Conscious Newborn Lambs Research Articles

Pulmonary vascular effects of vasoactive intestinal peptide in conscious newborn lambs.

Vasoactive intestinal peptide (VIP) may be a neurotransmitter in a peptidergic nervous system and is found in nerves in pulmonary blood vessels. Information regarding its pulmonary vascular effects is limited. We therefore studied VIP's effect on pulmonary vascular tone in the immature lung. Normoxic and hypoxic unsedated newborn lambs with chronically implanted flow probes around the right and left pulmonary arteries were used. VIP was injected into one pulmonary artery only, and direct effects of this peptide on the pulmonary vessels were determined by comparing the flow changes in the injected vs. the uninjected lung. VIP was a powerful pulmonary vasodilator with a threshold of 0.3 microgram/kg. It also was a systemic vasodilator (after 1 microgram/kg, aortic pressure fell 27% and cardiac output increased 29%, both P less than 0.01), with a threshold of 0.1 microgram/kg. Pretreatment with propranolol (1 mg/kg iv) did not abolish pulmonary or systemic vasodilation after VIP. On the other hand, pretreatment with indomethacin (3 mg/kg per day for 3 days) abolished VIP-induced pulmonary vasodilation but probably did not affect systemic vasodilation. We conclude that VIP is a powerful pulmonary vasodilator in the newborn lamb and that this dilation can be blocked by the cyclooxygenase inhibitor indomethacin. VIP is also a powerful systemic vasodilator in the newborn lamb but this effect is not blocked by either propranolol or indomethacin.

Action of PGI 2 analogs on the pulmonary and systemic circulations in the conscious newborn lamb

Previous work (Lock et al. , J. Pharm. Exp. Ther. 215 :156, 1980) has shown that conventional screening procedures for vasoactive PGI 2 analogs have little value in predicting pulmonary vasodilator activity in the newborn lamb. To gain a better insight into the structural requirements for pulmonary vasoactivity and possibly identify useful compounds for the management of neonatal pulmonary hypertensive disorders, we have tested the following PGI 2 analogs in normoxic and hypoxic newborn lambs: 15(S)-9-deoxy-15-methyl1–9α, 6-nitrilo-PGF 1 (analog I); 9-deoxy-9α, 5-nitrilo-PGF 1 (analog II); (6S, 15S)-15-methyl-PG1 1 (analog III); and (6R, 15S)-15-methyl-PGI 1 (analog IV). A prostaglandin analog mimicking PGI 2 (compound BW245C; (±)-5-(6-carboxyhexyl)-1-(3-cyclohexyl-3-hydroxypropyl)hydantoin) was tested as well. Compounds were injected into a branch pulmonary artery and any local pulmonary effect could be assessed from the change in the ratio of blood flow to the injected lung over total flow. None of the analogs tested proved to be a selective pulmonary dilator. BW245C was a potent peripheral vasodilator (threshold around 0.5 μg/kg) and indirectly lowered pulmonary vascular resistance through its systemic effects. Analog I also dilated the systemic circulation, but only at the highest dose tested (100 μg/kg). The latter finding is surprising because it was previously shown that the parent, non-methylated compound is a fairly potent and selective pulmonary vasodilator. Analog II and IV were inactive at a dose up to, respectively, 30 and 20 μg/kg. Analog III, on the other hand, weakly constricted the systemic circulation at a dose of 10 μg/kg. These findings suggest that the neonatal pulmonary vasculature is endowed with specific receptor sites which can discriminative between closely related PGI 2 analogs.

Pulmonary vascular effects of bombesin and gastrin-releasing peptide in conscious newborn lambs.

A growing body of anatomic findings has led investigators to suggest that peptide-containing pulmonary neuroendocrine cells may control pulmonary vascular tone. One such peptide, bombesin, has been found in increased quantities in the lungs of infants with pulmonary disorders that cause pulmonary hypertension. Therefore we studied the effect on pulmonary vascular tone of bombesin, and its C-terminal analog, gastrin-releasing peptide (GRP). Normoxic and hypoxic unsedated newborn lambs with chronically implanted flow probes around the right and left pulmonary arteries were used. Bombesin and GRP were injected into one pulmonary artery only, and direct effects of these peptides were determined by comparing the flow changes in the injected vs. the uninjected lung. Bombesin had no measurable effect on pulmonary vascular resistance under any condition or at any dose (0.1-60 micrograms/kg). Systemic blood pressure increased significantly (12%) after a 10-micrograms/kg dose. GRP was devoid of any measurable hemodynamic effects, even at a dose of 10 micrograms/kg. If pulmonary neuroendocrine cells help regulate vascular resistance, bombesin does not appear to play a role.

