Phenytoin interacts with calcium channels in brain membranes

Drug-induced gingival overgrowth--a review

Drug-induced gingival overgrowth is a side effect associated with 3 types of drugs: anticonvulsants (phenytoin), immunosuppressive agents (cyclosporine A), and various calcium channel blockers for cardiovascular diseases. Gingival overgrowth is characterized by the accumulation of extracellular matrix in gingival connective tissues, particularly collagenous components with various degrees of inflammation. Although the mechanisms of these disorders have not been elucidated, recent studies suggest that these disorders seem to be induced by the disruption of homeostasis of collagen synthesis and degradation in gingival connective tissue, predominantly through the inhibition of collagen phagocytosis of gingival fibroblasts. The integrins are a large family of heterodimeric transmembrane receptors for extracellular matrix molecules. alpha2beta1 integrin serves as a specific receptor for type I collagen on fibroblasts, and alpha2 integrin has been shown to play a crucial role in collagen phagocytosis. Actin filaments, which are assembled from monomers and oligomers, are involved in collagen internalization after binding to integrins. Furthermore, the implication of intracellular calcium in the regulation of integrin-mediated binding activity and gelsolin activity, known as a calcium-dependent actin-severing protein, is also described. In this review, we focus on collagen metabolism in drug-induced gingival overgrowth, focusing on the regulation of collagen phagocytosis in fibroblasts.

Phenytoin/isradipine interaction causing severe neurologic toxicity

OBJECTIVE

To report a young man on phenytoin who developed acute neurologic symptoms after isradipine was introduced to his treatment regimen and discuss the possible causes of this drug interaction.

CASE SUMMARY

A 21-year-old white man, with propionic acidemia and seizures treated with phenytoin and carbamazepine, was started on isradipine for essential hypertension. Soon thereafter, he developed acute and severe lethargy, ataxia, dysarthria, and weakness that resolved once isradipine was withheld. Phenytoin concentrations were within normal limits or elevated, despite sequential reductions of phenytoin dosage, during concomitant isradipine administration.

DISCUSSION

Isradipine is a known inhibitor of the CYP450 isoenzyme family. Although the daily dose of phenytoin was decreased significantly, phenytoin blood concentrations remained high, suggesting a pharmacokinetic interaction. Previously, the patient had never had neurologic symptoms associated with increased phenytoin concentrations. This also indicates a likely pharmacodynamic interaction between phenytoin and the calcium-channel blocker. Both phenytoin and isradipine have been shown to bind to calcium channels and to inhibit calcium entry into the cells. Binding of isradipine to the brain has been described in humans and animals, and calcium-channel blockers have been shown to cause potentiation of anticonvulsant action of phenytoin.

CONCLUSIONS

Acute pharmacokinetic and pharmacodynamic interactions between phenytoin and isradipine were probably responsible for the lethargy, dysarthria, ataxia, and weakness our patient developed. The combination of phenytoin and calcium-channel blockers should be used with caution.

Discrimination reversal conditioning of an eyeblink response is impaired by NMDA receptor blockade

In the present study we examined the effects of the specific NMDA receptor antagonist CPP on discrimination reversal learning in rabbits. We report two primary findings. First, the institution of NMDA receptor blockade had no effect on a learned discrimination. Second, after stimulus reversal, CPP treatment impaired acquisition of the discrimination reversal. This impairment manifested itself early in training as a retardation in acquisition of a CR to the new CS+ and late in training as an inability to suppress responsiveness to the new CS-. Given the comparability of the present results with previously published results for phenytoin-treated rabbits, we suggest that the effects of phenytoin on learning in this paradigm is at least in part mediated by its effects on NMDA receptors. We further suggest that these findings emphasize the need to better define the role of NMDA receptor activation and hippocampally-mediated circuits in a variety of associative learning paradigms.

