Biochemical aspects of cardiac muscle differentiation. Possible control of deoxyribonucleic acid synthesis and cell differentiation by adrenergic innervation and cyclic adenosine 3':5'-monophosphate

New Modalities of 3D Pluripotent Stem Cell-Based Assays in Cardiovascular Toxicity

The substantial progress of the human induced pluripotent stem cell (hiPSC) technologies over the last decade has provided us with new opportunities for cardiovascular drug discovery, regenerative medicine, and disease modeling. The combination of hiPSC with 3D culture techniques offers numerous advantages for generating and studying physiological and pathophysiological cardiac models. Cells grown in 3D can overcome many limitations of 2D cell cultures and animal models. Furthermore, it enables the investigation in an architecturally appropriate, complex cellular environment in vitro. Yet, generation and study of cardiac organoids-which may contain versatile cardiovascular cell types differentiated from hiPSC-remain a challenge. The large-scale and high-throughput applications require accurate and standardised models with highly automated processes in culturing, imaging and data collection. Besides the compound spatial structure of organoids, their biological processes also possess different temporal dynamics which require other methods and technologies to detect them. In this review, we summarise the possibilities and challenges of acquiring relevant information from 3D cardiovascular models. We focus on the opportunities during different time-scale processes in dynamic pharmacological experiments and discuss the putative steps toward one-size-fits-all assays.

Optical Interrogation of Sympathetic Neuronal Effects on Macroscopic Cardiomyocyte Network Dynamics

Cardiac stimulation via sympathetic neurons can potentially trigger arrhythmias. We present approaches to study neuron-cardiomyocyte interactions involving optogenetic selective probing and all-optical electrophysiology to measure activity in an automated fashion. Here we demonstrate the utility of optical interrogation of sympathetic neurons and their effects on macroscopic cardiomyocyte network dynamics to address research targets such as the effects of adrenergic stimulation via the release of neurotransmitters, the effect of neuronal numbers on cardiac behavior, and the applicability of optogenetics in mechanistic in vitro studies. As arrhythmias are emergent behaviors that involve the coordinated activity of millions of cells, we image at macroscopic scales to capture complex dynamics. We show that neurons can both decrease and increase wave stability and re-entrant activity in culture depending on their induced activity-a finding that may help us understand the often conflicting results seen in experimental and clinical studies.

Myocardial Slices: an Intermediate Complexity Platform for Translational Cardiovascular Research

Myocardial slices, also known as “cardiac tissue slices” or “organotypic heart slices,” are ultrathin (100–400 μm) slices of living adult ventricular myocardium prepared using a high-precision vibratome. They are a model of intermediate complexity as they retain the native multicellularity, architecture, and physiology of the heart, while their thinness ensures adequate oxygen and metabolic substrate diffusion in vitro. Myocardial slices can be produced from a variety of animal models and human biopsies, thus providing a representative human in vitro platform for translational cardiovascular research. In this review, we compare myocardial slices to other in vitro models and highlight some of the unique advantages provided by this platform. Additionally, we discuss the work performed in our laboratory to optimize myocardial slice preparation methodology, which resulted in highly viable myocardial slices from both large and small mammalian hearts with only 2–3% cardiomyocyte damage and preserved structure and function. Applications of myocardial slices span both basic and translational cardiovascular science. Our laboratory has utilized myocardial slices for the investigation of cardiac multicellularity, visualizing 3D collagen distribution and micro/macrovascular networks using tissue clearing protocols and investigating the effects of novel conductive biomaterials on cardiac physiology. Myocardial slices have been widely used for pharmacological testing. Finally, the current challenges and future directions for the technology are discussed.

Use of Embryonic Heart Grafted In Oculo to Assess Neurohumoral Controls of Cardiac Development*

Culture of embryonic heart in the anterior eye chamber allows neurohumoral and genetic controls of cardiac development to be separated from the influence of hemodynamic load. Hearts from 12-day gestation rat embryos grafted into the anterior eye chamber of an adult host rat attach to the iris and become vascularized and innervated by collaterals from the host iris. The spontaneous beating of grafts is pacemaker-driven and under functional neural control. Grafts do not beat against a pressure load, allowing the influence of neurohumoral factors to be separated from altered hemodynamic load. In oculo, embryonic heart differentiates into mature myocardium by most morphologic and biochemical criteria. Mature intercalated disks and myofibrils with well-defined Z-lines and M-lines are observed. Mature grafts express the high levels of α-myosin heavy chain characteristic of young adult myocardium. Surgical sympathetic denervation of the anterior eye chamber prior to grafting of embryonic hearts compromises growth and increases the intrinsic pacemaker rate. Since the grafts are perfused by the host circulation, the hormonal milieu of the graft can be altered by treatment of the host. Thus, the interaction between hormones and innervation of grafts can be studied using the in oculo model system.

Preparation of viable adult ventricular myocardial slices from large and small mammals

This protocol describes the preparation of highly viable adult ventricular myocardial slices from the hearts of small and large mammals, including rodents, pigs, dogs and humans. Adult ventricular myocardial slices are 100- to 400-μm-thick slices of living myocardium that retain the native multicellularity, architecture and physiology of the heart. This protocol provides a list of the equipment and reagents required alongside a detailed description of the methodology for heart explantation, tissue preparation, slicing with a vibratome and handling of myocardial slices. Supplementary videos are included to visually demonstrate these steps. A number of critical steps are addressed that must be followed in order to prepare highly viable myocardial slices. These include identification of myocardial fiber direction and fiber alignment within the tissue block, careful temperature control, use of an excitation-contraction uncoupler, optimal vibratome settings and correct handling of myocardial slices. Many aspects of cardiac structure and function can be studied using myocardial slices in vitro. Typical results obtained with hearts from a small mammal (rat) and a large mammal (human) with heart failure are shown, demonstrating myocardial slice viability, maximum contractility, Ca²⁺ handling and structure. This protocol can be completed in ∼4 h.

Fibroblast-myocyte electrotonic coupling: does it occur in native cardiac tissue?

Heterocellular electrotonic coupling between cardiac myocytes and non-excitable connective tissue cells has been a long-established and well-researched fact in vitro. Whether or not such coupling exists in vivo has been a matter of considerable debate. This paper reviews the development of experimental insight and conceptual views on this topic, describes evidence in favour of and against the presence of such coupling in native myocardium, and identifies directions for further study needed to resolve the riddle, perhaps less so in terms of principal presence which has been demonstrated, but undoubtedly in terms of extent, regulation, patho-physiological context, and actual relevance of cardiac myocyte–non-myocyte coupling in vivo. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

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Electrical coupling of cardiomyocytes and fibroblasts is well-established in vitro

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Whether such hetero-cellular coupling exists in vivo has been a matter of debate

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We review the development of experimental and conceptual insight into the topic

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Conclusion 1: hetero-cellular coupling in heart tissue has been shown in principle

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Conclusion 2: extent, regulation, context, and relevance remain to be established

Early maternal separation has mild effects on cardiac autonomic balance and heart structure in adult male rats

Early life adverse experiences have long-term physiologic and behavioral effects and enhance stress sensitivity. This study examined the effects of maternal separation (MS) on cardiac stress responsivity and structure in adulthood. Male Wistar rats were separated from the dams for 3 h per day from postnatal days 2 through 15. When exposed to 5-day intermittent restraint stress (IRS) as adults, MS, and control rats showed similar acute modifications of cardiac sympathovagal balance, quantified via heart rate variability analysis. In addition, MS had no effect on cardiac pacemaker intrinsic activity (as revealed by autonomic blockade with scopolamine and atenolol) and did not affect the circadian rhythmicity of heart rate, neither before nor after IRS. However, MS differed from control rats in cardiac parasympathetic drive following IRS, which was heightened in the latter but remained unchanged in the former, both during the light and dark phases of the daily rhythm. The evaluation of adult cardiac structure indicated that stress experienced during a crucial developmental period induced only modest changes, involving cardiomyocyte hypertrophy, increased density of vascular structures, and myocardial fibrosis. The mildness of these functional-structural effects questions the validity of MS as a model for early stress-induced cardiac disease in humans.

