Polyamine inhibition preserves somatosensory evoked potential activity after transient cerebral ischaemia.

We tested the hypothesis that the increase in polyamines observed after cerebral ischaemia is related to deficits in electrocortical function as measured by somatosensory evoked potential (SEP). Adult Mongolian gerbils were anaesthetized with ketamine and prepared for monitoring SEP, cerebral blood flow (CBF) in parietal and frontal regions by H2 clearance, and for bilateral carotid artery occlusion (BCO). Seven animals served as controls and received saline. Another 7 animals were treated with the ornithine decarboxylase inhibitor, difluoromethylornithine (DFMO) (100 mg/kg I.P.) just prior to 40 min BCO followed by 4 h reperfusion. With BCO, both CBF and SEP declined significantly. In control animals, CBF fell from basal 37.8 +/- 4.7 cc/100 g/min to 2.9 +/- 1.2 cc/100 g/min and recovered to 22.7 +/- 3.5 cc/100 g/h over the 4 h reperfusion period. DFMO treatment did not alter this CBF pattern. SEP amplitude declined to 11.3 +/- 3.2% basal during occlusion. DFMO preserved SEP during ischaemia (35.5 +/- 16.8% basal) and remained significantly more preserved during reperfusion (p less than 0.05). These results suggest that polyamines are involved in the progressive decline in neuroelectrical function which occurs during occlusion/reperfusion in the Mongolian gerbil. The observation that polyamine inhibition preserves electrical function despite not altering blood flow indicates that the effects of polyamines are not manifested at the level of the vasculature but perhaps at the neuronal membrane.

Thromboxane synthetase inhibition with imidazole increases blood flow in ischemic penumbra.

Previous studies have indicated that the regional distribution of the arachidonic acid metabolites around a focal ischemic lesion may be important in the pathogenesis of cerebral ischemia. To determine the functional significance of this regionalization, we examined the effect of imidazole (a thromboxane synthetase inhibitor) on the distribution of the vasoconstrictor thromboxane and the vasodilators prostacyclin and prostaglandin E2 (PGE2) and on the distribution of cerebral blood flow (CBF) around a focal ischemic lesion, middle cerebral artery (MCA) occlusion in the cat. The study was conducted in two phases. The first phase examined regional distribution of tissue arachidonic acid metabolites and the effect of imidazole treatment on that distribution. The second phase examined the effect of imidazole treatment on the distribution of blood flow about the focal ischemic lesion as well as on electrocortical function and edema production. MCA occlusion resulted in increased thromboxane, prostacyclin, and PGE2 levels in the ipsilateral hemisphere. These increases were greatest in the region of marginal ischemia and were present both 3 and 6 hours after occlusion. Imidazole pretreatment (50 mg/kg i.p.) significantly inhibited thromboxane production, but augmented production of prostacyclin and PGE2. In the blood flow studies, imidazole was without effect on regions of dense cerebral ischemia (CBF less than 20 ml/minute/100 g for more than 12 of 24 postocclusion hours). In regions of marginal ischemia (20 less than CBF less than 30 ml/minute/100 g for more than 12 of 24 postocclusion hours), imidazole pretreatment significantly increased blood flow in both gray and white matter compared with saline-treated controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Moderate hypothermia reduces postischemic edema development and leukotriene production.

Using the bilateral carotid artery occlusion model of cerebral ischemia in the gerbil, we studied the effect of moderate hypothermia (30 to 31 degrees C) on the postischemic production of prostanoids (cyclooxygenase pathway) and leukotrienes (lipoxygenase pathway) and accompanying changes in cerebral edema formation. Hypothermia capable of slowing central evoked potential conduction time was studied over the course of 40 minutes of cerebral ischemia and for up to 2 hours of reperfusion. The successful induction of cerebral ischemia was confirmed by somatosensory evoked potential amplitude changes. Measurements of 6-ketoprostaglandin F1 alpha (PGF1 alpha) and leukotriene B4 (LTB4) (radioimmunoassay) and cerebral edema (specific gravity) were made at early (10 minutes) and late (2 hours) reperfusion times. Although both white and gray matter showed no early significant difference in edema accumulation between normothermic and hypothermic gerbils at 10 minutes of reperfusion, hypothermic animals demonstrated significantly less white matter edema (specific gravity, 1.0397 +/- 0.0010 vs. 1.0341 +/- 0.0012, P less than 0.01) and gray matter edema (specific gravity, 1.0408 +/- 0.0009 vs. 1.0365 +/- 0.0008, P less than 0.01) by 2 hours of reperfusion. Production of PGF1 alpha was not significantly different between normothermic and hypothermic animals during the reperfusion period; however, hypothermic gerbils demonstrated significantly lower production of LTB4 at 10 minutes reperfusion time compared to normothermic animals (1.49 +/- 0.79 vs. 5.28 +/- 1.49 pg/mg of protein, P less than 0.05). This difference between the two groups in LTB4 levels was no longer detectable at 2 hours of reperfusion time.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of children and young adults with tethered spinal cord syndrome. Utility of spinal and scalp recorded somatosensory evoked potentials.

