULTS: Dilator responses to iloprost, which are BK(Ca) channel mediated, were reduced in the IR compared with control arteries (19+/-2% versus 33+/-2% at 10(-6) mol/L). Similarly, relaxation to the K(ATP) opener pinacidil was diminished in the IR MCAs (17+/-2%) compared with controls (38+/-2% at 10(-5) mol/L). IR also reduced the K(ATP) channel-dependent component in calcitonin gene-related peptide-induced dilation; however, the magnitude of the relaxation remained unchanged in IR because of a nonspecified K+ channel-mediated compensatory mechanism. In contrast, K(ir) channel-mediated relaxation elicited by increases in extracellular [K+] (4 to 12 mmol/L) was similar in the control and IR arteries. Blockade of the K(ir) and K(v) channels with Ba2+ and 4-aminopyridine, respectively, constricted the MCAs in both experimental groups with no significant difference. Pretreatment of arteries with superoxide dismutase (200 U/mL) plus catalase (150 U/mL) restored the dilatory responses to iloprost and pinacidil in the IR arteries. Immunoblots showed that the expressions of the pore-forming subunits of the examined K+ channels are not altered by IR. CONCLUSIONS: IR induces a type-specific K+ channel dysfunction mediated by reactive oxygen species. The alteration of K(ATP) and BK(Ca) channel-dependent vascular responses may be responsible for the increased risk of cerebrovascular events in IR.