Yaksh, Tony L.

The time-dependent release of prostaglandin E 2 (PGE 2 ), prostaglandin F 2α (PGF 2α ), thromboxane (Tx) B 2 , and 6-keto-PGF 1α (6-keto) from brain was measured before, during, and after a 15-min interval of total ischemia (four-vessel occlusion) in halothane-anesthetized cats using the technique of cerebroventricular perfusion. Resting levels of PGE 2 , PGF 2α , 6-keto, and Tx were: 253 ± 75, 953 ± 300, 650 ± 200, and 550 ± 170 pg/ml, respectively. During the 15-min ischemia, all prostanoids rose significantly, yet the highest levels were not observed until the first 15–60 min of the reflow at which time levels of PGE 2 , PGF 2α , 6-keto, and Tx, as compared with the preischemic baseline, rose ∼8, 3.4, 3, and 55-fold, respectively. Significantly, although all prostanoids showed increases relative to baseline, the ratios of PGF 2α /6-keto and PGE 2 /6-keto remained stable throughout the experiment in both groups of animals. In contrast, the Tx/6-keto ratio rose from ∼1 to ∼ 30 during the 60 min after reflow in untreated cats. Treatment with zomepirac sodium (5 mg/kg, i.v.), a cyclooxygenase inhibitor, resulted in highly significant reductions in the levels of all prostanoids during the preischemic period. In zomepirac sodium-treated animals, there were also highly significant reductions in the prostanoid response to ischemia. Of particular interest was that the degree of inhibition of Tx as compared with 6-keto, during the post-ischemic period of peak secretion was such that the Tx/6-keto ratio fell from 30 in the control ischemic animal to 0.3 in the zomepirac sodium-treated animal, suggesting a selective blockade of the formation of Tx as compared with prostacyclin.

We report that regional CBF determined by the initial slope technique using 133 Xe and 85 Kr in cats and rabbits can be significantly influenced by the size of the field of measurement. The clearance curves of umbelliferone, a lipid-soluble intracellular pH fluorescent indicator, from a visually avascular 80-fm field were used to cross-correlate rCBF with focal cortical perfusion. Our findings indicate that in the cat, as the gamma or beta detector's field of volume was reduced, regional CBF (rCBF) measured by intraarterially injected 133 Xe or 85 Kr decreased in value by 33% and 28%, respectively, and the slope of the rCBF-P a co 2 response curve became less steep by 56% and 45%, respectively. Umbelliferone, measuring a much smaller volume of tissue, showed a lower normocapnic flow and a more oblique P a co 2 response curve. In the rabbits studied, the normocarbic rCBFs and the rCBF-P a co 2 response curves measured with the three techniques corresponded to those measured in the cat. These results suggest that large field/volume measurements assess a measure of flow that is a weighted average of several distinct flow compartments and that these compartments differ in their response to changes in P a co 2 .

Zomepirac sodium (ZS) (5 mg/kg i.v.) was used to evaluate the effects of preischemia cyclooxygenase inhibition on CBF (as assessed by 133 Xe clearance), CBF–P a CO 2 responsiveness, and electrophysiologic (EEG) parameters before and after a 15-min period of complete global ischemia produced by four-vessel occlusion and mild hypotension. During the 15-min period of ischemia, CBF was essentially zero. Following reflow all groups displayed an initial hyperemia as compared with control (92 ± 11 vs. 141–146 ml/100 g/min). Saline-treated animals during reflow displayed a delayed hypoperfusion (26 ± 3 ml/100 g/min), which showed no improvement during the 2-h reflow period prior to death. In contrast, ZS-treated animals during reflow displayed significantly higher flows during the hypoperfusion phase (72 ± 9 ml/100 g/min). The CBF–P a CO 2 response displayed an approximately sevenfold reduction in slope at 2 h after reflow in saline-treated animals. This decrease in P a CO 2 reactivity was not observed in the ZS-pretreated animals. With regard to EEG, all animals showed a total flattening during the 15 min of ischemia. In saline-treated animals only one of seven showed any sign of even marginal recovery. In ZS-treated animals EEG activity showed prominent recovery in seven of seven. Brainstem auditory evoked potentials were monitored and showed prominent recovery of amplitude and latency in ZS but not saline-treated animals during reflow.

