Influence of hypoxia/ischemia on cerebrovascular responses to oxytocin in piglets.

We examined the effects of hypoxic/ischemic stress on cerebral arteriolar responses to oxytocin in anesthetized piglets. Pial arteriolar diameters were measured using a cranial window and intravital microscopy. First, we evaluated arteriolar responses to topical application of oxytocin during normoxic conditions. We then determined whether 5-10 min of arterial hypoxia, ischemia, or asphyxia alters oxytocin-induced responses. Arterial hypoxia was produced by inhalation of 7.5% O2-92.5% N2 for 10 min. Ischemia was achieved by increasing intracranial pressure for 10 min. Asphyxia was achieved by turning off the ventilator for 5 min. During normoxic conditions, oxytocin dilated pial arterioles by 9 +/- 1% at 10(-8) and by 16 +/- 1% at 10(-6) mol/l (n = 47, p < 0.05). Arteriolar responses to oxytocin did not change with repeated applications (n = 10). Following hypoxia, dilator effect of oxytocin was not changed at 10(-8) (8 +/- 2%) but it was reduced at 10(-6) mol/l (7 +/- 2%; p < 0.05, n = 8). After asphyxia or ischemia, oxytocin did not dilate arterioles at 10(-8) mol/l, whereas 10(-6) mol/l resulted in a mild vasoconstriction (-4 +/- 3 to -6 +/- 4%, n = 6 and 8). Topically applied superoxide dismutase did not preserve arteriolar responses to oxytocin after asphyxia although the arterioles did not constrict to 10(-6) mol/l oxytocin (n = 5). Dilatation of cerebral arterioles in response to oxytocin was reversed to constriction by N(omega)-nitro-L-arginine methyl ester (L-NAME) (15 mg/kg, i.v.; n = 5) and by endothelial impairment by intra-arterial infusion of phorbol ester (10[-5] mol/l; n = 5). We conclude that the absence of pial arteriolar dilation to oxytocin after ischemia and asphyxia indicates endothelial dysfunction which may be involved in the pathology of perinatal brain injury.

Interaction between ATP-sensitive K+ channels and nitric oxide on pial arterioles in piglets.

The interaction between ATP-sensitive K+ channels (KATP) and nitric oxide (NO) was studied in pial arterioles of piglets. We examined the effects of N omega-nitro-L-arginine methyl ester (L-NAME), a general inhibitor of nitric oxide synthase (NOS), and 7-nitroindazole (7-NI), a selective inhibitor of neuronal NOS, on aprikalim-induced cerebral vasodilation. Topically applied, aprikalim, a selective activator of KATP, dilated arterioles by 11 +/- 7% at 10(-8) M and 17 +/- 6% at 10(-6) M. After L-NAME treatment (15 mg/kg, i.v.), the response was reduced (4 +/- 4% and 12 +/- 7%, respectively; n = 8, p < 0.05). Administration of 7-NI (50 mg/kg, i.p.) did not change pial arteriolar responsiveness to aprikalim. However, both L-NAME and 7-NI reduced the vasodilator responses to 10(-4) M N-methyl-D-aspartate (NMDA) (by 73% and by 36%, respectively). Furthermore, 7-NI treatment abolished the glutamate-induced dilatation of pial arterioles. Administration of L-NAME reduced the NOS activity in the cerebral cortex by 88%, whereas the reduction after the 7-NI treatment was 44%. Pre-treatment and coadministration of 10(-5) M glibenclaminde, a specific inhibitor of KATP or L-NAME administration, did not change the dilatory response to sodium nitroprusside. We conclude that NO may be involved in aprikalim-induced dilation of pial arterioles.

Differential effects of short-term hypoxia and hypercapnia on N-methyl-D-aspartate-induced cerebral vasodilatation in piglets.

