Effects of adenosine and 2-chloroadenosine on cerebral collateral vessels

Glycoprotein Matrix Zinc Exhibits Improved Absorption: A Randomized Crossover Trial

Biotransformation of minerals via glycosylation by microorganisms such as yeast and/or probiotics yields nutrients bound to a food matrix, resulting in increased bioavailability. The purpose of this study was to compare the effects of glycoprotein matrix-bound zinc (GPM) on absorption compared to inorganic zinc oxide. Sixteen participants ingested 11 mg of zinc as either GPM™ Soy-Free Zinc (GPM, Ashland, Kearny, NJ, USA) or zinc oxide (USP). Blood samples were taken at 0 (i.e., baseline), 30, 60, 90, 120, 180, 240, 300, 360, 420, and 480 min post-ingestion. GPM zinc concentrations were significantly higher at 120 min (p = 0.02; 12.4 ± 5.1 mcg/dL), 180 min (p = 0.002; 16.8 ± 5.1 mcg/dL), and 240 min (p = 0.007; 14.6 ± 5.1 mcg/dL) in comparison to USP zinc oxide. In addition, GPM zinc significantly increased iAUC by 40% (5840 ± 2684 vs. 4183 ± 1132 mcg/dL * 480 min, p = 0.02), and Cmax values were 10% higher in GPM compared to USP (148 ± 21 mcg/dL vs. 135 ± 17.5 mcg/dL, p = 0.08). Tmax was 12% slower in GPM compared to USP (112.5 ± 38.7 min vs. 127.5 ± 43.1 min); however, differences in Tmax failed to reach statistical significance (p = 0.28). Zinc bound to a glycoprotein matrix significantly increased absorption compared to zinc oxide.

Caffeine withdrawal, acute effects, tolerance, and absence of net beneficial effects of chronic administration: cerebral blood flow velocity, quantitative EEG, and subjective effects

RATIONALE

Although the subjective effects of caffeine abstinence, acute and chronic administration, and tolerance are well described, the corresponding neurophysiological effects are not.

OBJECTIVES

Caffeine withdrawal, acute caffeine effects, caffeine tolerance, and net beneficial effects of chronic caffeine administration were investigated using cerebral blood flow velocity, quantitative electroencephalography (EEG), and subjective effects.

MATERIALS AND METHODS

Sixteen regular caffeine users participated in this double-blind, within-subject study during which they received acute caffeine and placebo challenges (1) while maintained on 400 mg caffeine daily for > or =14 days and (2) while maintained on placebo for > or =14 days. Blood flow velocity was determined for the middle (MCA) and anterior (ACA) cerebral arteries using pulsed transcranial Doppler sonography. EEG was recorded from 16 scalp sites. Subjective effects were assessed with questionnaires.

RESULTS

Acute caffeine abstinence (evaluated 24 h after placebo substitution) increased mean, systolic, and diastolic velocity in the MCA and ACA and decreased pulsatility index in the MCA. Acute caffeine abstinence increased EEG theta and decreased beta 2 power. Acute caffeine abstinence also increased measures of Tired, Fatigue, Sluggish, and Weary and decreased ratings of Energetic, Friendly, Lively, and Vigor. Acute caffeine effects were demonstrated across a wide range of measures, including cerebral blood flow, EEG, and subjective effects. Tolerance and "complete" tolerance were observed on subjective but not physiological measures. Chronic caffeine effects were demonstrated only on the measure of EEG beta 2 power.

CONCLUSION

Acute caffeine abstinence and administration produced changes in cerebral blood flow velocity, EEG, and subjective effects. Tolerance to subjective but not physiological measures was demonstrated. There was almost no evidence for net effects of chronic caffeine administration on these measures. Overall, these findings provide the most rigorous demonstration to date of physiological effects of caffeine withdrawal.