The diagnosis of tethered spinal cord syndrome should be considered in young patients with progressive orthopedic deformities, lower extremity weakness, urinary and fecal incontinence, low back pain, or combinations of these symptoms. Myelographic, computed tomographic, and urodynamic studies are useful for establishing a diagnosis, but contribute little to the evaluation of lower extremity sensory function or to the assessment of electrophysiologic impairment of the spinal cord itself. To determine the diagnostic usefulness of the somatosensory evoked potential after posterior tibial nerve stimulation (posterior tibial nerve somatosensory evoked potential) in tethered spinal cord syndrome, 22 consecutive patients with symptoms of tethered spinal cord syndrome (aged 18 months to 22 years) underwent recording of posterior tibial nerve somatosensory evoked potential; results were correlated with clinical, myelographic, and operative findings. In patients with clinical symptoms but no myelographically demonstrable lesions, posterior tibial nerve somatosensory evoked potentials were within normal limits, suggesting normal physiologic function. In patients with myelographically and operatively confirmed tethering dysraphic lesions, posterior tibial nerve somatosensory evoked potential was predictive of the level and laterality of the lesion. Similarly, ranking the severity of neurological impairment and extent of dysraphism at operation, as well as the extent of abnormality of posterior tibial nerve somatosensory evoked potential, revealed a significant (r = 0.81, p less than 0.001) correlation between clinical severity and posterior tibial nerve somatosensory evoked potential abnormalities. Postoperatively, in 8 patients, posterior tibial nerve somatosensory evoked potential also reflected improved function in relation to the level and type of dysraphic lesion present. These findings indicate that posterior tibial nerve somatosensory evoked potential is a sensitive indicator of neurophysiologic status in patients with tethered spinal cord, and is useful for determining the level of the conus medullaris, degree of spinal cord displacement, and severity of neurological impairment associated with this congenital disturbance of neuraxis formation. Recording of posterior tibial nerve somatosensory evoked potential is noninvasive and offers a more sensitive diagnostic tool than the clinical testing of sensation for detection of the development of neurologic deficits in patients with tethered cord syndrome.

Lipoxygenase metabolites of arachidonic acid and the development of ischaemic cerebral oedema.

This study examined the changes in cerebral blood flow, water content, and lipoxygenase metabolites (leukotrienes) following bilateral carotid artery occlusion (BCO) and reperfusion in the gerbil. The effect of inhibiting lipoxygenase with nordihydroguaretic acid (NDGA) was also examined. BCO caused cerebral blood flow (measured using H2 clearance) to decline from 23.5 +/- 1.9 to 4.5 +/- 1.9 ml/min/100 gm. Reperfusion increased flow to 27.9 +/- 4 ml/min/100 gm at 10 min, which declined to 13.7 +/- 1.3 ml/min/100 gm at 50 min. Concomitant oedema measurement revealed brain specific gravity decreasing to 1.0402 +/- 0.0014 at 10 min and to 1.0325 +/- 0.0006 at 50 min reperfusion (nonoccluded controls). Leukotriene B4 (LTB4) increased from 26.8 +/- 4.6 to 33.5 +/- 2.1 pg/mg protein 10 min after reperfusion (p less than 0.05), but declined to 21.8 +/- pg/mg protein by 100 min (vs nonischaemic control = 21.3 +/- 2.9 pg/mg protein). Activation of arachidonate metabolism was confirmed by significantly increased 6 keto PGF1 alpha. Pretreatment of the animals with NDGA did not alter CBF, but increased specific gravity above saline-treated controls at 50 min of reperfusion (NDGA = 1.0370 +/- 0.002 vs control = 1.0325 +/- 0.0006, p less than 0.05). Similarly, NDGA blunted the increase in LTB4 formation 10 min after reperfusion (control = 26.8 +/- 4.6 pg/mg protein vs NDGA = 29.7 +/- 2.9 pg/mg protein, p = N.S.). These findings indicate that LTB4 production is stimulated by BCO and reperfusion in the gerbil, and that this stimulation occurs early on in the reperfusion. Further, we observe that the lipoxygenase inhibitor NDGA limits the formation of ischaemic cerebral oedema.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of cyclooxygenase and lipoxygenase metabolites of arachidonic acid after transient cerebral ischemia.

