Correlation of pulmonary ACE activity and capillary surface area during postnatal development

Although a considerable amount of information is available regarding the remodeling and growth of the pulmonary arterial circulation, relatively little is known regarding postnatal development of the pulmonary microcirculation. We hypothesized that the maximal velocity (Vmax) of pulmonary angiotensin-converting enzyme (ACE) activity, measured from indicator-dilution outflow curves using a synthetic substrate, 3H-labeled benzoyl-phenylalanyl-alanyl-proline (BPAP), is directly related to the capillary endothelial cell surface area in the lungs of developing lambs. Accordingly we measured apparent kinetics of pulmonary ACE activity in 22 anesthetized ventilated lambs (2-171 days old) and compared our functional assessment to simultaneous in vivo determinations of CO diffusing capacity (DLCO) and postmortem structural assessment of alveolar septal dimensions using stereology and electron microscopy. There was a progressive increase in Vmax of ACE in this age group, with little change in apparent affinity for BPAP. Similar functional manifestation of growth was noted by an age-dependent increase in DLCO. Neither Vmax nor DLCO was significantly affected by an increase in left atrial pressure to 19 Torr (via inflation of a balloon in the left atrium), suggesting little recruitment of vessels under conditions of the present protocol. A close correlation was observed when either Vmax for ACE activity or DLCO was plotted vs. capillary endothelial cell surface area. Double logarithmic transformation of capillary endothelial cell surface area, Vmax-ACE and DLCO vs. lung volume revealed power functions with slopes all greater than that predicted from isotropic growth, suggesting selective differential postnatal development of the endothelium of the alveolar septum in lambs from 2-171 days of age.     

sGC and PDE5 are elevated in lambs with increased pulmonary blood flow and pulmonary hypertension

Utilizing aortopulmonary vascular graft placement, we established a lamb model of pulmonary hypertension that mimics congenital heart disease with increased pulmonary blood flow. We previously demonstrated that endothelial nitric oxide synthase (eNOS) is increased in lambs at age 4 wk. However, these lambs display a selective impairment of endothelium-dependent pulmonary vasodilation that is suggestive of a derangement downstream of NO release. Thus our objective was to characterize potential alterations in the expression and activity of soluble guanylate cyclase (sGC) and phosphodiesterase type 5 (PDE5) induced by increased pulmonary blood flow and pulmonary hypertension. Late-gestational fetal lambs (n = 10) underwent in utero placement of an aortopulmonary vascular graft (shunt). Western blotting analysis on lung tissue from 4-wk-old shunted lambs and age-matched controls showed that protein for both subunits of sGC was increased in shunted lamb lungs compared with age-matched controls. Similarly, cGMP levels were increased in shunted lamb lungs compared with age-matched controls. However, PDE5 expression and activity were also increased in shunted lambs. Thus although cGMP generation was increased, concomitant upregulation of PDE5 expression and activity may have (at least partially) limited and accounted for the impairment of endothelium-dependent pulmonary vasodilation in shunted lambs.     

Altered carnitine homeostasis is associated with decreased mitochondrial function and altered nitric oxide signaling in lambs with pulmonary hypertension

Utilizing aortopulmonary vascular graft placement in the fetal lamb, we have developed a model (shunt) of pulmonary hypertension that mimics congenital heart disease with increased pulmonary blood flow. Our previous studies have identified a progressive development of endothelial dysfunction in shunt lambs that is dependent, at least in part, on decreased nitric oxide (NO) signaling. The purpose of this study was to evaluate the possible role of a disruption in carnitine metabolism in shunt lambs and to determine the effect on NO signaling. Our data indicate that at 2 wk of age, shunt lambs have significantly reduced expression (P < 0.05) of the key enzymes in carnitine metabolism: carnitine palmitoyltransferases 1 and 2 as well as carnitine acetyltransferase (CrAT). In addition, we found that CrAT activity was inhibited due to increased nitration. Furthermore, free carnitine levels were significantly decreased whereas acylcarnitine levels were significantly higher in shunt lambs (P < 0.05). We also found that alterations in carnitine metabolism resulted in mitochondrial dysfunction, since shunt lambs had significantly decreased pyruvate, increased lactate, and a reduced pyruvate/lactate ratio. In pulmonary arterial endothelial cells cultured from juvenile lambs, we found that mild uncoupling of the mitochondria led to a decrease in cellular ATP levels and a reduction in both endothelial NO synthase-heat shock protein 90 (eNOS-HSP90) interactions and NO signaling. Similarly, in shunt lambs we found a loss of eNOS-HSP90 interactions that correlated with a progressive decrease in NO signaling. Our data suggest that mitochondrial dysfunction may play a role in the development of endothelial dysfunction and pulmonary hypertension and increased pulmonary blood flow.     

