The role of phospholipase, lipoxygenase, and cyclooxygenase pathways in the mechanism of the cerebrovascular response to CO2 and H+ was investigated in newborn piglets. Responsiveness of pial arterioles, 48-206 micron diameter, to inhalation of 6% CO2 and to suffusion of acidic cerebrospinal fluid (CSF, pH = 6.84), adenosine (10(-4) M), or theophylline (10(-2) M) was studied using a closed cranial window. Pial arteriolar diameter was measured using intravital microscopy. Phospholipase inhibitors quinacrine hydrochloride (10(-4) M in CSF) and p-bromophenacyl bromide (10(-4) M in CSF) abolished the CO2 vasodilation from delta diameter = 27 +/- 5% and 28 +/- 3% during baseline to 0 +/- 4% and -1 +/- 1% following the respective inhibitors. Following administration of the cyclooxygenase inhibitor indomethacin (5 mg/kg i.v.), the CO2 response was converted from vasodilation, 31 +/- 6%, to constriction, -4 +/- 1% (p less than 0.001), while the lipoxygenase inhibitor nordihydroguaiaretic acid (2 mg/kg i.v. or 10(-4) M in CSF) augmented the pial arteriolar response to CO2 from 21 +/- 4% to 34 +/- 7% (p less than 0.005). Topical application of superoxide dismutase (40 units/ml CSF) plus catalase (40 units/ml CSF) also appeared to augment the CO2 response. Suffusion of the cortical surface with acidic CSF at constant PCO2 increased pial arteriolar diameter by 11 +/- 2% that was also abolished by indomethacin. Vasodilatory responses to topical adenosine and theophylline were not affected by indomethacin, suggesting specificity for H+ ion-related vasodilation.(ABSTRACT TRUNCATED AT 250 WORDS)

To investigate the role of vasodilator prostanoids in the CO2-induced relaxation of cerebral arterioles, the present study examined the effect of exogenous prostaglandin (PG) E2 and nonprostanoid vasodilators, adenosine and sodium nitroprusside, on the indomethacin-impaired pial arteriolar response to CO2 in newborn piglets. Reactivity of pial arterioles (52-131 microns diam) was determined using a closed cranial window with intravital microscopy. Cortical prostanoid synthesis was assessed by analyzing for select prostanoids in cerebrospinal fluid sampled from under the cranial window. Inhalation of 7% CO2 caused an elevation of cortical 6-keto-PGF1 alpha and thromboxane (Tx) B2 and increased pial arteriolar diameter by 34 +/- 5%. Two cyclooxygenase inhibitors, indomethacin (5 mg/kg i.v.) and ibuprofen (30 mg/kg i.v.), abolished the CO2-induced elevation of cortical prostanoids. Indomethacin, but not ibuprofen, blocked the CO2-induced increase in pial arteriolar diameter. The indomethacin-impaired vasodilator response to CO2 was restored when PGE2 (0.1-1 microM) was applied topically to the cortical surface. Adenosine (1-100 microM) and sodium nitroprusside (0.5 microM) only partially restored the vasodilator response to CO2. The data suggest that vasodilator prostanoids facilitate cerebrovascular relaxation to CO2 and may play a permissive role in the relaxation response of vascular smooth muscle. The fact that adenosine (adenosine 3',5'-cyclic monophosphate-mediated dilator) and sodium nitroprusside (guanosine 3',5'-cyclic monophosphate-mediated dilator), were partially effective suggests a role for those intracellular signaling pathways. We speculate that receptor activation of intracellular pathways may alter Ca2+ sensitivity of the contractile apparatus in such a way that the relaxation response to CO2 can occur.(ABSTRACT TRUNCATED AT 250 WORDS)

