Cyanide intoxication induced exocytotic epinephrine release in rabbit myocardium

Cyanide intoxication, which has been used as a model of energy depletion at cardiac sympathetic nerve terminals, causes non-exocytotic release of norepinephrine (NE). However, the effect of cyanide intoxication on cardiac epinephrine (Epi) release remains unknown. Using cardiac microdialysis in the rabbit, we measured dialysate Epi and NE concentrations as indices of myocardial interstitial Epi and NE levels, respectively. Local administration of sodium cyanide (30 mM) through the dialysis probe increased both Epi and NE levels (from 11.3+/-2.3 to 32.3+/-4.4 pg/ml and from 33.6+/-6.1 to 389.0+/-71.8 pg/ml, respectively, mean+/-S.E., P<0.01). Local desipramine (100 microM) administration suppressed the cyanide induced NE response without affecting the Epi response. In contrast, local omega-conotoxin GVIA (10 microM) administration partially suppressed the cyanide induced NE response and totally abolished the Epi response. In conclusion, cyanide intoxication causes N-type Ca(2+) channel dependent exocytotic Epi release as well as inducing N-type Ca(2+) channel independent non-exocytotic NE release.     

Norepinephrine efflux evoked by potassium chloride in cat sympathetic nerves: dual mechanism of action

Using a dialysis technique, prominent efflux of norepinephrine (NE) from cardiac sympathetic nerve endings was observed under local administration of potassium chloride (KCl, 100 mM). KCl induced NE efflux was suppressed by ω-conotoxin GVIA or desipramine but residual efflux of NE was still detectable. In the presence of ω-conotoxin GVIA, KCl induced efflux of NE was augmented by pretreatment with reserpine, indicating that this efflux of NE was derived from axoplasma with neurotransporter. These data suggest that a KCl induced brisk increase in dialysate NE levels might occur as a consequence of exocytotic NE release and carrier mediated outward NE transport from nerve endings.     

Effects of nociceptin on cardiac norepinephrine and acetylcholine release evoked by ouabain

We investigated whether the novel peptide, nociceptin, modulates neuronal transmission at autonomic nerve endings. Using a cardiac dialysis technique, the effects of locally applied nociceptin on cardiac acetylcholine (ACh) and norepinephrine (NE) release were examined in anesthetized cats. Dialysis probes were implanted in the left ventricular wall, with the concentration of dialysate NE or ACh serving as an indicator of NE or ACh output at cardiac sympathetic or parasympathetic nerve endings. Locally applied ouabain evoked increases in NE and ACh output. Nociceptin suppressed the ouabain induced ACh increment. The ouabain induced NE release was not altered by nociceptin. However, in the presence of desipramine (a NE uptake inhibitor), nociceptin suppressed the ouabain-induced NE release. Inhibition by nociceptin of ouabain-induced release of NE or ACh was blocked by pretreatment with nocistatin (a nociceptin action blocking peptide). Nociceptin-induced inhibition of ACh or NE release is attributable to pre-synaptic modulation rather than a reversal of the ouabain effect. These findings demonstrate that nociceptin inhibits cardiac autonomic neurotransmission via a presynaptic opioid receptor-like1(ORL1) receptor.     

Effects of ω-conotoxin GVIA on cardiac sympathetic nerve function

Using a cardiac dialysis technique, the effects of ω-conotoxin GVIA (N-type Ca²⁺ channel blocker) on cardiac sympathetic nerve function was examined in anesthetized cats. Dialysis probes were implanted in the left ventricular wall and the concentration of dialysate norepinephrine (NE) served as an indicator of NE output at cardiac sympathetic nerve endings. Administration of ω-conotoxin GVIA (10 μg/kg i.v.) suppressed dialysate NE responses to the nerve stimulation. The ouabain (1 μM) induced NE increment was less markedly suppressed by ω-conotoxin GVIA. Furthermore, ω-conotoxin GVIA neither influenced neuronal NE uptake nor tyramine induced release of NE from stores. These findings suggest that the neuronal effect of ω-conotoxin GVIA is attributable to a reduction of exocytotic NE release without alterations of neuronal NE uptake or storage. Cardiac dialysis with ω-conotoxin GVIA offers a new approach for the discrimination between Ca²⁺ dependent exocytotic and non-exocytotic NE release.     