Intracerebral adenosine infusion improves neurological outcome after transient focal ischemia in rats

Second Institute of New Drug Research, Otsuka Pharmaceutical Co., Ltd., Tokushima, Japan In order to elucidate the role of adenosine in brain ischemia, the possible protective effects of adenosine on ischemic brain injury were investigated in a rat model of brain ischemia both in vitro and in vivo. Exogenous adenosine dose-dependently rescued cortical neuronal cells from injury after glucose deprivation in vitro. Adenosine (1 mM) also significantly reduced hypoglycemia/hypoxia-induced glutamate release from the hippocampal slice. In a rat model of transient middle cerebral artery occlusion (MCAO), extracellular adenosine concentration was increased immediately after occlusion, and then returned to the baseline by 30 min after reperfusion. Adenosine infusion through a microdialysis probe into the ipsilateral striatum (1 mM adenosine, 2 microl min(-1), total 4.5 h from the occlusion to 3 h after reperfusion) showed a significant improvement in the neurological outcome, and about 25% reduction of infarct volume, although the effect did not reach statistical significance, compared with the vehicle-treated group at 20 h after 90 min of MCAO. These results demonstrated the neuroprotective effect of adenosine against ischemic brain injury both in vitro and in vivo, suggesting the possible therapeutic application of adenosine regulating agents, which inhibit adenosine uptake or metabolism to enhance or maintain extracellular endogenous adenosine levels, for stroke treatment.

The feasibility of intracarotid adenosine for the manipulation of human cerebrovascular resistance

To assess the feasibility of manipulating human cerebrovascular resistance with adenosine, we measured cerebral blood flow (CBF) by determining the initial slope (IS) of tracer washout 20-80 s after intracarotid 133Xe injection (standard IS) during sequential 3-min intracarotid infusions of (a) saline; (b) adenosine 1.2-mg bolus followed by an infusion of 1 mg/min (bolus + infusion); (c) saline; and (d) nicardipine (0.1 mg/min). During 133Xe washout, adenosine caused a rapidly clearing compartment. Therefore, tracer washout was also analyzed 5-25 s after injection (early IS). Nicardipine (n = 8) increased both standard IS (from 39+/-12 to 53+/-16 mL 100g.min(-1); P < 0.005) and early IS (from 40+/-9 to 55+/-20 arbitrary units; P < 0.02) to a similar degree. Adenosine bolus + infusion increased early IS (from 33+/-6 to 82+/-43 arbitrary units; P < 0.02) but did not increase standard IS (from 41+/-12 to 43 +/-16 mL 100g(-1) min(-1)). Standard and early IS values were then determined before and after adenosine delivered either by infusion alone (2 mg/min for 3 min, n = 5) or bolus alone (2 mg in 1 s, n = 3). Neither standard nor early IS changed after adenosine infusion alone. Early IS increased after adenosine bolus alone. Increase in early IS, but not standard IS, suggests a transient (<30 s) increase in CBF.

IMPLICATIONS

Intracarotid adenosine, in the 1- to 2-mg dose range, may cause a transient, but not a sustained, increase in cerebral blood flow. Intracarotid adenosine in such a dose range does not seem to be an appropriate drug for sustained manipulation of cerebrovascular resistance.

Delayed treatment with an adenosine kinase inhibitor, GP683, attenuates infarct size in rats with temporary middle cerebral artery occlusion

BACKGROUND AND PURPOSE

Brain ischemia is associated with a marked increase in extracellular adenosine levels. This results in activation of cell surface adenosine receptors and some degree of neuroprotection. Adenosine kinase is a key enzyme controlling adenosine metabolism. Inhibition of this enzyme enhances the levels of endogenous brain adenosine already elevated as a result of the ischemic episode. We studied a novel adenosine kinase inhibitor (AKI), GP683, in a rat focal ischemia model.

METHODS

Four groups of 10 adult Sprague-Dawley rats were exposed to 90 minutes of temporary middle cerebral artery (MCA) occlusion. Animals were injected intraperitoneally with vehicle, 0.5 mg/kg, 1.0 mg/kg, or 2.0 mg/kg of GP683 30, 150, and 270 minutes after the induction of ischemia by a researcher blinded to treatment group. The animals were euthanatized 24 hours after MCA occlusion, and brains were stained with 2,3,5-triphenyltetrazolium chloride. We measured brain temperatures in a separate group of 6 rats before and after administration of 1.0 mg/kg GP683.

