Effect of beta-adrenergic receptor blockade on responses to acute hypoxemia in lambs

Effects of hypoxia-ischemia and inotropes on expression of cardiac adrenoceptors in the preterm fetal sheep

Preterm infants frequently suffer cardiovascular compromise, with hypotension and/or low systemic blood flow, leading to tissue hypoxia-ischemia (HI). Many preterm infants respond inadequately to inotropic treatments using adrenergic agonists such as dobutamine (DB) or dopamine (DA). This may be because of altered cardiac adrenoceptor expression because of tissue HI or prolonged exposure to adrenergic agonists. We assessed the effects of severe HI with and without DB/DA treatment on cardiac adrenoceptor expression in preterm fetal sheep. Fetal sheep (93-95 days) exposed to sham surgery or severe HI induced by umbilical cord occlusion received intravenous DB or saline for 74 h (HI + DB, HI, Sham + DB, Sham). The HI groups were also compared with fetal sheep exposed to HI and DA. Fetal hearts were collected to determine β-adrenoceptor numbers using [¹²⁵I]-cyanopindolol binding and mRNA expression of β₁-, β₂-, α_1A-, α_2A-, or α_2B-adrenoceptors. The HI group had increased β-adrenoceptor numbers compared with all other groups in all four heart chambers ( P < 0.05). This increase in β-adrenoceptor numbers in the HI group was significantly reduced by DB infusion in all four heart chambers, but DA infusion in the HI group only reduced β-adrenoceptor numbers in the left atria and ventricle. DB alone did not affect β-adrenoceptor numbers in the sham animals. Changes in β₁-adrenoceptor mRNA levels trended to parallel the binding results. We conclude that HI upregulates preterm fetal cardiac β-adrenoceptors, but prolonged exposure to adrenergic agonists downregulates adrenoceptors in the preterm heart exposed to HI and may underpin the frequent failure of inotropic therapy in preterm infants. NEW & NOTEWORTHY This is the first study, to our knowledge, on the effects of hypoxia-ischemia and adrenergic agonists on adrenoceptors in the preterm heart. In fetal sheep, we demonstrate that hypoxia-ischemia increases cardiac β-adrenoceptor numbers. However, exposure to both hypoxia-ischemia and adrenergic agonists (dobutamine or dopamine) reduces the increase in β-adrenoceptor numbers, which may underpin the inadequate response in human preterm infants to inotropic therapy using adrenergic agonists. Dobutamine alone does not affect the cardiac adrenoceptors in the sham animals.

Influence of prematurity on postnatal maturation of heart rate and arterial pressure responses to hypoxia in lambs

BACKGROUND

While hypoxic events with bradycardias are a frequent occurrence during the first weeks of life in preterm infants, the impact of preterm birth on maturation of the cardiovascular response to hypoxia in early postnatal life is unknown.

OBJECTIVES

In the present study, we tested the hypothesis that preterm birth influences postnatal maturation of cardiovascular responses to hypoxia.

METHODS

6 preterm lambs (term 132 days, birth weight 2.9 kg) and 6 full-term lambs (term 147 days, birth weight 4.3 kg) were studied without sedation on days 3, 7, 14, 21 and 28 during acute, steady-state hypoxia (FIO(2) = 0.08, 15 min).

RESULTS

Results show that full-term lambs increased their heart rate (HR) in response to hypoxia from the first day of life, with no postnatal maturation thereafter. In preterm lambs, HR did not change before day 14 and the full-term-like response was not acquired before 2 weeks after theoretical term. In both groups, mean arterial pressure tended to decrease below baseline, but the amplitude of variations remained of small magnitude.

CONCLUSION

The present results bring unique evidence that preterm birth is associated with a blunted heart response to steady-state hypoxia during the first postnatal week and a delay in the maturation of HR response.