Phenytoin blocks the reversal of a classically conditioned discriminative eyeblink response in rabbits

PURPOSE

Cognitive deficits associated with chronic treatment with phenytoin (PHT) have been reported. PHT blocks transfer from a signaled appetitive bar press to an active avoidance response in rats. We investigated the effects of PHT and the prodrug fosphenytoin in rabbits required to learn a discrimination and reversal of a classical eyeblink conditioning paradigm.

METHODS

Before drug treatment was started, rabbits were trained to produce a discriminated eyeblink response. PHT (n = 7) was administered centrally or the prodrug fosphenytoin (n = 2) was given systemically. Control animals were similarly treated centrally with either saline (n = 3) or no drug treatment (n = 13). Rabbits were then challenged with a stimulus reversal while being maintained on the respective drug.

RESULTS

On the first day of reversal training, control animals typically displayed high response rates to both tones, followed by a reduction in responsiveness to the new conditioned stimulus (CS-) in the ensuing days. In contrast, PHT-treated animals failed to suppress responsiveness to the new CS-.

CONCLUSIONS

The response patterns observed are similar to those observed in rabbits with hippocampal ablations, leading us to suggest that the adverse effects of phenytoin may be due to actions in the hippocampus.

Upregulation of L-type Ca2+ channels in reactive astrocytes after brain injury, hypomyelination, and ischemia

Anti-peptide antibodies that specifically recognize the alpha1 subunit of class A-D voltage-gated Ca2+ channels and a monoclonal antibody (MANC-1) to the alpha2 subunit of L-type Ca2+ channels were used to investigate the distribution of these Ca2+ channel subtypes in neurons and glia in models of brain injury, including kainic acid-induced epilepsy in the hippocampus, mechanical and thermal lesions in the forebrain, hypomyelination in white matter, and ischemia. Immunostaining of the alpha2 subunit of L-type Ca2+ channels by the MANC-1 antibody was increased in reactive astrocytes in each of these forms of brain injury. The alpha1C subunits of class C L-type Ca2+ channels were upregulated in reactive astrocytes located in the affected regions in each of these models of brain injury, although staining for the alpha1 subunits of class D L-type, class A P/Q-type, and class B N-type Ca2+ channels did not change from patterns normally observed in control animals. In all of these models of brain injury, there was no apparent redistribution or upregulation of the voltage-gated Ca2+ channels in neurons. The upregulation of L-type Ca2+ channels in reactive astrocytes may contribute to the maintenance of ionic homeostasis in injured brain regions, enhance the release of neurotrophic agents to promote neuronal survival and differentiation, and/or enhance signaling in astrocytic networks in response to injury.

Effects of phenytoin, carbamazepine, and gabapentin on calcium channels in hippocampal granule cells from patients with temporal lobe epilepsy

PURPOSE

The anticonvulsants phenytoin (PHT), carbamazepine (CBZ), and gabapentin (GBP) are commonly used in the treatment of temporal lobe epilepsy. Ca2+ current modulation has been proposed to contribute to the antiepileptic activity of these drugs. The purpose of this study was to determine the effects of these anticonvulsants on voltage-dependent calcium channels in pathologically altered neurons from patients with chronic temporal lobe epilepsy.

METHODS

Acutely isolated human hippocampal granule cells were examined by using the whole-cell configuration of the patch-clamp technique.

RESULTS

PHT and CBZ produced a reversible, concentration-dependent inhibition of high-voltage-activated (HVA) Ca2+ currents without affecting voltage-dependent activation. The concentration-response curves of PHT and CBZ indicated maximal inhibition of 35 and 65%, respectively, with half-maximal inhibition being obtained at 89 and 244 microM, respectively. At therapeutic cerebrospinal fluid (CSF) concentrations, HVA currents were not significantly altered by PHT and CBZ. However, PHT but not CBZ showed a reduction of HVA currents of 16% at a therapeutic whole-brain concentration of 80 microM. In contrast to CBZ, PHT produced a small hyperpolarizing shift in the voltage dependence of steady-state inactivation. PHT, 80 microM, shifted the potential of half-maximal inactivation by -3.1 +/- 0.5 mV (p < 0.05). GBP, which was recently found to bind to the alpha2delta subunit of a neuronal Ca2+ channel, showed no modulation of Ca2+ conductances.