RESULTS

All treated groups showed a reduction in infarct volumes, but a significant effect was observed only in the 1.0 mg/kg-dose group (44% reduction, P=0.0077). Body weight, physiological parameters, neurological scores, and mortality did not differ among the 4 groups. No apparent behavioral side effects were observed. Brain temperatures did not change after drug injection.

CONCLUSIONS

Our results indicate that the use of AKIs offers therapeutic potential and may represent a novel approach to the treatment of acute brain ischemia. The therapeutic effect observed was not caused by a decrease in brain temperature.

Effects of etomidate administration on cerebral collateral flow

OBJECTIVE

Augmentation of blood flow to collateral-dependent tissue (CDT) as a result of selective vasodilation of collateral vessels has been shown to occur with various stimuli after middle cerebral artery occlusion. Etomidate, a carboxylated imidazole derivative, is a nonbarbiturate anesthetic that is used clinically both as an anesthetic and as a neuroprotective agent. The effect etomidate has on collateral cerebral vessels is unknown. The purpose of our studies was to test whether etomidate selectively augmented cerebral blood flow (CBF) to CDT during ischemia as an additional mechanism of neuroprotection.

METHODS

A left craniotomy was performed in each of 14 dogs, with the animals under halothane anesthesia. A branch of the middle cerebral artery was occluded and cannulated distally for determination of CDT using a "shadow flow" technique. CBF and vascular pressures were measured and used to calculate vascular resistance. An etomidate infusion (0.1 mg/kg of body weight/min administered intravenously) was started, and CBF and vascular pressures were measured at 10 and 40 minutes. Hypotension was then induced, and CBF and pressures were again measured.

RESULTS

CBF was significantly reduced in all regions of the brain, including CDT, when etomidate was infused. CDT showed a 53.7% reduction in flow, whereas normal CBF was reduced by at least 63.4%. During hypotension, blood flow to CDT was reduced by an additional 42.7%, whereas normal cerebrum was reduced by at least 22.7%. Vascular resistance was increased in all vessels during etomidate infusion.

CONCLUSION

The neuroprotective effects of etomidate do not seem to be through the augmentation of collateral or global CBF.

Effect of N-methyl-D-aspartate and inhibition of neuronal nitric oxide on collateral cerebral blood flow after middle cerebral artery occlusion

OBJECTIVE

To study the role of N-methyl-D-aspartate (NMDA) receptor activation and selective inhibition of neuronal nitric oxide synthase with 7-nitroindazole (7-NI) on blood flow to collateral-dependent tissue (CDT) after middle cerebral artery (MCA) occlusion.

METHODS

A left craniotomy was performed in each of 11 dogs with the animals under halothane anesthesia. A branch of the MCA was occluded and cannulated distally for determination of CDT, using a "shadow flow" technique. Cerebral blood flow (CBF) and vascular pressures were measured and used to calculate vascular resistance.

TECHNIQUE

Our shadow flow model has the ability to identify an area of CDT, with minimal contamination from overlap flow within a morphologically identified "risk area" for MCA branch occlusion.

RESULTS

NMDA increased blood flow to CDT by 56.2%, while normal ipsilateral and contralateral cerebrum increased by at least 35% from baseline. 7-NI caused a significant drop in regional CBF, with the greatest drop of 41.7% occurring in the CDT. Normal ipsilateral and contralateral CBF was reduced by 31.7 and 23.9%, respectively. The dilator response to NMDA was significantly attenuated after 7-NI administration, except in CDT where flow increased ("inverse steal"). Cerebral vascular resistance decreased in response to NMDA and increased with 7-NI.

CONCLUSION

Neuronal nitric oxide production seems to play an important role in regulating vascular tone and CBF to CDT after MCA occlusion. Selective preservation of blood flow to CDT seems to be mediated by NMDA receptor activation but independent of neuronal nitric oxide production.