Adrenergic regulation of conduction velocity in cultures of immature cardiomyocytes

During cardiac maturation, increased exposure of the heart to circulating catecholamines correlates with increased conduction velocity and growth of the heart. We used an in vitro approach to study the underlying mechanisms of adrenergic stimulation induced changes in conduction velocity. By combining functional measurements and molecular techniques, we were able to demonstrate that the increased conduction velocity after beta-adrenergic stimulation is probably not caused by changes in intercellular coupling. Instead, RT-PCR experiments and action potential measurements have shown an increased excitability that may well explain the observed increase in conduction velocity. Apart from being relevant to cardiac maturation, our findings are relevant in the context of stem cells and cardiac repair. Preconditioning of stem cell derived cardiomyocytes may help to enhance electrical maturation of de novo generated cardiomyocytes and consequently reduce their proarrhythmogenic potential. (Neth Heart J 2008;16:106-9.).

Beta-, Not Alpha-Adrenergic Stimulation Enhances Conduction Velocity in Cultures of Neonatal Cardiomyocytes

BACKGROUND

During both cardiac maturation and myopathy, elevated levels of circulating catecholamines coincide with alterations in impulse propagation. An in vitro model of cultured cardiomyocytes was used to study the effects of adrenergic stimulation on the conduction characteristics of immature heart cells.

METHODS AND RESULTS

Neonatal rat cardiomyocytes were cultured on preparations designed to measure conduction velocity (CV). CV was measured on the same preparation twice at t=0 and at t=24 h. Under control conditions (n=7), CV at t=0 (30.9+/-1.9 cm/s) and t=24 (32.4+/-4.4 cm/s) was similar (p=0.70). Immunohistochemistry revealed expression of the gap junction proteins connexin (Cx) 40, Cx43 and Cx45, with Cx43 being highly predominant. Stimulation for 24 h with the beta-adrenergic agonist isoproterenol (ISO) significantly increased CV from 28.0 +/-2.0 cm/s at t=0 to 34.8+/-2.2 cm/s at t=24 (p=0.002, n=5). Microelectrode recordings showed a faster upstroke of the action potential (AP) of ISO-treated cells. Reverse transcribed-polymerase chain reactions (RT-PCR) showed that ISO increased expression of SCN5A and alpha(1c) (alpha-subunit of the cardiac sodium and L-type calcium channel, respectively). Stimulation of cells with ISO did not induce alterations in distribution or expression of Cx40, Cx43 and Cx45 (both mRNA and protein), but slightly increased the phosphorylation of Cx43. Stimulation for 24 h with the alpha-adrenergic agonist phenylephrine did neither affect CV nor the expression of the connexin isoforms, SCN5A and alpha(1c).

CONCLUSIONS

Alpha- and beta-adrenergic stimulation differently affect propagation of the electric impulse, which is primarily not caused by a differential effect on intercellular coupling. RT-PCR analysis and an enhanced AP upstroke velocity indicate a higher functional expression level of alpha(1c) and SCN5A in beta-adrenergic stimulated cells, which may explain the observed increase in CV.

Disruption of rat forebrain development by glucocorticoids: critical perinatal periods for effects on neural cell acquisition and on cell signaling cascades mediating noradrenergic and cholinergic neurotransmitter/neurotrophic responses

Glucocorticoids are the consensus treatment for the prevention of respiratory distress in preterm infants, but there is evidence for increased incidence of neurodevelopmental disorders as a result of their administration. We administered dexamethasone (Dex) to developing rats at doses below or within the range of those used clinically, evaluating the effects on forebrain development with exposure in three different stages: gestational days 17-19, postnatal days 1-3, or postnatal days 7-9. At 24 h after the last dose, we evaluated biomarkers of neural cell acquisition and growth, synaptic development, neurotransmitter receptor expression, and synaptic signaling mediated by adenylyl cyclase (AC). Dex impaired the acquisition of neural cells, with a peak effect when given in the immediate postnatal period. In association with this defect, Dex also elicited biphasic effects on cholinergic presynaptic development, promoting synaptic maturation at a dose (0.05 mg/kg) well below those used therapeutically, whereas the effect was diminished or lost when doses were increased to 0.2 or 0.8 mg/kg. Dex given postnatally also disrupted the expression of adrenergic receptors known to participate in neurotrophic modeling of the developing brain and evoked massive induction of AC activity. As a consequence, disparate receptor inputs all produced cyclic AMP overproduction, a likely contributor to disrupted patterns of cell replication, differentiation, and apoptosis. Superimposed on the heterologous AC induction, Dex impaired specific receptor-mediated cholinergic and adrenergic signals. These results indicate that, during a critical developmental period, Dex administration leads to widespread interference with forebrain development, likely contributing to eventual, adverse neurobehavioral outcomes.

Targeted insertion of the Cre-recombinase gene at the phenylethanolamine n-methyltransferase locus: A new model for studying the developmental distribution of adrenergic cells

To evaluate the developmental distribution of adrenergic cells in vivo, we inserted the Cre-recombinase gene into the locus encoding for the epinephrine biosynthetic enzyme phenylethanolamine n-methyltransferase (Pnmt) and crossed these Pnmt-Cre mice with ROSA26 reporter (R26R) mice to activate LacZ (encoding beta-galactosidase) expression in cells that were selectively derived from the adrenergic lineage. Our data show the following: (1) Insertion of Cre-recombinase into the Pnmt locus created a functional knockout of Pnmt expression with concomitant loss of epinephrine in homozygous Pnmt(Cre/Cre) mice; (2) Despite the reduction in Pnmt expression and epinephrine production in Pnmt(Cre/Cre) mice, these mice were viable and fertile, with no apparent developmental defects; (3) When crossed with R26R mice, Pnmt-Cre activation of LacZ expression faithfully recapitulated Pnmt expression in vivo; and (4) LacZ expression was activated in substantial numbers of pacemaking, conduction, and working cardiomyocytes.

Transiently overexpressed alpha2-adrenoceptors and their control of DNA synthesis in the developing brain

During brain development, neurotransmitters act as trophic factors controlling the patterns of cell replication and differentiation. Alpha2-adrenoceptors (alpha2ARs) are transiently overexpressed in zones with high mitotic activity and we evaluated whether these receptors are linked to DNA synthesis in the perinatal rat brain. Acute administration of clonidine (2 mg/kg), an alpha2AR agonist, elicited dramatic decreases in DNA synthesis in the forebrain, brainstem, and cerebellum whether given on gestational day (GD) 21, or on postnatal days (PN) 1 or 8. However, alpha2AR blockade elicited by yohimbine (2.5 mg/kg) also resulted in decreased DNA synthesis on GD21 and PN8, albeit to a smaller extent than with clonidine. Yohimbine was able to blunt the effects of clonidine, verifying that both drugs are acting through the same receptor population. Because betaARs are also known to regulate DNA synthesis, we used propranolol (10 mg/kg) blockade of betaARs to evaluate whether the alpha2AR effects were mediated by presynaptic autoreceptors that regulate the release of norepinephrine and consequent betaAR responses; the effects of yohimbine were still discernible in the presence of propranolol. Accordingly, transiently overexpressed alpha2ARs in the developing brain participate in the control of DNA synthesis in a biphasic manner, with promotional actions at low, endogenous levels of stimulation, but inhibitory effects when stimulation is high. Effects on alpha2ARs are likely to contribute to long-term consequences of adrenergic agents used in obstetrics or neurotoxicants that affect adrenergic activity.

Does terbutaline damage the developing heart?

BACKGROUND

Beta(2)-Adrenoceptor (betaAR) agonists, such as terbutaline, are widely used to arrest preterm labor. They also cross the placenta where they stimulate receptors in fetal tissues, which in turn use betaAR input for trophic control of cell replication and differentiation.

METHODS

As rats are altricial, we administered terbutaline in two different postnatal exposure periods (10 mg/kg given daily on Days 2-5 or 11-14).