Centrally administered ghrelin activates cardiac vagal nerve in anesthetized rabbits

Although central ghrelin has cardioprotective effect through inhibiting sympathetic nerve activity, the effects of central ghrelin on cardiac vagal nerve remain unknown. We investigated the effects of centrally administered ghrelin on cardiac autonomic nerve activities using microdialysis technique. A microdialysis probe was implanted in the right atrial wall adjacent to the sinoatrial node of an anesthetized rabbit and was perfused with Ringer's solution containing a cholinesterase inhibitor, eserine. After injection of ghrelin (1 nmol) into the right lateral cerebral ventricle, norepinephrine (NE) and acetylcholine (ACh) concentrations in the dialysate samples were measured as indices of NE and ACh release from nerve endings to the sinoatrial node using high-performance liquid chromatography. Heart rate was 270±4 bpm at baseline and decreased gradually after ghrelin injection to 234±9 bpm (P<0.01) at 60-80 min, followed by gradual recovery. Dialysate ACh concentration was 5.5±0.8 nM at baseline and increased gradually after ghrelin injection to 8.8±1.2 nM (P<0.01) at 60-80 min; the concentration started to decrease gradually from 100 to 120 min after injection reaching 5.6±0.8 nM at 160-180 min. Central ghrelin did not change mean arterial pressure or dialysate NE concentration. The elevated dialysate ACh concentration declined rapidly after transection of cervical vagal nerves. These results indicate that centrally administered ghrelin activates cardiac vagal nerve.     

In vivo direct monitoring of interstitial norepinephrine levels at the sinoatrial node

We assessed in vivo interstitial norepinephrine (NE) levels at the sinoatrial node in rabbits, using microdialysis technique. A dialysis probe was implanted adjacent to the sinoatrial node of an anesthetized rabbit and dialysate was sampled during sympathetic nerve stimulation. Atrial dialysate NE concentration correlated well with heart rate. Desipramine significantly increased dialysate NE concentrations both before and during sympathetic nerve stimulation compared with the absence of desipramine. However, desipramine did not affect the relation between heart rate and dialysate NE concentration. These results suggest that atrial dialysate NE level reflects the relative change of NE concentration in the synaptic cleft. Microdialysis is a powerful tool to assess in vivo interstitial NE levels at the sinoatrial node.     

New role of nerve growth factor--an inhibitory neuromodulator of adrenergic transmission.

In the present study, we investigated the effects of nerve growth factor (NGF) on norepinephrine (NE) release from peripheral sympathetic nerve endings of rat mesenteric artery. We made isolated mesenteric artery-intestinal loop preparations, by the modified method of Castelluci et al., from 4- and 8-week-old Wistar rats. NGF produced a dose-dependent inhibition of NE overflow from sympathetic nerve endings evoked by electrical nerve stimulation in the range of 0.1-10 ng/ml. Inhibition of NE overflow also occurred in the presence of a neuronal uptake blocker, desipramine (5 x 10(-8) M). NGF showed no effect on pressor response to exogenous NE (1 micrograms). These results suggest that NGF inhibits NE release from sympathetic nerve endings, in other words, NGF acts as an inhibitory neuromodulator of adrenergic transmission. This function of NGF might be considered as an inhibitory feedback mechanism against catecholamine-stimulated NGF synthesis.     

Alteration in sympathetic nerve activity during liver regeneration in rats after partial hepatectomy.

To determine if the sympathetic nerve has a role in liver regeneration, the alterations of tissue norepinephrine (NE) content and its turnover were measured in rats after partial hepatectomy. NE content per unit liver mass decreased progressively to about one-fourth of controls by the ninth day after partial hepatectomy. Since the total amount of NE in the whole liver did not change during nine days of liver regeneration, it was supposed that sympathetic innervation could make slower progress than proliferation of hepatocytes. Fractional turnover rate of NE was reduced transiently in regenerating liver 24-48 h after partial hepatectomy and recovered to normal 8-9 days after the operation. Such a transient reduction of NE turnover was observed specifically in the liver. These results suggest that sympathetic nerve activity of the liver is suppressed at the early stage of regeneration.     

