Effects of kininase II inhibitors on the vasomotor response to bradykinin of feline intracranial and extracranial arteries in vitro and in situ

Bradykinin B₂ receptors increase hippocampal excitability and susceptibility to seizures in mice

Bradykinin (BK) and its receptors (B1 and B2) may exert a role in the pathophysiology of certain CNS diseases, including epilepsy. In healthy tissues, B2 receptors are constitutively and widely expressed and B1 receptors are absent or expressed at very low levels, but both receptors, particularly B1, are up-regulated under many pathological conditions. Available data support the notion that up-regulation of B1 receptors in brain areas like the amygdala, hippocampus and entorhinal cortex favors the development and maintenance of an epileptic condition. The role of B2 receptors, instead, is still unclear. In this study, we used two different models to investigate the susceptibility to seizures of B1 knockout (KO) and B2 KO mice. We found that B1 KO are more susceptible to seizures compared with wild-type (WT) mice, and that this may depend on B2 receptors, in that (i) B2 receptors are overexpressed in limbic areas of B1 KO mice, including the hippocampus and the piriform cortex; (ii) hippocampal slices prepared from B1 KO mice are more excitable than those prepared from WT controls, and this phenomenon is B2 receptor-dependent, being abolished by B2 antagonists; (iii) kainate seizure severity is attenuated by pretreatment with a non-peptide B2 antagonist in WT and (more effectively) in B1 KO mice. These data highlight the possibility that B2 receptors may have a role in the responsiveness to epileptogenic insults and/or in the early period of epileptogenesis, that is, in the onset of the molecular and cellular events that lead to the transformation of a normal brain into an epileptic one.

Animal models of migraine headache and aura

PURPOSE OF REVIEW

Over the past 30 years, animal models of migraine have led to the identification of novel drug targets and drug treatments as well as helped to clarify a mechanism for abortive and prophylactic drugs. Animal models have also provided translational knowledge and a framework to think about the impact of hormones, genes, and environmental factors on migraine pathophysiology. Although most acknowledge that these animal models have significant shortcomings, promising new drugs are now being developed and brought to the clinic using these preclinical models. Hence, it is timely to provide a short overview examining the ways in which animal models inform us about underlying migraine mechanisms.

RECENT FINDINGS

First generation migraine models mainly focused on events within pain-generating intracranial tissues, for example, the dura mater and large vessels, as well as their downstream consequences within brain. Upstream events such as cortical spreading depression have also been modeled recently and provide insight into mechanisms of migraine prophylaxis. Mouse mutants expressing human migraine mutations have been genetically engineered to provide an understanding of familial hemiplegic migraine and possibly, by extrapolation, may reflect on the pathophysiology of more common migraine subtypes.

SUMMARY

Animal models of migraine reflect distinct facets of this clinically heterogeneous disorder and contribute to a better understanding of its pathophysiology and pharmacology.

Vasomotor and permeability effects of bradykinin in the cerebral microcirculation

All components of an intracerebral kallikrein-kinin system have been described. Thus, bradykinin (BK) acting from the parenchymal site as well as from the blood site may influence cerebral microcirculation. BK is a potent dilator of extra- and intraparenchymal cerebral arteries when acting from the perivascular site. The vasomotor effect of BK is mediated by B2 receptors which appear to be located at the abluminal membrane of the endothelial cell. The effect of BK is mediated by NO. prostanoids, free radicals, H2O2 or leukotrienes depending on the animal species and on the location of the artery. Selective opening of the blood-brain barrier for small tracers (Na(+)-fluorescein; MW, 376) has been found in cats during cortical superfusion or intraarterial application of BK. This leakage is mediated by B2 receptors located at the luminal and abluminal membrane of the endothelial cells. Formation of brain edema has been found after ventriculo-cisternal perfusion or interstitial infusion of BK. This can be explained by increase of vascular permeability and cerebral blood flow due to arterial dilation thus enhancing driving forces for the extravasation. An increase of the BK concentration in the interstitial space of the brain up to concentrations which induce extravasation, dilatation and oedema formation has been found under several pathological conditions. Thus, BK may be involved in oedema formation after cold lesion, concussive brain injury, traumatic spinal cord and ischemic brain injury. The mediator role of BK in brain edema is further supported by therapeutic results. Brain swelling due to cold lesion or ischemia could be diminished by treatment with kallikrein-inhibitors. Similarly, dilatation of cerebral arterioles after concussive brain injury was reduced by blockade of B2 receptors. Thus, all criteria favour BK as one mediator of vasogenic oedema.

