Reduction of catecholamine-induced cardiovascular malformations in the chick embryo with metoprolol

A cluster of hypoplastic left heart malformation in Baltimore, Maryland

Congenital cardiovascular malformations (CCVMs) of the left side of the heart show familial recurrence of various forms of obstructive malformations, including hypoplastic left heart (HLH), interrupted aortic arch, coarctation of the aorta, and aortic stenosis. In a previous population-based study in the Baltimore-Washington region, these malformations were associated with parental reports of occupational or leisure solvent exposure, overt diabetes, and family history of CCVM in first-degree relatives. Spatial analysis in this well-characterized study population may augment self-reported data by revealing additional associations with potential environmental risk factors. We used spatial analysis to identify clusters of HLH as a group. The study population included all live-born cases of hypoplastic left heart syndrome diagnosed in the first year of life between 1981 and 1989 and a random sample of unaffected infant controls matched by year and hospital of birth. The nested case-control cohort in this spatial analysis included 77 HLH cases and 1894 controls in Maryland and the District of Columbia. Nonparametric and regression analyses included personal variables from the interview data set as well as spatial variables. A region of Baltimore was identified that contained HLH at twice the expected frequency based on the distribution of population younger than 5 years of age. The region included 30 of 77 geocoded cases of HLH in the cohort and is significant by spatial scanning at p = 0.056. Within this region, male cases of HLH were disproportionately present compared to females. This cluster is in a region of Baltimore with industrial release of solvents, dioxin, and polychlorinated biphenyls in air. Outside the cluster, HLH is associated with family history of CCVM in a first-degree relative, maternal exposure to miscellaneous solvents, paternal anesthesia, maternal art painting, aspirin ingestion, and maternal diabetes. Inside the cluster, father's painting and exposure to sympathomimetic drugs were associated risk factors. Spatial analysis of HLH cases delineated an urban region with increased prevalence of this left heart malformation. Within this region, excess male cases of HLH occurred, and industrial release to air of solvents, dioxin, and polychlorinated biphenyl compounds was documented. We propose that both genetic and environmental factors contribute to the phenotype of HLH.

Cardiovascular teratogenicity of terbutaline and ritodrine in the chick embryo

OBJECTIVE

We determined the teratogenic effects of terbutaline and ritodrine, both beta 2-sympathomimetic agonists, on the stage 24 (4-day) chick embryo.

STUDY DESIGN

We used a topical method of application of terbutaline or ritodrine to the stage 24 chick embryo in ovo. Doses of terbutaline ranged from 5.5 x 10(-10) to 6.5 x 10(-9) mol per embryo, and ritodrine doses ranged from 4.6 x 10(-11) to 4.6 x 10(-8) mol per embryo. To further determine the pharmacologic nature of the teratogenic potential of terbutaline or ritodrine, the experiments were repeated after pretreatment with butoxamine hydrochloride, a preferential beta 2-antagonist, or metoprolol tartrate, a preferential beta 1-antagonist, 4 hours before application of terbutaline or ritodrine.

RESULTS

Terbutaline treatment was associated with significantly higher rates of anomalies than in controls at all dosages used, whereas ritodrine induced significantly more anomalies at or above doses of 4.6 x 10(-9) mol per embryo. At an equimolar dose pretreatment with butoxamine hydrochloride significantly reduced the cardiovascular teratogenic effects of terbutaline and ritodrine. Pretreatment with metoprolol tartrate at any dose did not significantly reduce terbutaline's potential. Metoprolol, at doses tenfold or 100-fold higher than ritodrine, was able to significantly reduce the teratogenic effects of ritodrine.

CONCLUSIONS

Our data suggest that terbutaline and ritodrine are teratogenic in the chick and that these agents exert their teratogenic effects primarily through stimulation of the beta 2-adrenergic receptor.

Teratogenesis associated with oxydemeton-methyl in the stage 12 chick embryo

The teratogenic potential of oxydemeton-methyl (ODM) was investigated using a stage specific localized topical method of application to the stage 12 chick embryo. A dose of 0.01, 0.05, 0.10, 0.50, 1.0, or 2.0 mg/embryo was applied to the vitelline membrane directly above the stage 12 embryo. The embryo was then returned to the incubator and monitored daily until stage 41 (15 day). At stage 41 the embryo was autopsied and examined for gross external and internal malformations, wet weight, and crown-rump length. Experimental data was compared to unopened and saline treated controls. At doses less than 0.50 mg/embryo, survival rates were high (> 80%) but when that dose was exceeded, the survival rate fell significantly (P < 0.001). A dose-dependent increase in malformation rate was seen in all treatment groups with 0.10 mg/embryo, producing a maximum malformation rate (19/33) with minimum mortality (3/36). Crown-rump lengths and wet weights were significantly less than controls in all treatment groups (P < 0.001). Anomalies were primarily seen in the musculoskeletal (ventral midline, limb, and neck) and cardiovascular (ventricular septum and aortic arches) systems. Thoracogastroschisis and ventricular septal defects were the most common combination of malformations. Our data suggest that ODM is teratogenic when topically applied to the stage 12 chick embryo.