Developmental changes in synthesis of and responsiveness to prostaglandins I2 and E2 in hypoxic lamb lungs

Previous studies have shown that the attenuated hypoxic pulmonary vasoconstriction (HPV) of young newborn lamb lungs was enhanced by cyclooxygenase inhibition. We sought to determine whether this reflected greater synthesis of and (or) responsiveness to dilator prostaglandins (PG). Protocol 1 measured responses to graded hypoxia and perfusate concentrations of 6-keto-PGF1alpha (the stable metabolite of PGI2) and PGE2 in isolated lungs from 1-day- and 1-month-old lambs. Protocol 2 compared dose responses and segmental vascular resistances during infusion of PGI2 and PGE2 in hypoxic, cyclooxygenase-inhibited, lungs from 1- to 2-day-old and 1- to 3-month-old lambs. Lungs of 1-day-old lambs with attenuated responses to 4% O2 had significantly higher perfusate concentrations of 6-keto-PGF1alpha and PGE2, but responses to both PGE2 and the more potent vasodilator, PGI2 did not differ with age. These data support the hypothesis that attenuated HPV in young newborn lamb lungs is due to increased synthesis of dilator PG, particularly PGI2.     

Chronic endothelin A receptor blockade in lambs with increased pulmonary blood flow and pressure

Endothelin receptor blockade is an emerging therapy for pulmonary hypertension. However, hemodynamic and structural effects and potential changes in endogenous nitric oxide (NO)-cGMP and endothelin-1 signaling of chronic endothelin A receptor blockade in pulmonary hypertension secondary to congenital heart disease are unknown. Therefore, the objectives of this study were to determine hemodynamic and structural effects and potential changes in endogenous NO-cGMP and endothelin-1 signaling of chronic endothelin A receptor blockade in a lamb model of increased pulmonary blood flow following in utero placement of an aortopulmonary shunt. Immediately after spontaneous birth, shunt lambs were treated lifelong with either an endothelin A receptor antagonist (PD-156707) or placebo. At 4 wk of age, PD-156707-treated shunt lambs (n = 6) had lower pulmonary vascular resistance and right atrial pressure than placebo-treated shunt lambs (n = 8, P < 0.05). Smooth muscle thickness or arterial number per unit area was not different between the two groups. However, the number of alveolar profiles per unit area was increased in the PD-156707-treated shunt lambs (190.7 +/- 5.6 vs. 132.9 +/- 10.0, P < 0.05). Plasma endothelin-1 and cGMP levels and lung NOS activity, cGMP, eNOS, preproendothelin-1, endothelin-converting enzyme-1, endothelin A, and endothelin B receptor protein levels were similar in both groups. We conclude that chronic endothelin A receptor blockade attenuates the progression of pulmonary hypertension and augments alveolar growth in lambs with increased pulmonary blood flow.     

rhVEGF treatment preserves pulmonary vascular reactivity and structure in an experimental model of pulmonary hypertension in fetal sheep

