Functional role of the Na+/H+ exchanger in the regulation of cerebral arteriolar tone in rats

Mechanism of alkalosis-induced constriction of rat cerebral penetrating arterioles

Cerebral arterioles are in close contact with the supplied tissue and are strong regulators of cerebrovascular tone. Transient ischemia can cause brain intracellular alkalosis producing vasoconstriction. However, the mechanisms of alkalosis-induced cerebral arteriolar constriction are poorly understood. Here, we determined the vascular responses to alkalosis under different conditions by monitoring the internal diameter of pressurized penetrating arterioles isolated from the rat cerebrum with an operating microscope. The roles of Na+/H+ exchanger (NHE), Na+/Ca²+ exchanger (NCX), Na+/K+-adenosine triphosphatase (NKA), and potassium (K+) channels during alkalosis were examined using specific inhibitors. Our results indicated that the extent of constriction of the penetrating arterioles was dependent on alkaline pH. Moreover, the alkalosis-induced vasoconstriction was significantly attenuated by inhibitors of NHE, NCX, and NKA, but not K+ channel inhibitors. Therefore, we concluded that NHE, NKA, and NCX are important regulators involved in alkalosis-induced vasoconstriction of rat cerebral penetrating arterioles.

Hyperammonemia acts synergistically with lipopolysaccharide in inducing changes in cerebral hemodynamics in rats anaesthetised with pentobarbital

BACKGROUND/AIMS

The aim was to determine the effect of ammonia (NH(3)) and lipopolysaccharide (LPS) alone or in combination, on cerebral blood flow (CBF) and intracranial pressure (ICP) in the rat. Since amiloride-sensitive-ion-pathways in the blood-brain barrier (BBB) modulate CBF, we also aimed to test if Na(+)/H(+)-inhibitors could prevent this possible synergism between NH(3) and LPS.

METHODS

In experiment A, four groups of rats received ammonium acetate (140 micromol/kg/min) or saline, each of them associated with either vehicle or LPS (2 mg/kg). In experiments B and C, rats received similar treatments after having received amiloride (30 mg/kg) or 5-(N-methyl-N-isobutyl)-amiloride (MIA, 5 mg/kg). Plasma tumor-necrosis-factor-alpha (TNF-alpha), ICP (via a cisterna magna catheter) and CBF (by laser-Doppler flowmetry) were measured.

RESULTS

An increase in ICP and CBF within 60 min was observed only in rats that received NH(3) together with LPS as compared to any other group (P<0.01), which could be prevented by amiloride (P<0.05), but not by MIA. Both amiloride and MIA decreased the plasma TNF-alpha concentration.

CONCLUSIONS

In rats anaesthetised with pentobarbital NH(3) infusion aggravates a LPS induced rise in ICP and induces an increase in CBF less clearly seen with LPS alone. This effect is prevented by the non-specific Na(+)/H(+) inhibitor amiloride, but not by MIA, a specific inhibitor of Na(+)/H(+) exchanger. Thus, the synergistic effect of NH(3) and LPS seems mediated by other amiloride-sensitive-ion-pathways in the BBB than the Na(+)/H(+) exchanger.

SEA0400: A Novel Sodium-Calcium Exchange Inhibitor with Cardioprotective Properties1

The cardiac sodium-calcium exchanger (NCX) plays an important role in calcium homeostasis. It is the primary mechanism for removing calcium ions that enter myocytes through L-type calcium channels on a beat-to-beat basis. Its direction of transport is determined by the membrane potential and the ionic concentrations of Na+ and Ca2+, with the forward (or Ca2+-efflux) mode of transport being the dominant mode under physiological conditions. In contrast, the Ca2+-influx mode (or reverse mode) of NCX becomes important in certain pathophysiological conditions, such as myocardial ischemia and reperfusion. Recent discovery of compounds that inhibit the Ca2+-influx mode (or reverse mode) of NCX has generated intense research interest in the pharmacology of NCX. Among the newer NCX inhibitors described to date, 2-[4-[(2,5-difluorophenyl)methoxy]-phenoxy]-5-ethoxyaniline (SEA0400) appears particularly promising in attenuating cardiac, renal, and cerebral ischemia/reperfusion injuries in various experimental models. Moreover, the mixed results that have emerged from clinical trials evaluating the efficacy and safety of inhibitors of the sodium-hydrogen exchanger (an upstream target in relation to the sodium-calcium exchanger) in myocardial protection stimulated interest in evaluating NCX as an alternative therapeutic target. This article reviews the pharmacological profile of SEA0400, as presented in the published literature, and discusses the therapeutic potential of this compound in attenuating myocardial ischemia/reperfusion injury.

Astrocytic and neuronal localization of the scaffold protein Na+/H+ exchanger regulatory factor 2 (NHERF-2) in mouse brain

The Na+/H+ exchanger regulatory factor 2 (NHERF-2) is a scaffold protein that regulates cellular signaling by forming protein complexes. Several proteins known to interact with NHERF-2 are abundantly expressed in the central nervous system, but little is known about NHERF-2 localization in the brain. By using immunohistochemistry combined with light and electron microscopy, we found that many populations of astrocytes, as well as some populations of neurons, were immunopositive for NHERF-2 throughout the mouse brain. Quantitative analysis of the subcellular distribution of NHERF-2 immunostaining in four brain structures, cerebral cortex, hippocampus, striatum, and cerebellar cortex, showed that NHERF-2 was expressed mainly in astrocytic processes but was also sometimes observed in both pre- and postsynaptic neuronal elements. NHERF-2 immunostaining was associated mainly with the plasma membrane of neurons and astrocytes. However, NHERF-2 immunoreactivity was also observed in association with synaptic vesicles in putative glutamatergic axon terminals. The subcellular localization of NHERF-2 in brain is consistent with a role for NHERF-2 in forming complexes between cell surface and cytosolic proteins, and the preferential expression of NHERF-2 in astrocytes suggests that this scaffold protein may play an important role in astrocytic physiology.

Diabetes induces Na/H exchange activity and hypertrophy of rat mesenteric but not basilar arteries

Experimental hyperglycemia produces a marked hypertrophic response in rat mesenteric arteries, accompanied by activation of Na/H exchange (NHE) in medial smooth muscle. This study asked if other vascular beds are similarly affected by examining the hypertrophic and NHE response of the basilar artery. Sections of mesenteric and basilar arteries from adult rats were analysed by standard morphometric techniques at 1 and 3 weeks after streptozotocin injection. NHE activity was assessed as changes in intracellular pH in isolated intact vessels using concurrent myography and fluorescence spectroscopy. Mesenteric arteries showed a significant increase in lumenal (47%), medial (51%) and adventitial (17%) area. In contrast, these parameters were not increased in basilar arteries from the same set of animals. Maximal NHE activity was significantly increased at 1 week (24%) and 3 weeks (20%) in mesenteric arteries, but in basilar arteries there was no change in basal intracellular pH, maximal NHE activity or kinetic properties of the transporter. NHE plays a central role in vascular changes in diabetes. As the mesenteric hypertrophy is amenable to therapeutic intervention these findings add further to the potential of NHE as a therapeutic target for ameliorating vascular disease in diabetes.