Cerebrovascular effects of angiotensin converting enzyme inhibition involve large artery dilatation in rats

BACKGROUND AND PURPOSE

The aim of the study was to selectively examine the effects of converting enzyme inhibition on the large brain arteries by using concomitant inhibition of carbonic anhydrase to cause severe dilatation of mainly parenchymal resistance vessels.

METHODS

Cerebral blood flow was measured using the xenon-133 injection technique in three groups of Wistar rats either during carbonic anhydrase inhibition with acetazolamide (treatment A, n = 8), during carbonic anhydrase inhibition followed by converting enzyme inhibition with captopril 40 minutes later (treatment B, n = 10), or during carbonic anhydrase inhibition preceded by converting enzyme inhibition 20 minutes earlier (treatment C, n = 7).

RESULTS

After treatment A, cerebral blood flow rose rapidly and stabilized within 20 minutes at an average of 220 ml/100 g.min; flow remained stable until at least 60 minutes. After treatment B, cerebral blood flow increased by a further 17.4%, from an average of 219 ml/100 g.min to an average of 257 ml/100 g.min (p less than 0.01). After treatment C, cerebral blood flow stabilized at an average of 238 ml/100 g.min, with flow from 20 to 60 minutes always being higher (from 5% to 17%) than during carbonic anhydrase inhibition alone (p less than 0.02). Thus the additional inhibition of converting enzyme resulted in higher cerebral blood flow than during inhibition of carbonic anhydrase alone.

CONCLUSIONS

These results suggest that converting enzyme inhibition reduced resistance of large brain arteries and support the hypothesis that there is some angiotensin II-induced tone in large cerebral arteries.

The role of glucose uptake and metabolism in hyperglycemic exacerbation of neurological deficit in the paraplegic rat

Previous studies indicate that hyperglycemia, particularly that induced by exogenous glucose administration, exacerbates neurological deficits in the rat spinal cord ischemic model. The effect of inhibition of glucose uptake (glucose transporter) and initial metabolism (hexokinase) on neurological outcome was evaluated in the present investigation using the competitive inhibitors 2-deoxyglucose (2-DG) and 3-O-methylglucose (3-OMG). Sprague-Dawley rats, weighing 200 to 300 gm each, received either 0.25, 1, or 2 gm/kg 2-DG; 2 gm/kg 3-OMG; 2 gm/kg glucose; or an equivalent volume of 0.9% saline intraperitoneally. Rats were intubated and ventilated with 1% to 1.5% halothane. The aortic arch was exposed and snares were placed on the right and left subclavian arteries and the aorta distal to the left subclavian artery. The three vessels were occluded for 10, 11, 12, or 13 minutes. Lower-extremity neurological deficits were evaluated at 1, 4, 18, and 24 hours postocclusion based on a 15-point scale (normal = 0, severe deficit = 15). Lower-extremity neurological deficits were significantly less severe in the groups treated with 2-DG (0.25 and 1 gm/kg) at 18 and 24 hours postocclusion (p less than 0.05 for 0.25 gm/kg and p less than 0.005 for 1 gm/kg, Student's t-test with Bonferroni correction). The lower 2-DG dose of 0.25 gm/kg did not significantly increase the plasma glucose level, suggesting that the glucose transporter was not markedly inhibited, and that the improved neurological outcome was more likely due to inhibition of hexokinase. The higher 2-DG dose of 1 gm/kg afforded protection despite significantly increasing the plasma glucose level, implying a strong inhibition of both the glucose transporter and hexokinase. Administration of 3-OMG, which only inhibits glucose uptake and not hexokinase, actually worsened the neurological deficit in a manner similar to that observed in rats treated with glucose. The authors conclude that the activity of the glucose transporter by itself does not significantly contribute to hyperglycemic exacerbation of neurological deficits. In contrast, the hexokinase step, at least in combination with the transporter and possibly alone, plays a significant role in hyperglycemic exacerbation of the lower-extremity neurological deficit in the paraplegic rat.

