Eörs Dóra

Contractile and endothelium-dependent dilatory responses of cerebral arteries at various extracellular magnesium concentrations

To clarify the effect of extracellular magnesium (Mg2+) on the vascular reactivity of feline isolated middle cerebral arteries, the effects of slight alterations in the Mg2+ concentration on the contractile and endothelium-dependent dilatory responses were investigated in vitro. The contractions, induced by 10−8-10−5 M norepinephrine, were significantly potentiated at low Mg2+ (0.8 mM v. the normal, 1.2 mM). High (1.6 and 2.0 mM) Mg2+ exhibited an inhibitory effect on the contractile responses. No significant changes, however, in the EC50 values for norepinephrine were found. The endothelium-dependent relaxations induced by 108–10−5 M acetylcholine were inhibited by high (1.6 and 2.0 mM) Mg2+. Lowering of the Mg2+ concentration to 0.8 mM or total withdrawal of this ion from the medium failed to alter the dilatory potency of acetylcholine. The changes in the dilatory responses also shifted the EC50 values for acetylcholine to the right. The present results show that the contractile responses of the cerebral arteries are extremely susceptible to the changes of Mg2+ concentrations. In response to contractile and endothelium-dependent dilatory agonists, Mg2+ probably affects both the calcium influx into the endothelial and smooth muscle cells as well as the binding of acetylcholine to its endothelial receptor. Since Mg2+ deficiency might facilitate the contractile but not the endothelium-dependent relaxant responses, the present study supports a role for Mg2+ deficiency in the development of the cerebral vasospasm.

Effect of adenosine and its stabile analogue 2-chloroadenosine on cerebrocortical microcirculation and NAD/NADH redox state

In the present study, the effects of topically applied adenosine (ADO) and its stabile analogue 2-chloroadenosine (CADO) on cerebrocortical microcirculation and NAD/NADH redox state (oxidized/reduced nicotinamide adenine dinucleotide) were investigated. Vascular volume (CVV), mean transit time of blood flow (tm), blood flow (CBF), and NADH fluorescence of the cat brain cortex were measured through a cranial window with a microscope fluororeflectometer. The reference values of CVV,tm, and CBF, measured in the artificial cerebrospinal fluid (mock CSF) which superfused brain cortex, were regarded as 100%. Adenosine and 2-chloroadenosine, in the concentration range of 10−6–10−3 M, resulted in concentration-dependent increases in CBF and NAD reduction. 10−5 M adenosine and 2-chloroadenosine increased CBF by 49.6±5.6% and 80.4±10.3%, respectively. At a pharmacologically high concentration (10−3 M), ADO increased CBF by 164.6±13.5%, CADO by 333±44%. At the same time, 10−3 M ADO and CADO shifted the cortical NAD/NADH redox state by 7.9±0.4% and 12.4±0.7%, respectively toward a more reduced state. Our results, concerning the vasodilator potency of adenosine and 2-chloroadenosine, accord with available data in the literature. However, the pronounced NAD reduction obtained with these adenosine nucleosides suggests that, besides an action on vascular adenosine receptors, some other changes, such as increased substrate mobilization and possibly cAMP production, may contribute to the vasodilator effect of adenosine and 2-chloroadenosine.

Endothelium-dependent influence of small changes in extracellular magnesium concentration on the tone of feline middle cerebral arteries.

The aim of this study was to investigate the effect of small alterations in the extracellular magnesium concentration on the tone of feline middle cerebral arteries and to examine the role of the endothelium in these responses. We measured the isometric tension of isolated arterial rings placed between two stainless steel wires in a tissue chamber containing Krebs-Henseleit solution aerated with a gas mixture containing 95% O2 and 5% CO2 at 37 degrees C. After precontraction with noradrenaline, a decrease of the extracellular magnesium concentration from 1.2 mM to 1.0 and 0.8 mM resulted in sustained relaxations, whereas elevation of the extracellular magnesium concentration from 0.8 mM to 1.2 mM caused an increase in vascular tone when the endothelium was intact. The magnesium deficiency-related dilations were absent in endothelium-denuded vessels and were inhibited by 5 x 10(-6) M oxyhemoglobin and 10(-5) M methylene blue, suggesting the involvement of an endothelium-derived relaxing factor in this vascular response. However, 5 x 10(-7) M nifedipine or 3 x 10(-5) M dichlorobenzamil did not affect the magnesium deficiency-related relaxations. Therefore, nifedipine-sensitive calcium channels or the sodium/calcium antiport system are not involved in this vascular action of magnesium. We conclude that small alterations in the extracellular magnesium concentration, possibly within the physiological range, are able to modify the basal formation and release of endothelium-derived relaxing factor and thus alter arterial smooth muscle tone in this vascular bed.(ABSTRACT TRUNCATED AT 250 WORDS)