Presynaptic modulation of sympathetic neurotransmission in rat left ventricle slices: effect of pressure overload

Summary. The purpose of this study was to establish the rat left ventricle (LV) tissue slice system for examination of norepinephrine (NE) release from sympathetic nerve terminals. Moreover, initial experiments were performed to use the LV tissue slice system to examine differences in NE uptake and release following cardiac pressure overload induced by abdominal aortic constriction (AC). Kinetic parameters (Vmax, Km) for the specific uptake of [³H]-NE demonstrated high affinity (Km, 1.94 ± 0.83 μM) and moderate capacity uptake (Vmax, 182 ± 6 fmol/mg/weight/min). Following 10 days of pressure overload, the Vmax for [³H]-NE uptake was significantly reduced (by 46%) in LV slices from AC rats compared to sham-operated (SO) controls. In control rat LV slices preloaded with [³H]-NE, electrically evoked [³H]-overflow was calcium- and stimulus pulse number-dependent. The neuronal uptake inhibitor, desipramine (DMI), increased (by 60%) evoked [³H]-overflow from LV slices. The α₂-agonist, UK14304, decreased evoked [³H]-overflow from LV slices in a concentration-dependent manner (maximal reduction of 75%). The β₂-agonist, salbutamol, increased evoked [³H]-overflow from LV slices in a concentration-dependent manner (maximal increase of 200%). In separate experiments, the LV tissue slice system was used to examine the effect of pressure overload on evoked [³H]-overflow. Following 10 days of pressure overload, evoked [³H]-overflow from LV slices of AC rats was increased (by 50%) compared to SO control. Increases in evoked [³H]-overflow from LV slices of AC rats compared to SO controls remained evident in the presence of DMI. These results demonstrate the relative importance of NE release and uptake using an in vitro LV tissue slice system. Sympathetic nerve terminals innervating rat LV were demonstrated to possess functional presynaptic α₂- and β₂-adrenergic receptors. Finally, using this LV tissue slice system, reductions in the uptake velocity and increases in evoked NE release were demonstrated in response to acute cardiac pressure overload. Received August 12, 1999; accepted January 12, 2000     

Autonomic dysfunction in Machado-Joseph disease assessed by iodine123-labeled metaiodobenzylguanidine myocardial scintigraphy

Iodine¹²³-labeled metaiodobenzylguanidine, a radioiodinated analogue of norepinephrine, is a tracer for evaluating sympathetic function. We used iodine¹²³-labeled metaiodobenzylguanidine myocardial scintigraphy and sympathetic skin response to study autonomic nervous functions in 19 patients with Machado-Joseph disease (MJD) and 20 control subjects. Planar imaging of all the participants was done to evaluate myocardial scintigraphy. The ratio of average counts in the heart to average counts in the mediastinum was calculated for both early and delayed images, the latter of which reflects the cardiac neural uptake of the tracer. Single photon emission computed tomography also was done on 12 patients with MJD to examine regional tracer uptake to the heart. The mean ratio of counts in the heart to counts in the mediastinum in the delayed images was lower for the patients with MJD than for the control subjects (p <0.01). Abnormal sympathetic skin response was present in 6 patients with MJD whose mean ratio of counts in the heart to counts in the mediastinum was lower than that of patients with MJD who had normal sympathetic skin response (p <0.01). A single photon emission computed tomography study showed significantly lower accumulation of the tracer in patients with MJD than in the control subjects in the anterior lateral sectors predominantly innervated by sympathetic nerves but not in the inferior septal sectors reported to be under main innervation by parasympathetic fibers. These results show that MJD is accompanied by cardiac sympathetic dysfunction, as detected by iodine¹²³-labeled metaiodobenzylguanidine myocardial scintigraphy, which appears to be correlated with sudomotor sympathetic dysfunction.     

Outcome of epilepsy surgery correlates with sympathetic modulation and neuroimaging of the heart