Angiotensin converting enzyme inhibition and cerebral circulation--a review

1. The identification of a vascular wall renin angiotensin system and of angiotensin converting enzyme on the luminal surface of the endothelium in many tissues, including the brain, has stimulated research on the influence of the renin angiotensin system on regional blood flows. 2. In experimental studies inhibition of the angiotensin converting enzyme shifts the limits of cerebral blood flow autoregulation towards lower blood pressure values. 3. In patients with chronic arterial hypertension and in patients with chronic heart failure cerebral blood flow is not changed by acute or chronic angiotensin converting enzyme inhibition, despite in some cases pronounced reductions in the mean arterial blood pressure. Angiotensin converting enzyme inhibition does not change ischaemic regional cerebral blood flow in acute stroke. 4. It is concluded that following angiotensin converting inhibition cerebral blood flow is maintained at an unchanged level. The mechanism may include inhibition of locally produced angiotensin II leading to a selective dilation of larger cerebral arteries with a compensatory constriction of the smaller cerebral arteries.

Kallidin and bradykinin metabolism by isolated cerebral microvessels

Although kinins have been reported to affect cerebral vascular tone and permeability, their actions are not potentiated by angiotensin converting enzyme inhibitors. To investigate cerebral vascular kinin metabolism, porcine cerebral microvessels were isolated by differential sieving and centrifugation and characterized by microscopic examination and marker enzyme enrichment. Purified microvessels contained a membrane-bound carboxypeptidase which hydrolyzed the C-terminal Phe-Arg bond of both kallidin and bradykinin. Hydrolysis was optimal at pH 7.0, was activated more than 300% by 0.1 mM CoCl2, and was inhibited by o-phenanthroline and the carboxypeptidase N (EC 3.4.17.3) inhibitor DL-2-mercaptomethyl-3-guanidino-ethylthiopropanoic acid (MERGETPA) (IC50 = 2 microM). Conversely, inhibitors of angiotensin I converting enzyme (captopril), neutral endopeptidase (phosphoramidon), post proline cleaving enzyme (Z-Pro-prolinal), dipeptidyl(amino)peptidase IV (diprotin A) and amino-peptidase M (amastatin) had no effect. When the rates of C-terminal hydrolysis of kallidin by detergent-solubilized cerebral microvasculature were determined over a range of substrate concentrations (16.6 to 250 microM), the Km and Vmax values obtained were 26.0 +/- 3.0 microM and 14.7 +/- 1.3 nmol/min/ml (N = 4) respectively. These data suggest that a cerebral microvascular carboxypeptidase may play a role in vivo in modulating the effects of kinins on cerebral blood flow and permeability and in preventing circulating kinins from crossing the blood-brain barrier.

Vasomotor responses of rat intracerebral arterioles to vasoactive intestinal peptide, substance P, neuropeptide Y, and bradykinin

The effect of vasoactive peptides on vascular smooth muscle in the cerebral microcirculation was examined using an isolated intracerebral arteriole preparation. Extraluminally applied vasoactive intestinal peptide (VIP) dilated the spontaneous tone of intracerebral arterioles to 118.9 +/- 3.1% of control diameter at pH 7.30, with an EC50 of 7.27 X 10(-8) M. Similar degrees of dilation to VIP were seen in vessels preconstricted by changing bath solution to pH 7.60. Substance P had no effect on vessel diameter at pH 7.30. However, in vessels precontracted by pH 7.60, significant dose-dependent dilation was observed with an EC50 of 2.55 x 10(-10) M. Neuropeptide Y constricted intracerebral arterioles to 81.22 +/- 2.7% of control diameter, with an EC50 of 6.23 x 10(-10) M. Bradykinin dilated intracerebral arterioles at pH 7.30 and pH 7.60 to 130 +/- 3.0% of control diameter. VIP and bradykinin are potent vasodilators of intracerebral arterioles. Neuropeptide Y is a vasoconstrictor. The effect of substance P appeared to be either pH-dependent or dependent on some degree of precontraction by another agonist, but no effect on vessel diameter was seen at pH 7.30.

