Brain excitatory amino acid concentrations are lower in the neonatal pig: a buffer against excitotoxicity?

Hypoglycemic brain damage has been associated with high levels of the excitatory amino acids (EAA) aspartate and glutamate in the newborn and adult. We hypothesized that newborn piglet EAA would be different from those of older pigs when stressed with severe insulin-induced hypoglycemia (<30 mg/dl). Brain EAA were measured in piglets and adolescent pigs via microdialysis. Eleven of 12 newborn normoglycemic piglets had no detectable baseline levels (<0.5 microM) of EAA, while pigs had aspartate and glutamate concentrations of 1.78 +/- 0.44 and 3.43 +/- 1.14 microM (mean +/- SEM), respectively. Piglet aspartate and glutamate concentrations reached but did not significantly exceed normoglycemic pig levels after 2 h with plasma glucose values < or =20 mg/ml. Elevations in EAA were only detected in piglets whose EEG activity ceased. Aspartate and glutamate concentrations did not increase in insulin-treated pigs nor in control animals. We speculate that newborns with blood glucose less than clinically acceptable values (35 mg/dl) may be protected from EAA-associated neuronal damage during acute hypoglycemia. Lower normoglycemic and hypoglycemic levels of EAA in newborns when compared to older pigs provide this protection.

Nitric oxide modulation of retinal, choroidal, and anterior uveal blood flow in newborn piglets.

The purpose of the present study was to investigate the role of nitric oxide (NO) in modulating the resting vascular tone of the choroidal and anterior uveal circulations and the autoregulatory gain of the retina. Blood flow (ml/min/100 gm dry weight) to tissues was determined in 23 anesthetized piglets (3-4 kg) using radiolabelled microspheres. Ocular Perfusion Pressure (OPP) was defined as mean arterial pressure minus intraocular pressure (IOP) which was manipulated hydrostatically by cannulation of the anterior eye chamber. The OPP was decreased during intravenous infusion (30 mg/kg/hr) of either the NO-synthase inhibitor L-NAME or the inactive enantiomer D-NAME. Blood flows were determined at OPP of 60, 50, 40, 30, and 20 mmHg following initial ocular blood flow measurements. Mean initial choroidal and anterior uveal blood flows with L-NAME showed a 47+/-12% and a 43+/-6% reduction (p <.001), respectively. Mean choroidal blood flows were significantly reduced (p<.01) in the L-NAME treated animals at an OPP of 60 and 50 when compared to D-NAME. Uveal blood flows were linearly correlated with OPP in the L-NAME and D-NAME treated groups. Uveal blood flow was greater following exogenous administration of L-arginine (180 mg/kg). Mean initial retinal blood flow did not differ significantly in either group. Retinal blood flow with L-NAME was reduced at OPP of 60 mmHg and below compared to D-NAME (p<.05). The degree of compensation in the autoregulatory gain of the retinal vasculature was reduced in the presence of L-NAME at an OPP of 50 mmHg and below compared to D-NAME. These data support the hypothesis that NO may be a primary mediator in maintaining resting vascular tone to the choroid and anterior uvea in vivo and that NO blockade reduces the degree of compensation in the autoregulatory gain of the retinal vasculature within a specific range of ocular perfusion pressures.

Voice of the victims--the key to consensus and support for alcoholism research.

The US National Institute on Alcohol Abuse and Alcoholism (NIAAA) recognizes two forms of problematic drinking: 'willful alcohol abuse', a behavioural problem, and 'alcohol dependence', a true medical disorder, which includes a genetic component, that can be scientifically understood and medically treated. Current biomedical research has linked specific neurotransmitters to certain effects of alcohol that are unique to alcoholics. An inadequate flow of information between the victims of alcoholism, researchers, and the public has impeded further exploration of the genetic and neurochemical underpinnings of alcohol dependence. This is due in part to continuing misconceptions about alcohol dependence, not only among the general public, but within the scientific and medical communities as well. Consequently, compared to other diseases, research in alcohol dependence is proceeding with less urgency despite its relatively high economic and social costs. Incorporating the input of recovering alcoholics into future research agendas can help to ensure relevant scientific investigation and the delivery of a more accurate and consistent message to the public with regard to alcoholism.