RESULTS

Hearts were examined twenty-four hours after the last dose and on postnatal day 30 for cardiac damage. Neither treatment paradigm caused an increase in cardiac abnormalities compared to controls but quantitative analysis of the number of nuclei indicated reductions in females.

CONCLUSIONS

These findings do not support earlier case reports of outright myocardial necrosis after terbutaline tocolysis in human infants. Nevertheless, the significant statistical association between terbutaline and cardiac anomalies in epidemiological studies suggest that terbutaline may sensitize the developing heart to other insults that affect development.

Ontogenesis of beta-adrenoceptor signaling: implications for perinatal physiology and for fetal effects of tocolytic drugs

G-Protein-coupled receptors play an instrumental role in cellular development and function. In the mature organism, receptor signaling is controlled through the processes of desensitization and down-regulation. Recent evidence suggests that these regulatory mechanisms are not inherent properties, however, but rather are acquired during ontogenesis. This review focuses on beta-adrenoceptors (betaARs), which are found in fetal and neonatal tissues and are effectively linked through adenylyl cyclase (AC) to the production of cAMP. Agonist-induced stimulation of betaARs in the immature organism fails to produce desensitization, and instead, responsiveness increases. The unique mechanisms underlying this anomalous response involve induction of AC, a switch to more catalytically efficient AC isoforms, an increase in the ratio of stimulatory to inhibitory G-proteins, and interference with the expression and/or function of other G-protein-linked receptors that provide offsetting, inhibitory inputs. These adjustments are thus heterologous, influencing signaling mediated by a host of other G-protein-coupled neurotransmitter and hormone receptors. The net effect is to maintain and augment betaAR signaling in the face of continued stimulation, properties that disappear with maturation. The unique regulatory mechanisms for betaAR signaling in the fetus and neonate provide the necessary physiological adjustments required for the perinatal transition from intrauterine to extrauterine life. At the same time, however, the inability to restrict betaAR function may underlie adverse effects of betaAR-agonist tocolytics that are used in the treatment of preterm labor.

Developmental toxicity of terbutaline: critical periods for sex-selective effects on macromolecules and DNA synthesis in rat brain, heart, and liver

beta-Adrenoceptors (betaARs) control cell replication/differentiation, and during development, signaling is not subject to desensitization. We examined the effects of terbutaline, a beta(2)AR agonist used as a tocolytic, on development in rat brain regions and peripheral tissues with high betaAR concentrations. Prenatal terbutaline (gestational days 17-20) decreased cell numbers (DNA content) in the fetal brain and liver. Early postnatal exposure (PN2-5) reduced DNA synthesis in early-developing brain regions of females, with sensitization of the effect upon repeated terbutaline administration; after multiple terbutaline injections, DNA content was reduced in male cerebellum. The cerebellum was targeted later (PN11-14), exhibiting decreased DNA synthesis in both sexes; in contrast, cardiac DNA synthesis decreased after one injection but increased after the fourth daily injection. Our results suggest that excessive betaAR stimulation by terbutaline alters cell development in brain regions and peripheral tissues, with the net effect depending on sex and the timing of exposure. These effects may contribute to neuropsychiatric, cognitive, cardiovascular, and metabolic abnormalities reported in the offspring of women treated with beta-agonist tocolytics.

Beta-adrenoceptor control of G protein function in the neonate: determinant of desensitization or sensitization

Neonatal beta-adrenoceptors (beta-ARs) are resistant to agonist-induced desensitization. We examined the functioning of G(i) and G(s) after repeated administration of beta-AR agonists to newborn rats. Isoproterenol (beta(1)/beta(2) agonist) obtunded G(i) function in the heart but not the liver; in contrast, terbutaline, a beta(2)-selective agonist, enhanced G(i) function. Isoproterenol, but not terbutaline, increased membrane-associated G((s)alpha), which would enhance receptor function. In addition, isoproterenol increased and terbutaline maintained the proportion of the short-splice (S) variant of G((s)alpha) in the membrane fraction; G((s)alpha)S is functionally more active than the long-splice variant. Either isoproterenol or terbutaline treatment increased G((s)alpha) in the cytosolic fraction, a characteristic usually associated with desensitization in the adult. Decreased G(i) activity, coupled with increased membrane-associated G((s)alpha) concentrations and maintenance or increases in membrane G((s)alpha)S, provide strong evidence that unique effects on G protein function underlie the ability of the immature organism to sustain beta-AR cell signaling in the face of excessive or prolonged stimulation; these mechanisms also contribute to tissue selectivity of the effects of beta-agonists with divergent potencies toward different beta-AR subtypes.

Are developing beta-adrenoceptors able to desensitize? Acute and chronic effects of beta-agonists in neonatal heart and liver

During fetal and neonatal development, beta-adrenergic receptors (beta-ARs) appear to be resistant to desensitization by beta-agonist drugs. To determine the mechanisms underlying the regulatory differences between adults and neonates, we administered isoproterenol, a mixed beta(1)/beta(2)-AR agonist, and terbutaline, a beta(2)-selective agonist. Effects were examined in the ensuing 4 h after a single injection, or after the last of four daily injections. We prepared cell membranes from heart (predominantly beta(1)-ARs) and liver (predominantly beta(2)-ARs) and assessed signal transduction in the adenylyl cyclase (AC) pathway. In the first few hours after a single administration of isoproterenol to adult rats, cardiac beta-ARs showed activation of G proteins (elevated AC response to forskolin) and desensitization of beta-AR-mediated responses; after the fourth injection, heterologous desensitization emerged, characterized by a loss of signaling mediated either through beta-ARs or glucagon receptors. Terbutaline evoked an increase in the forskolin response but no desensitization of receptor-mediated responses. When we gave the same treatments to neonatal rats, we observed cardiac G protein activation, but there was neither homologous nor heterologous desensitization of beta-ARs or glucagon receptors. In the adult liver, isoproterenol and terbutaline both failed to evoke desensitization, regardless of whether the drugs were given once or for 4 days. In neonates, however, acute or chronic treatment elicited homologous desensitization of beta-AR-mediated AC signaling, while sensitizing the response to glucagon. These results show that neonatal beta-ARs are inherently capable of desensitization in some, but not all, cell types; cellular responses can be maintained through heterologous sensitization of signaling proteins downstream from the receptor. Differences from adult patterns of response are highly tissue selective and are likely to depend on ontogenetic differences in subtypes of beta-ARs and AC.

Beta-adrenoceptor-mediated cell signaling in the neonatal heart and liver: responses to terbutaline

Terbutaline, a beta(2)-adrenoceptor (beta(2)-AR) agonist, is a widely used tocolytic that also crosses the placenta to stimulate fetal beta-ARs. The current study examines the effects of terbutaline administered to neonatal rats. Terbutaline (10 mg/kg sc) given on postnatal day (PN) 2-5 or PN 11-14 elicited significant downregulation of both cardiac and hepatic beta-ARs, with a much greater effect in the liver. Despite the reduction in cardiac beta-ARs, receptor desensitization was absent as evidenced by the maintained ability of isoproterenol to stimulate adenylyl cyclase (AC) in membrane preparations. The underlying mechanism was dissected by using stimulants that operate at different points in the AC signaling pathway, NaF, forskolin, and Mn(2+). When administered in the early neonatal period, terbutaline failed to evoke any changes in cardiac AC activity; however, treatment on PN 11-14 evoked heterologous sensitization downstream from the receptor, evidenced by increases in the response to NaF and forskolin. In the liver, neonatal terbutaline administration elicited a small (approximately equal to 10%) decrease in the AC response to isoproterenol, an effect much smaller than the downregulation of beta-ARs (>40%). In this tissue, desensitization was again offset by heterologous sensitization of AC signaling. These results indicate that, in the developing organism, beta-AR-mediated cell signaling responses are maintained in the face of receptor downregulation through heterologous induction of downstream signaling elements. These unique responses serve to sustain beta-AR signaling in the perinatal period.

