Brain injury impairs ATP-sensitive K+ channel function in piglet cerebral arteries

Relationship among NO, the KATP channel, and opioids in hypoxic pial artery dilation

Nitric oxide (NO), opioids, and ATP-sensitive K+ (KATP) channel activation contribute to hypoxia-induced pial artery dilation. NO releasers and cGMP analogs increase opioid concentration in cerebrospinal fluid (CSF) and elicit dilation via KATP channel activation. Opioids themselves also elicit dilation via KATP channel activation. This study was designed to investigate the relationships among the above mechanisms in hypoxic pial artery dilation using newborn pigs equipped with a closed cranial window. Cromakalim (10(-8) and 10(-6) M), a KATP agonist, produced dilation that was unchanged by the NO synthase inhibitor N-nitro-L-arginine (L-NNA, 10(-6) and 10(-3) M): 13 +/- 1 and 31 +/- 1 vs. 14 +/- 1 and 31 +/- 1% before and after 10(-3) M L-NNA. Cromakalim dilation also was not associated with increased CSF cGMP and was unchanged by the Rp diastereomer of 8-bromoguanosine 3',5'-cyclic monophosphothioate, a cGMP antagonist. Glibenclamide (10(-6) M), a KATP antagonist, attenuated hypoxic dilation but hypoxia-associated CSF cGMP release was unchanged: 457 +/- 12 and 935 +/- 30 vs. 458 +/- 11 and 921 +/- 22 fmol/ml. Coadministration of L-NNA with glibenclamide had no further effect on the already diminished hypoxic dilation but blocked the hypoxia-associated rise in CSF cGMP. Cromakalim had no effect on CSF methionine enkephalin: 1,012 +/- 28 and 1,062 +/- 32 pg/ml. These data show that KATP channel agonists do not elicit dilation via NO/cGMP and do not release opioids. NO release during hypoxia also is independent of KATP channel activation. These data suggest that hypoxic dilation results from the sequential release of NO, cGMP, and opioids, which in turn activate the KATP channel.

Brain injury impairs prostaglandin cerebrovasodilation

Previous studies have observed that ATP- and calcium-sensitive K+ (KATP and Kca) channel function is impaired after fluid percussion brain injury (FPI). The present study was designed to characterize the effect of FPI on prostaglandin (PG)E2 and 12 pial artery dilation and the role of activation of these K+ channels in that dilation in newborn pigs equipped with a closed cranial window. FPI of moderate severity (1.9-2.1 atm) was produced by using a pendulum to strike a piston on a saline-filled cylinder that was fluid coupled to the brain via a hollow screw inserted through the cranium. PGE2 vasodilation was blunted by FPI (9+/-1%, 13+/-1%, and 19+/-1% vs. 2+/-1%, 5+/-1%, and 9+/-1%, for 1, 10, and 100 ng/ml PGE2 before and after FPI, respectively). PGE2 dilation was associated with increased CSF cGMP and cAMP concentration and such changes in cyclic nucleotides were blunted by FPI (448+/-10 and 793+/-38 vs. 316+/-11 and 403+/-27 fmol/ml for control and PGE2 induced change in cGMP before and after FPI, respectively). PGI2-induced dilation and associated changes in CSF cyclic nucleotide concentration were similarly blunted by FPI. PGE2 dilation was attenuated by either glibenclamide or iberiotoxin, KATP and K,ca channel antagonists, and coadministration of both K+ channel antagonists further decremented the dilator response (9+/-1%, 14+/-1%, and 21+/-1%; vs. 4+/-1%, 7+/-1%, and 12+/-1%; vs. 2+/-1%, 4+/-1%, and 7+/-1%, for 1, 10, and 100 ng/ml PGE2 during control, after glibenclamide, and after combined glibenclamide and iberiotoxin, respectively). Glibenclamide and iberiotoxin had similar effects on PGI2 dilation. These data show that prostaglandin dilation is attenuated after FPI. These data also show that prostaglandin dilation is dependent on activation of both KATP and Kca channels. Further, these data suggest that attenuated prostaglandin dilation following FPI results from diminished prostaglandin-associated elevation in cyclic nucleotide concentration and impaired KATP and Kca channel function.

Altered function of inward rectifier potassium channels in cerebrovascular smooth muscle after ischemia/reperfusion

BACKGROUND AND PURPOSE

Several recent studies have demonstrated that inward rectifier potassium channels (K(ir)s) are located on vascular smooth muscle of cerebral arteries in the rat. Activation of the K(ir)s dilates the arteries by relaxing the vascular smooth muscle. We tested the following hypothesis in the present study: function of inward rectifier potassium channels is altered after ischemia/reperfusion (I/R).

METHODS

Temporary (2-hour) focal ischemia was induced in male Long-Evans rats (3% isoflurane anesthesia) by the intraluminal filament model. After 24 hours of reperfusion, ipsilateral and contralateral middle cerebral arteries (MCAs) were harvested and mounted on micropipettes, pressurized to 85 mm Hg, and luminally perfused.