Voice of the victims--the key to consensus and support for alcoholism research.

The US National Institute on Alcohol Abuse and Alcoholism (NIAAA) recognizes two forms of problematic drinking: 'willful alcohol abuse', a behavioural problem, and 'alcohol dependence', a true medical disorder, which includes a genetic component, that can be scientifically understood and medically treated. Current biomedical research has linked specific neurotransmitters to certain effects of alcohol that are unique to alcoholics. An inadequate flow of information between the victims of alcoholism, researchers, and the public has impeded further exploration of the genetic and neurochemical underpinnings of alcohol dependence. This is due in part to continuing misconceptions about alcohol dependence, not only among the general public, but within the scientific and medical communities as well. Consequently. compared to other diseases, research in alcohol dependence is proceeding with less urgency despite its relatively high economic and social costs. Incorporating the input of recovering alcoholics into future research agendas can help to ensure relevant scientific investigation and the delivery of a more accurate and consistent message to the public with regard to alcoholism.

Production of pulmonary vasodilation by tolazoline, independent of nitric oxide production in neonatal lambs.

OBJECTIVE: To determine whether tolazoline reduces pulmonary vascular resistance (PVR) by means of endogenous nitric oxide production. DESIGN: Thirty newborn lambs (2 to 7 days of age) were anesthetized with pentobarbital, and their lungs were ventilated through an endotracheal tube. Intravascular catheters were placed in the left ventricle, descending aorta, right atrium, and pulmonary artery for continuous monitoring of intravascular pressures. Cardiac output was measured with radiolabeled microspheres. Arterial carbon dioxide pressure and pH were maintained in a normal range throughout the experiments. Animals were randomly assigned to the following groups: group 1, lungs ventilated with a hypoxic gas mixture and administered tolazoline; group 2, given N omega-nitro-L-arginine (L-NA) (5 mg/min intravenously for 60 minutes) and tolazoline; group 3, given L-NA with hypoxia and tolazoline. Acetylcholine (0.5 microgram/kg) was injected into the right atrium to assess pulmonary nitric oxide synthase activity before and after the L-NA infusion. Data were analyzed by analysis of variance. RESULTS: L-NA inhibited the acetylcholine-induced reduction in mean pulmonary artery pressure (MPAP) by more than 75%. Hypoxia and L-NA increased both MPAP and PVR. Tolazoline produced immediate reductions in both MPAP and PVR in all three groups (group 1, 27% +/- 3% and 50% +/- 5%; group 2, 34% +/- 5% and 50% +/- 6%; and group 3, 31% +/- 4% and 46% +/- 5%, respectively). CONCLUSIONS: These results suggest that tolazoline produces vasodilation independent of nitric oxide production. Understanding the mechanism by which tolazoline produces pulmonary vasodilation may provide insight into the clinical use of this drug and information regarding other potential endogenous mediators of pulmonary vasomotor tone in the neonate.

Cerebral vascular response to hemorrhagic hypotension in newborn lambs: the influence of developing anemia.

The ability of newborn animals to autoregulate cerebral blood flow (CBF) has been documented. Most studies of the cerebral vascular response to hypotension utilize hemorrhage, generally confounded with anemia. We studied the cerebral blood flow and metabolic response of chloralose and urethane anesthetized newborn lambs to regulated hypotension. Lambs (< or = 7 days old) were catheterized for radioactive microsphere determinations of CBF. The dorsal sagittal sinus was catheterized to obtain cerebral blood samples for the calculation of oxygen uptake. Cerebral perfusion pressure was reduced in a step-wise fashion with hemorrhagic hypotension. Animals spontaneously became anemic with hypotension (AH; n = 8). In a group of animals (NH; n = 6), anemia was prevented by infusion of autologous red blood cells. Arterial pressure was reduced from control to 50, 40, and 30 mm Hg. In the AH group hematocrit fell 37% but was not different from control in the NH group. Total CBF was maintained in all groups. The lowest perfusion pressures studied were 25 +/- 1 and 22 +/- 1 mm Hg in AH and NH groups respectively. Oxygen delivery decreased (37%) only in the AH group, secondary to anemia. Calculated oxygen consumption was maintained in the AH group but increased (approximately 50%) in the NH group at 50 and 40 mm Hg. The ratio of oxygen uptake to oxygen delivery (fractional oxygen extraction) increased linearly in both groups as arterial pressure decreased. The major findings of these experiments are (i) The anesthetized newborn lamb can maintain CBF when perfusion pressure falls to 25 mm Hg and this autoregulatory capacity (classically defined) is not dependent on a change in hematocrit and, presumably, viscosity; (ii) Cerebral hypotension, anemic or not, appears to be accompanied by an increase in fractional extraction of oxygen.