Cerebrovascular reactivity to angiotensin and angiotensin-converting enzyme activity in cerebrospinal fluid

The purpose of the present study was to test the vasomotor effect of angiotensin I (A I) and angiotensin II (A II) in feline cerebral arteries and to examine the presence of angiotensin converting enzyme (ACE) activity in the vessel wall and cerebrospinal fluid (CSF). A II (10(-8) -10(-5) M) induced concentration-dependent contractions of feline pial arteries (resting diameter, 98-286 microns) in situ with a maximum of 34% at 10(-4) M A II. A I produced dose-related contractions being approximately 20 times less potent than A II. The action of A I was significantly attenuated by the ACE inhibitor captopril (10(-5) M). These findings demonstrate the presence of ACE activity in the vessel wall and/or its surroundings. ACE activity was also found in feline CSF sampled from the cisterna cerebellomedullaris. Bradykinin (BK) was broken down and A I converted to A II by CSF, both effects being inhibited by captopril. This was demonstrated using bioassay and high-performance liquid chromatography. Considering the present and previous studies we conclude that the presence of ACE in the vessel wall and CSF is necessary for the conversion of A I to A II. Although ACE in CSF is able to degrade BK it appears not to be important for the metabolism of BK acting from the perivascular side of pial arteries in situ.

Inhibition of bradykinin- and kallikrein-induced cerebral arteriolar dilation by a specific bradykinin antagonist

We have previously shown that topical brain application of kallikrein, an enzyme which converts kininogen to bradykinin, induces rabbit pial arteriole dilation. The purpose of the present investigation was to utilize a newly developed competitive kinin receptor antagonist to test the hypothesis that kallikrein-induced dilation was due to the conversion of brain kininogen to vasoactive kinins. As in our previous study, we measured rabbit pial arteriole diameter with a microscope using the closed cranial window technique. The kinin antagonist (6 microM) reduced the dose-dependent dilation produced by bradykinin and blocked the dilation induced by kallikrein. In addition, the kinin antagonist was specific since it did not alter the cerebral arteriole dilation produced by adenosine, acetylcholine, or vasoactive intestinal polypeptide. These experiments provide further evidence for a possible role of the endogenous brain kallikrein-kinin system in the modulation of the cerebral circulation and provide the necessary pharmacologic foundation for future use of this antagonist in testing the role of kinins in the normal or altered cerebral circulation.

Glycolytic inhibition by 2-deoxyglucose reduces hyperglycemia-associated mortality and morbidity in the ischemic rat

Numerous laboratories have shown that hyperglycemia increases cerebral ischemic damage. This presumably results from increased lactate production and accumulation during ischemia. Although increased tissue lactic acidosis is associated with increased ischemic brain damage, this damage has not been directly linked to glycolytic flux. Because 2-deoxyglucose (2-DG) is a competitive inhibitor of glycolysis we tested its ability to reduce hyperglycemia-exacerbated ischemic brain damage. Severe forebrain ischemia was produced by the four-vessel occlusion model in rats. Four rats received 3 g/kg glucose and saline while a second group (n = 5) was injected with 3 g/kg glucose plus 1.6 g/kg 2-DG. A third group (n = 5) was treated with 1 g/kg glucose plus saline and a fourth group (n = 5) received 1 g/kg glucose and 1.6 g/kg 2-DG. All rats were injected i.p. 10 minutes prior to the ischemic insult with the same volume/kg body weight. All rats receiving the high dose of glucose alone (3 g/kg) were dead within 24 hours postischemia. Rats who received 2-DG in addition to 3 g/kg glucose showed only 40% mortality (p = 0.119 Fisher's Exact). 2-DG completely eliminated convulsions during the initial two hours of recovery which was significant (p = 0.008), however, all rats in both groups showed some convulsions by 24 hours postischemia. Among rats receiving the low glucose dose (1 g/kg), none of the rats receiving 2-DG died or convulsed by 24 hours postischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a possible role of the brain kallikrein-kinin system in the modulation of the cerebral circulation

