Pulmonary clearance of norepinephrine in lambs.

The lungs play an important role in the metabolism of vasoactive substances including endogenous amines. The role of pulmonary clearance of circulating norepinephrine has not been well defined in the young lamb (7-8 d of age). Using radiolabeled tracer norepinephrine in acutely instrumented lambs, we determined the in vivo pulmonary clearance and spillover rate of norepinephrine under baseline and hypoxic conditions. The fractional extraction of norepinephrine, the percent removed on a single pass through the pulmonary circulation, was 23 +/- 2%. The corresponding pulmonary clearance rate was 61 +/- 10 mL/kg/min and the net pulmonary norepinephrine removal rate was 0.41 +/- 0.14 nmol/kg/min. This clearance represented over 70% of whole body norepinephrine clearance. The spillover of synaptic norepinephrine was 0.22 +/- 0.13 nmol/kg/min. During hypoxia, animals showed significant increases in pulmonary artery pressure and resistance. Fractional extraction and norepinephrine decreased to 16 +/- 3%, p less than 0.005. Pulmonary clearance decreased to 31 +/- 7 mL/kg/min, and net pulmonary norepinephrine removal rate decreased to 0.27 +/- 0.07 nmol/kg/min. These results demonstrate that pulmonary clearance plays a significant role in norepinephrine clearance in 1-wk-old lambs. Alteration of norepinephrine clearance during physiologic states such as hypoxia may be important in the pathophysiology of altered pulmonary vascular resistance in newborn animals.

Plasma catecholamines and their physiologic thresholds during the first ten days of life in sheep.

We studied serial plasma catecholamine levels in healthy newborn sheep over the first ten days of life. The results show that plasma norepinephrine values in newborn sheep are 3-4 fold higher, and plasma epinephrine values are two-fold higher than values in term fetal sheep. These elevations are sustained over the first 10 days of life. Cardiovascular (heart rate and blood pressure) and metabolic parameters (glucose and free fatty acids) are also significantly elevated above fetal levels. We performed graded catecholamine infusions in newborn animals and adult ewes to determine the minimum plasma catecholamine concentrations necessary for discernible physiologic effects. In response to step-wise increases in epinephrine or norepinephrine infusion rates, there were immediate increases in blood pressure and other physiologic responses. This pattern was seen in both newborn and adult animals, and differed from previous observations in fetal sheep where log-linear, dose response curves characteristic of a threshold response were seen. These results suggest that during the first two weeks of life plasma catecholamine levels are elevated above the threshold value for physiologic responses. These sustained elevations in circulating catecholamines are important in the maintenance of physiologic homeostasis.

Free and sulfoconjugated catecholamine responses to hypoxia in fetal sheep.

Plasma catecholamines circulate either in conjugated or unconjugated forms. In adult humans, sulfoconjugated catecholamines predominate; however, there is considerable variation between species. In a variety of pathophysiological states catecholamine conjugation is believed to represent an important mechanism of inactivation of high circulating catecholamine levels. To date, there have been few data in developing animals or humans on catecholamine sulfoconjugation. We studied the differences in free and sulfoconjugated catecholamines in full term (141 +/- 1 days) and preterm (123 +/- 1 days) chronically catheterized fetal sheep and determined the changes in free and sulfoconjugated catecholamines in response to hypoxia. The results demonstrate that term and preterm animals have a comparable percentage of basal circulating sulfoconjugated catecholamines (free-to-total ratio 50-60%). In response to hypoxia, both free and sulfoconjugated catecholamines were promptly elevated with significant increases in each by 5 min of hypoxia. This was true for both term and pretern animals. The proportion of free and total catecholamines remained relatively constant during hypoxia despite a 5- to 10-fold increase in circulating levels of each. These data demonstrate that fetal sheep, as early as 80% gestation, have a well developed mechanism for sulfoconjugation and subsequent inactivation of the high circulating levels of catecholamines seen during fetal and newborn life.

Thresholds for physiological effects of plasma catecholamines in fetal sheep.

To clarify the physiological role for the marked increases in circulating norepinephrine (NE) and epinephrine (E) that occur at birth, we performed graded infusions of NE and E in preterm (131 days) and full-term (142 days) fetal sheep. A variety of hemodynamic, metabolic, and endocrine responses to stepwise increases in plasma catecholamine levels were analyzed by computer-based graphical analysis of the dose-response curves. We determined the "threshold" value or minimum plasma concentration necessary to produce discernible effects. We observed increases in systolic blood pressure, diastolic blood pressure, and dP/dt beginning at plasma concentrations of 500-800 pg/ml of NE or E. In contrast, increases in plasma free fatty acid and glucose levels were observed at E concentrations as low as 50-100 pg/ml. Full-term animals had generally lower thresholds and higher peak responses than preterm animals. Because these thresholds for infused NE and E are well within the range of plasma catecholamine values seen at birth, these results underscore the importance of circulating catecholamines in the events of neonatal adaptation.

The effect of chemical sympathectomy on catecholamine release at birth.

