Biphasic effects of NMDA on the motility of the rat portal vein

Astrocytes drive cortical vasodilatory signaling by activating endothelial NMDA receptors

Astrocytes express neurotransmitter receptors that serve as sensors of synaptic activity and initiate signals leading to activity-dependent local vasodilation and increases in blood flow. We previously showed that arteriolar vasodilation produced by activation of cortical astrocytes is dependent on endothelial nitric oxide synthase (eNOS) and endogenous agonists of N-methyl-D-aspartate (NMDA) receptors. Here, we tested the hypothesis that these effects are mediated by NMDA receptors expressed by brain endothelial cells. Primary endothelial cultures expressed NMDA receptor subunits and produced nitric oxide in response to co-agonists, glutamate and D-serine. In cerebral cortex in situ, immunoelectron microscopy revealed that endothelial cells express the GluN1 NMDA receptor subunit at basolateral membrane surfaces in an orientation suitable for receiving intercellular messengers from brain cells. In cortical slices, activation of astrocytes by two-photon flash photolysis of a caged Ca²⁺ compound or application of a metabotropic glutamate receptor agonist caused endothelial NO generation and local vasodilation. These effects were mitigated by NMDA receptor antagonists and conditional gene silencing of endothelial GluN1, indicating at least partial dependence on endothelial NMDA receptors. Our observations identify a novel astrocyte-endothelial vasodilatory signaling axis that could contribute to endothelium-dependent vasodilation in brain functional hyperemia.

High-glucose enhances a thromboxane A2-induced aortic contraction mediated by an alteration of phosphatidylinositol turnover

The effect of the thromboxane A(2) analogue U46619 (9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F(2)(alpha)) on sustained contraction in the mouse aorta was investigated. U46619 induced concentration-dependent (1 - 100 nM) increases in contraction. These contractile responses were enhanced significantly under high-glucose-physiological salt solution (HG-PSS) (2-fold greater than normal-PSS) conditions. This hyperactivation may be associated with aortic dysfunction in diabetes. However, the mechanisms remain unclear. HG-PSS enhanced U46619-induced accumulation of endogenous diacylglycerol (DG). Phospholipase C inhibitor (U73122) suppressed DG accumulation under normal conditions; however, suppression was not observed under high-glucose conditions. The HG-PSS-induced enhancement of contraction was inhibited by protein kinase C (PKC) inhibitor (calphostin C). This result indicated that accumulated DG might increase PKC activity, which then stimulates DG kinase activation as a feedback mechanism. DG kinase inhibition also suppressed HG-PSS-induced enhancement of contraction. Increased myo-inositol incorporation was detected under high-glucose conditions, indicating an acceleration of phosphatidylinositol (PI)-turnover. Moreover, rho kinase inhibitor (Y27632) suppressed U46619-induced contraction exclusively in normal-PSS. These findings indicated that HG-PSS treatment increases DG synthesis derived from incorporated glucose, PKC and DG kinase activation, and enhances the U46619-induced contraction via acceleration of PI-turnover. This series of responses may be involved in the dysfunction of aorta under high-glucose conditions occurring in association with diabetes.

Enhancement effect under high-glucose conditions on U46619-induced spontaneous phasic contraction in mouse portal vein

The effect of the thromboxane A(2) analog 9,11-dideoxy-11alpha, 9alpha-epoxymethanoprostaglandin F(2alpha) (U46619) on spontaneous phasic contractions in the mouse portal vein was studied. U46619 induced concentration-dependent (1-100 nM) increases in amplitude, frequency, and contractile period (ON-time) of the contraction. Both amplitude and ON-time were enhanced significantly under high-glucose (HG; 4-fold greater than normal) conditions. This hyperactivation may be associated with portal vein dysfunction in diabetes. However, the mechanisms remain unclear. HG enhanced the U46619-induced accumulation of endogenous diacylglycerol (DG). Phospholipase C inhibition suppressed accumulation under normal conditions; however, this suppression was not observed under HG conditions. The HG-induced enhancement of U46619-induced contraction was inhibited by protein kinase C (PKC) inhibition. This finding indicated that accumulated DG might increase PKC activity. Activated PKC stimulated DG kinase activation as a feedback mechanism. DG kinase inhibition also suppressed the HG-induced enhancement of contraction. Increased myo-inositol incorporation was detected under HG conditions, indicating an acceleration of phosphatidylinositol (PI) turnover. This acceleration was inhibited by PKC and DG kinase inhibitors. These findings indicated that HG treatments increased DG synthesis derived from incorporated glucose, PKC, and DG kinase activation. These responses induce hyperactivation of the amplitude and contractile period of contraction mediated by acceleration of PI turnover. This series of responses may be involved in the dysfunction of the portal vein under the HG conditions occurring with diabetes.