The dose-dependent effects of MK-801, a glutamate receptor antagonist, on changes in CBF, CBF-P a co 2 responsiveness ( 133 Xe clearance), EEG, and blood-brain barrier (methylene blue) were examined after a 15-min period of reversible complete global ischemia induced in halothane-anesthetized cats by occlusion of the vertebral and carotid arteries. Pretreatment with doses of MK-801 of ≥0.5 mg/kg had no effect on resting CBF measures and produced a dose-dependent slowing of the dominant EEG frequency. In animals receiving this agent, there was an almost immediate return of baseline EEG patterns upon reinstitution of flow, no hypoperfusion after 2 h of reflow, preservation of CBF and CBF-P a co 2 responsiveness, and maintenance of blood-brain barrier integrity. In contrast, parallel control animals and animals treated with MK-801 at a dose of 0.1 mg/kg exhibited poor recovery based on the above parameters. MK-801 also diminished in a dose-dependent manner the CSF levels of 6-keto-prostaglandin (PG) F 1α (stable metabolite of PGI 2 ) and thromboxane (Tx) B 2 (stable metabolite of TxA 2 ), which were otherwise elevated in vehicle-treated animals 2 h after reflow. Of particular interest, the CSF TxB 2 /6-keto-PGF 1α ratio in vehicle-treated animals was near 2. In animals pretreated with MK-801, at doses of ≥0.5 mg/kg, this ratio was nearly 1. These observations are consistent with a possible triggering role of glutamate release in initiating at least part of the acute sequelae of ischemia. Such release in an electrically silent cell would increase C a 2+ influx and activate free fatty acid metabolism, leading to probable changes in vascular function and changes in blood-brain barrier permeability.

CBF, as measured by the clearance of 133 Xe or 85 Kr in the pentobarbital-anesthetized cat, displays a monotonic increase as the P a co 2 is elevated over a range of 20–60 mm Hg (slope Xe, 1.65 ± 0.14 ml/100 g/min/mm Hg; slope Kr, 1.40 ± 0.11 ml/100 g/min/mm Hg). Clonidine (20 μg/kg i.v.), a centrally acting, α 2 -preferring agonist, reduced the slope of the P a co 2 -CBF response functions for Xe and Kr by 70 and 64%, respectively. Clonidine reduced normocarbic CBF-Xe by 36%, but had no effect on normocarbic CBF-Kr. ST-91, a polar structural analog of clonidine that does not cross the blood–brain barrier, did not reproduce the effects of clonidine when administered at an equivalent dose. This indicates that the effects of clonidine observed were secondary to its action on central rather than peripheral sites. In addition to the effects on the clearance of CBF markers, clonidine reduced the increased MABP otherwise evoked by elevated P a co 2 . Reduction in the MABP response to P a co 2 did not account for the lowering of CBF during hypercarbia. In separate experiments where MABP was elevated to correspond with the P a co 2 -MABP response observed in the absence of clonidine, a comparable reduction in the slope of the P a co 2 response was also observed. In addition, the pressure autoregulatory response was unaltered after clonidine treatment. These observations suggest that the central action of α 2 -receptors on the CBF-CO 2 response cannot be attributed to an altered perfusion pressure.

