Anesth Analg 2003;97:878-882
© 2003 International Anesthesia Research Society
OBSTETRIC ANESTHESIA
The Effect of Maternal Cocaine Exposure on Neonatal Rat Cardiac Function
Lena S. Sun, MD*,
,
Shin Takuma, MD
,
Rui Lui, MD, and
Shunichi Homma, MD
Departments of *Anesthesiology,
Pediatrics, and
Medicine, College of Physicians and Surgeons of Columbia University, New York, New York
Address correspondence and reprint requests to Lena S. Sun, MD, College of Physicians and Surgeons of Columbia University, Department of Anesthesiology, 630 W. 168th St., BH4-440 North, New York, NY 10032. Address e-mail to lss4{at}columbia.edu,
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Abstract
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Fetal cocaine exposure has been associated with a variety of cardiovascular dysfunctions in humans. We treated pregnant rats with either saline or cocaine at 60 mg/kg by gastric lavage for the entire gestational period and for 14 days after parturition. We then performed high-frequency transthoracic echocardiography to determine whether cocaine exposure affected neonatal cardiac contractile function in vivo in 7- and 14-day-old neonatal rats. All studies were performed in the unsedated, conscious state. Heart rate (HR) and systolic function, expressed as fractional area of change at the midpapillary muscle level, were calculated from two-dimensional images. Resting HR was faster in the cocaine-exposed group at both ages, but baseline contractile function was not different between control (CTL) and cocaine-exposed (COC) neonatal rats. Dobutamine induced a significant increase in HR in all groups at only the largest dose tested (Day 7 CTL HR increased from 438 ± 3 bpm to 462 ± 10 bpm; Day 7 COC HR increased from 466 ± 3 bpm to 493 ± 7 bpm; Day 14 CTL HR increased from 443 ± 4 bpm to 487 ± 4 bpm; Day 14 COC HR increased from 477 ± 4 bpm to 501 ± 5 bpm). Dobutamine elicited a significant increase in contractile response at both Day 7 (from 76.6% ± 0.6% to 81.5% ± 0.7%) and Day 14 in CTL (from 78.2% ± 0.7% to 81.9% ± 0.7%), but not in COC, animals (from 76.7% ± 0.8% to 78.9% ± 0.8% at Day 7 and from 76.8% ± 1.1% to 79.3% ± 0.8% at Day 14). Epinephrine induced a significant increase in contractile response in CTL, but not in COC, rats at Day 7 and had no effect on fractional area of change at 14 days of age in either CTL or COC animals. Our results indicate that perinatal cocaine exposure does not modify resting contractile function but attenuates the contractile response to ß-adrenoceptor stimulation in the neonatal rat. These results suggest that perinatal cocaine exposure may lead to decreased responsiveness to inotropic drugs during the early neonatal period.
IMPLICATIONS: Perinatal cocaine exposure decreases the cardiac response to adrenaline-like drugs often used to improve contractile function but has no effect on the ability of these drugs to increase heart rate in the neonatal rat.
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Introduction
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Fetal cocaine exposure has been associated with a variety of cardiovascular dysfunctions in humans (1). Cocaine-exposed human neonates have a faster resting heart rate and decreased heart rate variability (2,3). Several reports have also indicated that cardiac function may be impaired in children with prenatal cocaine exposure (1,2,4,5). Reduced cardiac output was noted in neonates born to mothers who abused cocaine either by itself or with other substances. Using color kinesis studies, Mehta et al. (4) documented that neonates with prenatal cocaine exposure showed evidence of abnormal cardiac contractile function. To further understand the mechanisms of cardiac dysfunction after maternal cocaine administration, we developed a rat model of perinatal cocaine exposure. We have successfully used this rat model in documenting changes in heart rate variability in the neonatal rat, similar to what has been reported in the human neonate (6). In this study, we examined neonatal cardiac contractile function in the rat model of perinatal cocaine exposure.
We have previously shown that, after intrauterine cocaine exposure, baseline and isoproterenol-stimulated cell-shortening amplitude and slope are reduced; this is suggestive of a reduction in cardiomyocyte contractile function. The attenuated cardiomyocyte contractile response to the ß-agonist isoproterenol occurred despite a documented increase in myocardial ß-adrenoceptor (ßAR) density in the 1-day-old neonatal rat (7). However, the upregulation in ßAR density did not persist beyond the immediate neonatal period (7). Our studies in Day 7 (D7) and Day 14 (D14) neonatal rats showed that perinatal cocaine exposure did not alter myocardial ßAR density but was associated with a significant reduction in ßAR-stimulated adenylyl cyclase activity compared with controls. On the basis of these observations, we hypothesized that cardiac contractile function and the response to ß-adrenergic receptor stimulation in neonatal rats during the first two postnatal weeks would be decreased after perinatal cocaine exposure. In this study, we used transthoracic echocardiography to determine in vivo cardiac contractile function in the neonatal rat. Because anesthetics used to sedate animals for these studies can alter the contractile functional response on their own (8), all of the studies were performed in unanesthetized, awake infant rats.