Vasoactive arachidonic acid metabolites are postulated to play a role in the pathogenesis of cerebral ischemia. In order to characterize the local generation of cyclooxygenase and lipoxygenase metabolites of arachidonic acid in transient ischemia with reperfusion, Mongolian gerbils were studied for regional cerebral blood flow (CBF), using the hydrogen clearance technique, and for cerebral levels of the thromboxane metabolite TXB2, and prostaglandins 6-keto-PGF1 alpha and PGE2, as well as the leukotriene LTB4. The gerbils were anesthetized with pentobarbital, and half of the animals were pretreated with the cyclooxygenase inhibitor indomethacin. All received 10 or 20 minutes of dense forebrain ischemia followed by reperfusion of 10 minutes, 50 minutes, or 100 minutes. A separate control group received no ischemic lesion. Regional CBF decreased significantly from 23.7 +/- 2.6 to 4.3 +/- 1.7 cc/100 gm/min during ischemia (p less than 0.01). Reperfusion resulted in initially normal flows (22.5 +/- 5.1 cc/100 gm/min) followed by a progressive hypoperfusion (11.3 +/- 2.7 cc/100 gm/min). All metabolites showed parallel significant (p less than 0.05) increases after transient ischemia and reperfusion compared to baseline levels (values (in pg/mg protein) were: TXB2 45.5 +/- 7.1 vs 23.3 +/- 3.6; 6-keto-PGF1 alpha 262.8 +/- 47.9 vs 175.8 +/- 26.8; PGE2 256.5 +/- 35.6 vs 112.5 +/- 11.2; and LTB4 37.8 +/- 4.6 vs 24.6 +/- 6). These levels were all significantly decreased (p less than 0.05) by pretreatment with indomethacin except for the leukotriene LTB4, which was increased. Transient cerebral ischemia results in a reperfusion abnormality and the local generation of cyclooxygenase products, which are reduced by pretreatment with indomethacin; however, cyclooxygenase inhibition may result in increased substrate availability for the lipoxygenase system. Studies of such an interaction may lead to new understandings of the pharmacological modification of detrimental vascular changes after transient cerebral ischemia.

Effects of verapamil and diltiazem on acute stroke in cats.

To test the effect of verapamil and diltiazem in acute stroke, three groups of mongrel cats of either sex underwent occlusion of the middle cerebral artery (MCA) via a transorbital approach under ketamine anesthesia. The first group served as controls, the second received an intravenous infusion of verapamil (0.1 microgram/kg/min), and the third received an intravenous infusion of diltiazem (0.1 to 1.0 microgram/kg/min). All drug infusions began 2 hours before MCA occlusion and continued for the remainder of the experiment. Before and for up to 24 hours after MCA occlusion, regional cerebral blood flow (rCBF), somatosensory evoked potentials (SSEP's), arterial blood gases, blood pressure, temperature, and hematocrit were measured at least every 2 hours. At the experiment's end, brains were perfused with India ink, removed, sliced, photographed for determination of nonperfused brain area, and weighed, dried, and reweighed for H2O content determination. In these studies, verapamil was associated with worsening of rCBF in ischemic regions and inappropriate increases in rCBF in nonischemic regions, indicating intracerebral steal. Diltiazem increased rCBF in marginally ischemic regions. Changes in SSEP's paralleled blood flow changes, with verapamil decreasing amplitude and conduction velocity while diltiazem slightly improved conduction in the ischemic brain. Verapamil increased the area of nonperfused brain and the content of cerebral H2O. Diltiazem-treated animals had decreased cerebral H2O content, but had a marked increase in the area of nonperfused brain, a finding associated with the high incidence of transtentorial herniation in the diltiazem-treated animals. These findings agree with in vitro studies demonstrating high sensitivity of cerebral blood vessels to calcium channel blockers. These studies further support the notion that calcium channel blockers probably affect several different classes of calcium channels, at different brain sites.