BACKGROUND AND PURPOSE: Recent studies in piglets show that either asphyxia or global cerebral ischemia, which combines effects of hypoxia and hypercapnia, transiently attenuates N-methyl-D-aspartate (NMDA)-induced pial arteriolar dilation. The purpose of this study was to determine individually the effects of hypoxic hypoxia and normoxic hypercapnia on NMDA-dependent cerebrovascular reactivity. In addition, we examined mechanisms involved in reduced cerebral vascular dilation to NMDA. METHODS: In anesthetized piglets, we examined pial arteriolar diameters using a cranial window and intravital microscopy. Arteriolar responses to topically applied NMDA were determined under control conditions and after arterial hypoxia or arterial hypercapnia. In addition, arteriolar responses to NMDA were examined in animals given indomethacin (10 mg/kg IV) or superoxide dismutase (100 U/mL, topical application) before hypoxia. RESULTS: Under control conditions, application of NMDA produced a dose-related dilation of pial arterioles (eg, 9 +/- 1% to 10(-5), 15 +/- 2% to 5 x 10(-5), and 28 +/- 5% to 10(-4) mol/L NMDA above baseline, respectively, in the hypoxic group; n = 6, P < .05). After transient exposure to 15 minutes of hypoxic hypoxia, arteriolar responses to NMDA were reduced at 30 minutes and at 60 minutes (10(-4) mol/L NMDA dilated by 12 +/- 5% and 18 +/- 5%, respectively; n = 6, P < .05). Five minutes of hypoxic hypoxia also reduced dilatation to NMDA. Indomethacin or superoxide dismutase preserved arteriolar responses to NMDA after 15 minutes of hypoxia. Pial arteriolar responses to NMDA remained unimpaired during and after hypercapnia. CONCLUSIONS: Short-term severe hypoxic hypoxia and reventilation impair the NMDA-induced dilatation of pial arterioles. Respiratory acidosis alone does not modify pial arteriolar reactivity to NMDA. The reduced responsiveness of the cerebral blood vessels to NMDA caused by hypoxia appears to be due to action of oxygen radicals.

Effects of ischemia on cerebrovascular responses to N-methyl-D-aspartate in piglets.

We examined the effects of total global ischemia on cerebral arteriolar responses to N-methyl-D-aspartate (NMDA) in anesthetized newborn pigs. Arteriolar responses to 10(-4) M NMDA were determined before and after 10 to 20 min of ischemia caused by increasing intracranial pressure. Before ischemia, NMDA dilated arterioles by 30 +/- 5% (baseline = 88 +/- 2 microns; n = 6). However, after 10 min of ischemia, arteriolar dilation was reduced to 10 +/- 3% at 1 h (P < 0.05). At 2 and 4 h, NMDA-induced dilation was not different from preischemia values. Twenty minutes of ischemia had similar effects. Coadministration of 100 U/ml of superoxide dismutase did not restore arteriolar dilation to NMDA at 1 h after ischemia. Sodium nitroprusside dilated by 14 +/- 3 and 40 +/- 5% at 10(-6) and 10(-5) M before ischemia, respectively, and arteriolar responsiveness was not changed by ischemia (n = 6). Cortical nitric oxide synthase (NOS) activity, measured by the in vitro conversion of L-[14C]arginine to L-[14C]citrulline, was unaffected by ischemia (n = 12). We conclude that decreases in cerebral arteriolar responsiveness to NMDA are not due to impairment of NOS activity, enhanced degradation or chelation of nitric oxide (NO), or reduced vascular smooth muscle responsiveness to NO.

Ischemia reduces CGRP-induced cerebral vascular dilation in piglets.

BACKGROUND AND PURPOSE: Effects of anoxic stress on cerebrovascular responses to calcitonin gene-related peptide (CGRP) have not been examined previously. We determined the effects of total global ischemia on cerebral arteriolar responses to CGRP in newborn pigs. METHODS: Piglets were anesthetized and ventilated with a respirator. Pial arteriolar diameter was determined using a closed cranial window and intravital microscopy. Baseline arteriolar diameters ranged from 80 to 100 microns. Arteriolar responses to 10(-9) and 10(-8) mmol/L CGRP applied topically were determined before and 1, 2, and 4 hours after a 10-minute period of total global ischemia. Ischemia was caused by increasing intracranial pressure. RESULTS: Before ischemia, CGRP dilated arterioles by 14 +/- 2% (n = 6) and 24 +/- 3% (n = 7) at 10(-9) and 10(-8) mmol/L, respectively. However, after ischemia, arteriolar responses to 10(-9) mmol/L CGRP were reduced at 1 hour to 4 +/- 1%, at 2 hours to 3 +/- 2%, and at 4 hours to 5 +/- 4% (P < .05 for all comparisons). Similarly, arteriolar responses to 10(-8) mmol/L CGRP were reduced to 5 +/- 2% at 1 hour, 5 +/- 2% at 2 hours, and 10 +/- 6% at 4 hours (P < .05 for all comparisons). In time control animals, arteriolar responses to CGRP did not change over time. In other animals, we examined effects of pretreatment with indomethacin (5 mg/kg IV) on ischemia-induced decreases in arteriolar responses to CGRP. Indomethacin administration did not preserve arteriolar dilation to CGRP at 1 hour after ischemia, but responses were normal at 2 hours. CONCLUSIONS: Total global ischemia leads to prolonged attenuated dilator responses of cerebral arterioles to CGRP. In addition, indomethacin treatment alters effects of ischemia on CGRP-induced dilation.