Endotracheal administration of tolazoline in hypoxia-induced pulmonary hypertension.

STUDY OBJECTIVE: To compare the pulmonary and systemic vascular responses to intravenously (IV) and endotracheally (ET) administered tolazoline (Tz) in newborn lambs with hypoxia-induced pulmonary hypertension. DESIGN: Randomized, controlled study design. METHODS: Twenty lambs, 2 to 7 days of age, were anesthetized, intubated, and surgically catheterized for continuous physiologic monitoring and cardiac output measurements using radiolabeled microspheres. After a postoperative stabilization period, the lambs were ventilated with a hypoxic gas mixture which was titrated to increase mean pulmonary artery pressure (MPAP) 30% to 50% above baseline. Each animal was randomly assigned to receive either IV-Tz (2 mg/kg), ET-Tz (4 mg/kg), or ET-saline (Sal, control group). RESULTS: ET-Tz significantly (P < .05) reduced MPAP, PVRI (pulmonary vascular resistance index), MPAP/mean artery pressure (MAP) and PVRI/systemic vascular resistance index (SVRI), but not SVRI. IV-Tz lowered (P < .05) MPAP, PVRI, and PVRI/SVRI but also produced significant reductions in MAP and SVRI while only transiently decreasing MPAP/MAP: MPAP/MAP and PVRI/SVRI ratios were consistently lower in the ET-Tz animals than either the IV-Tz or ET-Sal animals. CONCLUSIONS: Our results suggest that ET-Tz produced a more selective pulmonary vascular response than IV-Tz and may warrant further investigation for potential clinical applications.

Use of an intravascular endoprosthesis (stent) to establish and maintain short-term patency of the ductus arteriosus in newborn lambs.

The feasibility of stenting the ductus arteriosus with a balloon-expandable vascular endoprosthesis was tested in 8 newborn lambs. Tantalum wire and stainless steel mesh coronary stents were implanted antegrade or retrograde by percutaneous transfemoral catheterization. One lamb died during the procedure from perforation of the aorta. In 7 lambs, the ductus arteriosus was crossed using endhole catheters and wires, and stents mounted on angioplasty catheters were expanded in the ductus arteriosus. Six lambs had successful implantation and had maintained a sizeable patent ductus arteriosus at 2 h. We conclude that the feasibility of percutaneous stenting of the newborn ductus was demonstrated. By providing patency of the ductus arteriosus, stents may offer nonsurgical alternatives for palliation of cyanotic congenital heart disease and hypoplastic left heart syndrome.

Validation of laser-Doppler flowmetry in measurement of spinal cord blood flow.

Laser-Doppler flowmetry (LDF) is a non-invasive method for continuous on-line monitoring of microvascular blood flow. LDF has previously been validated with established methods in various tissues, yet its validity and resolution in the central nervous system (CNS) remain unclear. We compared LDF with the microsphere method (MS) using two independent laser probes placed on the dorsal lumbar spinal cord (L5 laminectomy) of anesthetized rabbits (n = 9). After base-line flow measurements, spinal cord blood flow (SCBF) was increased (up to 50%) with phenylephrine (10-80 micrograms.kg-1.min-1 iv) and decreased (up to 50%) with chlorisondamine (10 mg/kg iv) or other stimuli. The percentage changes of lumbar SCBF and vascular resistance (VR) from the base line obtained by LDF and MS excellently agreed (rBF = 0.86, rVR = 0.94, P less than 0.0001). LDF estimated also the absolute SCBF values parallel to MS (r = 0.77, P less than 0.001). In conclusion, the validity of LDF in estimating the SCBF and dynamic changes of BF and VR is confirmed. Therefore, LDF may prove useful for monitoring CNS microcirculation in normal or pathophysiological states.