Temporal lobe epilepsy (TLE) is frequently associated with sympathetic over-activity. Single photon emission computed tomography (SPECT) with 123iodine-meta-iodobenzylguanidine (MIBG), a norepinephrine analogue, showed reduced tracer uptake in cardiac sympathetic nerve endings, indicating myocardial catecholamine disturbance. We investigated whether outcome of epilepsy surgery correlates with cardiac autonomic function in TLE patients.We studied 16 TLE patients before and after epilepsy surgery. We recorded heart rate (HR) and determined sympathetic and parasympathetic cardiac modulation as powers of low (LF, 0.04-0.15 Hz) and high frequency (HF, 0.15-0.5 Hz) heart rate oscillations. The LF/HF-ratio was calculated as index of sympathovagal balance. Cardiac MIBG uptake was assessed with MIBG-SPECT and compared to control data.After surgery, eight patients were seizure-free and eight had persistent seizures. Sympathetic LF-power and LF/HF-ratio were higher in patients who had persistent seizures than in patients who became seizure-free. After surgery, both parameters decreased in seizure-free patients but increased in patients with persistent seizures. MIBG uptake was lower in patients than controls and even lower in the patient subgroup who had persistent seizures. In this subgroup, MIBG uptake further decreased after surgery (P<0.05).Sympathetic cardiac modulation decreased in TLE patients after successful surgery, but further increased if seizures persisted. Reduction of cardiac MIBG uptake progressed after surgery in patients with persistent seizures. Interference of epileptogenic discharges with autonomic neuronal transmission might account for sympathetic cardiac over-stimulation and reduced MIBG uptake. Both findings are possible risk factors for sudden unexplained death and might be relevant for risk stratification in epilepsy patients.     

Effect of insulin excess and deficiency on norepinephrine turnover in rats

Summary To examine whether insulin enhances norepinephrine (NE) turnover, an index of sympathetic nerve activity, the effects of excess insulin and streptozotocin (STZ) induced insulin deficiency were examined in Sprague-Dawley rats. Exogenous insulin caused hyperphagia and elevated (approximately 300%) urinary epinephrine excretion, but did not alter cardiac NE content or turnover. STZ-induced insulin deficiency caused hyperglycemia and hyperphagia, but also did not alter cardiac NE content or turnover. Insulin deficiency reduced hepatic NE content 18%, but did not affect NE turnover or content of kidney or spleen. These data do not support the hypothesis that insulin influences cardiac sympathetic nerve activity in rats.     

ω-Conotoxin GVIA and desipramine insensitive norepinephrine efflux from cardiac sympathetic nerve terminal

Using dialysis technique, prominent accumulation of norepinephrine (NE) in the myocardial interstitial space was observed under local administration of a monoamine oxidase inhibitor (pargyline, 10 mM), and a vesicle uptake inhibitor (reserpine, 10 μM). Accumulation of NE in the myocardial interstitial space was associated with a reduction of dihydroxyphenylglycol levels. Pretreatment with ω-conotoxin GVIA (10 μM) or desipramine (10 μM) did not suppress this response. A brisk increase in dialysate NE levels might occur as a consequence of exocytotic or neurotransporter insensitive efflux of NE.     

Metabolic inhibition potentiates AMPA-induced Ca fluxes and neurotoxicity in rat cerebellar granule cells

The effects of partial metabolic inhibition (induced by 2 h exposure to low concentrations of cyanide (NaCN)) on the glutamate receptor agonist α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-induced excitotoxicity and elevation of free cytoplasmic Ca²⁺ levels ([Ca²⁺]_i) were studied in glucose-deprived primary cultures of cerebellar granule cells. Co-application of AMPA plus NaCN caused a marked increase of cell death, with morphological features of both necrotic and apoptotic cell death as estimated by the capacity of cultured cerebellar granule cells to metabolize 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide into formazan (MTT method), and by measuring the amount of DNA fragmentation in neurons using an ELISA test for histone-bound DNA fragments, respectively. Cell morphology was assessed by confocal microscopy of propidium iodide-stained cultures. No toxic effects were observed when AMPA or a low concentration of NaCN (0.1–0.3 mM; in the presence of NMDA receptor antagonist MK-801; 10 μM) were applied alone. The neurotoxic actions induced by AMPA plus NaCN were preceded and accompanied by a significant elevation of [Ca²⁺]_i, as well as by depletion of neuronal ATP stores. The marked enhancement in the functional responsiveness of AMPA receptors in energetically compromised neurons suggests that at least under certain conditions AMPA receptors may play an important role in excitotoxic processes which might be of relevance for the slowly developing neuronal death seen in several neurodegenerative diseases.     

Intracellular calcium concentrations during "chemical hypoxia" and excitotoxic neuronal injury.