CONCLUSIONS

These results suggest that, in contrast to GBP and CBZ, modulation of postsynaptic Ca2+ channels can contribute to the anticonvulsant action of PHT in human hippocampal granule cells.

Effects of diphenylhydantoin on motor potentials evoked with magnetic stimulation

We studied the effect of an acute loading dose of diphenylhydantoin (DPH) on motor responses elicited with cortical magnetic stimulation in normal subjects. DPH increased significantly the motor threshold activation of ADM, APB, FDI and biceps. The motor threshold increase was of greater magnitude for the proximal muscle. Spinal soleus alpha-motoneuron pool excitability assessed by H-reflex was increased significantly suggesting that the motor threshold increase is related to a supraspinal effect of the drug. Our study demonstrates that the motor threshold increase observed after DPH administration occurs not only in epileptic patients but also in normal subjects.

Effects of antiepileptic drugs, calcium channel blockers and other compounds on seizures induced by activation of voltage-dependent L calcium channel in DBA/2 mice

1. The convulsant activity of the calcium voltage L-channel agonist Bay k 8644 was studied in genetically epilepsy prone DBA/2 mice. 2. Seizures were induced by intracerebroventricular injection of Bay k 8644. 3. These seizures were reversed by some calcium channel blockers such as dihydropyridines, some excitatory amino acid antagonists such as 2-amino-7-phosphonoeptanoate and CPPene, 2-chloro-adenosine, some anticonvulsant drugs such as magnesium valproate, diazepam and clonazepam and two kappa opioid agonists (U-50488H and U-54494A). 4. The remaining antiepileptic drugs (carbamazepine, phenytoin, phenobarbital and trimethadione) were ineffective in this respect. Other anticonvulsant compounds such as dizocilpine (MK 801), ketamine and drugs enhancing GABAergic transmission did not significantly affect the clonic phase of the seizures induced by Bay k 8644. 5. These results show that Bay k 8644 seizures are relatively resistant to some anticonvulsant compounds. The role of some neurotransmitters on seizures induced by Bay k 8644 is discussed.

Effects of some calcium antagonists upon the activity of common antiepileptic compounds on sound-induced seizures in DBA/2 mice

1. Flunarizine (2.65 mumol/kg, i.p.) and nimodipine (5.25 mumol/kg, i.p.) potentiated the anticonvulsant properties of phenytoin, phenobarbital and valproate against audiogenic seizures in DBA/2 mice. 2. Diltiazem (5.25 mumol/kg, i.p.) was able to potentiate the antiseizure activity of phenytoin but was not effective against the anticonvulsant action of phenobarbital and valproate. 3. Verapamil (5.25 mumol/kg, i.p.) was unable to potentiate the anticonvulsant properties of all antiepileptic drugs studied. 4. Bay K 8644 (1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluorophenyl)-pyridine- 5-carboxylic acid), a calcium agonist at a dose of 2.65 mumol/kg, i.p., induced a reduction of anticonvulsant potency of phenytoin, phenobarbital and valproate. 5. None of the calcium antagonists used significantly increased the plasma levels of antiepileptic compounds or significantly affected the body temperature changes induced by anticonvulsant drugs. 6. It may be concluded that some calcium antagonists enhance the anticonvulsant properties of some antiepileptic drugs against audiogenic seizures. A pharmacokinetic interaction does not seem responsible for these effects.