Pulmonary vascular effects of amrinone in conscious lambs.

The direct pulmonary vascular effects of amrinone, a nonglycoside, noncatechole cardiotonic agent were studied in conscious newborn lambs using a double-flow probe preparation that allows separation of direct and indirect pulmonary vascular effects. Amrinone was found to be a direct pulmonary vasodilator with a threshold dose of 0.3 mg/kg. Amrinone also increased cardiac output and decreased aortic pressure at somewhat higher threshold doses (1.0 mg/kg and 3.0 mg/kg, respectively). Amrinone's peak effect on pulmonary resistance appeared greater than its peak systemic effects. These effects were noted in both normoxia and hypoxia, and were not changed by pretreatment with propranolol.

Left ventricular oxygen consumption and function in hypoxemia in conscious lambs.

The effects of hypoxemia on left ventricular myocardial blood flow, myocardial oxygen consumption, and contractile function were studied in 12 conscious newborn lambs 4-24 days after birth. Through a left thoracotomy, we placed fluid-filled catheters in the ascending aorta, coronary sinus, and left atrium. An electromagnetic flow transducer was placed around the ascending aorta, and a solid-state pressure transducer was introduced into the left ventricle. Three to four days later we measured aortic and coronary sinus blood oxygen contents, left ventricular myocardial blood flow, heart rate, aortic and left ventricular blood pressures, ascending aortic blood flow velocity, and the first derivative of left ventricular pressure (dP/dt) and ascending aortic blood flow velocity (dV/dt) during a control period and during 50 and 75% reductions in ascending aortic oxygen content. Myocardial oxygen consumption was calculated. There was no significant change in aortic or coronary sinus blood pH or CO2 tension during the study. Coronary sinus blood oxygen content and the arteriovenous difference of oxygen across the left ventricle decreased as a linear function of the aortic blood oxygen content. Myocardial blood flow increased in proportion to the reduction in aortic blood oxygen content. Myocardial oxygen consumption increased during hypoxemia but not as a function of aortic blood oxygen content. There was no significant change in left ventricular end-diastolic pressure or aortic mean blood pressure. dP/dt and dV/dt doubled during hypoxemia, but the increases did not occur as a function of the aortic blood oxygen content. In conscious, unanesthetized newborn lambs, 50 and 75% reductions of aortic blood oxygen content were associated with significant increases of left ventricular myocardial blood flow, oxygen consumption, and contractile function.

Gastrointestinal tract O2 uptake and regional blood flows during digestion in conscious newborn lambs

We determined gastrointestinal tract O2 uptake, cardiac output, regional blood flows, and whole-body O2 uptake before and for 1-6 h after feeding in 10 chronically catheterized unanesthetized lambs (9-15 days of age). Total gastrointestinal blood flow (sum of blood flows to the stomach, small intestine, and colon, as calculated with the radioactive microsphere technique) increased 23% at 1 h postprandially. This increased flow at 1 h was due to a large increase in blood flow to the stomach, whereas blood flows to the small intestine and colon did not change significantly. By 2 h, stomach blood flow and thus total gastrointestinal blood flow had returned to fasting values. In contrast, total O2 uptake by the gastrointestinal tract organs (stomach, small intestine, and colon) increased 65% at 1 h, 51% at 2 h, and 28% at 3 h postprandially in association with increases in O2 extraction (O2 uptake/O2 delivery) of 41% at 1 h, 45% at 2 h, and 27% at 3 h. There were no digestion-related changes in whole-body O2 uptake or in cardiac output and its distribution to the brain, heart, kidney, liver (hepatic artery), and carcass. Our data indicate that postprandial increases in O2 demand by gastrointestinal tract organs of the newborn animal are met primarily by enhanced tissue O2 extraction, rather than by metabolic hyperemia, because the postprandial hyperemia observed in the neonate is of short duration and is confined to the stomach.

Enhanced beta-adrenergic-receptor responsiveness in hypoxic neonatal pulmonary circulation.