Endothelium-dependent production and liberation of kynurenic acid by rat aortic rings exposed to L-kynurenine

Rat aortic slices produced and liberated the endogenous antagonist of glutamate receptors, kynurenic acid, upon exposure to L-kynurenine. Endothelium-denuded slices did not synthesize any measurable amount of kynurenic acid, indicating its endothelial origin. Aortic kynurenic acid production was diminished by modification of the ionic milieu, hypoxia and hypoglycemia, as well as by L-glutamate and L-aspartate, endogenous glutamate receptor agonists, and aminooxyacetic acid, a non-selective inhibitor of aminotransferases and mitochondrial respiration. These data pave the way for future research aimed to clarify the role of kynurenic acid in the physiology and pathology of the endothelium and vasculature.

Evidence for a spontaneous nitric oxide release from the rat median eminence: influence on gonadotropin-releasing hormone release

The involvement of nitric oxide (NO) as a gaseous neurotransmitter in the hypothalamic control of pituitary LH secretion has been demonstrated. NO, as a diffusible signaling gas, has the ability to control and synchronize the activity of the neighboring cells. NO is secreted at the median eminence (ME), the common termination field for the antehypophysiotropic neurons, under the stimulation of other signaling substances. At the ME, NO stimulates GnRH release from neuroendocrine terminals. The present studies were undertaken to determine whether NO is secreted spontaneously from ME fragments ex vivo and whether its secretion is correlated to GnRH release. To accomplish this, female rats were killed at different time points of the day and/or of the estrous cycle. The spontaneous NO release was monitored in real time, with an amperometric probe, during 4 periods of 30 min, from individual ME fragments (for each time point, n = 4). GnRH levels were measured in parallel for each incubation-period by RIA. The results revealed that NO was released in a pulsatile manner from female ME fragments and, unambiguously, that the amplitude of NO secretion varied markedly across the estrous cycle. Indeed, though the NO pulse period (32 +/- 1 min, n = 36) and duration (21 +/- 2 min, n = 36) did not vary significantly across the estrous cycle, the amplitude of this secretion pulse was significantly higher on proestrus (Pro; 39 +/- 3 nM, n = 20), compared with diestrus (16 +/- 1 nM, n = 8) or estrus (23 +/- 3 nM, n = 8, P < 0.05). The GnRH levels in the incubation medium were positively correlated to NO secretion across the estrous cycle (r = 0.86, P < 0.003, n = 9), confirming that NO and GnRH release are coupled. Furthermore, 5 x 10(-7) M L-N(5)-(1-iminoethyl)ornithine (L-NIO), a NO synthase inhibitor, succeeded in inhibiting the strong NO-GnRH secretory coupling and GnRH release on PRO: Because at this concentration, L-NIO selectively inhibits endothelial NO synthase, the results further demonstrate that the major source of NO involved in GnRH release at the ME is endothelial in origin. Additionally, the induction of a massive NO/GnRH release in 15-day ovariectomized rat treated with estradiol benzoate strongly suggested that estradiol is participating in the stimulation of NO release activity between diestrus II and PRO: The present study is the first demonstrating that ME can spontaneously release NO and that NO's rhythm of secretion varies markedly across the estrous cycle. This pulsatile/cyclic ME NO release may constitute the synchronizing link to anatomically scattered GnRH neurons.