Phenytoin preferentially inhibits L-type calcium currents in whole-cell patch-clamped cardiac and skeletal muscle cells

The effect of the anticonvulsant diphenylhydantoin (phenytoin) was tested on the inward calcium currents of whole-cell patch-clamped cells from rat and human muscles and from frog atrium. A concentration of 10 microM phenytoin was required to obtain a threshold inhibitory effect and, even with high concentrations (100 microM), the inhibition was not complete. In skeletal muscle (rat and human cells in culture), phenytoin (30 microM) exerted a more potent effect on the high-threshold calcium current (ICa,L inhibition: 53 +/- 6% mean +/- SDn-1) rather than on the low-threshold one (ICa,T inhibition: 16 +/- 10%). Similar results were obtained on dissociated frog atrial cells. These data are to be contrasted with those previously reported on neuronal cells, where specific inhibition of ICa,T was reported. Thus, the action of phenytoin appears to be different in muscle and nerve so that phenytoin does not appear to be a specific inhibitor of ICa,T.

Anticonvulsant effects of dihydropyridine Ca2+ antagonists in electrocortical shock seizures

The dihydropyridine calcium antagonists nimodipine (NMD), PN200-110, and nicardipine were compared with phenytoin (PHT) as potential anticonvulsants in electrocortical shock (ECS)-induced seizures in the white New Zealand rabbit. Before treatment, seizure duration ranged from 43.8 +/- 5.1 to 49.6 +/- 5.2 s with an ECS stimulus of 10-V, 100-Hz, 0.1-ms pulses for 5 s. Each drug was administered into the right internal intracarotid artery 2 min before the ECS. A cumulative nimodipine dose of 440 micrograms/kg decreased seizure discharge to 6.6 +/- 5.0 s (p less than 0.001), whereas a total dose of 1.0 mg/kg PN200-110 was required to achieve a similar effect. Nicardipine was ineffective. A cumulative dose of 7 mg/kg phenytoin was required to suppress seizure discharge. These results indicate that Ca2+ channels modulated by dihydropyridines play a facilitating role in ECS-induced seizures. We propose that the anticonvulsant effects of nimodipine and PN200-110 are due to inhibition of neuronal calcium L-channels. Dihydropyridine Ca2+ antagonists that penetrate the blood-brain barrier (BBB) and bind to neuronal tissue may emerge as a novel class of anticonvulsants.

Effects of diltiazem on hyperthermia induced seizures in the rat pup

1. We studied the effects of a calcium antagonist diltiazem, as well as diazepam and phenytoin on hyperthermia induced seizures in unrestrained 15 day-old rats. 2. Saline injected animals exposed to an ambient temperature of 40 degrees C showed a gradual increase in body temperature reaching a maximum of 42 +/- 0.1 degree C at 50 min. 3. At this time all rats pups had generalized seizures. 4. Similar results were obtained when the animals were pretreated with phenytoin (100% showed seizures). 5. Animals receiving diltiazem had a temperature of 41.5 +/- 0.1 degree C at 90 min of exposure to 40 degrees C environment. 6. However, diltiazem completely prevented seizures. 7. The rats treated with diazepam showed lower temperature than in saline, diltiazem and phenytoin groups and no seizures were observed in this experimental group.

Activators and inactivators of calcium channels: effects in the central nervous system

The interactions of calcium antagonists or channel activators with the different classes of calcium channel are reviewed with particular emphasis on interactions with neuronal tissue; reasons for the failure of calcium antagonists to inhibit neurotransmitter release under normal circumstances are outlined. Calcium antagonists may be protective in several pathological situations and the possibilities of protection against ischaemic damage in the central nervous system are evaluated.