Regulation of fetal cardiac and hepatic beta-adrenoceptors and adenylyl cyclase signaling: terbutaline effects

Terbutaline (Ter), a beta(2)-adrenergic agonist used in preterm labor, stimulates fetal beta-adrenoceptors (beta-ARs). We administered Ter to pregnant rats on gestational days 17-20 and examined beta-ARs and adenylyl cyclase (AC) signaling in heart and liver. Ter produced less downregulation of cardiac beta-ARs than in adults, despite a higher proportion of the beta(2)-subtype, and failed to elicit desensitization of the receptor-mediated AC response. AC stimulants acting at different points indicated an offsetting of homologous desensitization at the level of the beta-AR by heterologous sensitization at the level of AC: induction of total AC catalytic activity and a shift in the catalytic profile or AC isoform. In fetal liver, Ter produced downregulation of beta-ARs, in keeping with the predominance of the beta(2)-subtype; hepatic receptor downregulation was equivalent in fetus and adult. Nevertheless, there was still no desensitization of beta-AR-mediated AC responses and again AC was induced. Our results indicate that, unlike in the adult, fetal beta-AR signaling is not desensitized by beta-agonists and, in fact, displays heterologous sensitization, thus sustaining responses during parturition. At the same time, the inability to desensitize beta-AR AC responses may lead to disruption of cardiac, hepatic, or neural cell development as a consequence of tocolytic therapy with beta-agonists.

Behavioral and neural consequences of prenatal exposure to nicotine

OBJECTIVE

To review evidence for the neurodevelopmental effects of in utero exposure to nicotine. Concerns about long-term cognitive and behavioral effects of prenatal exposure to nicotine arise from reports of increased rates of disruptive behavioral disorders in children whose mothers smoked during pregnancy. The relatively high rate of tobacco smoking among pregnant women (25% of all pregnancies in the U.S.) underlines the seriousness of these concerns.

METHOD

This review examines the largest and most recent epidemiological and clinical studies that investigated the association of prenatal nicotine exposure with health, behavioral, and cognitive problems. Because of the numerous potential confounding variables in human research, findings from animal studies, in which environmental factors are strictly controlled, are also discussed. Finally, neural and molecular mechanisms that are likely to underlie neurodevelopmental disruptions produced by prenatal nicotine exposure are outlined.

RESULTS

A dose-response relationship between maternal smoking rates and low birth weight (potentially associated with lower cognitive ability) and spontaneous abortion is consistently found, whereas long-term developmental and behavioral effects in the offspring are still controversial, perhaps because of the difficulty of separating them from other genetic and environmental factors. Despite the wide variability of experimental paradigms used in animal studies, common physical and behavioral effects of prenatal exposure to nicotine have been observed, including low birth weight, enhanced locomotor activity, and cognitive impairment. Finally, disturbances in neuronal pathfinding, abnormalities in cell proliferation and differentiation, and disruptions in the development of the cholinergic and catecholaminergic systems all have been reported in molecular animal studies of in utero exposure to nicotine.

CONCLUSIONS

Prenatal exposure to nicotine may lead to dysregulation in neurodevelopment and can indicate higher risk for psychiatric problems, including substance abuse. Knowledge of prenatal exposure to nicotine should prompt child psychiatrists to closely monitor at-risk patients.

Does the developmental neurotoxicity of chlorpyrifos involve glial targets? Macromolecule synthesis, adenylyl cyclase signaling, nuclear transcription factors, and formation of reactive oxygen in C6 glioma cells

The widespread use of chlorpyrifos (CPF) has raised major concerns about its potential to cause fetal or neonatal neurobehavioral damage, even at doses that do not evoke acute toxicity. CPF has been shown to inhibit replication of brain cells, to elicit alterations in neurotrophic signaling governing cell differentiation and apoptosis, and to evoke oxidative stress. However, the specific cell types targeted by CPF have not been clarified, an issue of vital importance in establishing the boundaries of the critical period in which the developing brain is vulnerable. In the current study, we evaluated the effects of CPF on C6 glioma cells, a well-established glial model. In undifferentiated C6 cells, CPF inhibited DNA synthesis in a concentration-dependent manner, with greater potency than had been seen previously with neuronal cell lines. Just as found after in vivo CPF treatment or with neuronal cell lines, the effects on cell replication were independent of cholinergic stimulation, as cholinergic antagonists did not block CPF-induced inhibition. CPF interfered with cell signaling mediated through adenylyl cyclase at the level of G-protein function; the effects again were greater in undifferentiated C6 cells but were still detectable in differentiating cells. In contrast, differentiation enhanced the ability of CPF to elicit the formation of reactive oxygen species and to evoke deficits in Sp1, a nuclear transcription factor essential for differentiation. These results indicate that glial-type cells are targeted by CPF through the same multiple mechanisms that have been demonstrated for the effects of CPF on brain development in vivo. Because glial development continues long after the conclusion of neurogenesis, and given that CPF targets events in both glial cell replication and the later stages of differentiation, the vulnerable period for developmental neurotoxicity of CPF is likely to extend well into childhood.

Distribution of antigen epitopes shared by nerves and the myocardium of the embryonic chick heart using different neuronal markers

We examined which neuronal elements and nonneuronal tissues in the embryonic myocardium are stained with antibodies traditionally used for staining nerve tissue. Furthermore, we studied whether nonneuronal myocardial staining was confined to regions determining initial nerve entry points and development of cardiac ganglia. The third focus was whether nerves preferentially distribute in regions of the conduction system. Different neuronal markers were used such as the HNK-1 antibody against neural crest and nerve tissue, Tyrosine Hydroxylase antibody (TH) against putative sympathetic nerve tissue, anti-GFAP against glia cells, antibodies against phosphorylated neurofilaments DO170, RMO270, 3A10, and RT97, and finally the antibody Snap25 against a synaptic protein. Chick embryonic hearts between stage HH25-44 where immunohistochemically evaluated. Transient HNK-1 staining in the basal region of the heart coincided with ingrowing vagal branches and crest-derived neuronal precursor cells seeding the region of the atrioventricular sulcus, suggesting a role for HNK-1 in the homing of the parasympathetic plexus. Transient TH staining was confined to regions of the atrial myocardium coincident with the localization of the few early TH-positive nerve fibers before stage HH40, whereas the second wave of TH-positive nerve fibers at HH42 was mainly localized around myocardial coronary arteries. This transient myocardial TH staining might be involved in early emergence of the catecholaminergic phenotype, while coronary arteries or blood borne factors might be involved in later differentiation. Some myocardial expression, not related with initial nerve ingrowth, using Snap25, TH, HNK-1, DO170, and RMO270 was confined to regions of the ventricular conduction system. HNK-1 is the only marker staining the region of the putative sinoatrial node. Just before hatching nerve fibers, including TH-positive nerve fibers, are uniformly distributed throughout the myocardium, without being specifically confined to regions containing the conduction system or coronary arteries.

Antimitotic and cytotoxic effects of theophylline in MDA-MB-231 human breast cancer cells

A variety of cancer cell lines, including MDA-MB-231 human breast cancer cells, exhibit mitotic inhibition by cAMP. In earlier work, we found that the phosphodiesterase inhibitor, theophylline, reduced the number of cells and altered cellular morphology. In the current study, we evaluated the effects of theophylline on macromolecule synthesis and indices of cell viability. Theophylline evoked a concentration- and time-dependent decrease in DNA synthesis. However, the net decrease in cell number was greater than that predicted solely from mitotic arrest. Assessment of protein synthesis indicated a second effect of theophylline separable from that on DNA synthesis. This was confirmed by decreased cell viability and adhesion. Exposure of the cells to the phosphodiesterase inhibitor, IBMX, in concentrations that produced inhibition of DNA synthesis equivalent to that seen with theophylline, elicited a smaller reduction in cell number. Theophylline also evoked specific changes in the expression or function of membrane-bound adenylyl cyclase activity, effects that are likely to contribute to sustained reactivity of these cells to other cAMP-related inhibitors of cell proliferation, such as isoproterenol. The multiple pharmacologic properties of theophylline, producing mitotic inhibition, cytotoxicity and altered signaling in MDA-MB-231 cells, may provide insight into novel therapeutic strategies.