Contribution of Adenosine to the Regulation of Cerebral Blood Flow: The Role of Calcium Ions in the Adenosine-Induced Cerebrocortical Vasodilatation

It is well established that cerebral blood flow (CBF) and adenosine concentration in the brain can be elevated severalfold during arterial hypoxia and epileptic seizures (Winn et al., 1981). Since perivascularly administered adenosine dilatates pial arteries (Wahl and Kuschinsky, 1976) and the stable analog of adenosine, chloroadenosine, is an even more efficient dilatator of cerebrocortical vessels (Winn et al., 1981), it was suggested that adenosine plays a central role in the regulation of CBF (Winn et al., 1981). However, since the adenosine concentration of the normal brain is very low (Winn et al., 1981: around 10−7 mol l−1), this assumption is valid only if adenosine is present exclusively in the extracellular fluid. Besides this, it is also not yet clear whether the vasodilatator action of adenosine involves changes in calcium availability of the vascular smooth muscle (Dutta et al., 1980; Fenton et al., 1982), or adenosine dilates the vessels by some other mechanism (Kukovetz et al., 1978). In the present study the following questions were addressed: a) How efficient is topically administered adenosine when compared with arterial hypoxia and epileptic seizures in altering cerebrocortical vascular volume (CVV) ? b) Is there a possible role of adenosine-induced cortical metabolic changes in the vasodilatatory mechanism? c) What is the importance of calcium availability in the vascular action of adenosine? To answer these questions the brain cortex was superfused with various concentrations of adenosine.

Correlation between the Redox State, Electrical Activity and Blood Flow in Cat Brain CORTEX During Hemorrhagic Shock

Owing to its well developed autoregulatory mechanisms, total cerebral blood flow is stable over a wide range of mean arterial blood pressures and decreases significantly only when the perfusion pressure is below 50 mmHg.

Relationship Between Steady Redox State and Brain Activation-Induced NAD/NADH Redox Responses

It is well recognized that activation of the brain leads to an overwhelming increase in cerebral blood flow (CBF), oxygen, and glucose consumption (Siesjo, 1978; Sokoloff, 1981). Because ATP usage is augmented, the ratio of ATP/ADP decreases, and according to the in vitro data of Chance and Williams (1955), the rate of mitochondrial electron transport and ADP phosphorylation will be accelerated. Consequently, mitochondrial NADH should be oxidized (Chance and Williams, 1955). The increased need of mitochondrial electron transport for reducing equivalents is matched by the increased production of pyruvate via stimulation of glycogenolysis and glycolysis (Siesjo, 1978; Sokoloff, 1981). Though it is unlikely that brain suffers from hypoxia under augmented electrical activity (Siesjo, 1978), a considerable amount of pyruvate is converted into lactate, and NADH accumulates in the cytosol (Howse and Duffy, 1975; Siesjo, 1978). This NADH reduction is explained as being due to the restriced capability of the so-called “H-shuttle” mechanisms to transfer H+ from cytosolic NADH to mitochondrial NAD (Howse and Duffy, 1975; Siesjo, 1978). Interestingly, when the mitochondrial NAD/NADH ratio has been determined with the oxidized-reduced substrate ratio method during epileptic seizures, discernible NADH oxidation was not obtained (Siesjo, 1978).