Calcium, neuronal hyperexcitability and ischemic injury

Due to tight regulatory controls, a 10,000-fold concentration gradient exists between intracellular and extracellular free Ca2+ concentrations. With appropriate stimulus Ca2+ will rapidly flow into neurons through various types of membrane channels including voltage-dependent and receptor-operated channels. Intracellular Ca2+ concentrations are then quickly restored primarily through Ca2+-ATPase, Na+/Ca2+ exchange, and endoplasmic reticulum sequestration. It is well-known that Ca2+ is essential for neurotransmitter release. More recent investigations indicate that Ca2+ influx is essential for neuronal excitability independent from synaptic function. In fact, abnormal Ca2+ metabolism may play a dominant role in both the initiation and propagation of seizure discharge. Accordingly, Ca2+ channel blockers may represent a new therapeutic modality to treat epilepsy. Analyzed in this article are the major mechanisms by which neurons control Ca2+ fluxes and the evidence supporting the role of Ca2+ in seizure phenomena. Thereafter, an integrative theory for the role of calcium in neuronal hyperexcitability and ischemic cell death is constructed.

Inhibition by phenytoin of in vitro secretion of calcitonin from rat thyroid glands and cultured rat C cells

Baby rat thyroid glands and cultured rat medullary carcinoma C cells were incubated acutely with phenytoin (38-100 microM), and the calcitonin (CT) secreted into the serum-free medium was measured by radioimmunoassay (RIA). Phenytoin did not alter CT release from glands or C cells incubated in 1 mM Ca, but, when Ca was raised to 1.75 or 2.5 mM, a marked inhibitory effect of phenytoin was apparent. The inhibitory effect could be negated by including 10 microM BAY-K-8644 in the medium. Inhibitory effects on CT release also were obtained with 100 microM trifluoperazine or 100 microM nitrendipine, and these inhibitory effects also were counteracted by 10 microM BAY-K-8644. The results show that clinically relevant amounts of phenytoin can inhibit CT release, perhaps by interfering with C-cell Ca channels or by inhibiting calmodulin-dependent processes.

Effects of chronic diphenylhydantoin on cerebral metabolism in the adult rat

Diphenylhydantoin is one of the most widely used anticonvulsant agents in humans. To examine its effects on brain metabolism, we used the quantitative autoradiographic [14C]deoxyglucose method to measure local cerebral glucose utilization in adult rats receiving 50 mg/kg/day diphenylhydantoin for 1 week. Thirty-three brain structures were analyzed to determine whether the drug has global or site-specific effects on cerebral metabolism. Chronic administration produced statistically significant decreases in 23 neocortical and subcortical structures compared with those in vehicle-injected control animals. Therefore, our data support the concept that diphenylhydantoin has widespread depressant effects on brain metabolism.

Anticonvulsant effects of some calcium entry blockers in DBA/2 mice

1. The behavioural and anticonvulsant effects of several drugs acting by various mechanisms on calcium-channels or affecting intracellular Ca2+ concentrations were studied after both systemic and intracerebroventricular administration in DBA/2 mice, a strain genetically susceptible to sound-induced seizures. 2. The anticonvulsant effects were evaluated on seizures evoked by means of auditory stimulation (109 dB) in animals placed singly under a perspex dome. 3. Flunarizine and dihydropyridine derivatives, belonging to class I of calcium entry blockers, administered intraperitoneally, were the most potent compounds. 4. Diltiazem, a benzothiazepine derivative belonging to class III, and HA 1004, a calcium antagonist, acting by inhibiting Ca2+ mobilization from intracellular stores, injected intraperitoneally, were 3-7.6 fold and 5.8-10.7 fold less potent than flunarizine respectively. 5. Verapamil and methoxyverapamil, two phenylalkylamine derivatives, given intraperitoneally, were completely ineffective in preventing sound-induced seizures in DBA/2 mice. In addition, high doses of verapamil and its methoxyderivative occasionally produced spontaneous tonic-clonic seizures. 6. After intracerebroventricular administration of the hydrosoluble calcium entry blockers, belonging to different classes, the anticonvulsant effects were similar to those observed after systemic administration. 7. The systemic administration of Bay K 8644, a dihydropyridine analogue, having the ability to stimulate calcium entry into cells produced a dose-dependent increase in clonic and tonic convulsions and other neurological side effects. 8. The present results strongly support the idea that some Ca2+ antagonists may be useful in human epilepsy.