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Methods
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Studies were approved by the Institutional Animal Care and Use Committee at College of Physicians and Surgeons of Columbia University. Timed-pregnant (gestational Day 0 or 1) female Sprague-Dawley rats, 90–110 days old, were purchased and housed individually in a temperature- and humidity-controlled room on a 12:12-h light/dark cycle with ad libitum access to Purina laboratory rodent chow (Purina Mills, St. Louis, MO) and drinking water. On arrival in our animal care facility, these rats were weighed and then transferred to a maternal cage within the first 24 h. Weights were obtained on Days 7, 14, and 21 of gestation. Saline (control) or cocaine at 60 mg/kg in an equal volume was given once a day by intragastric administration beginning on gestational Day 2 and continuing until birth. This dose of cocaine and the intragastric route of administration have been documented to simulate the plasma and tissue concentrations achieved in the human abusers, and the dose caused minimal changes in maternal weight gain and fetal waste (9). After birth, neonatal animals were nursed by mothers that continued to receive drug treatment until postnatal D14. Animals were studied on D7 and D14 of postnatal life.
To determine whether cocaine exposure may affect neonatal cardiac contractile function in vivo, we performed high-frequency echocardiography (Philips 5500 ultrasound equipment with a 15-MHz phased-array transducer; Philips, Andover, MA) in each rat at D7 and then at D14. Animals were brought to the animal echocardiographic laboratory and allowed to acclimate for 30 min before studies were begun. The index finger of a latex glove was filled with acoustic coupling gel (Aquasonic 100; Parker Laboratories Inc., Orange, NJ) and attached to the convex head of the transducer. Studies were performed with animals in the conscious state. Conscious experiments were performed as follows: animals were carefully held with the left hand while grasping the skin on the back of the neck and wrapping the tail to prevent movement during the studies. Care was taken to avoid excessive pressure on the animal. All studies were performed at a frame rate of 121 Hz at a depth setting of 2 cm and were recorded on an optical disk for later analysis.
Animals were studied to obtain data at baseline and after drug treatment. All studies after drug treatment were performed exactly 10 min after intraperitoneal injection of saline, dobutamine, 1, 10, 50 mg/kg, or epinephrine 1 mg/kg administered in an equivolume of 0.1 mL. Each animal received only one drug treatment at both D7 and D14.
Two-dimensional echocardiographic short-axis images of the left ventricle were obtained at the papillary muscle level. Following the recommendation of the American Society of Echocardiography for two-dimensional echocardiography, the endocardium was traced by covering the innermost edge of the endocardial surface. This was performed from the data recorded on an optical disk. Each study was analyzed by a single investigator who was unaware of the animal’s group assignment or drug treatment. The heart rate during each study was determined from M-mode tracings. End-diastolic area (EDA) and end-systolic area (ESA) were determined as the maximum and minimum values on these tracings. Systolic function was evaluated as the percentage of fractional area change (FAC). The percentage of FAC was calculated as equation
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Eight measurements were averaged during each examination to calculate EDA, ESA, and FAC. The accuracy, precision, and reproducibility of the studies have been previously validated (8).
All data are expressed as mean ± SEM. Included for the data analysis were 35 saline, 29 dobutamine, and 11 epinephrine in the control group and 34 saline, 20 dobutamine, and 14 epinephrine in the cocaine group. Student’s paired t-test was used to analyze differences between 7- and 14-day-old rats within each treatment group (controls or cocaine treated) and the effect of saline, dobutamine, or epinephrine on heart rate or FAC. Differences in heart rate and FAC between controls and cocaine-treated animals at each age group were analyzed by unpaired Student’s t-tests. Results were deemed statistically significant if P < 0.05.
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Results
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Animals were culled to similar litter sizes at 1 day of age so that the number of pups in each litter was the same for both controls and cocaine animals. We studied 8 different litters of controls and 8 different litters of cocaine animals at the age of 7 days then at the age of 14 days. Data from 75 controls and 68 cocaine animals were included in the analysis. Body weights in control and cocaine animals at D7 and D14 were comparable (19 ± 0.5 g and 19 ± 0.6 g at D7 for controls and cocaine animals, respectively; 32 ± 0.5 g and 34 ± 0.9 g at D14 for controls and cocaine animals, respectively).