We have previously shown that lung VEGF expression is decreased in a fetal lamb model of PPHN and that VEGF165 inhibition causes severe pulmonary hypertension in fetal lambs. Therefore, we hypothesized that treatment with rhVEGF165 would preserve endothelium-dependent vasodilation and reduce the severity of pulmonary vascular remodeling in an experimental model of PPHN. We studied the effects of daily intrapulmonary infusions of rhVEGF after partial ligation of the ductus arteriosus (DA). We performed surgery in 24 late-gestation fetal lambs and placed catheters in the main pulmonary artery, left atrium, and aorta for pressure measurements and in the left pulmonary artery for drug infusions. A pressure transducer was placed around the LPA to measure blood flow to the left lung (Qp), and the DA was surgically constricted to induce pulmonary hypertension. rhVEGF165 or vehicle was infused for 7 or 14 days. ACh or 8-BrcGMP was infused on days 2 and 13 to assess endothelium-dependent and -independent vasodilation, respectively. ACh-induced vasodilation was reduced in PPHN lambs after 14 days (change in Qp from baseline, 106% vs. 11%). In contrast, the response to ACh was preserved in lambs treated with rhVEGF (change in Qp, 94% vs. 90%). Pulmonary vasodilation to 8-BrcGMP was not altered in PPHN lambs or enhanced by VEGF treatment. rhVEGF treatment increased expression of lung eNOS protein and decreased pulmonary artery wall thickness by 34% vs. PPHN lambs. We conclude that VEGF165 preserves endothelium-dependent vasodilation, upregulates eNOS expression, and reduces the severity of pulmonary vascular remodeling in experimental PPHN.     

Breed, litter and parity effects on placental weight and placentome number, and consequences for the neonatal behaviour of the lamb

Lamb survival is impaired in low birth weight lambs, and those that are slow to stand and suck. Many of the factors that influence lamb vigour, such as parity, litter size, and breed, may exert their effects, at least partially, before birth by influencing placenta development. Our hypothesis was that retarded lamb behavioural development was due to differences in placentation in these animals. Data were collected from Blackface and Suffolk lambs in the first 2 h after birth and placentas were collected when delivered. Suffolk lambs, which were behaviourally slower and had lower rectal temperatures than Blackface lambs, were associated with larger but less efficient placentas (placental efficiency defined as foetal weight supported per g placenta) with fewer foetal cotyledons than Blackface placentas. Triplet lambs were significantly slower than twin or single lambs to suck and had lower rectal temperatures. Although placenta efficiency increased with litter size, placenta and cotyledon weight, and cotyledon number increased with twinning but not thereafter. It seemed likely that triplet lambs suffered some placental insufficiency in comparison to other litter sizes. Lambs born to first parity mothers were slower to stand and reach the udder than lambs of more experienced ewes, and first parity ewes also had smaller and less efficient placentas although cotyledon number was not affected. Male lambs tended to be slower than female lambs for most behaviours, although rectal temperatures were not affected. The sire of the lamb also influenced lamb behaviour and rectal temperature. Both lamb sex and lamb sire influenced the average weight of placental cotyledons, thus some of the sire effect on the behaviour and birth weight of his progeny might be mediated through placental development. Lamb neonatal vigour was correlated with placental efficiency suggesting that lamb behaviour immediately after birth is related to placental development and function.     

Inhaled nitric oxide-induced rebound pulmonary hypertension: role for endothelin-1

Clinically significant increases in pulmonary vascular resistance have been noted on acute withdrawal of inhaled nitric oxide (NO). Endothelin (ET)-1 is a vasoactive peptide produced by the vascular endothelium that may participate in the pathophysiology of pulmonary hypertension. The objectives of this study were to determine the effects of inhaled NO on endogenous ET-1 production in vivo in the intact lamb and to determine the potential role of ET-1 in the rebound pulmonary hypertension associated with the withdrawal of inhaled NO. Seven 1-mo-old vehicle-treated control lambs and six PD-156707 (an ET(A) receptor antagonist)-treated lambs were mechanically ventilated. Inhaled NO (40 parts per million) was administered for 24 h and then acutely withdrawn. After 24 h of inhaled NO, plasma ET-1 levels increased by 119.5 +/- 42.2% (P < 0.05). Western blot analysis revealed that protein levels of preproET-1, endothelin-converting enzyme-1alpha, and ET(A) and ET(B) receptors were unchanged. On acute withdrawal of NO, pulmonary vascular resistance (PVR) increased by 77.8% (P < 0.05) in control lambs but was unchanged (-5.5%) in PD-156707-treated lambs. Inhaled NO increased plasma ET-1 concentrations but not gene expression in the intact lamb, and ET(A) receptor blockade prevented the increase in PVR after NO withdrawal. These data suggest a role for ET-1 in the rebound pulmonary hypertension noted on acute withdrawal of inhaled NO.     