RESULTS

Resting diameters for contralateral (control) and ipsilateral (I/R) MCAs were not significantly different (215+/-4 microm and 211+/-5 microm [n = 6 and n = 7], respectively). Activation of the K(ir)s by abluminal administration of 15 mmol/L KCl to the control MCAs dilated the MCA by 34+/-4% (n = 8). Activation of the K(ir)s in I/R MCAs produced a dilation of only 11+/-3% (n = 8; P<0.001 compared with control). BaCl2 (75 micromol/L), a concentration-selective inhibitor of the K(ir)s, significantly attenuated the dilation produced by 15 mmol/L KCl in control MCAs but not in the I/R MCAs. Endothelial-mediated dilations elicited by the luminal administration of uridine triphosphate (10 micromol/L) produced similar dilations in both groups (32+/-5% for sham [n = 4] and 33+/-2% for I/R [n = 4]), indicating that dilator function in general was not altered in I/R vessels.

CONCLUSIONS

We conclude that Kir function is altered after I/R. The Kir altered function is likely to exacerbate the brain injury occurring after I/R.

ATP-dependent K+ channel activation reduces loss of opioid dilation after brain injury

ATP-dependent K+ (KATP) channel function is impaired after fluid percussion brain injury (FPI). Additionally, the nitric oxide (NO) releaser sodium nitroprusside and a cGMP analog elicit pial dilation via KATP channel activation, whereas opioids such as methionine enkephalin (Met) elicit pial dilation via NO and KATP channel activation. Decremented Met dilation contributes to reductions in pial artery diameter and altered cerebral hemodynamics after FPI. This study was designed to investigate the role of KATP channel activation before FPI in the loss of opioid dilation subsequent to FPI in newborn pigs equipped with a closed cranial window. FPI was produced by allowing a pendulum to strike a piston on a saline-filled cylinder that was fluid coupled to the brain via a hollow screw in the cranium. FPI blunted dilation to Met (7 +/- 1, 11 +/- 1, and 17 +/- 1% before FPI vs. 1 +/- 1, 4 +/- 1, and 6 +/- 1% after FPI for 10(-10), 10(-8), and 10(-6) M Met, respectively). Met-associated elevation in cerebrospinal fluid (CSF) cGMP was similarly blunted (350 +/- 12 and 636 +/- 12 fmol/ml before FPI vs. 265 +/- 5 and 312 +/- 17 fmol/ml after FPI for control and 10(-6) M Met, respectively). In piglets pretreated with cromakalim (10(-10) M) 20 min before FPI, Met dilation was partially restored (7 +/- 1, 10 +/- 1, and 15 +/- 1% before FPI vs. 4 +/- 1, 7 +/- 1, and 11 +/- 1% after FPI for 10(-10), 10(-8), and 10(-6) M Met, respectively). Met cGMP release was similarly partially restored (400 +/- 9 and 665 +/- 25 fmol/ml before FPI vs. 327 +/- 11 and 564 +/- 23 fmol/ml after FPI for control and 10(-6) Met, respectively). Cromakalim (10(-10) M) had no effect on pial diameter itself but prevented pial artery constriction by FPI (148 +/- 5 to 124 +/- 5 microns vs. 139 +/- 4 to 141 +/- 4 microns in the absence vs. presence of cromakalim pretreatment, respectively). In contrast, pretreatment with a subthreshold concentration of NS-1619, a calcium-dependent K+ channel agonist, did not restore vascular and biochemical parameters after FPI. These data indicate that prior KATP channel activation reduces the loss of opioid dilation after FPI.

Role of impaired cAMP and calcium-sensitive K+ channel function in altered cerebral hemodynamics following brain injury

Previous studies have shown that pial arteries constricted and responses to dilator opioids were blunted after fluid percussion injury (FPI) in newborn pigs. Membrane potential of vascular muscle is a major determinant of vascular tone and activity of K+ channels is a major regulator of membrane potential. Recent data show that opioids elicit dilation via the sequential production of cAMP and subsequent activation of calcium-sensitive K+ (K(Ca2+)) channels by this second messenger. The present study was designed to investigate the effect of FPI on cAMP and K(Ca2+) channel function. Chloralose-anesthetized piglets equipped with a closed cranial window were connected to a percussion device consisting of a saline-filled cylindrical reservoir and a metal pendulum. Brain injury of moderate severity (1.9-2.1 atm) was produced by allowing the pendulum to strike a piston on the cylinder. FPI blunted dilation to the cAMP analogs 8-Bromo cAMP and Sp 8-Bromo cAMPs (10(-8), 10(-6) M), (9 +/- 1 and 16 +/- 1 vs. 2 +/- 1 and 3 +/- 1% dilations to 8-Bromo cAMP before and after FPI, respectively, n = 8). Similarly, FPI attenuated dilation to pituitary adenylate cyclase activating peptide (PACAP), an endogenous activator of adenylate cyclase, and NS 1619, a K(Ca2+) channel agonist (9 +/- 1 and 16 +/- 1 vs. 3 +/- 1 and 5 +/- 1% for NS 1619 10(-8), 10(-6) M before and after FPI, respectively, n = 8). Moreover, FPI attenuated PACAP, methionine enkephalin, leucine enkephalin, and dynorphin induced elevations in CSF cAMP concentration (940 +/- 2, 1457 +/- 50, and 2191 +/- 53 vs. 810 +/- 17, 1033 +/- 36, and 1218 +/- 49 fmol/ml for control, PACAP 10(-8), 10(-6) M before and after FPI, respectively, n = 8). These data show that cAMP and K(Ca2+) channel function is impaired after FPI. Further these data suggest that impaired cAMP and K(Ca2+) channel function contribute to altered cerebral hemodynamics following FPI.