Regional and species differences in vascular reactivity to extracellular potassium

Validation of Pressure Reactivity and Pulse Amplitude Indices against the Lower Limit of Autoregulation, Part I: Experimental Intracranial Hypertension

The purpose of this study was to provide validation of intracranial pressure (ICP) derived continuous indices of cerebrovascular reactivity against the lower limit of autoregulation (LLA). Utilizing an intracranial hypertension model within white New Zealand rabbits, ICP, transcranial Doppler (TCD), laser Doppler flowmetry (LDF), and arterial blood pressure were recorded. Data were retrospectively analyzed in a cohort of 12 rabbits with adequate signals for interrogating the LLA. We derived continuous indices of cerebrovascular reactivity: PRx (correlation between ICP and mean arterial pressure [MAP]), PAx (correlation between pulse amplitude of ICP [AMP] and MAP), and Lx (correlation between LDF-based cerebral blood flow [CBF] and cerebral perfusion pressure [CPP]). LLA was derived via piecewise linear regression of CPP versus LDF or CPP versus systolic flow velocity (FVs) plots. We then produced error bar plots for PRx, PAx, and Lx against 2.5 mm Hg bins of CPP, to display the relationship between these indices and the LLA. We compared the CPP values at clinically relevant thresholds of PRx and PAx, to the CPP defined at the LLA. Receiver operating curve (ROC) analysis was performed for each index across the LLA using 2.5 mm Hg bins for CPP. The mean LLA was 51.5 ± 8.2 mm Hg. PRx and PAx error bar plots demonstrate that each index correlates with the LLA, becoming progressively more positive below the LLA. Similarly, CPP values at clinically relevant thresholds of PRx and PAx were not statistically different from the CPP derived at the LLA. Finally, ROC analysis indicated that PRx and PAx predicted the LAA, with areas under the curve (AUCs) of 0.795 (95% confidence interval [CI]: 0.731-0.857, p < 0.0001) and 0.703 (95% CI: 0.631-0.775, p < 0.0001), respectively. Both PRx and PAx generally agree with LLA within this experimental model of intracranial hypertension. Further analysis of clinically used indices of autoregulation across the LLA within pure arterial hypotension models is required.

Spreading Depression, Spreading Depolarizations, and the Cerebral Vasculature

Spreading depression (SD) is a transient wave of near-complete neuronal and glial depolarization associated with massive transmembrane ionic and water shifts. It is evolutionarily conserved in the central nervous systems of a wide variety of species from locust to human. The depolarization spreads slowly at a rate of only millimeters per minute by way of grey matter contiguity, irrespective of functional or vascular divisions, and lasts up to a minute in otherwise normal tissue. As such, SD is a radically different breed of electrophysiological activity compared with everyday neural activity, such as action potentials and synaptic transmission. Seventy years after its discovery by Leão, the mechanisms of SD and its profound metabolic and hemodynamic effects are still debated. What we did learn of consequence, however, is that SD plays a central role in the pathophysiology of a number of diseases including migraine, ischemic stroke, intracranial hemorrhage, and traumatic brain injury. An intriguing overlap among them is that they are all neurovascular disorders. Therefore, the interplay between neurons and vascular elements is critical for our understanding of the impact of this homeostatic breakdown in patients. The challenges of translating experimental data into human pathophysiology notwithstanding, this review provides a detailed account of bidirectional interactions between brain parenchyma and the cerebral vasculature during SD and puts this in the context of neurovascular diseases.

High-molecular-weight polyethylene glycol enhances hypothermic storage of feline kidney cells

Phosphate-buffered sucrose (PBSc) solution is effective for short-term hypothermic preservation of tissue during feline kidney transplantation. A high-molecular-weight polyethylene glycol (35,000 Da, PEG35) reportedly enhanced the protective effects against cold-induced tubular injuries in animal kidney transplantation models. We investigated the ability of PBSc solution containing PEG35 to preserve cultured feline kidney cells using in vitro WST-8 cell proliferation assays. PEG35 significantly improved cell viability during 24 hr of cold preservation. PBSc containing 20 g/l PEG35 achieved an effect almost equal to that of University of Wisconsin (UW) solution, the gold standard preservation solution used in human clinical kidney transplantation, for up to 24 hr of preservation. Our results suggest that PBSc containing PEG35 provides an excellent medium for graft cold storage during feline kidney transplantation.