Polyamine and prostaglandin markers in focal cerebral ischemia.

This study examines the pathophysiology of stroke secondary to focal cerebral ischemia. The interaction of arachidonic acid metabolites and polyamines, a class of ubiquitous ornithine-derived molecules with important membrane effects on edema, Ca++-dependent endocytosis, platelet function, and prostaglandin (PG) formation, are correlated with regional changes in H2 clearance, cerebral blood flow (rCBF), ischemic edema, and somatosensory evoked responses (SSERs) after middle cerebral artery (MCA) occlusion. Thirty cats were studied up to 3 hours before and 6 hours after right MCA occlusion. Four areas of brain showing different levels of perfusion after MCA occlusion were sampled for tissue levels of PGs: 6-keto-PGF1 alpha, PGE2, and as well as thromboxane B2 (TXB2), ornithine decarboxylase activity (ODC) (a measure of polyamine activity) and gravimetric determination of cerebral edema. After right MCA occlusion, right hemisphere SSER amplitude decreased and interpeak latency increased markedly. rCBF was distributed into zones of dense, partial, and no ischemia ranging from 12.6 to 59.4 ml/100 g/minute. Ischemic edema was distributed inversely to rCBF and was increased in areas of dense ischemia (85.2 +/- 0.5%) and ischemia (82.7 +/- 0.7%), but not in partially ischemic or control areas. 6-Keto-PGF1 alpha (1257.3 pg/mg), PGE2 (1628.5 pg/mg), and TXB2 (1572.8 pg/mg) were all significantly (P less than 0.05) increased in areas of partial ischemia that had not yet developed edema. ODC levels were significantly elevated (3812 pmole/g/hour, P less than 0.05) and increased with time in areas of slightly denser ischemia that showed an intermediate increase in edema, but not the presence of infarction. This is the first demonstration that ODC, the rate-limiting enzyme for polyamine synthesis, is stimulated by cerebral ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Indomethacin-mediated improvement following middle cerebral artery occlusion in cats. Effects of anesthesia.

Focal cerebral ischemia initiates multiple detrimental effects in the brain. Chief among these are the regional development of ischemic edema, decreased local perfusion, altered neuronal function, and eventual infarction. To determine if pretreatment with the cyclo-oxygenase inhibitor, indomethacin, would result in improvement in these parameters, adult cats were given indomethacin or control solvent (4 mg/kg intraperitoneally twice daily) and were studied for periods up to 24 hours after right middle cerebral artery occlusion. The interaction of anesthetic agents with indomethacin was also examined in separate groups of experimental animals using pentobarbital and ketamine. In cats allowed to recover from pentobarbital anesthesia, indomethacin reduced gray and white matter edema at 6 and 24 hours after occlusion (p less than 0.05). This was noted in densely areas (indomethacin = 84.3%, control = 87.5%), "penumbra" regions (indomethacin = 82.5%, control = 85.3%), and in nonischemic zones (indomethacin = 81.5%, control = 82.3%) at 24 hours. Somatosensory evoked potential amplitude and central latency were also improved in the indomethacin group (p less than 0.05), as was cerebral perfusion (p less than 0.05). In animals anesthetized with continuous ketamine administration, cerebral edema and perfusion as well as evoked potentials were not significantly improved in any region by indomethacin. Regional cerebral blood flow in the group was increased by indomethacin in the nonischemic opposite hemisphere (indomethacin = 64.7 cc/100 gm/min, control = 48.5 cc/100 gm/min, p less than 0.05), but not in the penumbra region of the ischemic hemisphere (indomethacin = 15.0 cc/100 gm/min, control = 18.6 cc/100 gm/min, p less than 0.05), when measured 4 hours after occlusion. This suggested a steal phenomenon. Beneficial effects of indomethacin were evident in the presence of pentobarbital, but not after ketamine anesthesia. This suggests a synergism dependent on decreased arachidonic acid production from pentobarbital-stabilized membranes coupled with diminished production of cyclic endoperoxides from available arachidonate due to inhibition of cyclo-oxygenase with indomethacin.