The time course of changes in cortical tissue pH (pHi) and blood flow during cortical seizures in halothane-anesthetized cats was examined. The clearance of the molecular form of umbelliferone (Um) was used to estimate focal cortical blood flow (CBFu), whereas the ratio of the molecular to the ionic form of the molecule was used to concurrently calculate the local pHi. Resting pHi and flow in normocarbic animals was 7.116 ± 0.008 and 46 ± 8 ml/100 g/min, respectively. Respiratory induced alterations of Paco 2 over a range of 20–60 torr revealed a correlated change in pHi from 7.39 ± 0.05 to 7.01 ± 0.03 and a monotonic increase in the rate of Um clearance (slope 0.89 ± 0.13 ml/100 g/min/torr). Focal electrical stimulation of the cortex resulted in a rapid vasodilation (50% dilation = 1–3 s) of pial arterioles and venules and an increase in Um clearance. pHi showed no significant change until around 10 s. The maximum fall in pHi occurred by 30–60 s (6.85 ± 0.054). Longer intervals of stimulation (10 min) resulted in no further decline in pHi, but upon cessation of stimulation, pHi remained acidotic for poststimulation periods up to 10 min, with a mild but statistically significant acidosis being observed at 20 min. The absolute decline in pH observed following stimulation appeared to be closely regulated, as comparable levels following stimulation were observed during hypocarbia and hypercarbia. These observations thus suggest that pHi regulation during intense cortical activation may be considered in three phases: (1) following the onset of activity, an initial acute regulation of pHi at control levels; (2) an intracellular acidosis of around 6.8, which is closely regulated and which can be readily reversed upon termination of stimulation; and (3) during continued stimulation, a change in state where in spite of no further change in pHi, the ability of the cortex to return to control pHi appears to be significantly impaired.

In chloralose-urethanized cats, vasoactive intestinal peptide (VIP), applied by superfusion in steady-state concentration (10 −10 –10 −6 M) onto cortical vessels in situ resulted in a rapid concentration-dependent vasodilatation in vessels that were mildly constricted by prostaglandin F 2α (PGF 2α ) (5 × 10 −5 M) or hypocarbia (P a CO 2 = 26). The maximum dilatation produced by VIP (10 −6 M) was about 60% over baseline in pial arteries and 40% in pial veins. Blockade of local neuronal activity with tetrodotoxin (TTX) (10 −5 M) had no effect on the VIP-evoked dilation of pial vessels. Activation of the cortex by either direct electrical stimulation or indirectly by stimulation of the mesencephalic reticular formation (MRF) resulted in a rapid dilatation of pial arterioles and venules. The vasodilatory effects of VIP and of cortical activation via direct cortical stimulation were not blocked by phentolamine (10 −4 M), propranolol (10 −4 M), atropine (10 −4 M), or naloxone (10 −4 M), indicating that the stimulated vasodilatation was not mediated by adrenergic, cholinergic, or opiate receptors. The dilatory effects of MRF, but not direct cortical stimulation, were not blocked by TTX. VIP antiserum (1:25) preincubated in cortical cups had no effect on resting vessel diameter, but resulted in a significant, though subtotal, reduction in the vasodilatation elicited by direct cortical and MRF stimulation. Normal rabbit sera or VIP antiserum preincubated with saturating amounts of VIP were ineffective. In similar experiments, pial arteriolar and venular dilation evoked by hypercarbia was not attenuated by cortically applied VIP antisera. These observations suggest that pial dilation evoked by local increases in neuronal activity may be mediated in part by the local release of VIP from intrinsic neurons. Such a substrate would define a close obligatory coupling between local neuronal activation and local perfusion, such that nutritive flow could be enhanced prior to the onset of any metabolic deficit.

Intracellular brain pH and indicator tissue perfusion were measured with a lipid-soluble, pH-sensitive fluorescent indicator in 10 rabbits who had either severe or moderate focal ischemia depending on whether the middle cerebral artery was occluded at its main trunk or bifurcation. Preocclusion tissue indicator perfusion was 50.1 ml/100 g/min and intracellular brain pH was 7.03. In severe focal ischemia, immediate postocclusion tissue perfusion was 12.7 ml/100 g/min and intracellular brain pH was 6.64. Four hours after occlusion, the perfusion was 5.2 ml/100 g/min and intracellular brain pH was 6.08. There was EEG and histological confirmation of infarction. In the moderate focal ischemia group, immediate postocclusion flow was 20.0 ml/100 g/min and intracellular brain pH was 6.92. At 3 h, postocclusion tissue perfusion was 22.6 ml/100 g/min and intracellular brain pH was 6.86. Therefore, for the first 3 h, this ischemic penumbra was stable. At the fourth hour, both cerebral tissue perfusion and intracellular brain pH worsened. This suggests that the ischemic penumbra is a dynamic state. The rabbit is a good experimental model for the production of both severe and moderate focal ischemia.