Depressor action and vasorelaxation of methylene chloride fraction extracted from Rubus coreanum

Introduction

The present study was designed to examine whether methylene chloride (CH₂Cl₂) fraction extracted from Rubus coreanum affects the contractility of the isolated thoracic aortic strips and blood pressure of normotensive rats.

Methods

One of the common carotid arteries or of the femoral arteries was catheterized with a polyethylene tubing. The tubing was connected to a pressure transducer, and pulse of the mean arterial blood pressure was recorded on a biological polygraph continuously.

Results

The CH₂Cl₂ fraction (range, 200 to 800 μg/mL) significantly depressed both phenylephrine (PE, 10 μM)- and high K⁺ (56 mM)-induced contractile responses of the isolated thoracic aortic strips in a concentration-dependent fashion. In the simultaneous presence of N^ω-nitro-L-arginine methyl ester hydrochloride (L-NAME) (an inhibitor of nitric oxide [NO] synthase, 300 μM) and the CH₂Cl₂ fraction (400 μg/mL), both PE- and high K⁺-induced contractile responses were recovered to the significant level of the corresponding control response in comparison with inhibition of CH₂Cl₂ fraction treatment alone. Moreover, in the simultaneous presence of the CH₂Cl₂ fraction after pretreatment with 0.4% CHAPS (3-[(3-cholamidopropyl) dimethylammonio]-1-propane sulfonate), both PE- and high K⁺-induced contractile responses were recovered to the significant level of the corresponding control response compared to the inhibitory response of CH₂Cl₂ fraction treatment alone. Also, in anesthetized rats, the CH₂Cl₂ fraction (range, 0.3 to 3.0 mg/kg) injected into a femoral vein dose-dependently produced depressor responses. This hypotensive action of the CH₂Cl₂ fraction was greatly inhibited after treatment with phentolamine (1 mg/kg), chlorisondamine (1 mg/kg), L-NAME (3 mg/kg/30 min), or sodium nitroprusside (30 μg/kg/30 min). Intravenous infusion of the CH₂Cl₂ fraction (range, 1.0 to 10.0 mg/kg/30 min) markedly inhibited norepinephrine-induced pressor responses.

Discussion

Taken together, these results demonstrate that the CH₂Cl₂ fraction causes vascular relaxation in the isolated rat thoracic aortic strips as well as hypotensive action in anesthetized rats. These vasorelaxation and hypotension of the CH₂Cl₂ fraction seem to be mediated at least by the increased NO production through the activation of NO synthase of the vascular endothelium and the inhibitory adrenergic modulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s40885-014-0006-1) contains supplementary material, which is available to authorized users.

The effects of potassium on the rat middle cerebral artery

After traumatic brain injury, extracellular K(+) in brain can dramatically increase. We studied the effects of increased K(+) on the isolated pressurized rat middle cerebral artery (MCA). MCAs (200-250 microm OD) were isolated, cannulated with glass micropipettes, and pressurized. K(+) was increased in the extraluminal bath using three paradigms: (1) isotonic K(+) (K(iso)) where increases in K(+) were offset by decreases in Na(+), (2) hypertonic K(+) (K(hyper)) where K(+) was increased without a concomitant adjustment of Na(+), and (3) K(suc), a solution using K(iso) but with the addition of sucrose to obtain a hypertonic solution. Increases in K(+) in the extraluminal bath produced significant dilations (approximately 20%) at 21 mM K(+) in all three groups (K(iso), K(hyper), and K(suc)). With the K(hyper) and K(suc) groups, the magnitude of the dilation diminished with further increases in K(+). L-NAME (10(-5) M), an inhibitor of nitric oxide synthase, had no effect on the response of the K(hyper) and K(suc) groups at 21 mM but significantly enhanced constrictions of the MCAs above 40 mM K(+) compared to the control. The K(iso) group was not affected by L-NAME at any K(+) concentration and showed profound constrictions above 40 mM K(+). We conclude that changes in the K(+) concentration and osmolality of the extracellular fluid may have profound effects on the cerebral vasculature.