The response of cerebral arteries to norepinephrine was examined in vivo in six dated preterm fetal (94-121 days gestation), eight term fetal (127-141 days gestation), five newborn (7-14 days), and five adult sheep, anesthetized and equipped with a closed cranial window. Norepinephrine (10(-8)-10(-4) M in cerebrospinal fluid) caused a dose-dependent decrease in pial arteriolar diameter in fetal and newborn lambs; however, preterm fetuses were 7- and 14-fold more sensitive to norepinephrine than term fetuses and newborn lambs, respectively. The effective concentration of norepinephrine inducing a 15% decrease in diameter (EC15) was 4.6 +/- 1.8, 33 +/- 11, and 64 +/- 23 microM, for the respective ages. Adult cerebral arterioles did not contract to norepinephrine. In preterm and term fetuses, the contractile response to norepinephrine was blocked by alpha 1-antagonist, prazosin (3 mg iv), and was enhanced by cocaine (10(-5) M; EC15 = 0.086 +/- 0.04 and 1.84 +/- 1.20 microM, respectively) indicating that alpha 1-adrenoceptors mediate the response and that the decrease in sensitivity is not caused by development of neuronal uptake processes. In seven fetuses (111-141 days; mean 123 days gestation), electrical stimulation of the superior cervical sympathetic ganglion constricted pial arterioles by 21 +/- 5%; this contractile response was also blocked by prazosin. The cerebral arterioles of the fetus in utero possess a functional sympathetic innervation capable of influencing cerebrovascular resistance. There is a loss of responsiveness of cerebral arterioles to norepinephrine during fetal and postnatal development, suggesting that the contribution of neuroadrenergic mechanisms to cerebrovascular regulation may be relatively unique to the immature brain.

The effect of lipid peroxidation on the affinity of specific active sites of Na+,K+-ATPase for ATP (substrate), K+ and Na+ (activators), and strophanthidin (a specific inhibitor) was investigated. Brain cell membranes were peroxidized in vitro in the presence of 100 microM ascorbate and 25 microM FeCl2 at 37 degrees C for time intervals from 0-20 min. The level of thiobarbituric acid reactive substances and the activity of Na+, K+-ATPase were determined. The enzyme activity decreased by 80% in the first min. from 42.0 +/- 3.8 to 8.8 +/- 0.9 mumol Pi/mg protein/hr and remained unchanged thereafter. Lipid peroxidation products increased to a steady state level from 0.2 +/- 0.1 to 16.5 +/- 1.5 nmol malonaldehyde/mg protein by 3 min. In peroxidized membranes, the affinity for ATP and strophanthidin was increased (two and seven fold, respectively), whereas affinity for K+ and Na+ was decreased (to one tenth and one seventh of control values, respectively). Changes in the affinity of active sites will affect the phosphorylation and dephosphorylation mechanisms of Na+, K+-ATPase reaction. The increased affinity for ATP favors the phosphorylation of the enzyme at low ATP concentrations whereas, the decreased affinity for K+ will not favor the dephosphorylation of the enzyme-P complex resulting in unavailability of energy for transmembrane transport processes. The results demonstrate that lipid peroxidation alters Na+, K+-ATPase function by modification at specific active sites in a selective manner, rather than through a non-specific destructive process.

The purpose of this study was to examine responses of pial arteries of newborn pigs to stimulation of sympathetic nerves and to exogenous norepinephrine. In the cerebral circulation, pial arteries are important resistance vessels. Diameters of pial arteries in anesthetized piglets, aged 1-6 days, were determined using the "closed" cranial window method. Electrical stimulation of the ipsilateral superior cervical ganglion (16 Hz; 2.5 msec; 10 V) reduced pial arterial diameter from 219 +/- 13 microns (mean +/- SEM) to 190 +/- 12 micron (n = 16) (p less than 0.05) without affecting arterial blood pressure. Pial arterial constriction during nerve stimulation was sustained over the 5-min stimulation period. Following cessation of stimulation, diameters returned to control levels. Exogenous norepinephrine in artificial cerebrospinal fluid constricted pial arteries from 149 +/- 19 to 133 +/- 18 microns at 2 X 10(-6) M (p less than 0.05) and from 159 +/- 20 to 123 +/- 16 microns at 2 X 10(-4) M (p less than 0.05) (18 arteries from nine piglets). Pial arterial responses to nerve stimulation and exogenous norepinephrine were not dependent on initial size of the vessels. The results of this study establish the existence of functional sympathetic innervation in the cerebral circulation at birth in pigs.