Hyperventilation, alkalosis, prostaglandins, and pulmonary circulation of the newborn

This study was designed to determine whether the effects of hyperventilation on the pulmonary circulation of the newborn lamb were 1) due to mechanical factors or to respiratory alkalosis; and 2) mediated by prostaglandins. Six control lambs were studied during normal ventilation and during hyperventilation with, and without, decreased carbon dioxide (CO2). Five lambs were given indomethacin and studied similarly. In control lambs, hyperventilation with decreased CO2 decreased pulmonary arterial pressure from 26 +/- 2.2 to 18 +/- 1.0 (SE) Torr (P less than or equal to 0.005) and pulmonary vascular resistance from 0.099 +/- 0.035 to 0.070 +/- 0.011 Torr X kg-1 X min-1 (P less than or equal to 0.015). Hyperventilation with normal CO2 did not affect the pulmonary circulation. Hyperventilation with decreased CO2 increased pulmonary arterial concentrations of 6-ketoprostaglandin F1 alpha, a major metabolite of prostacyclin, in control lambs but not in the indomethacin-treated lambs. However, it affected the pulmonary circulation of the control- and indomethacin-treated lambs similarly. In conclusion, hyperventilation affected the pulmonary circulation by respiratory alkalosis not by mechanical factors and prostaglandins did not mediate its effects.     

Spontaneous interferon production by pulmonary leukocytes is associated with lentivirus-induced lymphoid interstitial pneumonia

Ovine lentiviruses share genome sequence, structural features, and replicative mechanisms with HIV, the etiologic agent of AIDS. A lamb model of lentivirus-induced lymphoid interstitial pneumonia, comparable to lymphoid interstitial pneumonia associated with pediatric AIDS, was used to investigate production of leukocyte-soluble mediators. Lentivirus-infected lambs and adult sheep with severe lymphoid interstitial pneumonia had significantly elevated levels of spontaneous interferon (IFN) production from pulmonary leukocytes compared with ovine lentiviruses-infected animals with mild or no lesions of lymphoid interstitial pneumonia or non-infected controls. However, peripheral blood mononuclear cells from lentivirus-infected lambs did not spontaneously release significant amounts of IFN. IFN production by pulmonary lymph node lymphocytes was enhanced in the presence of lentivirus-infected alveolar macrophages. Animals with lentivirus-induced disease and spontaneous IFN production had enhanced virus replication within tissues. The ovine lentiviruses-induced IFN had a m.w. of between 25,000 and 35,000 and was resistant to freeze/thawing procedures. The IFN activity was sensitive to trypsin and stable to low pH and heat. IFN with similar physical and biochemical properties was produced when ovine lentiviruses was added to control leukocyte cultures. IL-2 and PGE2 production and responses to mitogen by pulmonary lymph node lymphocytes of lentivirus-diseased lambs were not statistically different from control animals. Increased local production of IFN in lentivirus-infected host tissues may serve to accelerate the entry of leukocytes into virus-induced lesions promoting cell-mediated tissue damage and also provide increased numbers of cells for virus replication.     