Effect of “flow anoxia” and “non flow anoxia” on the NAD/NADH redox state of the intact brain cortex of the cat

In the present study, we compared the nicotin-amide adenine dinucleotide (NAD) reducing potencies of “flow anoxia” and “non flow anoxia” in the cat brain cortex. In animals anaesthetized with alpha-D-glucochloralose “flow anoxia” and “non flow anoxia” were produced by ventilating for 2 and 25 min, respectively, with nitrogen gas. Following “non flow anoxia”, the brain cortices of dead animals were superfused with oxygen saturated artificial cerebrospinal fluid (mock CSF), and subsequently with CSF containing various concentrations (10−3–10−1 M) of potassium cyanide. NADH (reduced NAD) fluorescence of the brain cortex was measured through a cranial window with microscope fluororeflectometer. Ventilating the animals for 2 and 25 min with nitrogen gas increased cortical NADH fluorescence (NAD reduction) by 43.5±2.8% and 135.3±6.1%, respectively. Oxygen saturated CSF superfusion of the ischemic brain cortex restored the cortical NAD/NADH redox state to the preanocic level (oxidation of NADH). 10−1 M cyanide, applied after superfusion of the brain cortex with oxygen saturated CSF resulted in comparable NAD reduction to that produced by “non flow anoxia”. On the basis of these findings it is suggested that “non flow anoxia” leads to much greater cortical NAD reduction than “flow anoxia”, because oxygen tension in the cortex may not fall to zero mm Hg during nitrogen anoxia lasting for 2 min. Besides this, a more pronounced substrate mobilization and acidosis may also contribute to the greater NAD reducing potency of “non flow anoxia”. Finally, since 10–15 min after the death of the animal the cerebral carbohydrate reserves are completely exhausted, and in our experiments “non flow anoxia”, reoxygenation of the ischemic brain cortex and inhibition of the cortical mitochondrial electron transport by cyanide (10−1 M) resulted in comparable redox state changes (as far as their magnitude is concerned), it is concluded that the recorded changes in NADH fluorescence were of mitochondrial origin.

Transient Metabolic and Vascular Volume Changes Following Rapid Blood Pressure Alterations Which Precede the Autoregulatory Vasodilation of Cerebrocortical Vessels

It has been shown by surface fluoro-reflectometry that stepwise decrease of arterial blood pressure causes a biphasic cerebrocortical vascular volume response. After the arterial blood pressure decrease the vascular volume first decreased and later increased. In both parts of the biphasic reflectance change, the cerebrocortical NAD-NADH redox state shifted considerably towards reduction and there was no reoxidation after the onset of cortical vasodilatation. Since a very rapid NADH reduction occurred during the first 30 secs. of the arterial hypotension in parallel with the vascular volume decrease, it is suggested that in the transient phase of arterial hypotension cerebral hypoxia may occur. Furthermore it is suggested that anaerobic tissue metabolites or some unknown NAD-NADH dependent process might dilate the cerebrocortical arterial network during the autoregulatory adjustment of CBF. The participation of the sympathetico-adrenal system in transient brain hypoxia caused by bleeding is a possibility since both the early vasoconstriction and the steep NADH reduction were prevented by the administration of phenoxybenzamine (1 mg/kg) before bleeding.

Regional differences in the regulation of contraction-relaxation machinery of vascular smooth muscle.

In the present study we revealed substantial differences in the regulation of the contraction-relaxation machinery of the middle cerebral and mesenteric arteries. a. Although K+-Krebs solution resulted in similar contractile responses in both vascular beds, norepinephrine, serotonin and PGF2 alpha were more potent in inducing contraction in the mesenteric artery than in the middle cerebral artery. Contrary, extremely high concentrations of acetylcholine produced negligible contractions in the mesenteric artery as compared to the middle cerebral artery. b. In more than 50% of the cases a considerable spontaneous tone developed in the middle cerebral artery, which was not due to the activation of adrenergic alpha receptors or some arachidonic acid metabolite. A similar phenomenon never occurred in the mesenteric artery. c. Acetylcholine, adenosine triphosphate and adenosine brought about similar dilatory concentration-response curves in the middle cerebral artery, but greatly different ones in the mesenteric artery where acetylcholine was the most, adenosine triphosphate the least potent dilator.

Glycolysis and Regulation of Cerebral Blood Flow and Metabolism

In past decades several humoral agents and neural mechanisms were suggested for the coupling between cerebral blood flow (CBF), metabolism and functional activity in the brain (Siesjo, 1978). To explore the role of glycolysis in the regulation of CBF, and also to assess the importance of glucose as a substrate in the energy homeostasis of the brain, glycolysis was inhibited topically in the brain cortex by iodoacetate (IAA).