Effect of theophylline on renal vasoactivity of acetate and adenosine.

To examine the possible role of adenosine in the vasoactivity of acetate, acetate and adenosine were injected into the renal artery of the dog while measuring renal blood flow. Low (9-28 mg) doses of acetate produced vasodilation, but higher doses (28-94 mg), like adenosine, produced vasoconstriction followed by vasodilation. Theophylline, a competitive inhibitor of the vascular action of adenosine, inhibited the vasodilator action of adenosine but not that of acetate. Secondly, theophylline inhibited the vasoconstrictor effect of adenosine and acetate but the latter to a much lesser extent. These findings suggest that adenosine mediates the vasoconstriction effect of acetate in the canine kidney.

Regional peripheral and CNS hemodynamic effects of intrathecal administration of a substance P receptor agonist.

Regional CNS and peripheral hemodynamic effects of the intrathecal (i.t.) administration of a substance P receptor agonist, [pGlu5,MePhe8,MeGly9]-substance P5-11 ([DiMe]-SP), were studied in anesthetized rats with the radioactive microsphere technique. It was previously shown that [DiMe]-SP caused a sympathetically mediated increase in mean arterial pressure (MAP) by an action within the spinal cord. In this study, [DiMe]-SP (5 and 33 nmol, i.t.) increased MAP. The 5 nmol dose increased resistance in cutaneous, renal, splanchnic, and adrenal vascular beds but decreased resistance, and increased blood flow in some skeletal muscle beds. Total peripheral resistance was unchanged. The 33 nmol dose increased resistance in each peripheral vascular bed analyzed and increased total peripheral resistance. Whereas each dose increased heart rate, stroke volume and cardiac output were unchanged with the 5 nmol dose and were reduced with the 33 nmol dose. Neither dose of [DiMe]-SP significantly altered regional brain or spinal cord blood flows. These data show that the i.t. administration of the SP agonist, [DiMe]-SP, increased vascular tone to most peripheral vascular beds whereas the low dose caused a vasodilation of skeletal muscle. These effects are consistent with the notion of a dose-related activation of SP receptors in the spinal cord affecting sympathetic outflow to the adrenals and to the vasculature.

Intrathecal administration of a substance P receptor antagonist: studies on peripheral and central nervous system hemodynamics and on specificity of action.

Regional central nervous system and peripheral hemodynamic effects of the intrathecal (i.t.) administration of a substance P (SP) receptor antagonist, [D-Arg1, D-Pro2, D-Trp7,9, Leu11]-substance P ([D-Arg]-SP), were studied in anesthetized rats. It was found that [D-Arg]-SP (3.3 nmol i.t.) reduced mean arterial pressure and cardiac output due to a reduction in stroke volume. Total peripheral resistance was not altered. Whereas most vascular beds showed no alterations in vascular resistance, a renal vasoconstriction was noted. The hypotensive effect of [D-Arg]-SP was blocked by phentolamine (10 mg/kg i.v.) but not by propranolol (1 mg/kg i.v.). In the absence of changes in vascular arterial resistance due to [D-Arg]-SP, it appears that a change in venous return may contribute to the [D-Arg]-SP-induced reduction in stroke volume. These data provide evidence that a spinal cord SP system may tonically affect sympathetic neurons controlling venous, but not arterial, vasomotor tone. [D-Arg]-SP (i.t.) did not alter brain blood flow but significantly decreased blood flow in the thoracolumbar spinal cord 15 to 20 min after administration. The reduction in spinal cord flow did not appear to be responsible for the [D-Arg]-SP-induced hypotension because kainic acid (i.t.), an agent that interacts with glutamate receptors, produced similar pressor responses in the presence and absence of [D-Arg]-SP. In addition, whereas the pressor effect of low doses of a SP agonist [pGlu5, MePhe8, MeGly9]-substance P (5-11) were blocked by [D-Arg]-SP, a higher dose produced the typical pressor effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of norepinephrine on lymph flow and edema formation in the canine forelimb.