Nitric oxide and superoxide generation from endothelial NOS: modulation by HSP90

Previously, we have shown that pulmonary arterial endothelial cells (PAECs) isolated from fetal lambs produce significant levels of nitric oxide (NO) but minimal superoxide upon stimulation, whereas PAECs isolated from 4-wk-old lambs produce significant amounts of both NO and superoxide. These data indicated that a certain degree of uncoupling of endothelial NO synthase (eNOS) occurs in PAECs during postnatal development. In this study, we sought to extend these studies by investigating the potential role of heat shock protein 90 (HSP90) in eNOS coupling. Western blot analyses revealed higher HSP90 expression in PAECs isolated from fetal compared with 4-wk-old lambs, whereas the analysis of recombinant human eNOS activation in vitro in the presence of HSP90 indicated that HSP90 significantly augmented NO production while inhibiting superoxide generation from eNOS. To further investigate whether HSP90 could be involved in uncoupling of eNOS in PAECs isolated from 4-wk-old lambs, we utilized an adenovirus to overexpress HSP90. We found that overexpression of HSP90 significantly increased the shear-stimulated association of HSP90 with eNOS and led to significant increases in NO production and reduced NOS-dependent superoxide generation. Conversely, the exposure of PAECs isolated from fetal lambs to the HSP90 inhibitor radicicol led to significant decreases in eNOS-HSP90 interactions, decreased shear-stimulated NO generation, and increased NOS-dependent superoxide production indicative of eNOS uncoupling. Finally, we examined eNOS-HSP90 interactions in our lamb model of pulmonary hypertension associated with increased pulmonary blood flow (shunt). Our data indicate that HSP90-eNOS interactions were decreased in shunt lambs and that this was associated with decreased NO generation and an increase in eNOS-dependent generation of superoxide. Together, our data support a significant role for HSP90 in promoting NO generation and inhibiting superoxide generation by eNOS and indicate that the disruption of this interaction may be involved in the endothelial dysfunction associated with pulmonary hypertension.     

Effects of capsaicin pretreatment on expiratory laryngeal closure during pulmonary edema in lambs.

The present study, performed in nonsedated, conscious lambs, consisted of two parts. In the first part, we 1) examined for the first time whether a respiratory response to pulmonary C-fiber stimulation could be elicited in nonsedated newborns and 2) determined whether this response could be abolished by capsaicin pretreatment. Then, by using capsaicin-desensitized lambs, we studied whether pulmonary C fibers were involved in the sustained, active expiratory upper airway closure previously observed during pulmonary edema. Airflow and thyroarytenoid and inferior pharyngeal constrictor muscle electromyographic activities were recorded. In the first set of experiments, a 5-10 microg/kg capsaicin bolus intravenous injection in seven intact lambs consistently led to a typical pulmonary chemoreflex, showing that C fibers are functionally mature in newborn lambs. In the second series of experiments, eight lambs pretreated with 25-50 mg/kg subcutaneous capsaicin did not exhibit any respiratory response to 10-50 microg/kg intravenous capsaicin injection, implicating C fibers in the response. Finally, in the above capsaicin-desensitized lambs, we observed that halothane-induced high-permeability pulmonary edema did not cause the typical response of sustained expiratory upper airway closure seen in the intact lamb. We conclude that functionally mature C fibers are present and responsible for a pulmonary chemoreflex in response to capsaicin intravenous injection in nonsedated lambs. Capsaicin pretreatment abolishes this reflex. Furthermore, the sustained expiratory upper airway closure observed during halothane-induced pulmonary edema in intact nonsedated lambs appears to be related to a reflex involving stimulation of pulmonary C fibers.     

Fostering in sheep. I. Facilitation by use of textile lamb coats

Transfer of odour between lambs less than 1 week old was attempted by the use of rectangular coats of hessian (a jute textile) worn for 24 h by a ewe's own lamb and then transferred to a potential foster lamb. In the absence of their own lamb, 50–60% of ewes accepted the foster lamb immediately and more than 90% accepted the foster after confinement together over the next 2 days. Without exchange of coats, 12% of ewes accepted alien lambs immediately, and 50% after confinement.Breed differences in the success of fostering were observed, and a higher proportion of primiparas than multiparas accepted foster lambs. Foster lambs of similar appearance to the ewe's own lamb were more readily accepted than lambs of different appearance, and lambs 1–2 days old were more readily accepted than older lambs. There was a low level of success with fostering several weeks after lambing. Ewes that had accepted a foster lamb and then had their own lamb returned within 24 h of fostering initially tended to favour their own lamb.The success of the fostering method used in these studies appears to depend on both transfer and masking of lamb odour. The method should prove useful as an experimental tool, and as a practical means of saving lambs.     