Neonatal chlorpyrifos exposure targets multiple proteins governing the hepatic adenylyl cyclase signaling cascade: implications for neurotoxicity

Chlorpyrifos has been hypothesized to interact with receptors and transduction proteins involved in the production of cyclic AMP, contributing to adverse effects on cell replication and differentiation. We studied the effects of neonatal chlorpyrifos exposure on hepatic adenylyl cyclase (AC) activity, as the liver accumulates the highest concentrations of chlorpyrifos and is the site for generation of its active metabolite, chlorpyrifos oxon. Newborn rats were given 1 mg/kg of chlorpyrifos s.c. on PN1-4. On PN5, 24 h after the last dose, AC catalytic activity was induced as assessed by the response to the direct AC stimulant, Mn(2+). In contrast, AC activation dependent upon interaction of the enzyme with G-proteins (forskolin) did not show any enhancement, suggesting impairment of G-protein function. This conclusion was confirmed by impaired responsiveness to fluoride, which directly activates G-proteins. In addition, the response of AC to hormonal signals was altered in a receptor-selective manner, with an enhanced response to glucagon but not to the beta-adrenoceptor agonist, isoproterenol. The effects of chlorpyrifos on AC signaling displayed a critical developmental period of vulnerability, as treatment of older rats (PN11-14) failed to cause substantial induction of AC or interference with G-protein signaling, although it did still enhance the glucagon response. In all cases, the effects of chlorpyrifos disappeared within a few days of discontinuing treatment. These results stand in contrast to the delayed deterioration of AC signaling seen in the brain after the same chlorpyrifos treatment. The temporal and organ selectivity of chlorpyrifos' effects on the AC cascade suggest that disruption of membrane signaling occurs consequent to selective effects on cell development, rather than representing a direct interaction between chlorpyrifos and signaling proteins.

Beta-adrenoceptor signaling and its control of cell replication in MDA-MB-231 human breast cancer cells

MDA-MB-231 human breast cancer cells express high beta-adrenoceptor levels, predominantly the beta2 subtype. Receptor stimulation by isoproterenol evoked immediate reductions in DNA synthesis which were blocked completely by propranolol and were of the same magnitude as effects elicited by high concentrations of 8-Br-cAMP. Isoproterenol-induced inhibition of DNA synthesis was maintained throughout several days of exposure, resulting in a decrement in total cell number, and the effects were augmented by cotreatment with dexamethasone; an even greater effect was seen when cAMP breakdown was inhibited by theophylline, with or without addition of isoproterenol. Despite the persistent effect of isoproterenol, receptor downregulation was evident with as little as 1 h of treatment, and over 90% of the receptors were lost within 24 h. Receptor downregulation was paralleled by homologous desensitization of the adenylyl cyclase response to beta-adrenoceptor stimulation. Dexamethasone augmented the effects of isoproterenol on DNA synthesis but did not prevent receptor downregulation or desensitization. These results indicate that beta-adrenoceptors are effectively linked, through cAMP, to the termination of cell replication in MDA-MB-231 human breast cancer cells, and that activation of only a small number of receptors is sufficient for a maximal effect. Novel pharmacologic strategies that focus on cell surface receptors operating through adenylyl cyclase may offer opportunities to combat cancers that are unresponsive to hormonal agents, or that have developed multidrug resistance.

Factors involved in GLUT-1 glucose transporter gene transcription in cardiac muscle

Glucose constitutes a major fuel for the heart, and high glucose uptake during fetal development is coincident with the highest level of expression of the glucose transporter GLUT-1 during life. We have previously reported that GLUT-1 is repressed perinatally in rat heart, and GLUT-4, which shows a low level of expression in the fetal stage, becomes the main glucose transporter in the adult. Here, we show that the perinatal expression of GLUT-1 and GLUT-4 glucose transporters in heart is controlled directly at the level of gene transcription. Transient transfection assays show that the -99/-33 fragment of the GLUT-1 gene is sufficient to drive transcriptional activity in rat neonatal cardiomyocytes. Electrophoretic mobility shift assays demonstrate that the transcription factor Sp1, a trans-activator of GLUT-1 promoter, binds to the -102/-82 region of GLUT-1 promoter during the fetal state but not during adulthood. Mutation of the Sp1 site in this region demonstrates that Sp1 is essential for maintaining a high transcriptional activity in cardiac myocytes. Sp1 is markedly down-regulated both in heart and in skeletal muscle during neonatal life, suggesting an active role for Sp1 in the regulation of GLUT-1 transcription. In all, these results indicate that the expression of GLUT-1 and GLUT-4 in heart during perinatal development is largely controlled at a transcriptional level by mechanisms that might be related to hyperplasia and that are independent from the signals that trigger cell hypertrophy in the developing heart. Furthermore, our results provide the first functional insight into the mechanisms regulating muscle GLUT-1 gene expression in a live animal.

Expression of second messenger- and cyclin-dependent protein kinases during postnatal development of rat heart

During early postnatal development, cardiomyocytes, which comprise about 80% of ventricular mass and volume, become phenotypically developed to facilitate their contractile functions and terminally differentiated to grow only in size but not in cell number. These changes are due to the expression of contractile proteins as well as the regulation of intracellular signal transduction proteins. In this study, the expression patterns of several protein kinases involved in various cardiac functions and cell-cycle control were analyzed by Western blotting of ventricular extracts from 1-, 10-, 20-, 50-, and 365-day-old rats. The expression level of cAMP-dependent protein kinase was slightly decreased (20%) over the first year, whereas no change was detected in cGMP-dependent protein kinase I. Calmodulin-dependent protein kinase II, which is involved in Ca2+ uptake into the sarcoplasmic reticulum, was increased as much as ten-fold. To the contrary, the expressions of protein kinase C-alpha and iota declined 77% with age. Cyclin-dependent protein kinases (CDKs) such as CDK1, CDK2, CDK4, and CDK5, which are required for cell-cycle progression, abruptly declined to almost undetectable levels after 10-20 days of age. In contrast, other CDK-related kinases, such as CDK8 or Kkialre, did not change significantly or increased up to 50% with age, respectively. Protein kinases implicated in CDK regulation such as CDK7 and Wee1 were either slightly increased in expression or did not change significantly. All of the proteins that were detected in ventricular extracts were also identified in isolated cardiac myocytes in equivalent amounts and analyzed for their relative expression in ten other adult rat tissues.

Cellular mechanisms for developmental toxicity of chlorpyrifos: targeting the adenylyl cyclase signaling cascade

Developmental neurotoxicity caused by chlorpyrifos exposure is generally thought to target cholinesterase but chlorpyrifos may also act on cellular intermediates, such as adenylyl cyclase, that serve global functions in the coordination of cell development. In the current study, neonatal rats were exposed to apparently subtoxic doses of chlorpyrifos (no weight loss, no mortality) either on Postnatal Days 1-4 or on Postnatal Days 11-14, and the effects on components of the adenylyl cyclase cascade were evaluated in brain regions that are enriched (forebrain) or sparse (cerebellum) in cholinergic innervation, as well as in a nonneural tissue (heart). In all three, chlorpyrifos evoked deficits in multiple components of the adenylyl cyclase cascade: expression and activity of adenylyl cyclase itself, functioning of G-proteins that link neurotransmitter and hormone receptors to cyclase activity, and expression of neurotransmitter receptors that act through this cascade. Disruption of signaling function was not restricted to transduction of cholinergic signals but rather extended to adrenergic signals as well. In most cases, the adverse effects were not evident during the immediate period of chlorpyrifos administration, but appeared after a delay of several days. These results suggest that chlorpyrifos can affect cell development by altering the activity and reactivity of the adenylyl cyclase signaling cascade, a major control point for trophic regulation of cell differentiation. The effects are not restricted to cholinergic targets, nor even to the central nervous system. Hence, disruption of cell development by chlorpyrifos is likely to be more widespread than previously thought.