Transient peripheral warming accompanies the hypoxic metabolic response in the golden-mantled ground squirrel

The hypoxic metabolic response of mammals involves a reversible metabolic suppression, possibly brought about by a reduction in the body temperature set-point. In the present study we tested the hypothesis that this is accompanied by a transient increase in heat loss that facilitates the decline in body temperature and metabolic rate. Peripheral heat distribution was assessed using infrared thermography to measure the surface temperatures of the golden-mantled ground squirrel at three different ambient temperatures (10, 22 and 30 degrees C). During early hypoxic exposure, surface temperatures increased dramatically in the feet, ears and nose, and this increase was more dramatic and prolonged at 22 degrees C than at the other two temperatures. These increases were associated with a fall in metabolic rate. Following this initial increase, surface temperatures decreased back to control values, and at 10 degrees C, the surface temperatures of the eyes and body decreased below normoxic levels. Subsequent normoxic recovery was not accompanied by transient changes in surface temperatures, despite large increases in metabolic rate associated with post-hypoxic shivering and thermogenesis. The temporal changes in surface temperature suggest that peripheral blood flow is initially increased during hypoxia, shifting heat away from the core to the periphery and thus facilitating cooling. These results are consistent with the hypothesis that hypoxia leads to a regulated fall in body temperature.

Fetal myocardial responses to long-term hypoxemia

In fetal sheep subjected to high altitude hypoxemia for 110 days beginning on day 30 of gestation, cardiac output was decreased 24% compared to normoxic control fetuses. This decrease was due to a 33% reduction in right ventricular output, with only a 14% reduction in left ventricular output. There were no changes in preload or heart rate, but approximately 7% of the reduction in cardiac output could be explained by an increase in arterial blood pressure (afterload). In papillary muscle isolated from long-term hypoxemic fetal hearts, maximum developed tension in response to increasing concentrations of calcium was reduced in both the right and left ventricles, but sensitivity to calcium was increased in both. This finding suggests alterations in the calcium pathway for excitation-contraction coupling in the hypoxemic fetal hearts may be responsible for the reduction in contractility. The mechanism for the decrease in contractility could not be explained by changes in sarcolemmal L-type calcium channel number or sarcoplasmic reticulum calcium release channel number. In addition, there were no changes in the calcium-induced calcium release mechanism involving the sarcoplasmic reticulum, which could explain the reduced contractility. We speculate that the decreased calcium response may be due to other factors, such as the amount of calcium stored in the sarcoplasmic reticulum, myofilament calcium sensitivity, or cellular content of myofilaments.

Effect of three hours of hypoxia on atrial natriuretic factor gene expression in the ovine fetal heart

OBJECTIVES

The current study investigated the effects of 3 hours of hypoxia on atrial natriuretic factor gene expression and peptide content in each of the four cardiac chambers of the near-term ovine fetus.

STUDY DESIGN

Twenty-three chronically catheterized ovine fetuses at 125 to 129 days' gestation (term 145 days) were used for this study. Fetal hypoxia was induced for 3 hours in 12 fetuses by infusion of nitrogen into the maternal trachea. The remaining fetuses were used as controls. Fetal arterial PO2 and plasma atrial natriuretic factor concentrations were measured during hypoxia. At the end of the hypoxic period atrial natriuretic factor peptide contents and messenger ribonucleic acid levels in each cardiac chamber were determined by radioimmunoassay and Northern blot analysis, respectively.

RESULTS

With infusion of nitrogen into the maternal trachea, fetal arterial PO2 was reduced within 30 minutes by an average of 8.0 +/- 0.3 (SEM) mm Hg (p < 0.0001) and remained reduced at this level throughout the entire hypoxic period. Plasma atrial natriuretic factor concentrations increased by 1152 +/- 212 pg/ml (p < 0.003) and the increase was sustained for the duration of hypoxia. Atrial natriuretic factor peptide and messenger ribonucleic acid levels were much higher in the atria than in the ventricles. Hypoxia did not result in alterations of atrial natriuretic factor peptide content or messenger ribonucleic acid abundance in each cardiac chamber.

CONCLUSIONS

In the near-term ovine fetus, 3 hours of hypoxia resulted in greatly elevated plasma atrial natriuretic factor concentrations; this response was sustained for the duration of hypoxia. However, the increase was not associated with a detectable change in atrial natriuretic factor peptide content or an induction of atrial natriuretic factor gene expression in the atria and ventricles.

Neonatal hemodynamic responses to extreme ranges of controlled graded hypoxia

OBJECTIVES

To determine the hemodynamic responses to a wide range of specific, controlled, graded levels of hypoxic hypoxia over 120 mins in a neonatal porcine model and to identify the PaO2 threshold for altered hemodynamic homeostasis.