The precise source of circulating catecholamine (CA) at birth and their role in circulatory adaptation is unclear. In order to determine the contribution of increased postganglionic sympathetic nerve activity to the CA surge at birth, we induced complete sympathectomy in near term fetal lambs prior to delivery by giving 6-hydroxydopamine. Chronically catheterized fetal sheep received either 6-hydroxydopamine (n = 5) or control infusion (n = 6). Chemical sympathectomy was verified by tyramine infusion. Lambs were delivered at 142 +/- 1 days of gestation and serial plasma CA, heart rate, blood pressure, cardiac output, blood gases, blood glucose, and free fatty acids, were measured before and for 4 h after delivery. Myocardial beta-adrenergic receptors and tissue CA concentration were determined following sacrifice. Baseline circulating norepinephrine (NE) values were lower in sympathectomized animals (183 +/- 45 versus 373 +/- 125 pg/ml, p less than 0.05) and epinephrine values were slightly higher (118 +/- 89 versus 48 +/- 1 pg/ml, NS). There was only a 2-fold increase in NE after cord cutting in sympathectomized animals while control animals had a 4-fold increase (peak NE values 354 +/- 121 versus 1305 +/- 363 pg/ml respectively, p less than 0.001). Epinephrine increased significantly in both groups and there were no significant differences between sympathectomized and control animals. Heart rate and blood pressure rose abruptly in both groups after cord cutting and there were no significant differences.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic clearance and plasma appearance rates of catecholamines in preterm and term fetal sheep.

Plasma catecholamines increase markedly during labor and delivery. Moreover we have noted greater increases at birth in preterm than term lambs. It was unclear whether these differences were due to differences in secretion or clearance. We therefore designed experiments to compare the metabolic clearance rates (MCR) and plasma appearance rates of norepinephrine (NE) and epinephrine (E) in chronically catheterized term (143 +/- 1 days) and preterm (131 +/- 1 days) fetal sheep. Two different techniques, radioisotope tracer techniques and infusion of cold hormones for estimation of clearance rates, were compared systematically in the same group of animals. The results demonstrate that MCR of NE in term fetuses (178 +/- 28 ml X kg-1 min-1) is similar to preterm fetuses (205 +/- 22 ml X kg-1 min-1) as is MCR for E (193 +/- 28 versus 170 +/- 33 ml X kg-1 min-1, respectively). Estimates of MCR from cold hormone infusion were highly dependent on infusion rate with estimates as much as 150% above that determined by isotope tracer infusions. Plasma appearance rates for both NE and E in term and preterm animals were similar. There were no detectable physiologic alterations during the tracer isotope infusions whereas infusions of cold hormone were associated with marked alterations in heart rate and blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Continuous ovine fetal hemorrhage: sensitivity of plasma and urine arginine vasopressin response.

Intravascular hemorrhage of the ovine fetus is a potent stimulus for arginine vasopressin (AVP) secretion. However, the method (acute, continuous) and rate of blood withdrawal may influence the fetal response. To determine the hemorrhage threshold for AVP secretion in response to slow continuous hemorrhage, five chronically catheterized ovine fetuses were continuously hemorrhaged (0.6% blood vol/min) to 24-30% blood volume withdrawal. Immediately after hemorrhage fetal blood was reinfused at an equivalent rate. In addition to AVP measurements by radioimmunoassay, fetal urinary responses were monitored as an index of fetal AVP secretion. Significant increases in plasma AVP occurred during hemorrhage (1.0 +/- 0.1 to 8.0 +/- 2.0 pg/ml). The fetal plasma AVP-hemorrhage threshold, as defined by regression analysis, occurred at withdrawal of 13.0% blood volume. Fetal urine volume significantly decreased from a mean basal rate of 0.59 +/- 0.03 to 0.21 +/- 0.06 ml/min at the completion of hemorrhage. Urinary sodium, potassium, and osmolar excretion also significantly decreased at the completion of hemorrhage. Urinary AVP excretion, urine osmolality, sodium, and potassium concentrations did not change significantly during the hemorrhage period but increased significantly during the reinfusion period; the delay a result of renal and catheter dead space. Reinfusion of blood resulted in a return of plasma AVP to basal levels. These results define a threshold for AVP secretion and demonstrate significant urinary effects in response to slow continuous hemorrhage.

Detection of private and common tumor-associated antigens in murine sarcomas induced by different chemical carcinogens.

Two fibrosarcomas of similar histological type, induced in C3Hf mice by either methylcholanthrene or 3,4-benz(a)pyrene, were shown to have individually unique tumor-rejection antigens in classical transplantation-type experiments. By contrast, sera of autochthonous mice, which resisted only transplants of the immunizing sarcoma, were found to contain complement-dependent cytotoxic antibodies, specific for both sarcomas, in vitro. The existence of individually unique as well as common tumor-associated antigens in chemically-induced murine sarcomas is suggested. The private "tumor transplantation-type" antigens elicited tumor rejection responses in vivo. The common tumor-associated antigens, although immunogenic in autochthonous hosts, inducing the production of tumor-specific antibodies, failed to induce transplantation cross-resistance in vivo. This study supports the contention that, in carcinogen-induced murine tumors, and perhaps in human neoplasms as well the evaluation of humoral (and cell-mediated) immune responses in vitro may not reflect tumor rejection-type immune responses in vivo.