Neuronal control of cardiac and hepatic macromolecule synthesis in the neonatal rat: effects of sympathectomy

Neurotransmitters are thought to influence cell development in their target tissues. In the current study, neonatal rats were given 6-hydroxydopamine to produce permanent sympathetic denervation, and the effects on cardiac and hepatic DNA and protein synthesis were assessed. Lesioned animals showed deficits in cardiac DNA synthesis over the first 8 d postpartum, a period in which sympathetic innervation is sparse and synaptic norepinephrine concentrations are low; the effect of lesioning was also evident for protein synthesis. Subsequently, DNA synthesis in control animals declined precipitously during the second to third postnatal week, the phase associated with ingrowth of the majority of sympathetic terminals and sympathetic hyperactivity. Neonatal lesioning delayed the ontogenetic decline in DNA synthesis: this effect was not shared by protein synthesis. In the liver, a tissue whose cells, unlike the heart, maintain the ability to divide into adulthood, there was no effect of 6-hydroxydopamine on DNA synthesis and only minor changes in protein synthesis. These results suggest that neural input provides two distinct trophic signals to the developing heart: an early promotion of cell replication associated with low levels of stimulation, and a subsequent promotion of the switchover from cell replication, to cell differentiation and enlargement, associated with high levels of stimulation. In light of the precipitous rise in circulating catecholamines at parturition, and of the subsequent development of sympathetic innervation, catecholamines are likely to play a trophic role in the establishment of the proper pattern of cardiac cell development.

Chlorpyrifos interferes with cell development in rat brain regions

Chlorpyrifos, one of the most widely used pesticides, exhibits greater toxicity during development than in adulthood. We administered chlorpyrifos to neonatal rats in doses spanning the threshold for systemic toxicity and examined developing brain regions (brainstem, forebrain, cerebellum) for signs of interference with cell development using markers for cell packing density and cell number (DNA concentration and content) and cell size (protein/DNA ratio). Neonatal rats given 5 mg/kg of chlorpyrifos on postnatal days 1-4 showed significant mortality and the survivors exhibited severe cell loss in the brainstem; brainstem growth was maintained by enlargement of the remaining cells. This effect was not seen at 1 mg/kg, a dose that did not compromise survival or growth, nor was there any adverse effect at either dose in the forebrain, despite the fact that both brainstem and forebrain possess comparable cholinergic projections. When chlorpyrifos was administered later, on days 11-14, the major target for cell loss shifted from the brainstem to the forebrain and in this case, effects were seen at doses that did not compromise survival or growth. The loss of forebrain cell number occurred between 15 and 20 days of age rather than during the chlorpyrifos treatment. The cerebellum differed from the other regions in that it showed short-term elevations of DNA after chlorpyrifos exposure in either early or late postnatal periods; nevertheless, values then regressed to subnormal in parallel with the loss of cells in other regions. Thus, chlorpyrifos likely causes delayed cell death. Although regions rich in cholinergic projections, such as brainstem and forebrain, may be more affected than noncholinergic regions (cerebellum), the maturational timetable of each region (brainstem earliest, forebrain intermediate, cerebellum last) appears to be more important in setting the window of vulnerability. These results indicate that, even when growth or survival are unaffected, chlorpyrifos produces cellular deficits in the developing brain that could contribute to behavioral abnormalities.

Reduction by beta-adrenoceptor blockade of hypoxia-induced right heart hypertrophy in the rat

1. The study was undertaken to assess the role of beta-adrenoceptors in the induction of compensatory cardiac hypertrophy in an in vivo model. 2. In the rat, exposure to severe hypoxia (6% inspired oxygen for 8 h day) caused a 51% increase in right heart weight and a 75% increase in haematocrit. 3. The hypoxia-induced right ventricular hypertrophic response was reduced by 65% by oral treatment with a high dose of the non-selective beta-adrenoceptor antagonist, propranolol (80 mg kg-1 body weight); the drug treatment caused only a minor reduction (6%) in secondary polycythaemia. 4. With a less severe degree of hypoxia (7% inspired oxygen) there was only minimal secondary polycythaemia (+15%), and a lesser degree of compensatory right ventricular hypertrophy in untreated rats (+33%). 5. Treatment with the beta 1-adrenoceptor antagonist, atenolol, in a dose of 80 mg kg-1 body weight abolished right ventricular hypertrophy in response to 7% inspired oxygen, without affecting haematocrit and caused a small reduction in the ratio of heart weight to body weight in normoxic rats. 6. The results show that the effect of propranolol on hypoxic right ventricular hypertrophy is not secondary to any effect on secondary polycythaemia as has previously been suggested and that a marked reduction of compensatory cardiac hypertrophy can be obtained by a beta 1-selective adrenoceptor antagonist. Thus these findings support the view that noradrenaline released from cardiac sympathetic nerve terminals exerts a trophic effect on myocardial cells and demonstrates that in vivo, this trophic effect can be reduced by beta 1-adrenoceptor blockade.

Neural input and the development of adrenergic intracellular signaling: neonatal denervation evokes neither receptor upregulation nor persistent supersensitivity of adenylate cyclase

In the adult, denervation of adrenergic target tissues leads to compensatory upregulation of receptor sites and to supersensitive responses. When 6-hydroxydopamine (6-OHDA) was given to neonatal rats, cardiac beta-receptors failed to show significant upregulation throughout the first five postnatal weeks and alpha 1-receptors were unchanged except at 35 days of age, despite 70-95% depletion of norepinephrine. The failure to upregulate could not be attributed to the high background level of receptor expression commensurate with ontogenetic increases in receptor numbers, since the same deficiency was seen in the liver, a tissue in which beta-receptors decline with development; liver alpha 1-receptors also failed to upregulate after neonatal denervation. Examination of the linkage of beta-receptors to adenylate cyclase indicated major differences from mature regulatory mechanisms, as denervation supersensitivity was completely absent (liver) or emerged only transiently several weeks after 6-OHDA treatment (heart). In the heart, there was evidence for a defect in the G-protein-dependent component of the receptor/cyclase linkage that could contribute to the delayed appearance of supersensitivity. Because the fundamental patterns of receptor ontogeny and of adenylate cyclase responsiveness are still present after neonatal denervation, it is unlikely that neural input provides the major impetus for basal development. However, adult-type regulation of receptors and responses did not emerge even after a prolonged period; thus, neural input during a critical developmental stage may be required for the cell to learn how to adjust receptor expression and the receptor/cyclase link in response to stimulation.

In search of a mechanism for receptor-mediated neurobehavioral teratogenesis by nicotine: catecholamine release by nicotine in immature rat brain regions

Nicotine disrupts central nervous system development through interactions with nicotinic cholinergic receptors found in immature brain, leading to discoordination of target cell replication and differentiation. However, it is unclear whether the net result is achieved by nicotine's actions on its specific target cells, or indirectly through receptor-mediated release of other neurotransmitters, such as catecholamines, that possess neurotrophic properties. In the current study, developing rats (1, 7, 14 and 21 days old) were challenged acutely with nicotine (0.3 mg/kg) and the release of catecholamines was evaluated in vivo (AMPT method) in three brain regions that differ in nicotinic receptor concentrations. Nicotine did not stimulate catecholamine release at birth, but developed the capacity to do so in parallel with the ontogeny of nicotinic cholinergic receptors in the midbrain+brainstem and in the forebrain. In the cerebellum, which remains poor in nicotinic receptors, no response was obtained at any age. Superimposed on this general pattern, changes in sensitivity to nicotine were also seen that corresponded to ontogenetic changes in endogenous cholinergic tone, suggesting that receptor desensitization occurs normally during developmental stages in which neuronal activity is high. The absence of a catecholamine response to nicotine at birth in the rat indicates that neurobehavioral teratology associated with fetal nicotine exposure does not reflect secondary actions mediated through catecholamines. However, because brain development in the neonatal rat corresponds to fetal stages in man, the onset of these mechanisms may be relevant to human fetal exposure.