Behavioural patterns in ewe–lamb pairs and vulnerability to predation by wolverines

In Norway domestic sheep Ovis aries range unattended in mountainous areas during the summer season. Wolverines Gulo gulo re-established in the alpine regions of southern Norway during recent decades and are viewed as a substantial predator on lambs. Reducing predation on sheep by wolverines would not only reduce the economic loss to farmers but also promote the acceptance of wolverines in their summer ranges. We hypothesized that male lambs would be more prone to wolverine predation, because of higher locomotor activity, lower behavioural ewe–lamb synchrony and larger ewe–lamb distance. We studied ewe and lamb behavioural patterns, synchrony and ewe–lamb distance on a summer range in Knutshøi, south-central Norway. Although no differences were found in ewe–lamb distance or locomotor activity in gender, female lambs synchronized their behaviour more with their mother than males. Only for female lambs, increased synchrony resulted in closer ewe–lamb distances. Overall losses to wolverines based on long-term data indicate that male lambs are more prone to predation than females later in the season. These sex-specific behavioural differences in lambs affect the spatial and social relationships between ewe and lamb, and are likely to increase with age eventually leading to sexual segregation. Male lambs can therefore be expected to be more prone to wolverine predation towards the end of the season, when lambs become independent from the ewe.     

Hydrogen sulfide acts as an inflammatory mediator in cecal ligation and puncture-induced sepsis in mice by upregulating the production of cytokines and chemokines via NF-kappaB

Increasing data suggest that oxidative stress, due to an increased production of reactive oxygen species and/or a decrease in antioxidants, is involved in the pathophysiology of pulmonary hypertension. Several antioxidant systems regulate the presence of oxidant species in vivo, and of primary interest are the superoxide dismutases (SOD) and catalase. However, little is known about the expression of antioxidant enzymes during the development of pulmonary hypertension. This study uses our lamb model of increased postnatal pulmonary blood flow, secondary to in utero aortopulmonary graft placement (shunt lambs), to investigate the expression patterns as well as activities of antioxidant enzymes during the early development of pulmonary hypertension. Protein levels of catalase, SOD1, SOD2, and SOD3 were evaluated by Western blot, and the activities of catalase and SOD were also quantified. In control lambs, protein expression and activities of catalase and SOD2 increased postnatally (P < 0.05). However, SOD1 and SOD3 protein levels did not change. In shunt lambs, catalase, SOD1, and SOD2 protein levels all increased over the first 8 wk of life (P < 0.05). However, SOD3 did not change. This was associated with an increase in the activities of catalase and SOD2 (P < 0.05). Compared with control lambs, catalase and SOD2 protein levels were decreased in 2-wk-old shunt lambs and this was associated with increased levels of hydrogen peroxide (H(2)O(2)) and superoxide (P < 0.05). Developmentally superoxide but not H(2)O(2) levels significantly increased in both shunt and control lambs with levels being significantly higher in shunt compared with control lambs at 2 and 4 but not 8 wk. These data suggest that the antioxidant enzyme systems are dynamically regulated postnatally, and this regulation is altered during the development of pulmonary hypertension secondary to increased pulmonary blood flow. An increased understanding of these alterations may have important therapeutic implications for the treatment of pulmonary hypertension secondary to increased pulmonary blood flow.     