Because hypoxic/ischemic neurodegeneration appears to be in part linked to glutamate neurotoxicity, we measured intracellular calcium (Ca2+i) levels in cultured hippocampal neurons during exposure to toxic doses of glutamate (GLU) and to an anoxic environment simulated by sodium cyanide (NaCN). Changes in Ca2+i produced by cyanide greatly exceeded those induced by GLU. The NaCN response was mimicked when oxidative metabolism was also disrupted by sodium azide, oligomycin, or dinitrophenol. Noncompetitive NMDA receptor antagonists and enzymatic GLU degradation abolished the GLU-induced Ca2+i increases and attenuated those produced by NaCN. Only NaCN-induced increases were blocked when dantrolene and ruthenium red were applied to prevent release from intracellular pools. All responses were reduced proportionally in the absence of added external calcium. These results suggest that extracellular GLU accumulation and subsequent activation of GLU receptors were involved in the NaCN response. During such metabolic compromise, however, GLU-induced elevations of Ca2+i were enormously amplified. In parallel toxicity studies, NaCN was not neurotoxic despite the large elevations in Ca2+i, indicating that a general elevation in cytoplasmic calcium does not necessarily predict neurodegeneration.     

Nerve sprouting suppresses myocardial I(to) and I(K1) channels and increases severity to ventricular fibrillation in rat

Nerve sprouting in healed myocardial infarction has been associated with increased incidences of ventricular tachyarrhythmia and sudden cardiac death. However, the underlying electrophysiological mechanisms are unclear. To investigate the linkage between nerve sprouting and potassium channel function, we developed a rat model of cardiac sympathetic nerve sprouting by chronic subcutaneous injection of 4-methylcatechol, a potent stimulator of nerve growth factor (NGF) synthesis. Cardiac sympathetic nerves were visualized by immunohistochemical staining. Myocardial necrotic injury was created by focal cold shock across intact diaphragm to mimic infarction. Transient outward current (I(to)) and inward rectifier current (I(K1)) of cardiomyocytes were recorded with the whole-cell patch clamp technique. We found that chronic 4-MC administration 1) increased cardiac NGF level and the density of cardiac sympathetic innervation; 2) decreased the expressions of Kv4.2, Kv channel-interacting protein 2 (KChIP2), Kir2.1, and the current densities of I(to) and I(K1); 3) reduced the phosphorylation of extracellular signal-regulated kinase 1/2 (pERK1/2); and 4) decreased heart rate variability and increased the susceptibility to ventricular fibrillation. Myocardial necrotic injury exerted similar effects as 4-methylcatechol, and 4-methylcatechol plus myocardial necrotic injury intensified the cardiac effects of 4-methylcatechol alone and decreased the phosphoralation of cAMP response element-binding protein (CREB). We conclude that nerve sprouting suppressed the expressions and functions of myocardial I(to) and I(K1) channels and increased the susceptibility to ventricular fibrillation. These effects are associated with decreased phosphorylation of ERK and CREB and reduced expression of KChIP2.     

Renal sympathetic responses to changes in arterial pressure in conscious rabbits

Renal sympathetic nerve activity (RSNA) at rest and in response to changes in mean arterial pressure (MAP) were examined by measuring norepinephrine (NE) spillover rate across the kidney in 14 conscious rabbits. NE spillover rate was calculated from veno-arterial difference in plasma NE concentrations across the kidney corrected by the fractional extraction of infused tritiated NE. Renal NE spillover rate (mean +/- S.E.M.) at rest in 14 conscious rabbits was 14.7 +/- 0.9 ng/min. During sodium nitroprusside infusions at 10 and 20 micrograms/kg/min, renal NE spillover rate significantly increased by 55 and 101% in response to falls in MAP of 15 and 19% respectively. During phenylephrine infusion at 2 micrograms/kg/min, renal NE spillover rate significantly decreased by 40% in response to a 14% rise in MAP. The relative contribution of renal to total NE spillover rate decreased during sympathetic stimulation, while this ratio was unchanged during sympathetic inhibition. This study demonstrates that the renal NE spillover method is sufficiently sensitive and reliable to detect the responses in RSNA to physiological stimuli in conscious rabbits. The difference observed in the degree of stimulation of renal versus total NE spillover rate during hypotension produces confirmation that sympathetic responses are not uniformly distributed in the body.     

Characteristics of sympathetic reflexes evoked by electrical stimulation of phrenic nerve afferents.