The influence of alveolar hypoxia on pulmonary vascular adrenergic receptors was studied in conscious newborn lambs. In control animals, pulmonary vessels were directly constricted by epinephrine and norepinephrine, but were unaffected by isoproterenol. Pulmonary resistance (PVR) was also unaffected by propranolol, thus implying minimal beta-receptor activity under normoxic conditions. Hypoxia raised PVR but also modified the pulmonary vascular responses to catecholamines: isoproterenol became a dilator, whereas the constrictor effects of epinephrine and norepinephrine were abolished. Although beta-blockade did not alter base-line PVR, propranolol increased the constrictor response to hypoxia, implying that hypoxia increases beta-adrenergic activity or reactivity in the pulmonary circulation. Consistent with this hypothesis are the following: 1) in alpha-blocked lambs, epinephrine was without local effects during normoxia, but caused vasodilation during hypoxia; 2) the absent constrictor response to epinephrine during hypoxia is fully restored by propranolol; and 3) although alpha-blockade blunts the hypoxic constrictor response, the full response is restored when beta-blockade is added. These results indicate that the hypoxic constrictor response is partially opposed by increased beta-mediated vasodilation. These enhanced beta-receptor effects are due, at least in part, to increased beta-receptor reactivity of unknown mechanism.

Oxygen consumption by the gastrointestinal tract and liver in conscious newborn lambs.

We determined blood flow to and O2 consumption (VO2) by the gastrointestinal tract (GI) and liver and also measured cardiac output and whole-body VO2 in nine chronically catheterized unanesthetized lambs (7-16 days of age). Blood flows were calculated with the radionuclide-labeled microsphere technique, and blood O2 contents were measured with an O2 content analyzer. During the fasting state, GI blood flow was 58 +/- 4 (means +/- SE) ml.min-1.kg body wt-1; GI VO2 was 1.4 +/- 0.1 ml O2.min-1.kg-1. Neonatal GI VO2 was linearly related to both GI blood flow and O2 delivery (DO2). GI O2 extraction [(VO2/DO2).100] averaged 28% and did not vary with blood flow or DO2. Liver blood flow was 73 +/- 4 ml.min-1.kg-1 (271 +/- 23 ml.min-1.100 g liver-1), and liver VO2 was 2.0 +/- 0.1 ml O2.min-1.kg-1 (7.3 +/- 0.5 ml O2.min-1.100 g-1). Hepatic O2 extraction varied from 18 to 81% . VO2 by the neonatal liver did not correlate with liver blood flow or DO2. Hepatic O2 extraction, however, was inversely related to liver DO2. Our data indicate that the gastrointestinal tract and liver of the unanesthetized newborn animal exhibit O2 demands 1.5-3 times those reported in the adult. The neonatal gastrointestinal tract meets its O2 demands with a comparatively large blood flow and O2 delivery, whereas the neonatal liver provides for its O2 requirements by varying its O2 extraction.

Direct pulmonary vascular responses to prostaglandins in the conscious newborn lamb.

The effects of prostaglandins PGD2, PGE1, PGE2, PGF2 alpha, and PGI2 on pulmonary vascular tone were investigated in normoxic and hypoxic unsedated newborn lambs with chronically implanted flow probes around right and left pulmonary arteries. Prostaglandins were injected into only one pulmonary artery, and direct effects of the compounds were determined by comparing the flow changes in the injected vs. the uninjected lung. PGD2 and PGF2 alpha were direct pulmonary vasoconstrictors, whereas PGE1, PGE2 and PGI2 were direct pulmonary dilators. Hypoxia augmented the vasodilation of PGE1 and PGI2 and diminished the constriction of PGD2 and PGF2 alpha; it also lowered the threshold for PGI2-induced dilation. While the E-type prostaglandins and PGI2 were both local dilators, they differed in that the systemic dilatory effects of the E-type agents exceeded their pulmonary effects, whereas the opposite occurred with PGI2. These results demonstrate that prostaglandins can act locally on pulmonary vascular smooth muscle tone in vivo and that the level of oxygenation or base-line vascular tone influences the pulmonary vascular responses to the PGs. Among the dilator prostaglandins, PGI2 appears to be more effective on the pulmonary rather than the systemic vascular bed under hypoxic conditions.

Indomethacin-induced pulmonary vasoconstriction in the conscious newborn lamb.