Developmental expression of sarcomeric and ubiquitous mitochondrial creatine kinase is tissue-specific

Creatine kinase (CK) isoenzymes play prominent roles in myocardial energy metabolism. Two nuclear genes encode mitochondrial creatine kinase (MtCK), are tissue-specific in their expression, and are thus designated as sarcomeric MtCK (sMtCK) and ubiquitous MtCK (uMtCK). Quantitative analysis of the mRNA expression of both MtCKs in developing rat tissues demonstrates tissue-specific developmental regulation. sMtCK mRNA in heart is undetectable prenatally but is dramatically upregulated by 28 d postnatally. sMtCK mRNA in skeletal muscle is also extremely low prenatally but is markedly upregulated at birth and doubles by 28 d postnatally. uMtCK mRNA expression is present at low levels in fetal brain and intestine. Brain uMtCK mRNA continues to rise from -4 d prenatally until 28 d postnatally (6-fold increase), but intestinal uMtCK mRNA increases immediately prior to birth, falls, and is upregulated again at 28 d (20-fold). uMtCK mRNA is undetectable in fetal skeletal muscle or heart, but increases to low levels in skeletal muscle at birth and remains at this level into adulthood. uMtCK is not detectable in heart, lung, testes, or liver at any stage examined. We conclude that sMtCK and uMtCK are developmentally regulated in a tissue-specific manner. Unlike cytosolic muscle CK and brain CK, there is no isoenzyme switch between sMtCK and uMtCK in the developing animal. Our results suggest that specific trans-acting factors regulate the different developmental and tissue-specific expression of the MtCK genes.

Expression of the mitochondrial creatine kinase genes

Mitochondrial Creatine Kinase (MtCK) is responsible for the transfer of high energy phosphate from mitochondria to the cytosolic carrier, creatine, and exists in mammals as two isoenzymes encoded by separate genes. In rats and humans, sarcomere-specific MtCK (sMtCK) is expressed only in skeletal and heart muscle, and has 87% nucleotide identity across the 1257 bp coding region. The ubiquitous isoenzyme of MtCK (uMtCK) is expressed in many tissues with highest levels in brain, gut, and kidney, and has 92% nucleotide identity between the 1254 bp coding regions of rat and human. Both genes are highly regulated developmentally in a tissue-specific manner. There is virtually no expression of sMtCK mRNA prior to birth. Unlike cytosolic muscle CK (MCK) and brain CK (BCK), there is no developmental isoenzyme switch between the MtCKs. Cell culture models representing the tissue-specific expression of either sMtCK or uMtCK are available, but there are no adequate developmental models to examine their regulation. Several animal models are available to examine the coordinate regulation of the CK gene family and include 1) Cardiac Stress by coarctation (sMtCK, BCK, and MCK), 2) Uterus and placenta during pregnancy (uMtCK and BCK), and 3) Diabetes and mitochondrial myopathy (sMtCK, BCK, and MCK). We report the details of these findings, and discuss the coordinate regulation of the genes necessary for high-energy transduction.

Opioid antagonist modulation of rat heart development

Endogenous opioids are known to regulate morphogenesis in both neural and non-neural systems. This study examined whether endogenous opioids influence cardiac development. Naltrexone, a potent opioid antagonist that blocks the interaction of opioid peptides and opioid receptors, was administered acutely (50 mg/kg) to 1-day old rats. The numbers of myocardial and epicardial cells in the ventricles and atria that synthesized DNA, as determined by [3H]-thymidine incorporation and autoradiography, were markedly increased from control levels. Labeling indices were significantly elevated for at least 12 hr following a single injection of naltrexone. Examination of 10-day old rats exposed to naltrexone from birth revealed higher labeling indices, as well as increases in body and heart weights and in areal measurements of the entire heart and the ventricles. The effects of naltrexone were not mediated through the sympathetic nervous system or thyroid hormone. These results lead one to suggest that an opioid peptide is tonically acting as a negative regulatory factor in the formation of the heart. Alterations in the endogenous opioid system in early life may contribute to cardiac dysmorphogenesis. Moreover, these data indicate that opioid antagonists could act as an important therapeutic influence with regard to cardiac malformations.

Prenatal cocaine exposure affects the development of aortic adrenergic innervation and contractile responses

This study examines the effects of prenatal cocaine administration on the development of vascular sympathetic innervation and contractile responsiveness. Rabbits received cocaine (4 mg/kg, iv, bid) or saline during gestational days 8 to 29. Aortas were obtained on postnatal days 10, 20, 30 and 50. Vascular smooth muscle responsiveness was assessed by measuring aortic contractile responses to norepinephrine (NE) and to other vasoconstrictors. Vascular adrenergic innervation was evaluated by measuring desipramine sensitive [3H]-NE uptake into aortic ring segments and aortic NE content. [3H]-NE uptake and NE content were reduced at postnatal days 10 and 20 in the rabbits exposed prenatally to cocaine. Differences were not observed at postnatal days 30 or 50. The contractile response to NE was reduced in rabbits exposed to cocaine prenatally. Maximal response and potency were decreased at postnatal day 10 and potency was still decreased at day 20, but not at the older ages. Contractile responses to serotonin (5-HT) and angiotensin II (AII) were not affected by prenatal cocaine exposure. These results suggest that prenatal cocaine exposure delays the development of aortic adrenergic innervation and alpha adrenoceptor responsiveness.

Development of neuropeptide Y and tyrosine hydroxylase immunoreactive innervation in postnatal rat heart

Neuropeptide Y (NPY), immunoreactive (IR), and tyrosine hydroxylase (TH)-IR nerve fibers were scarce at birth in rat heart, but increased rapidly during the first 2 postnatal weeks, reaching approximately adult levels by the third week. The sequence of development was: interatrial septum and atrial wall, free ventricular wall starting from the epicardium, and finally the atrial appendages and interventricular septum. In ventricles and atrial appendages both fiber types developed similarly. In interatrial septum and atrial walls more NPY-IR than TH-IR fibers were evident, and NPY-IR, but not TH-IR, neurons were detected in intrinsic ganglia. Double-label immunohistochemistry provided further evidence that NPY is located in ventricular and atrial noradrenergic nerves, but is also located in nonnoradrenergic nerves in atria.

Effects of fetal dexamethasone exposure on postnatal control of cardiac adenylate cyclase: beta-adrenergic receptor coupling to Gs regulatory protein

In the adult, glucocorticoids have been shown to upregulate beta-adrenergic control of adenylate cyclase by a variety of mechanisms; glucocorticoids are also thought to play a role in development of cardiac adrenergic function. In the current study, pregnant rats were given 0.2 mg/kg of dexamethasone on gestational days 17, 18, and 19 and the effects on the development of cardiac beta-receptors and their linkage to the stimulatory G-protein, Gs, were examined at 4 days postpartum. beta-Receptor numbers and affinity were unaffected by dexamethasone exposure, nor was there any change in the ability of the GTP analog, Gpp(NH)p, to shift the affinity state of the receptor. Addition of Gpp(NH)p to cardiac membranes enhanced basal and isoproterenol-stimulated adenylate cyclase activity, but the total response to isoproterenol, with or without Gpp(NH)p, represented a very small fraction of total enzymatic activity. Quantitative analysis of Gs indicated no changes attributable to dexamethasone treatment. Although prenatal dexamethasone has been shown to increase adenylate cyclase reactivity to beta-adrenergic input, the effect appears to be at the level of the catalytic subunit of adenylate cyclase, rather than at receptor or G-protein stages.

Glucocorticoids regulate the development of intracellular signaling: enhanced forebrain adenylate cyclase catalytic subunit activity after fetal dexamethasone exposure

Although glucocorticoids cause growth retardation and interfere with cell development, selective promotion of some aspects of cell function also has been reported. The current study examines whether glucocorticoids enhance intracellular transduction mechanisms mediated by adenylate cyclase in the developing forebrain, a region in which steroids have been shown to interfere with cell replication, maturation, and growth. Pregnant rats were given dexamethasone at doses spanning the threshold for growth impairment (0.05, 0.2, and 0.8 mg/kg) on gestational days 17, 18, and 19, and development of adenylate cyclase was evaluated in membrane preparations, using four different activity measures; basal adenylate cyclase in the absence or presence of GTP, maximal G-protein activation by fluoride in the presence of GTP, and stimulation mediated by forskolin-Mn2+, which bypasses the G-proteins. Prenatal exposure to dexamethasone produced a dose-dependent impairment of body growth, with smaller deficits in forebrain weights (brain sparing) indicative of systemic toxicity. Basal adenylate cyclase activity was unaffected by dexamethasone treatment, regardless of whether GTP was present in the assay. Similarly, fluoride stimulation developed normally in all dexamethasone groups. However, forskolin-Mn(2+)-stimulated activity was significantly enhanced in a dose-dependent fashion. These results suggest that glucocorticoids serve as positive factors for the development of adenylate cyclase catalytic subunit activity, independently of their adverse effects on general growth and development; thus, these hormones may be a primary regulator of cell signaling during early development.