In chloralose-anaesthetized cats, sympathetic reflex responses were recorded in left cardiac and renal nerve during stimulation of afferent fibres in the ipsilateral phrenic nerve. In cardiac nerve, a late reflex potential with a mean onset latency of 75.6 +/- 13.8 ms was regularly recorded which, in 20% of the experiments, was preceded by an early, very small reflex component (latency between 35 and 52 ms). In contrast, in renal nerve only a single reflex component after a mean latency of 122.1 +/- 13.1 ms was observed. Bilateral microinjections of the GABA-agonist muscimol into the rostral ventrolateral medulla oblongata resulted in a nearly complete abolition of sympathetic background activity and in an 88% reduction of the late reflex amplitude with only small effects on the latency of the evoked potentials. Under this condition, an early reflex component was never observed to appear. After subsequent high cervical spinalization, the residual small potentials which persisted after bilateral muscimol injections were completely abolished and in cardiac nerve an early reflex potential with a mean latency of 45 +/- 10 ms was observed in all but one experiment. The early reflex was therefore referred to as a spinal reflex component which, however, is suppressed in most animals with an intact neuraxis. In the renal nerve a spinal response was only observed in one experiment after spinalization. The results suggest that sympathetic reflexes evoked by stimulation of phrenic nerve afferent fibres possess similar spinal and supraspinal pathways as previously described for somato-sympathetic and viscero-sympathetic reflexes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Membrane depolarization-mediated survival of sympathetic neurons occurs through both phosphatidylinositol 3-kinase- and CaM kinase II-dependent pathways

It has been well established that the NGF-mediated survival of sympathetic neurons in culture occurs through the phosphatidylinositol (PI) 3-kinase/Akt-dependent pathway. In contrast, the mechanism by which membrane depolarization promotes neuronal survival independently of NGF remains unresolved. Here we show that LY294002, a specific inhibitor of PI 3-kinase, induced cell death of sympathetic neurons under depolarizing conditions with elevated K(+) (IC(50)= approximately 30 microM). Interestingly, lower concentrations of this agent (< or =10 microM) were sufficient to suppress Akt phosphorylation at Ser-473, a putative downstream target of PI 3-kinase, under these conditions. We also show that KN-62, a specific inhibitor of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) suppressed depolarization-mediated survival in a does-dependent manner (IC(50)= approximately 2 microM) that paralleled attenuation of sustained levels of intracellular Ca(2+) evoked by depolarization. This IC(50) value is greater than that for CaMKII ( approximately 0.8 microM). These findings led us to hypothesize that depolarization-mediated survival occurs through both the PI 3-kinase/Akt and the CaMKII pathways. Indeed, combined treatment with LY294002 (25 microM) and KN-62 (0.5 microM) dramatically abolished depolarization-mediated survival, whereas each alone did not significantly attenuate it. Under these conditions, KN-62 neither impaired sustained levels of intracellular Ca(2+), nor inhibited the phosphorylation of Akt. It is thus likely that PI 3-kinase and CaMKII independently promote the membrane depolarization-mediated survival of sympathetic neurons in culture.     

Visualization of mitochondrial oxygen fixation in brain slices by gas–tissue autoradiography

We have developed a novel autoradiographic method of visualizing oxygen fixation with sufficient delivery of [¹⁵O]O₂/O₂. Brain slices (400 μm) were preincubated in Krebs–Ringer phosphate buffer and exposed to [¹⁵O]O₂ in a chamber. Fixation of [¹⁵O]O₂ correlated with the polarographically measured oxygen consumption among tissue slices from various organs (r=0.84). The fixation of [¹⁵O]O₂ by brain slices was significantly reduced (7.2% of the control) by heat-treatment or dose dependently by NaCN (18.2% of the control on 50 mM NaCN pretreatment). The ¹⁵O radioactivity in the brain slices prepared from rotenone injected rats was also reduced compared to the control (56.8% of the control side). In an autoradiographic study, ¹⁵O radioactivity showed a heterogeneous distribution both in coronal and sagittal sections. Autoradiography of young and senescent rat brain sections showed reduction of oxygen uptake with aging in the cerebrum, the senescent being 77.4% of the young. This method provides information regarding basic oxygen consumption of tissue slices under condition of sufficient O₂ delivery, which reflects mitochondrial electron transport.