Potentiating effects of forskolin on the cardiovascular teratogenicity of ephedrine in chick embryos

The effects of forskolin on the teratogenicity of ephedrine in the developing chick heart were studied. Forskolin was administered to 4-day chick embryos (Hamburger-Hamilton stage 24) together with ephedrine at doses at which each agent alone caused minimal embryotoxicity. The embryos were examined for malformations on day 14 of incubation. The frequency of malformed embryos exposed to ephedrine (0.5 or 5 mumol) alone was 8 and 26%, respectively, and significantly increased to 47-72% in the presence of 1 nmol forskolin. Forskolin (1 nmol) alone did not induce a significant number of cardiac malformations. These results suggest that the increase in cAMP through stimulation of adenylate cyclase by forskolin is associated with the potentiation of ephedrine-induced cardiovascular malformations in the chick.

Tissue and plasma levels of a teratogenic dose of dopamine in the chick embryo following pretreatment with metoprolol or phosphate buffered saline

Metoprolol pretreatment has been shown to reduce the cardiovascular malformation rate produced by topical doses of dopamine in the stage 24 chick embryo. We report on the tissue and plasma levels and teratogenic effect of dopamine hydrochloride following topical application of a teratogenic dose in stage 24 chick embryos pretreated with either metoprolol tartrate or phosphate buffered saline (PBS). Pretreatment with either metoprolol or PBS resulted in similar patterns of dopamine distribution in the head, body, and heart, with peak levels occurring at 12 hours after dopamine treatment. Plasma concentrations of dopamine were similar for both PBS and metoprolol pretreated embryos, with plasma levels exceeding tissue concentrations, but also peaking at 12 hours after dopamine treatment. Pretreatment with PBS followed by a teratogenic dose of dopamine resulted in a decrease in the teratogenic effect of dopamine similar to that found in previous work in our lab with pretreatment with metoprolol. The developing chick cardiovascular system experiences peak susceptibility to the teratogenic effects of dopamine at stage 24 during development, which represents a time frame of about 12 hours. A delay in the peak levels of dopamine to 12 hours after dopamine treatment as compared to previous work in our lab reporting peak levels of dopamine at 1 hour, suggests that the previously reported antiteratogenic effects of metoprolol may be due, at least in part, to a delayed absorption of dopamine past the time of peak susceptibility of the embryo to the teratogen.

Ventricular blood pressure and cardiac output changes in epinephrine- and metoprolol-treated chick embryos

The effects of a teratogenic dose (5 micrograms) of epinephrine on mean ventricular blood pressure (MVBP) and cardiac output (CO) at one and two hours after treating stage 24 chick embryos were investigated. Previous work demonstrated that a differential response in terms of cardiac rhythm during the first hour after epinephrine treatment was related to pathogenesis of two contrasting types of aortic arch malformations. Absence of one or more aortic arches occurred more frequently in embryos which developed a characteristic dysrhythmia, while persistence of the left fourth aortic arch (PL4AA) occurred more frequently in nondysrhythmic embryos. In this study, dysrhythmic epinephrine-treated embryos exhibited reductions in both MVBP and CO at one hour after treatment when compared to control values. Nondysrhythmic epinephrine-treated embryos exhibited elevated MVBP and no change in CO at one hour after treatment. MVBP and CO in recovered dysrhythmic and nondysrhythmic embryos were similar to control values at two hours following epinephrine treatment. MVBP and CO measurements were obtained from embryos which were pretreated with metoprolol and then subsequently treated with epinephrine. Metoprolol is a beta 1-adrenoreceptor antagonist which was previously shown to block the teratogenic effects of epinephrine and other catecholamines with beta 1-adrenoreceptor agonist properties. Pretreating embryos with metoprolol in this study reduced the dysrhythmogenic potential of epinephrine and also blocked the MVBP and CO changes observed in embryos treated with epinephrine alone. We conclude that pathogenesis of 1) abnormally absent aortic arches is related to dysrhythmogenesis, reduced MVBP, and reduced CO, and 2) an abnormally persistent left fourth aortic arch is related to elevated MVBP in the epinephrine model.

The antidysrhythmic effect of metoprolol in the epinephrine treated chick embryo

It was confirmed through electrocardiography that within two hours after epinephrine treatment, four day chick embryos either maintained normal rhythm or developed a severe cardiac dysrhythmia (22/93, 24% dysrhythmic). The ECG dysrhythmia in epinephrine treated embryos were characterized by periods of bradycardia, asystole, and various supraventricular or ventricular dysrhythmias. Within four hours after treatment, dysrhythmic embryos either reestablished normal rhythmicity or died. Electrocardiographic data also demonstrated that metoprolol pretreatment will block epinephrine induced dysrhythmias (0/46, 0% dysrhythmic). We conclude that metoprolol possesses antidysrhythmic properties in the epinephrine treated chick embryo.