In the present study we sought evidence for the hypothesis that norepinephrine (NE) can cause constriction of lymph vessels in the canine forelimb perfused at constant flow. Mechanical venous compression (small-vein pressure approximately equal to 45 mmHg), intra-arterial infusion of histamine (16 micrograms base/min), and intra-arterial infusion of NE (16 micrograms base/min) caused the limbs to gain weight at similar rates due to edema formation; the first two maneuvers caused a sustained increase in skin lymph flow, but the increase in lymph flow with NE was only transient. Similar changes were seen during infusion of NE at 1 and 2 micrograms/min, and studies with radioactive microspheres indicated that NE increased capillary blood flow. When venous pressure was elevated and held constant at 45 mmHg. NE (4 micrograms base/min ia) antagonized and phentolamine (400 micrograms/min ia) potentiated the increase in skin lymph flow. Finally, NE (16 micrograms base/min ia) caused a reduction in histamine (16 micrograms base/min)-elevated skin lymph flow, despite the fact that capillary pressure was greatly increased due to active venous constriction. We conclude that intra-arterial infusion of norepinephrine in the dog forelimb perfused at constant flow causes constriction of lymphatic trunk vessels in skin, resulting in increased resistance to the flow of lymph.

Effects of positive end expiratory pressure on shunt flow in atelectasis.

We studied the effects of positive end expiratory pressure (PEEP) on vascular pressure flow relationships in atelectatic lobes in the closed-chest pigs and compared our results to measurements we previously obtained in sublobar atelectasis. Regional hemodynamic responses to lung inflation were significantly different between lobes and sublobar regions. PEEP caused marked increase in the fraction of cardiac output perfusing the atelectatic lobe from 16.5 +/- 2.0% (SE) to 32.5 +/- 2.0% at similar pulmonary vascular transmural pressures. In contrast, similar levels of PEEP failed to redistribute blood flow to the atelectatic sublobar regions. We propose that distortion of the sublobar region with inflation of the surrounding lung may be responsible for the failure of redistribution of pulmonary blood flow with application of PEEP.

Vasoactive intestinal polypeptide and the canine cerebral circulation.

A potential role for cerebrovascular nerves containing vasoactive intestinal polypeptide (VIP) was examined in 24 anesthetized, ventilated dogs. Cerebral blood flow (CBF) was measured by either the cerebral venous outflow or microsphere method. Plasma VIP concentration was measured by radioimmunoassay. Hypercapnia (5% and 10% CO2) and hypoxia (7% O2) produced significant increases in cerebral venous outflow, but had no affect on arterial or cerebral venous VIP concentrations. Measurements of VIP in cerebrospinal fluid (CSF) made during 5% and 8% CO2 breathing also were not different from control values. VIP produced large dose-dependent increases in common carotid artery and temporalis muscle blood flow when injected or infused intraarterially; however, VIP had no effect on total or regional cerebral blood flow (rCBF) within the brain when administered in a similar manner. Unilateral perfusion of the cerebral ventricles with VIP produced significant increases (range: 11-80%) in rCBF. These data are consistent with the possibility that local release of VIP from perivascular nerve endings could affect CBF. The unresponsiveness of canine cerebral vessels to blood-borne VIP may be due to the blood-brain barrier, since VIP dilates cerebral vessels when the barrier is bypassed by intraventricular infusion. These studies do not support the hypothesis that CBF changes induced during hypercapnia or hypoxia are mediated by VIP.

Sublobar atelectasis and regional pulmonary blood flow.

We studied the effects of sublobar atelectasis on regional blood flow in anesthetized paralyzed pigs. Following the washout of nitrogen from the lung with oxygen, a sublobar airway was obstructed and a peripheral segment of lung allowed to collapse. Blood flow to the atelectatic region fell from a control of 140.0 +/- 20.5 to 16.6 +/- 2.9 ml/(min . g). Basilar regions that became atelectatic spontaneously during the course of the studies had similar decreases in blood flow. Inflation of the surrounding lung by the application of positive end-expiratory pressure failed to increase blood flow and vascular conductance within the atelectatic regions. These results indicate that mechanical and hypoxic effects on vessels perfusing sublobar atelectatic regions limit the effects of interdependence from the surrounding lung. Furthermore, with inflation of the surrounding lung, increases in pulmonary shunting of blood are small.