Somatosensory evoked potentials as a measure of experimental cerebral ischemia.

Somatosensory evoked potentials (SEP's) reflect the integrity of the central neuronal pathway, and as such may be used to assess function that remains during a variety of cerebral insults. To evaluate the natural history and utility of SEP's during experimental cerebral ischemia and infarction, SEP's were measured in 17 adult cats at 24 hours and 1 hour prior to right middle cerebral artery (MCA) occlusion, and again immediately afterward and at either 6 hours (five cats) or 24 hours (six cats) post-occlusion. Before occlusion of the right MCA, the SEP's were identical in the right and left hemispheres. After occlusion, there was a significant slowing of the interpeak latency of the first positive peak (P1) in the right hemisphere (3.53 +/- 0.6 msec before compared to 3.99 +/- 0.6 msec after occlusion, p less than 0.001). Maximal slowing in right hemisphere P1 latency was seen in those animals in which the stroke extended into the thalamus (4.38 +/- 0.1 msec). This was significantly slower than left hemisphere values (3.92 +/- 0.32 msec, p less than 0.01). The ipsilateral cortical components of the SEP's, the second positive peak (P2), and the major negative deflection (MN) were slowed in all cats immediately after right MCA occlusion compared to preocclusion measurements (p less than 0.001). Severe infarcts in the mid-suprasylvian and posterior ectosylvian gyri (including the somatosensory cortex) resulted in a greater slowing of the latency of MN compared to less severe infarcts in that region (20.6 +/- 3.9 msec versus 16.4 +/- 1.1 msec, p less than 0.05). There was a precipitous decrease in the amplitude or voltage of the ipsilateral P2-MN complex immediately after occlusion (5.32 +/- 0.4 microV before compared to 0.98 +/- 0.3 microV after occlusion, p less than 0.001). Therefore, the central latencies and cortical amplitudes of the SEP's are sensitive experimental tools as indicators of the onset and extent of a cerebral ischemic insult.

Mechanism for inhibition of renin release by acute plasma volume expansion in the dog.

Plasma volume expansion alters renal tubular sodium chloride transport and renal nerve activity. The purpose of this study was to determine the mechanism(s) for inhibition of renin secretion by acute volume expansion with albumin in the anesthetized dog. In dogs with a single intact kidney, albumin infusion decreased renin release by 86% and significantly increased renal blood flow, glomerular filtration rate, and sodium excretion. Albumin volume expansion inhibited renin secretion to a lesser extent in dogs with denervated filtering kidneys and in dogs with innervated nonfiltering kidneys. In dogs with denervated nonfiltering kidneys, albumin infusion did not change renin secretion. Comparable volume expansion was produced in all groups. Thus, inhibition of renin release by acute plasma volume expansion is dependent on both a renal tubular mechanism and the integrity of the renal nerves. Partial inhibition of renin release was observed with interruption of either one of the mechanisms, whereas interruption of both mechanisms totally abolished the effect of acute plasma volume expansion on renin secretion.

Neural control of pancreatic insulin and somatostatin secretion.

This study had a dual purpose. First, the effects of pancreatic buffer flow on whole organ hormone output were investigated. Second, the receptor mechanisms by which sympathetic nerve stimulation alters the secretion rates of somatostatin and insulin were assessed. Pancreata of mongrel puppies were perfused in situ with nonrecirculated Krebs-Ringer bicarbonate buffer (150 mg/dl glucose). Buffer flow was adjusted between 0.2 and 4 ml/min X g pancreas. Insulin secretion rate (nanograms per min/g pancreas; ISR) as well as pancreatic O2 and glucose consumption increased as flow increased between 1 and 2 ml/min X g, where each reached a maximum plateau. Thus, ISR was shown to be dependent on flow over the middle range of flow investigated. In separate experiments, bilateral stimulation of the splanchnic nerves or pancreatic arterial infusion of norepinephrine to a final concentration of 60 microM decreased ISR and the somatostatin secretion rate (SSR). Adrenergic suppression of ISR was antagonized by phentolamine and phenoxybenzamine. Adrenergic inhibition of SSR was blocked only by phenoxybenzamine. Propranolol had no effects. We conclude that norepinephrine is sufficient to account for sympathetic inhibition of ISR and SSR (e.g. it is not necessary to postulate another transmitter) and that this inhibition may be transmitted through an effect on the islet vasculature or an effect on the islet cells themselves. The types of alpha-adrenoceptors mediating the adrenergically induced decrease in ISR differ from those causing the decrease in SSR.