Uptake and blood pressure studies in the chick embryo following treatment with a teratogenic dose of dopamine

We report on the ventricular pressor response and regional (head, heart, and body) uptake of dopamine hydrochloride (DA) following topical application of a teratogenic dose to the stage 24 chick embryo. Embryos treated with DA exhibited a significant increase in mean ventricular blood pressure (MVBP) at 0.5 and 1 hour after treatment when compared to saline-treated control embryos. The time of the elevated MVBP correlated with the time of the peak uptake of DA in the embryonic heart. Even though the level of DA in the heart remained high for 9 hours, there was no measurable increase in blood pressure beyond 1.5 hours, suggesting unresponsiveness to DA or perhaps conversion of DA to its metabolites. These experiments have correlated a physiologic response of a known cardiovascular teratogen to its uptake in the embryo.

The effect of isoproterenol on cardiovascular function in the stage 24 chick embryo

The developing cardiovascular system of the chick embryo is susceptible to teratogenic effects of catecholamines. Yet the mechanism for the teratogenetic action is unclear. Since catecholamines affect cardiovascular physiology, we studied the acute effect of the beta-agonist isoproterenol on mean atrial pressure, heart rate, mean dorsal aortic blood flow, mean arterial pressure and vascular resistance in stage 24 chick embryos. Dorsal aortic blood velocity was measured with a 20-MHz pulsed-Doppler velocity meter and intravascular pressure was measured with a servo-null pressure system. Isoproterenol in doses of 2 X 10(-4) micrograms (2.5 micrograms/kg), 8 X 10(-4) micrograms (10 micrograms/kg), and 1.2 X 10(-3) micrograms (15 micrograms/kg) was injected intravenously in 5-microliters aliquots of chick Ringer's solution. Additional groups of embryos were treated with the beta-antagonist propranolol, and isoproterenol plus propranolol. Control embryos received 5 microliters chick Ringer's solution to assess the hemodynamic effects of a volume injection. We found that isoproterenol caused no change in mean atrial pressure, heart rate, or mean arterial pressure. However, isoproterenol caused a dose-related decrease in dorsal aortic blood flow and a 2.5-fold increase in vascular resistance. The effects of isoproterenol were blocked by propranolol, which suggested that the increase in vascular resistance was mediated by beta-receptor stimulation.

The role of dysrhythmic heart function during cardiovascular teratogenesis in epinephrine-treated chick embryos

Chick embryos were treated with a teratogenic dose (5 micrograms) of epinephrine hydrochloride at 4 days of development (stage 24). Heart rates were determined at 20, 40, and 60 minutes after treatment. The mean heart rate values for epinephrine-treated embryos were significantly less than values obtained for untreated and saline-treated control embryos. The decrements in mean heart rate could be explained by severe cardiac dysrhythmias that occurred in approximately 40% of epinephrine-treated embryos. The remaining 60% of embryos exhibited heartbeat patterns similar to controls. This finding enabled us to separate epinephrine-treated embryos into two physiologically distinct groups: dysrhythmia-positive and dysrhythmia-negative. Dysrhythmias were characterized by periods of bradycardia alternating with periods of asystole and were confirmed by electrocardiography. EKG data suggest that epinephrine induces cardiac conduction disturbances in some chick embryos. An additional experiment was conducted to identify the frequencies at which abnormally obliterated aortic arches, abnormally persistent aortic arches, ventricular septal defects, and embryonic death occurred in dysrhythmia-positive and negative embryos. Abnormally obliterated vessels and embryonic death occurred with significantly greater frequencies in dysrhythmia-positive embryos. Abnormally persistent vessels and ventricular septal defects occurred with significantly greater frequencies in dysrhythmia-negative embryos. We conclude that the production of specific malformations and decreased probability for survival in epinephrine-treated embryos are related to dysrhythmogenesis and bradycardia.

The spontaneous occurrence of aortic arch and cardiac malformations in the white Leghorn chick embryo (Gallus domesticus)

Spontaneous aortic arch and cardiac malformations occur in White Leghorn chick embryos at a relatively high rate. Although this breed of Gallus domesticus is widely used for biomedical and biological research, no previous study has recorded the incidence of these defects. We found aortic arch malformations in 7.1% (14 of 196) and ventricular septal defects in 11.7% (23 of 196) of living embryos. Defects occurred alone or as a combined pattern. Our findings suggest that the cardiovascular defects in the chick embryo documented in past studies may, in some cases, have been part of normal spontaneous occurrence, rather than the major result of experimental manipulation.