In both controls and cocaine animals, an increase in heart rate was observed from D7 to D14. However, resting heart rate was much faster in neonatal rats that had exposure to maternal cocaine treatment at both D7 and D14. Baseline contractile function, as measured by FAC, was similar between control and cocaine animals at both ages (Table 1).
Separate groups of animals were treated with 1, 10, or 50 mg/kg of dobutamine by intraperitoneal injection. Because we did not detect any chronotropic or inotropic response at the two smaller doses (data not shown), only the response at 50 mg/kg is reported. At both ages, dobutamine, but not vehicle injection, significantly increased the heart rate in controls (n = 29) and cocaine (n = 20) animals (Fig. 1).
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Figure 1. Heart rate (HR) at rest (baseline) before intraperitoneal dobutamine injection was significantly higher in cocaine animals at both Day 7 (D7) and Day 14 (D14) (cocaine [n = 20] compared with age-matched saline-treated controls [CTL; n = 29]. In all animals, dobutamine induced a significant increase in heart rate from baseline (post Dobutamine). *P < 0.05 versus baseline; P < 0.05 versus CTL of the same age.
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Baseline FAC was comparable between neonatal control and cocaine animals, indicating that resting contractile function was similar in both groups. Although saline injection had no effect on controls at either age or on cocaine animals at D14, saline injection caused a small but significant increase in FAC in cocaine animals at D7. The ß1-agonist dobutamine induced a significant increase in FAC at both D7 (from 76.6% ± 0.6% to 81.5% ± 0.7%) and D14 (from 78.2% ± 0.7% to 81.9% ± 0.7%) in controls, but not in cocaine animals (from 76.7% ± 0.8% to 78.9% ± 0.8% at D7 and from 76.8% ± 1.1% to 79.3% ± 0.8% at D14) (Fig. 2).
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Figure 2. FAC (fractional area of change), as an index of systolic function, was comparable between controls (CTL; n = 29) and cocaine animals (n = 20) at both Day 7 (D7) and Day 14 (D14) at rest (pre-Dobutamine). In response to dobutamine, FAC increased significantly in controls (CTL), but not in cocaine animals, at both D7 and D14. *P < 0.05 versus pre-Dobutamine.
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Epinephrine, a mixed ß and 
agonist, was administered at 1 mg/kg by the intraperitoneal route in 7- and 14-day-old rats. Epinephrine failed to significantly alter heart rate in either group at both ages (data not shown). In controls (n = 11), epinephrine significantly increased FAC at D7 but had no effect at D14 (Fig. 3). In cocaine animals (n = 14), epinephrine was without effect on contractile function at either age.
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Figure 3. The inotropic effect of epinephrine was different between controls (CTL; n = 11) and cocaine (n = 14) animals at Day 7 (D7). Epinephrine stimulated systolic function in controls, but not in cocaine animals, at D7. Epinephrine did not change (fractional area of change; FAC) at Day 14 (D14) for either controls or cocaine animals. #P < 0.05 versus cocaine animals of the same age.
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Discussion
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The major finding of our study was that maternal cocaine treatment during the perinatal period increased resting heart rate but had no significant effect on baseline contractile function at D7 and D14 in the neonatal rat. However, the ability of a large-dose ßAR agonist to increase contractility was significant only in control neonatal rats and was attenuated in neonatal rats that had perinatal cocaine exposure.
Most studies assessing contractile function in the neonatal rat have used in vitro isolated heart preparations. Our study is the first to use transthoracic echocardiography to examine in vivo contractile function in the neonatal rat. Echocardiographic studies in rodents have documented that the use of anesthetics in the course of performing these studies can significantly alter the data obtained (8). Our studies were performed without the aid of sedatives or anesthesia to avoid the confounding influence of anesthetics on cardiac contractile function. Because the animals we studied weighed 10–40 g, the technical requirement for obtaining intravascular access for drug administration would require prolonged periods of handling of the animals. Therefore, we chose to administer the drugs by using the intraperitoneal route (10). This method is reliable and easy and does not entail substantial manipulation of the neonatal animals.
Cocaine-exposed neonatal rats had a much faster resting heart rate compared with saline-exposed controls. We had previously reported that maternal IV cocaine treatment also resulted in a faster resting heart rate in animals at 7 days of age, but the study did not examine animals at 14 days of age (6). Although the mode of cocaine exposure was different, this study extended our prior observation that prenatal and early postnatal exposure to cocaine significantly altered automaticity in the neonatal rat. Our previous study also found that heart rate variability was altered in cocaine-exposed neonatal rats, consistent with changes in autonomic neural activity. In this study, we did not perform heart rate variability analysis. However, the faster resting heart rate may be attributable to an increase in resting sympathetic activity. In several models of perinatal cocaine exposure, increased sympathetic activity has been demonstrated (11,12). In cocaine-exposed human neonates, circulating catecholamines were reported to be increased (13,14). In infant rabbits after gestational cocaine treatment, the cardiac adrenergic neurosecretory response was shown to be enhanced (12).