Effects of verapamil on renin and aldosterone in the dog and rat.

The purpose of this study was to evaluate the effects of calcium channel blockade on renin secretion and plasma aldosterone. Verapamil infusion (0.004 mg X kg-1 X min-1) into the renal artery of uninephrectomized dogs with an intact kidney resulted in significant increases (P less than 0.05) in renal blood flow, creatinine clearance, and the excreted fractions of Na+ and Cl-; renin secretion decreased (P less than 0.05) and plasma aldosterone did not change. Conversely, renal arterial infusion of verapamil in dogs with nonfiltering, denervated, papaverine-treated kidneys resulted in no change in renal blood flow and a significant increase (P less than 0.05) in renin secretion. In the rat, compared with untreated control animals, dietary verapamil (12.5 mg X kg-1 X day-1) did not effect plasma renin activity but significantly suppressed plasma aldosterone (P less than 0.001) in animals on NaCl-restricted [14.7 +/- 5.4 vs. 41.6 +/- 7.8 ng/dl (SE)] and NaCl-free [103.7 +/- 7.5 vs. 156.7 +/- 18.7 ng/dl (SE)] diets. In addition, verapamil suppressed (P less than 0.0003) plasma corticosterone [16.1 +/- 5.4 vs. 52.8 +/- 7.1 microgram/dl (SE)]. Thus, acute but not chronic verapamil administration stimulates renin release in the nonfiltering kidney, and chronic verapamil inhibits adrenal mineralocorticoid and glucocorticoid secretion. The disparate effects of verapamil on renin secretion from intact and nonfiltering kidneys may be due to actions of the Ca2+ channel blocker on renal hemodynamic and/or renal tubular mechanisms in the intact kidney that mask a direct effect of verapamil on renin-secreting cells.

Cerebral vascular response to moderate blood loss: modification by hypertension.

To study the effect of non-hypotensive hemorrhage on cerebral blood flow in normo- and hypertensive states, chloralose anesthetized cats were subjected to graded blood loss (5 ml/kg) every 30 min. Cerebral blood flow was measured using radiolabelled microspheres or H2 clearance. Hypertension was produced by infusion of phenylephrine to a diastolic blood pressure of 100 mm Hg. Control animals suffered no net blood loss. PCO2 was between 28 and 32 mm Hg for all groups over the entire experiment. In normotensive cats, cerebral blood flow increased following withdrawal of 10 ml/kg of blood. In hypertensive cats, cerebral blood flow increased after withdrawal of 20 ml/kg of blood. These findings were consistent for all brain regions examined. Animals without blood loss, whether normo- or hypertensive showed no consistent change in cerebral blood flows. Possible explanations for these findings, particularly neurally mediated responses, are discussed.

Pancreatic somatostatin secretion is suppressed by splanchnic nerve stimulation.

The pancreases of fasted mongrel puppies (n = 4) were isolated and perfused in a non-recirculating system with an oxygenated Krebs Ringer bicarbonate buffer. The gland remained in situ and innervated. Bilateral stimulation of the splanchnic trunks (20 V, 5 Hz, 1 msec) resulted in a significant decrease in somatostatin (57 +/- 10%, p less than 0.01) and insulin (71 +/- 3%, p less than 0.01), secretion rates, together with a significant increase in pancreatic perfusion pressure (53 +/- 6%, p less than 0.05). Perfusate glucose concentration remained constant during stimulation. No difference was found between three, three minute stimulations in any one dog, but significant differences were found among the dogs in the prestimulation somatostatin secretion rate in spite of the similarities in the buffer glucose concentration.