Pre- and postischemic effects of the NMDA receptor antagonist dizocilpine maleate (MK-801) on collateral cerebral blood flow

The authors studied the effects of pre- and postischemic administration of dizocilpine maleate (MK-801) on collateral and regional cerebral blood flow (CBF). The ischemic penumbra appears to benefit most from the neuroprotective effects of MK-801. The precise mechanism by which MK-801 provides this neuroprotection remains controversial. Alterations in CBF have been demonstrated with MK-801 administration, but whether the response is an increase or decrease in flow has remained unclear. A left-sided craniectomy was performed in 20 dogs. A branch of the middle cerebral artery (MCA) was cannulated and collateral blood supply-dependent tissue (CDT) was identified using the "shadow flow" technique. Regional CBF was measured using radiolabeled microspheres. Six dogs received MK-801 (1 mg/kg administered intravenously) before they underwent MCA branch occlusion; the remaining 14 dogs received MK-801 after they underwent MCA occlusion. Cerebral blood flow and vascular pressures were measured 30 and 60 minutes after MK-801 administration. In animals that received MK-801 before MCA occlusion, CBF did not change significantly from baseline values before or after occlusion. In contrast, in animals that received MK-801 after MCA occlusion, CBF was significantly reduced in all regions of the brain, including the CDT. Collateral blood supply-dependent tissue showed a 51.7% reduction in flow, whereas normal CBF was reduced by 29.7%. The MK-801 induced cerebral vasoconstriction in both groups. The neuroprotective effects of MK-801 do not appear to be caused by the augmentation of collateral or global cerebral circulation and, in fact, may block the glutamate-mediated vasodilation that occurs during ischemia.

Effects of serotonin on cerebral circulation after middle cerebral artery occlusion

Serotonin (5-HT) produces constriction of peripheral collateral blood vessels. Using an animal model, the authors tested the hypothesis that 5-HT constricts collateral vessels in the cerebrum. A branch of the middle cerebral artery (MCA) was occluded proximally and cannulated distally in anesthetized dogs. Blood flow to the area at risk for infarction was detected by perfusing the cannulated MCA branch with microsphere-free blood during systemic injection of radioactive microspheres (shadow flow technique). Blood flow to collateral-dependent and normal cerebrum was measured during intravenous infusion of 5-HT (10 and 40 mg/kg/minute). Serotonin produced a dose-related reduction of blood flow to collateral-dependent cerebrum, increased collateral vessel resistance in large cerebral arteries and collateral vessels, and decreased cerebral artery perfusion pressure. In contrast, blood flow to normal cerebrum was not altered because a decrease in small vessel resistance effectively compensated for a decrease in MCA perfusion pressure. These findings indicate that 5-HT produces constriction of collateral vessels in the cerebrum. This response is clearly different from normal small cerebral vessels, which dilate during 5-HT infusion.

Superselective intraarterial papaverine administration: effect on regional cerebral blood flow in patients with arteriovenous malformations

In this study the authors determined the effect of papaverine on regional cerebral blood flow (rCBF) in the angiographically normal arteriolar beds of patients with arteriovenous malformations (AVMs) who underwent transfemoral superselective angiography. Middle cerebral artery (MCA) branch vessels were catheterized during 10 procedures performed in nine patients. The mean (+/- standard deviation) largest AVM diameter was 4.4 +/- 1 cm. Regional CBF was measured by recording the washout of a bolus of xenon-133 injected through the microcatheter. In a dose-ranging study. rCBF and MCA pressure in two patients were repeatedly measured after 3-minute infusions of papaverine at 0.07, 0.7, and 7 mg/minute. In a single-dose study, an additional eight patients received only the highest dose of papaverine administered over a 3-minute period. In the dose-ranging study, CBF increased from baseline in a dose-dependent fashion. In the single-dose study, papaverine increased in rCBF 103%, from 48 +/- 11 to 95 +/- 23 ml/100 g/minute at an MCA pressure of 55 +/- 23 mm Hg. Increase in rCBF was linearly related (y = 2.2x - 17, r2 = 0.84; p = 0.001) to baseline MCA pressure (range 22-84 mm Hg). Papaverine increases rCBF in a direct proportion to baseline MCA pressure, even at low baseline pressures. Selective infusion of vasodilators should be investigated in acute cerebral hypotension to facilitate either primary or collateral recruitment of CBF by aiding spontaneous autoregulatory vasodilation. In addition, rCBF monitoring may be useful in determining the most effective intraarterial dose of papaverine while minimizing complications due to hyperemia.