The pulmonary vascular effects of indomethacin were studied in chronically-instrumented conscious newborn lambs, 2-12 wk old. Flows were measured in right and left pulmonary arteries; indomethacin was injected into only one lung; and constriction or dilation was assessed from the proportion of pulmonary flow directed to the injected vs. the noninjected lung. Indomethacin was a pulmonary vasoconstrictor in all animals, its threshold dose being 0.01 mg/kg. The constriction after a dose of 0.1 mg/kg was associated with plasma indomethacin levels sufficient to inhibit prostaglandin (PG) synthesis. However, chronic indomethacin therapy (3 mg/kg per day orally) for 3 days, though resulting in similar plasma indomethacin levels, altered neither base-line pulmonary tone nor the pulmonary vascular responses to hypoxia, acetylcholine, bradykinin, or histamine. Pretreatment with indomethacin did abolish the pulmonary effects of a further intravenous dose of the drug. Our results indicate that production of a pulmonary PG may help to maintain normal vessel relaxation. However, the neonatal lung adapts to chronic inhibition of PG synthesis, maintaining normal vascular tone and homeostasis. Pulmonary PGs may play a role in, but are not essential for, normal neonatal pulmonary vascular control.

Influence of indomethacin on renal function in conscious newborn lambs.

Transient renal dysfunction has been observed in some premature infants who received indomethacin to constrict patent ductus arteriosi, and it has been suggested that this complication may be dose related. Accordingly, the present study was designed to investigate the influence of high (7.5 mg/kg) and low (0.2 mg/kg) doses of indomethacin on effective renal blood flow, glomerular filtration rate, urine flow, sodium and potassium excretion and plasma levels of indomethacin in conscious newborn lambs studied continously over a 24-hour period. When compared to control lambs, both indomethacin doses reduced effective renal blood flow significantly at 2--4 h, but only high doses significantly reduced renal blood flow at 22--24 h (6.5 +/- 0.9 vs. 13.4 +/- 1.4 ml/min/kg, respectively; p less than 0.001). At 8-16 and 16-24 h after drug administration, urine flow was significantly lower than in the control group only in high-dose lambs (p less than 0.05). During the 16 to 24-hour period, urine flow for control, low- and high-dose animals was 0.15 +/- 0.01, 0.14 +/- 0.01, and 0.08 +/- 0.02 ml/min/kg, respectively. Glomerular filtration rate and sodium and potassium excretion rates were not affected by either indomethacin dose.

Baroreflex activity in conscious fetal and newborn lambs.

The immediate transient baroreceptor sensitivity was measured in 9 conscious fetal and 7 conscious newborn lambs for periods of at least 35 days following bolus injections of phenylephrine (20-50 mug/kg). Mean sensitivities were unchanged throughout gestation from 105 days at 6.7 +/- 0.4 msec/cm H2O (n = 45) and were insignificantly different from those in the newborn period, 5.9 +/- 0.4 msec/cm H2O (n = 78). In contrast, baroreflex sensitivities were less in 2 fetuses and 2 newborn lambs when pressures were increased by chronically implanted thoracic aortic balloon cuff; they were 3.03 +/- 0.11 (n = 127) and 0.91 +/- 0.11 msec/cm H2O (n =61), respectively. "Steady-state" heart period-arterial pressure curves indicate that the baroreflex operates down to levels of 40 cm H2O in the fetus which is lower than that achieved in the adult of other species, rabbit and man.

Temperature responses of lambs after centrally injected prostaglandins and pyrogens.

It has been proposed that pyrogens may produce their febrile response by the release of prostaglandins in the hypothalamus. To test this theory, prostaglandin E1 (PGE1) was injected into a lateral ventricle in dosages of 2-200 ug into conscious newborn lambs, ages 4-168 h. Fiifteen of 40 injections were followed by rises in rectal temperature but the remainder were followed either by no change or by falls. Temperature responses did not appear to be related to age and a variation in responses to the same dosage of PGE was often observed. Some lambs were able to develop fevers in response to intravenous bacterial pyrogen yet did not develop fever after intraventricular PGE 1. Intraventricular bacterial pyrogen (3 ng) produced no change in body temperature, whereas three of four injections of 300 ng pyrogen caused fever. The results suggest that the newborn lamb may be able to develop a fever independently of the central involvement of PGE1. Alternatively, the intraventricular approach may not be useful for the study of the central control of body temperature in the newborn lamb.