Experiments by others have shown that exogenous bradykinin dilates cerebral arterioles and that the brain contains kininogen and kallikrein, the latter being the enzyme which converts kininogen to bradykinin. The objective of these experiments was to determine if bradykinin produced from endogenous brain kininogen can affect the cerebral microcirculation. Rabbit pial arteriolar diameter was measured with a microscope using the closed cranial window technique. Topical application of bradykinin (10(-8)-10(-5) M) induced a dose-dependent vasodilation (8-46%) which was completely inhibited by the cyclooxygenase enzyme inhibitors indomethacin and meclofenamic acid. Topical application of 1 U of tissue kallikrein per milliliter of artificial cerebrospinal fluid induced 43% dilation, which could be prevented by local treatment with indomethacin or the proteinase inhibitor aprotinin. The action of aprotinin and indomethacin was specific, since aprotinin did not affect the dilation produced by bradykinin, and indomethacin did not affect dilation produced by adenosine. A second application of kallikrein had no effect on cerebral diameter, yet the arterioles still responded normally to exogenous bradykinin, indicating that the first application of kallikrein depleted brain kininogen. We suggest that activation of brain kallikrein and subsequent formation of kinin from brain kininogen may be important in modulation of cerebral blood flow or generation of cerebral edema.

Effects of bradykinin on permeability and diameter of pial vessels in vivo

The effect of bradykinin on the permeability and vasomotor response of pial vessels has been studied to enhance our understanding of the pathophysiological role of the kallikrein-kinin system in cerebral tissue. Intravital fluorescence microscopy of the pia arachnoidea was conducted using Na+-fluorescein, FITC-dextran, and FITC-albumin as low and high molecular weight blood-brain barrier indicators. Massive arterial dilatation evolved immediately upon administration of bradykinin by superfusion of the exposed cerebral surface. An increase of the arterial diameter by 40% was the maximal response found at bradykinin concentrations of 4 x 10(-5) M. Arterial dilatation became attenuated with continuous superfusion of the preparation with bradykinin. In pial veins, a moderate reduction of the vessel diameter was observed, however, only after prolonged superfusion of the preparation. Bradykinin led to selective opening of the blood-brain barrier for Na+-fluorescein at superfusate concentrations of greater than or equal to 4 x 10(-7) M, but not for FITC-dextran or FITC-albumin. Topical administration of l-isoproterenol (10(-4) M) was found to prevent extravasation of Na+-fluorescein in the presence of bradykinin concentrations of 4 x 10(-6) M. Protection of the blood-brain barrier by isoproterenol was not observed when higher concentrations of bradykinin were employed. Intracarotid infusion of bradykinin were employed. Intracarotid infusion of bradykinin led also to a selective opening of the blood-brain barrier for Na+-fluorescein, but not for FITC-dextran or FITC-albumin. In contrast to superfusion, this route of administration did not induce changes of the vasomotor behavior of the arteries or veins. Additional experiments with B1-agonists and -antagonists suggest that bradykinin causes the openings of the blood-brain barrier th rough an interaction with B2-receptors on endothelial cells, and arterial dilatation via interaction with B2-receptors on vascular smooth muscle cells. Our findings support the concept that the release of kinins in the brain during an acute cerebral lesion mediates secondary damaging processes by the enhancement of blood-brain barrier dysfunction.

The effect of kininase II inhibitors on the response of feline cerebral arteries to bradykinin and angiotensin

Bradykinin (BK) produced concentration-related relaxations of cat middle cerebral arteries and was ineffective on cat basilar arteries in vitro under resting tension and when contracted with 5-hydroxy-tryptamine (5-HT). Angiotensin I and II (AI, AII) produced concentration-related contraction on both arteries, AII being approximately 10 times more potent than AI. The angiotensin-converting-enzyme inhibitors, SQ, 14225, BPP5a and BPP9a had no effect on the concentration effect curves to BK, AII or 5-HT but responses to AI were inhibited. These results suggest that angiotensin-converting-enzyme is present in both preparations and is important for the conversion of AI to AII but apparently not for the degradation of BK.