Adenovirus-mediated gene transfer to human cerebral arteries

Gene therapy is being investigated as a putative treatment option for cardiovascular diseases, including cerebral vasospasm. Because there is presently no information regarding gene transfer to human cerebral arteries, the principal objective of this study was to characterize adenovirus-mediated expression and function of recombinant endothelial nitric oxide synthase (eNOS) gene in human pial arteries. Pial arteries (outer diameter 500 to 1,000 microm) were isolated from 30 patients undergoing temporal lobectomy for intractable seizures and were studied using histologic staining, histochemistry, electron microscopy, and isometric force recording. Gene transfer experiments were performed ex vivo using adenoviral vectors encoding genes for bovine eNOS (AdCMVeNOS) and Escherichia coli beta-galactosidase (AdCMVLacZ). In transduced arteries, studied 24 hours after exposure to vectors, expression of recombinant beta-galactosidase and eNOS was detected by histochemistry, localizing mainly to the adventitia (n = 4). Immunoelectron microscopy localized recombinant eNOS in adventitial fibroblasts. During contractions to U46619, bradykinin-induced relaxations were significantly augmented in AdCMVeNOS-transduced rings compared with control and AdCMVLacZ-transduced rings (P < 0.01; n = 6). The NOS inhibitor L-nitroarginine methylester (L-NAME) caused significantly greater contraction in AdCMVeNOS-transduced rings (P < 0.001; n = 4) and inhibited bradykinin-induced relaxations in control and transduced rings (P < 0.001; n = 6). The current findings suggest that in AdCMVeNOS-transduced human pial arteries, expression of recombinant eNOS occurs mainly in adventitial fibroblasts where it augments relaxations to NO-dependent agonists such as bradykinin. Findings from the current study might be beneficial in future clinical applications of gene therapy for the treatment or prevention of cerebral vasospasm.

Does PGE1 attenuate potassium-induced vasoconstriction in initial pulmonary artery flush on lung preservation?

The standard clinical protocol for lung transplantation employs cold single pulmonary artery flush with Euro-Collins solution or the University of Wisconsin solution. Prostaglandin E1 (PGE1) is usually given by direct injection into the pulmonary artery to reduce pulmonary vasoconstriction caused by these intracellular, high-potassium solutions, however, the efficacy of PGE1 on lung preservation remains controversial. In this study we demonstrated that vasodilator effects of PGE1 were markedly reduced under a high-potassium condition, and that potassium-induced pulmonary vasoconstriction were inhibited by calcium channel blocker nifedipine. There are three therapeutic options in the cold single pulmonary artery flush for optimal lung transplantation, including the use of a higher dose of PGE1, use of the calcium channel blocker instead of PGE1, or the use of the extracellular, low-potassium solution such as low-potassium dextran solution for initial pulmonary artery flush before the lung harvest.

Impact of initial flush potassium concentration on the adequacy of lung preservation