Despite the increased resting heart rate at D7 in cocaine animals, a developmental increase in heart rate from D7 to D14 was observed that was indistinguishable from the pattern in controls. The mechanism for the developmental increase in heart rate has not been clearly established. Developmental change in the pacemaker current (15), maturational change in the autonomic control of the sinoatrial node, and ontogenetic differences in the responsiveness of autonomic neurotransmitters (16) may all play a role. Specifically, sympathetic innervation in the developing heart may be important in the control of heart rate. Our results seem to suggest that the factors that govern the maturational increase in heart rate, including autonomic neural development, may be unperturbed by perinatal cocaine exposure. However, we have previously documented autonomic control of heart rate to be modified by perinatal cocaine exposure (6). Therefore, the changes in autonomic tone as a result of perinatal cocaine exposure may have been compensated for by other elements mediating the developmental increase in heart rate.
We previously provided evidence that perinatal cocaine exposure led to modified myocardial adrenergic receptor function in the developing rat at the cellular level (7). To further examine adrenergic responsiveness in the immature cardiovascular system, we first determined the effect of dobutamine on heart rate and contractile function in neonatal rats. Dobutamine is often used in the clinical setting in echocardiographic studies, and it is a selective ß1 agonist. We tested the response to dobutamine at 1, 10, and 50 mg/kg. At the two smaller doses, dobutamine had no effect on heart rate at either D7 or D14 in controls and cocaine animals. At the largest dose (50 mg/kg), dobutamine increased the heart rate in controls and cocaine rats at both ages. There was an apparent difference in the chronotropic and inotropic response to dobutamine in the cocaine animals. Although the large dose of dobutamine increased heart rates in cocaine animals at D7 and D14, it failed to significantly increase FAC at either age. In contrast, the large-dose dobutamine induced a significant chronotropic and inotropic response in controls at D7 and D14. Our results thus indicate that the ßAR response in ventricular tissues, but not in atrial tissues, was blunted in the cocaine-exposed animals. We have previously reported that myocardial ßAR-stimulated adenylyl cyclase activity was attenuated at D7 and D14 without any significant alteration in ßAR density (7). These findings indicate that perinatal cocaine exposure led to depressed ßAR-stimulated cardiac contractility as a functional consequence of "uncoupling" of the ßAR-adenylyl cyclase pathway. However, it is worth noting that the change in FAC in response to either dobutamine or epinephrine was relatively small, although it was significant in controls. This is consistent with the idea that effects of inotropic drugs acting via the ßAR signaling pathway are less robust in the neonatal heart than in adult hearts because the ßAR-cyclic adenosine monophosphate (cAMP) signaling pathway becomes better coupled with advancing maturational age (17). Although epinephrine was a positive inotropic drug in controls at D7, it did not elicit a positive inotropic response at D14 in either controls or cocaine animals. A possible explanation could be that epinephrine exerted a greater ß effect at D7, but at D14 its effects were predominantly .
Most reports on the cardiac outcome of human neonates with maternal cocaine exposure have been limited to observations of cardiac function at rest (2,4,5). No study has specifically examined whether gestational cocaine exposure causes a significant change in "stressed" cardiac contractile function. Our results in the rat model suggest that although resting function is unchanged after perinatal cocaine exposure, the contractile functional reserve in these animals may be diminished. Many prenatally cocaine-exposed neonates have cardiac and other congenital anomalies that require surgical intervention or medical management in the critical care environment (1). Our results underscore the fact that although these children may not have abnormal cardiac function at rest, their response to inotropic drugs that activate the ßAR/cAMP signaling pathway may be reduced. We did not specifically examine the effect of myocardial depressant drugs on cardiac contractile function in cocaine-exposed neonatal rats. Given the changes in the myocardial ßAR/cAMP signaling pathway after perinatal cocaine exposure and their correlation with an attenuated ßAR-stimulated inotropic response, future studies will need to be directed toward investigating whether perinatal cocaine exposure could lead to enhanced sensitivity to myocardial depressants.
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Acknowledgments
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Supported by National Institute on Drug Abuse Grant RO1DA12962.
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References
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Accepted for publication April 16, 2003.
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Abstract
Introduction