Effects of flunarizine on Metrazol-induced seizures in developing rats

Antimetrazol action of flunarizine (5, 10, 20 and 40 mg/kg i.p.) was tested during ontogenesis on male Wistar rats aged 7, 12, 18, 25 and 90 days. The latencies and incidences of jerks, minimal Metrazol seizures and major Metrazol seizures remained unchanged by flunarizine in all age groups. A specific action (an abolition of the tonic phase of major seizures) was seen throughout the development and was reflected in lower scores of seizures.

Common anticonvulsants inhibit Ca2+ uptake and amino acid neurotransmitter release in vitro

Phenytoin (PHT), phenobarbital (PB), and carbamazepine (CBZ) dose-dependently inhibited veratrine-stimulated calcium influx and evoked amino acid neurotransmitter release in rat cortical slices at relatively low concentrations. The action on Ca2+ influx was in the clinical effective dose range for these anticonvulsant drugs, whereas the action on amino acid release was mostly well above this range. Neither ethosuximide (ESM) nor sodium valproate (VPA) had any effect on the veratrine-stimulated Ca2+ influx. Stimulated amino acid release was not affected by ESM, whereas VPA specifically inhibited the release of aspartate in preference to glutamate and GABA at concentrations well within the clinically effective dose range. The actions of VPA and ESM on other parameters measured were detectable only at very high concentrations, which are likely to be irrelevant in defining their clinical mode of action.

Calcium channels and calcium channel antagonists

Changes in free intracellular Ca2+ levels provide signals that allow nerve and muscle cells to respond to a host of external stimuli. A major mechanism for elevating the level of intracellular Ca2+ is the influx of extracellular Ca2+ through voltage-dependent channels in the cell membrane. Recent research has yielded new insights into the physiological properties, molecular structure, biochemical regulation, and functional heterogeneity of voltage-dependent Ca2+ channels. In addition, Ca2+ channel antagonist drugs have been developed that are valuable both as probes of channel structure and function and as therapeutic agents. Preliminary evidence suggests that these drugs may be useful in the treatment of diverse neurological disorders, including headache, subarachnoid hemorrhage, stroke, and epilepsy.

Induction of seizures in mice by intracerebroventricular administration of the calcium channel agonist BAY k 8644

The calcium channel agonist BAY k 8644 was used to investigate the role of the calcium ion (Ca2+) in epileptogenesis. Intracerebroventricular administration of the compound induced murine seizures that were reversed by calcium channel inhibitors (CCIs) but not by anticonvulsants such as carbamazepine, pentobarbital, and diazepam. The seizures were exacerbated by phenytoin and valproic acid. Chronic administration of CCI's, previously shown to produce down-regulation of the binding of the CCI [3H]nitrendipine, resulted in augmentation of BAY k 8644-induced seizures.

Nifedipine in the treatment of myotonia in myotonic dystrophy

Abnormal calcium transport may be implicated in the membrane defect in myotonic dystrophy. A single blind crossover trial of placebo (t.i.d.), nifedipine 10 mg (t.i.d.) and nifedipine 20 mg (t.i.d.), was performed in 10 patients with myotonic dystrophy. The severity of myotonia was assessed by measuring finger extension time after maximum voluntary finger flexion. A significant improvement in myotonia, after nifedipine, was recorded by this technique and supported by a subjective improvement in 50% of patients and clinical improvement of greater than 20% in five patients. Initial grip strength and muscle fatiguability measured by grip strength ergometry were not significantly altered.