The effects of initial lung flushing with intracellular and extracellular fluid type solutions were studied in lungs stored with the University of Wisconsin solution. Excised Sprague-Dawley rat lungs (n = 39) were flushed first with one of the following solutions: (1) the University of Wisconsin solution (K+ = 140 mmol/L), (2) modified (low potassium) University of Wisconsin solution (K+ = 20 mmol/L), (3) phosphate buffered saline solution (K+ = 3.9 mmol/L), (4) modified low-potassium phosphate-buffered saline solution (K+ = 20 mmol/L), (5) modified high-potassium phosphate-buffered saline solution (K+ = 40 mmol/L), and (6) Euro-Collins solution (K+ = 115 mmol/L) followed by secondary flush with storage solution and cold (4 degrees C) storage in University of Wisconsin solution for 24 hours. The lungs were then reperfused in the isolated, pulsatile, blood-perfused working lung system for 2 hours or until lung failure. Blood gas analysis and shunt fraction, aerodynamic parameters (airway resistance, lung compliance, elastic work, and flow resistive work), and total pulmonary vascular resistance were measured throughout the perfusion period. The mean oxygen tensions (in millimeters of mercury) at 30 minutes after the onset of reperfusion for University of Wisconsin solution, modified University of Wisconsin solution, phosphate-buffered saline solution, modified phosphate-buffered saline solutions (20 and 40 mmol/L), and Euro-Collins solution were 56.1 +/- 4.2, 72.7 +/- 9.1, 87.7 +/- 6.9 (p < 0.01 versus University of Wisconsin solution; p < 0.01 versus Euro-Collins solution), 86.0 +/- 9.6 (p < 0.01 versus University of Wisconsin solution; p < 0.01 versus Euro-Collins solution), 87.9 +/- 7.7 (p < 0.01 versus University of Wisconsin solution; p < 0.01 versus Euro-Collins solution), and 53.5 +/- 6.0, respectively. All aerodynamic parameters in the lungs flushed with extracellular fluid type solutions were superior to those flushed with intracellular fluid type solutions. We conclude that the efficacy of initial flushing was essential for successful lung preservation and that extracellular fluid type solutions were superior to intracellular fluid type solutions, at least for flushing the lung before storage with University of Wisconsin solution. Potassium concentration in flushing solution should be 20 mmol/L or less to obtain appropriate flushing and subsequent adequate distribution of the storage solution.

Eliminating the strong pulmonary vasoconstriction caused by Euro-Collins solution

Single-flush perfusion with Euro-Collins solution (ECS), after pretreatment with prostaglandin E1 or prostacyclin, is at most centers the standard procedure for preservation of lungs for transplantation. In a previous study, we showed that the high potassium content of ECS causes strong pulmonary vasoconstriction at temperatures higher than 20 degrees C. In the present study, five drugs used as pretreatment and added to the perfusate were compared for their ability to counteract ECS-induced constriction of porcine pulmonary arteries: papaverine reduced the vasoconstrictive effect by 92% +/- 4%; nifedipine, by 62% +/- 6%; the thromboxane A2 receptor antagonist daltroban, by 15% +/- 4%; and prostaglandin E1, by 12% +/- 4%. On the other hand, prostacyclin not only failed to reduce ECS-induced vasoconstriction but at the highest concentration tested, enhanced it by 37% +/- 7%. The combination of papaverine (10(-4) mol/L) and nifedipine (10(-6.5) mol/L) was the only pretreatment to abolish ECS-induced vasoconstriction; moreover, it has no adverse effect on endothelial function. Neither prostaglandin E1 nor prostacyclin effectively counteracts ECS-induced vasoconstriction, though they may have other beneficial effects.

High potassium contents in organ preservation solutions cause strong pulmonary vasocontraction