DESIGN

Prospective, experimental, animal study.

SETTING

University cardiovascular research laboratory.

SUBJECTS

Three-day-old domestic swine.

INTERVENTIONS

Anesthetized, intubated, and ventilated 3-day-old pigs (n = 88) were assigned to one of five predetermined graded PaO2 groups: Group I (normoxia, PaO2 = 80 torr [10.7 kPa]); group II (PaO2 = 60 torr [8.0 kPa]); group III (PaO2 = 40 torr [5.3 kPa]); group IV (PaO2 = 30 torr [4.0 kPa]); or group V (PaO2 = 20 torr [2.7 kPa]).

MEASUREMENTS AND MAIN RESULTS

Hemodynamic parameters including heart rate, systolic blood pressure, diastolic blood pressure, mean arterial pressure (MAP), and pulse pressure were evaluated. Acid-base status (arterial pH and lactate) was monitored in each experimental group over the 120-min study period. Hemodynamic and acid-base parameters were unaltered in animals in groups I and II. In group III animals, blood pressure was maintained (partly by increased heart rate), and acid-base balance was unaltered. In contrast, group IV animals had a gradual and progressive decrease in systolic blood pressure, diastolic blood pressure, and MAP, and slightly decreased pulse pressure, despite sustained tachycardia. Group IV animals also developed mild lactic acidosis. Group V animals exhibited a biphasic hemodynamic response, while the heart rate response was characterized by tachycardia at the induction of hypoxia, which was reduced in magnitude by 120 mins. The biphasic hemodynamic response in this group of animals included an initial increase in systolic and pulse pressures, followed by a gradual and progressive decrease in systolic and diastolic blood pressures, MAP, and pulse pressure. In addition, group V animals also developed profound progressive lactic acidosis.

CONCLUSIONS

In anesthetized neonatal pigs, tachycardia occurred in response to a PaO2 of 40 torr (5.3 kPa), and thus marked the threshold for altered hemodynamic homeostasis. Beyond this threshold, both the 30 torr (4.0 kPa) and 20 torr (2.7 kPa) groups had a PaO2- dependent "late" hypotension, while only the 20 torr (2.7 kPa) group had a significant biphasic hemodynamic response characterized by "early" hypertension. The "late" hypotension which occurred in these two profound hypoxia groups indicates an inability to adequately adjust hemodynamics during prolonged hypoxic hypoxia.

Differential regulation of right and left ventricular beta-adrenergic receptors in newborn lambs with experimental cyanotic heart disease

To determine whether chronic hypoxemia secondary to an intracardiac right-to-left shunt alters regulation of the myocardial beta-adrenergic receptor/adenylate cyclase system, we produced chronic hypoxemia in nine newborn lambs by creating right ventricular outflow obstruction and an atrial septal defect. Oxygen saturation was reduced to 65-74% for 2 wk. Eight lambs served as normoxemic controls. beta-receptor density (Bmax) and ligand affinity (KD) were determined with the radio-ligand [125I]iodocyanopindolol and adenylate cyclase activity determined during stimulation with isoproterenol, sodium fluoride (NaF), and forskolin. During chronic hypoxemia, Bmax decreased 45% (hypoxemic, 180.6 +/- 31.5 vs. control, 330.5 +/- 60.1 fmol/mg) in the left ventricle (exposed to hypoxemia alone) but was unchanged in the right ventricle (exposed to hypoxemia and pressure overload). KD was not different from control in either ventricle. Left ventricular isoproterenol-stimulated adenylate cyclase activity was decreased by 39% (30.0 +/- 4.3% increase vs. 44.1 +/- 9.5% increase) whereas right ventricular adenylate cyclase activity was unchanged. Stimulation of adenylate cyclase with NaF or forskolin was not different from control in either ventricle. Circulating epinephrine was increased fourfold whereas circulating and myocardial norepinephrine were unchanged. These data demonstrate a down-regulation of the left ventricular beta-adrenergic receptor/adenylate cyclase system during chronic hypoxemia secondary to an intracardiac right-to-left shunt.