Thyroid hormone differentially regulates cellular development in neonatal rat heart and kidney

The role of thyroid hormone in the control of cardiac and renal cell development was examined in neonatal rats made hyperthyroid by administration of triiodothyronine (T3, 0.1 mg/kg s.c. on postnatal days 1-5) or hypothyroid by administration of propylthiouracil (PTU, 20 mg/kg s.c. given to dams on gestational day 17 through postnatal day 5 and to pups on postnatal days 1-5). Indices of total cell number (total DNA per tissue), cell packing density (DNA per g tissue), and relative cell size (protein/DNA ratio) were evaluated from birth through young adulthood. PTU administration led to primary shortfalls in cell number that were of similar magnitude in both tissues, but persisted somewhat longer in the kidney than in the heart. Deficits in cell packing density and cell size in the hypothyroid animals were secondary to the effect on cell number, displaying smaller magnitudes of effect and a lag in appearance and disappearance of the deficits compared to that for total DNA; indeed, the phase in which tissues were restoring their cell numbers was accompanied by increased cell packing density, reflecting a more rapid restitution of cell numbers than tissue weight or cell size. In contrast to the relatively similar effects of PTU on developing cardiac and renal cells, the effects of T3 were selective for the heart. Although T3 caused general growth impairment, it evoked marked cardiac overgrowth that was accompanied by a striking increase in cell number and a small increase in cell size. The cardiac hyperplasia is unique to the developing animal, as post-replicative heart cells in adult animals show only hypertrophy in response to thyroid hormone.(ABSTRACT TRUNCATED AT 250 WORDS)

Application of stereological analysis of cell volume to isolated myocytes in culture with and without adrenergic innervation

A three-dimensional analysis to evaluate structural changes in cultured cardiac myocytes following adrenergic innervation was performed using stereological techniques formerly limited to cells in tissue and organs. Cell volumes were calculated for two groups of cells at 96 hours in culture: isolated myocytes and myocytes innervated with adrenergic neurons. Relative and absolute volumes of the nucleus, cytoplasm, and cell were quantified by systematically sampling sections throughout the cell and by point count sampling techniques. Volumetric estimates were similarly determined for the mitochondria, sarcomeres, and other cellular components in the cytoplasm. Data were analyzed with ANOVA and randomized block design to control for variation among the cultures. Adrenergic innervation produced a 44% increase in cell volume, X +/- SEM, (3,344 +/- 196 microns3 to 4,816 +/- 400 microns3, P = 0.007). The absolute volume of mitochondria significantly increased after innervation (521 +/- 42 microns3 to 744 +/- 54 microns3, P less than 0.01). Absolute sarcomere volume did not change significantly (750 +/- 92 microns3 to 642 +/- 1061 microns3, P = 0.14). Other cellular components, defined as all cytoplasmic components except mitochondria and sarcomeres, significantly increased with innervation (1,739 +/- 166 microns3 to 3,097 +/- 338 microns3, P = 0.02). The relative volume of the nucleus and the cytoplasm in the cell remained unchanged following innervation. However, the relative volume of mitochondria decreased by 6%, the percent of the cytoplasm occupied by the sarcomeres decreased by 44%, and the volume occupied by the other cellular components increased by 22%. These findings support the use of stereological analysis as a means to quantify cell volumes of cultured myocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental expression of dystrophin on the rat myocardial cell membrane

The Duchenne muscular dystrophy gene product, dystrophin, is expressed on the cell membrane of skeletal and cardiac muscles. We examined the developmental time course of dystrophin expression in the rat ventricular myocardium. Dystrophin was immunohistochemically undetectable on the 15th embryonic day, but a small amount was expressed on the cell membrane on the 17th embryonic day. The amount increased during the perinatal period reaching adult level at 2 weeks after birth. These findings were supported by immunoblot analysis. Chemical sympathectomy in newborn rats by 6-hydroxydopamine (prevention of innervation) had no detectable influence on myocardial dystrophin expression at 2 weeks after birth. Our results show that dystrophin lining on the rat myocardial cell membrane increases during the perinatal period, and that this process is little influenced by the development of sympathetic innervation.

Role of sympathetic innervation in cardiac development in oculo

Embryonic heart cultured in oculo differentiates into adult-like heart tissue, increases in size, and maintains spontaneous activity driven by a defined pacemaker. The growth and beating rate of embryonic heart maturing in oculo are modulated by its neural milieu. Culture in a sympathetically denervated eye chamber compromised growth and increased intrinsic beating rate. Exposure to an additional source of sympathetic neurons increased parasympathetic control of heart rate but did not alter growth or intrinsic beating rate. Because eye chamber sympathectomy alters neurotransmitter expression in parasympathetic and sensory neurons, it is possible that the growth inhibition observed in heart grafts is not a direct consequence of preventing sympathetic innervation. This possibility is being tested in additional experiments.

Genetic, neurohumoral, and hemodynamic influences on spontaneously hypertensive rat heart development in oculo

To distinguish among genetic, neurohumoral, and hemodynamic explanations for structural and functional differences in the hearts of young spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) control rats, embryonic SHR and WKY rat heart tissue was cultured in the anterior eye chamber of adult SHR and WKY rats. In study 1, atria from E-12 WKY rat embryos grafted into anterior eye chambers of either SHR or WKY host rats achieved a larger size than did SHR grafts by 8 weeks in oculo (2.98 +/- 0.75 and 2.55 +/- 0.32 mm2 vs. 1.80 +/- 0.20 and 2.04 +/- 0.44 mm2). Beating rates did not differ between SHR and WKY rat atria implanted into SHR or WKY host rats. In study 2, ventricles from E-13 embryonic SHR and WKY rat hearts grew to similar size and weight when implanted into SHR or WKY host rats (e.g., SHR hearts, 1.81 +/- 0.32 vs. 1.74 +/- 0.33 mm2; WKY rat hearts, 1.75 +/- 0.29 vs. 2.29 +/- 0.32 mm2). Ventricle grafts from SHR embryos into SHR host rats beat more rapidly (165 +/- 19 beats/min) during weekly measurements than either WKY rat ventricles (92 +/- 9 beats/min in SHR hosts and 99 +/- 9 beats/min in WKY host rats) or SHR ventricles grafted into WKY host rats (109 +/- 7 beats/min, p less than 0.001). In study 3, atria from E-13 SHR and WKY rat embryos were grafted into sympathectomized and intact eye chambers of SHR or WKY host rats. Sympathectomy of the eye chamber compromised growth of grafts into WKY host rats (1.54 +/- 0.24 vs. 0.90 +/- 0.14 mm2) but not SHR hosts (1.54 +/- 0.25 vs. 1.73 +/- 0.24 mm2). Grafts into sympathectomized eye chambers of WKY host rats beat more slowly than grafts into eye chambers with sympathetic innervation intact (282 +/- 14 vs. 202 +/- 14 beats/min); sympathectomy did not alter beating rate of grafts in SHR hosts (266 +/- 14 vs. 255 +/- 18 beats/min). These results suggest that the growth and beating rate of SHR atrial grafts may be less sensitive to sympathetic innervation than WKY rat atrial grafts. In these studies, SHR grafts did not grow larger than WKY heart grafts and did not show an increased intrinsic beating rate, suggesting that the cardiac hypertrophy and increased intrinsic beating rate observed in intact SHR are unlikely to result from direct genetic programming.