Euro-Collins (ECS) and UCLA-formula organ preservation solutions induced strong vasocontraction in porcine pulmonary arteries when studied in organ baths at temperatures of 37 degrees C and 30 degrees C. At 20 degrees C ECS induced a 30% contraction, but at 6 degrees C no contraction (n = 5) or a weak contraction (n = 1) was elicited. Neither prostaglandin E1 nor nifedipine caused any significant reduction of the vasocontraction elicited by ECS and UCLA. Krebs solution, enriched with potassium in amounts corresponding to those in ECS (115 mmol/L) or UCLA (30 mmol/L), induced vasocontraction comparing well with those induced by ECS or UCLA, indicating that it is the high potassium content that causes the vasocontraction. In a second experiment lung segments were stored at 4 degrees C for 9 hours in ECS, UCLA, or Krebs solution. Pulmonary arterial segments were then studied in organ baths at 37 degrees C. The choice of preservation solution did not significantly affect the contractile properties of potassium, noradrenaline, or the thromboxane mimic U-46619. To conclude, high potassium contents in organ preservation solutions induce strong pulmonary vasocontraction in lung temperatures greater than 20 degrees C but not in temperatures less than 10 degrees C. These vasocontractions are not significantly reduced by prostaglandin E1 or nifedipine. We suggest that the initial preservation solution used to cool down the lungs should contain 4 mmol/L or no potassium. When the lung temperature is less than 10 degrees C, a second perfusion might be done, and then a high potassium content (if thought to be essential) will not cause vasocontraction.

Evidence against parenchymal metabolites directly promoting pial arteriolar dilation during cortical spreading depression in rabbits

The role of parenchymal metabolic factors in directly promoting pial arteriolar dilation during cortical spreading depression (CSD) in anesthetized rabbits was examined by direct measurement of periarachnoid cerebrospinal fluid (CSF) levels of a representative metabolite (i.e., K+) or superfusion of the cerebral cortical surface with artificial CSF. CSD was induced by KCl microinjection or tissue puncture and its movement was monitored electrophysiologically. Pial arteriolar diameter was determined using a closed cranial window and intravital microscopy. CSD propagated across the cortex under the window with a velocity of 2.9 +/- 0.2 mm/min, and caused pial arteriolar diameter to increase from 87 +/- 9 microns to 133 +/- 11 microns (53%, n = 23) for 1.6 +/- 0.1 min. At the same time, K+ concentration increased from 3.0 +/- 0.2 mM to a maximum of 4.6 +/- 0.3 mM. Topical application of 6 mM K+ increased pial arteriolar diameter by only 8%. Continuous superfusion of the cortical surface with aCSF at a rate of 3.0-4.5 ml/min (window volume = 0.5 ml) did not affect pial arteriolar dilation during CSD, but virtually abolished pial arteriolar dilation during inhalation of 10.2% CO2. These results suggest that pial arterioles dilate via a mechanism which does not involve diffusion of vasoactive metabolites released from the parenchyma during CSD.

Cerebral hemodynamics during cortical spreading depression in rabbits

Effects of a single cortical spreading depression (CSD), elicited by KCl microinjection, on diameter of pial arterioles and venules in the parieto-occipital cortex were examined in urethane-anesthetized adult rabbits using a closed cranial window. The velocity of CSD propagation was 2.7 +/- 0.1 mm/min (mean +/- S.E.M.). All arterioles (n = 39) except for those in the retrosplenial region (n = 6) increased their diameter significantly during CSD. The arteriolar dilation lasted for 1.5 +/- 0.1 min. Location of dilating arteriole and propagating CSD showed that they were always closely associated temporally. As a percentage change, diameters of smaller arterioles significantly increased (from 60 +/- 1 to 103 +/- 2 microns, 71%, n = 12) more than those of larger ones (from 82 +/- 2 to 129 +/- 3 microns, 57%, n = 27). While venules with initial diameter of 85 +/- 4 microns (n = 5) did not dilate, those with initial diameter of 49 +/- 3 microns increased to 57 +/- 3 microns (16%, n = 8) for 1.4 +/- 0.2 min during CSD. The majority of the dilated venules started to increase their diameter after nearby arterioles had dilated maximally. Pial arterioles, which dilated during ipsilateral CSD, decreased their diameter significantly from 78 +/- 2 to 72 +/- 3 microns (8%, n = 11) during contralateral CSD for 13.8 +/- 3.6 min with similar onset latencies as those observed for the dilation. Indomethacin pretreatment significantly enhanced arteriolar dilation during CSD (from 73 +/- 4 to 138 +/- 6 microns, 89%, n = 4).(ABSTRACT TRUNCATED AT 250 WORDS)