Expression of VEGF and its receptors Flt-1 and Flk-1/KDR is altered in lambs with increased pulmonary blood flow and pulmonary hypertension

Utilizing in utero aortopulmonary vascular graft placement, we developed a lamb model of congenital heart disease and increased pulmonary blood flow. We showed previously that these lambs have increased pulmonary vessel number at 4 wk of age. To determine whether this was associated with alterations in VEGF signaling, we investigated vascular changes in expression of VEGF and its receptors, Flt-1 and KDR/Flk-1, in the lungs of shunted and age-matched control lambs during the first 8 wk of life. Western blot analysis demonstrated that VEGF, Flt-1, and KDR/Flk-1 expression was higher in shunted lambs. VEGF and Flt-1 expression was increased at 4 and 8 wk of age (P <0.05). However, KDR/Flk-1 expression was higher in shunted lambs only at 1 and 4 wk of age (P <0.05). Immunohistochemical analysis demonstrated that, in control and shunted lambs, VEGF localized to the smooth muscle layer of vessels and airways and to the pulmonary epithelium while increased VEGF expression was localized to the smooth muscle layer of thickened media in remodeled vessels in shunted lambs. VEGF receptors were localized exclusively in the endothelium of pulmonary vessels. Flt-1 was increased in the endothelium of small pulmonary arteries in shunted animals at 4 and 8 wk of age, whereas KDR/Flk-1 was increased in small pulmonary arteries at 1 and 4 wk of age. Our data suggest that increased pulmonary blood flow upregulates expression of VEGF and its receptors, and this may be important in development of the vascular remodeling in shunted lambs.     

Birthdate and survival in bighorn lambs (Ovis canadensis)

The relationship between parturition date and lamb survival was investigated among individually marked bighorn ewes in south-western Alberta, Canada. Lambs were born from 17 May to 21 July. Most births occurred in the first two weeks of the lambing period. Lambs born in May enjoyed greater survival than lambs born in June and July. The viability of lambs born after 10 June was extremely low. Inadequate nutrition is suggested as the cause of greater mortality of lateborn lambs. Ewes that give birth late are at an earlier stage of lactation when forage quality declines and may not produce sufficient milk to ensure lamb survival. Their lambs have access to high-quality forage for a shortened period. The proportion of late lambs appeared to be correlated with ewe density in the winter range.     

Respiratory effects of a patent ductus arteriosus in premature newborn lambs

We examined the respiratory effects of a patent ductus arteriosus in 29 premature lambs (131-135 days gestational age) after infiltrating the ductal wall with formaldehyde solution (Formalin) and placing a snare around the ductus to regulate its patency. The lambs were given sheep surfactant, paralyzed, and mechanically ventilated at birth. We first compared 8 lambs with open ductus and 13 lambs with closed ductus during the 12 h after birth. Although lambs with open ductus had greater pulmonary blood flow (301 +/- 36 vs. 188 +/- 11 ml.min-1.kg-1, mean +/- SE, at 12 h of age) and mean pulmonary arterial (44 +/- 3 vs. 33 +/- 2 mmHg) and left ventricular end-diastolic (6 +/- 0.6 vs 4 +/- 0.7 mmHg) pressures, we found no differences in dynamic respiratory compliance (Cdyn = 0.55 +/- 0.07 vs. 0.55 +/- 0.03 ml.cmH2O-1.kg-1), midtidal volume resistance (62 +/- 5 X 10(-3) vs. 62 +/- 7 X 10(-3) cmH2O.ml-1.s), or functional residual capacity (FRC = 27 +/- 3 vs. 26 +/- 2 ml.kg-1). Alveolar-arterial PO2 difference was lower in the lambs with open ductus (238 +/- 65 vs. 362 +/- 37 Torr). Next, we challenged eight lambs with two separate saline infusions (50 ml.kg-1 over 3 min), each given with the ductus alternately closed or open. When the ductus was closed, FRC was unchanged, but Cdyn increased by 18% immediately after the infusion. When the ductus was open, FRC decreased by 16% and Cdyn decreased by 12%. We conclude that the premature lamb is surprisingly resistant to changes in respiratory function from ductal patency during the immediate neonatal period.