Motor cortical epileptic foci in vivo: actions of a calcium channel blocker on paroxysmal neuronal depolarizations

Focal epileptiform activity was induced by local application of penicillin to the surface of the rat motor cortex. Neurons located within the epileptic focus displayed typical paroxysmal depolarization shifts (PDS). The participation of membrane calcium currents in the generation of PDS was examined by injecting the quaternized calcium entry blocker D890 into single neurons by iontophoresis or by pressure pulses. After intracellular injections of D890, PDS were depressed in amplitude by up to 55%. In a few cases the depression of PDS following intracellular application of D890 was preceded by a transient increase. Similar increases of PDS amplitude were obtained by injections of the calcium chelator EGTA. Control experiments in preparations without epileptic activity revealed that excitatory potentials elicited by thalamic stimulation and Cl(-)-dependent inhibitory postsynaptic potentials evoked by epicortical stimulation were not affected by intracellular D890. In these experiments successful intracellular drug application was verified by monitoring the transient shift of the Cl(-)-equilibrium potential induced by injection of KCl together with D890. It is concluded that in the penicillin-induced epileptic focus of the motor cortex Ca2+ inward currents participate in the generation of neuronal PDS.

Diphenylhydantoin blocks cardiac calcium channels and binds to the dihydropyridine receptor

Diphenylhydantoin was studied for its effects on Ca currents in single isolated guinea pig ventricular cells. The whole-cell patch-clamp technique was used, and Ca currents were studied after suppressing Na and K currents. At low frequencies (0.1 Hz) and negative holding potentials (-50 mV), diphenylhydantoin produced a concentration-dependent decrease in Ca currents without any significant change in the current-voltage relations. Half blocking effect occurred at 2 X 10(-4) M. The effects of diphenylhydantoin on Ca currents were dependent upon the holding potential. Inactivation curves for Ca currents were shifted to more negative potentials by the drug. The recovery of Ca currents from inactivation was prolonged by diphenylhydantoin, and the repriming of the current displayed an additional component, attributed to slow release of the drug from the channels. The voltage-dependent block was attributed to preferred binding by the inactivated channel state. Diphenylhydantoin also blocked specific [3H]-nitrendipine binding to guinea pig ventricular membrane preparations. The inhibition of [3H]-nitrendipine binding by diphenylhydantoin was competitive. Diphenylhydantoin also blocks cardiac Na channels in a voltage-dependent manner. We suggest that diphenylhydantoin binding sites exist on both Ca and Na channels.

Kinetics of microcirculatory, NAD/NADH, and electrocorticographic changes in cat brain cortex during ischemia and recirculation

Changes in microcirculation, the NAD/NADH redox state, and electrical activity during 2 hours of ischemia and 4 hours of reperfusion produced by middle cerebral artery occlusion and release were studied in cats. Twelve animals were classified into three groups of ischemia (mild, moderate, and severe) based on the severity of electrocorticographic (ECoG) depression at the end of the recovery period. Four animals were studied as controls. Occlusion of the middle cerebral artery (MCAO) resulted in a marked but similar degree of reduction in cerebral blood flow (CBF) in all three groups. After this initial change, CBF increased continuously during occlusion in the mild group. CBF in the moderate and severe groups remained at the same low level during the entire period of MCAO. Immediately after MCAO, NAD reduction was increased by approximately 50% in all groups. At the end of MCAO, while the NAD/NADH redox state returned to its pre-ischemic reference level in the severe group, it remained markedly reduced in the mild and moderate groups. Removal of the clip led to slight reactive hyperemia in the mild and severe groups but not in the moderate group. Immediately after recirculation, NAD/NADH redox was shifted toward oxidation in all groups. However, this reoxidation of NADH was only partial in the mild and moderate groups, and a pronounced hyperoxidation occurred in the severe group. In spite of the similar behavior of CBF in the recovery period, a marked secondary NAD reduction occurred in the moderate group during the recirculation period. It is suggested that this represents cessation of mitochondrial electron transport in the dying cells, accompanied by stimulated anaerobic glycolysis in other cells.(ABSTRACT TRUNCATED AT 250 WORDS)