[3H]dofetilide binding to cardiac myocytes: modulation by extracellular potassium

The radioligand [3H]dofetilide binds specifically to the delayed rectifier potassium channel and provides a biochemical approach to study interactions of Class III drugs with this channel. However, previous studies have examined the binding of [3H]dofetilide to cardiac myocytes only at extracellular potassium of 135 mM. Because previous electrophysiological studies have shown that hyperkalemia could alter the pharmacological responses to I(Kr) channel blockers, the hypothesis tested in this study was that changing ionic conditions would alter characteristics of [3H]dofetilide binding.

RESULTS

under physiological conditions (Na+ 135 mM, K+ 5 mM), [3H]dofetilide bound to two sites on guinea-pig ventricular myocytes (a high-affinity site, K(d) 26+/-8 nM, B(max) 81+/-12 fmol/10(6) cells: and a low-affinity site, K(d) 1.6+/-0.8 microM, B(max) 1003+/-173 fmol/10(6) cells, n=11). Binding properties were not altered by changes in osmolarity or extracellular sodium. However, when extracellular K+ was increased to 20 mM, a single binding site was observed with an affinity K(d) of 120+12 nM and a B(max) of 303+/-57 fmol/10(6) cells (P<0.05; n=6). To establish whether this effect was mediated at the high-affinity site we assessed the effects of elevated extracellular potassium on a biological model, neonatal mouse myocytes, that expressed solely the high-affinity binding sites. The K(d) values for binding to fetal mouse cardiac myocytes at an extracellular K+ of 5 mM and 20 mM were also significantly different, 29+/-10 and 230+/-46 nM, respectively. In conclusion, [3H]dofetilide binding to its high-affinity site is modulated by extracellular potassium.

Ontogeny of insulin-like growth factor-I and -II gene expression in ovine fetal heart

OBJECTIVE

Insulin-like growth factors (IGF)-I and -II have been implicated in growth and differentiation during embryonic and fetal development. To examine the role of the IGFs in growth of the fetal heart, we determined the gene expression of IGF-I and IGF-II in the four cardiac chambers of the ovine fetus from 58 to 146 days' gestation (term = 147 days).

METHODS

Total RNA was obtained from the cardiac chambers, analyzed by Northern blot, and hybridized to ovine specific cDNA probes for IGF-I and IGF-II. The resulting autoradiograms were subjected to light densitometry, and the intensity of the IGF signals was normalized to the respective 28S ribosomal RNA signals.

RESULTS

In the atria and ventricles, IGF-I mRNA abundance was very low throughout the gestational period studied, whereas IGF-II mRNA levels were higher and readily detectable. In the atria, IGF-I mRNA was very low at 60 days' gestation and appeared to increase gradually toward term. Abundance of IGF-II mRNA was high at 60 days, increased further until 120 days, and decreased slightly toward term. In contrast, in the ventricles, IGF-I mRNA increased from 60 to 100 days and then declined moderately at term. Levels of IGF-II mRNA in the ventricles were high at 60 days and decreased progressively to low levels at term. No difference in IGF-I or IGF-II mRNA levels was noted between the right and left atria or right and left ventricles.

CONCLUSION

These results suggest that developmental patterns for IGF-I and IGF-II gene expression exist in the ovine fetal heart, and the patterns differ between the atria and ventricles. Further, these gestational trends differ from those for atrial natriuretic factor (ANF) found in our previous studies, indicating that expression of the ANF gene in the fetal heart may not be associated with cardiac growth and differentiation.

Cardiomyocyte DNA synthesis and binucleation during murine development

Cardiomyocyte DNA synthesis and binucleation indexes were determined during murine development. Cardiomyocyte DNA synthesis occurred in two temporally distinct phases. The first phase occurred during fetal life and was associated exclusively with cardiomyocyte proliferation. The second phase occurred during early neonatal life and was associated with binucleation. Collectively, these results suggest that cardiomyocyte reduplication ceases during late fetal life. Northern and Western blot analyses identified several candidate genes that were differentially expressed during the reduplicative and binucleation phases of cardiomyocyte growth.

Differential expression of alpha A- and alpha B-crystallin during murine ocular development

PURPOSE

To compare the temporal and spatial expression patterns of alpha A- and alpha B-crystallin mRNA during ocular development.

METHODS

Tissue samples from embryonic day 9.5 (E9.5) through postnatal day 14 were collected from FVB/N strain mice. The specimens were fixed in paraformaldehyde, histologically processed, and assayed for alpha A- and alpha B-crystallin mRNA expression by in situ hybridization.

RESULTS

During ocular development, alpha B-crystallin transcripts are present in the lens placode at E9.5. Transcripts of alpha A-crystallin are first observed in the lens cup at E10 to 10.5. During subsequent development of the lens, alpha A crystallin transcripts are most abundant in the fiber cells, and alpha B crystallin mRNA is preferentially expressed in epithelial cells. Transcripts of alpha A-crystallin were detected only in the lens. In contrast, alpha B-crystallin transcripts are present in retinal pigment epithelium, optic nerve, extraocular muscle, iris, ciliary body, cornea, and several nonocular sites, such as heart and nasal epithelium.

CONCLUSIONS

Transcription of alpha B-crystallin precedes the expression of alpha A-crystallin during murine ocular development. Furthermore, the patterns of alpha A- and alpha B-crystallin expression in the lens are distinctive: alpha A is upregulated and alpha B is downregulated during prenatal fiber cell differentiation. These results indicate that the alpha-crystallin genes are not identically regulated either within or outside the lens.

Expression of intermediate filament desmin and vimentin in the human fetal heart

BACKGROUND

The intermediate filament (IF) desmin provides support for contractile machinery in muscle cells, and vimentin plays an important role in maintaining the stability of mesenchymal cells and in signal transduction. However, development of IFs in heart tissue during intrauterine life in human is not well established.

METHODS

In the present study, development of desmin and vimentin in human fetal hearts aged 9-28 weeks of gestation (n = 41) were investigated by immunohistochemistry with monoclonal antibodies against desmin and vimentin. Relative density of fluorescence of each sample was determined by densitometry. Left ventricle (LV) tissues from a 1-year-old child (n = 1) were examined by immunohistochemistry for postnatal comparison. Western blot analyses were done with only a few randomly selected LV tissues from fetuses of 9, 20, and 28 weeks gestation to assess trends of desmin and vimentin expression.

RESULTS

By Western blot analyses, 53-kDa desmin and 54-kDa vimentin were present in all fetal heart tissues examined. Desmin intensity was progressively increased with increasing fetal age, whereas vimentin intensity decreased. Desmin was present only in cardiomyocytes. In the earlier period (10-14 weeks gestation), desmin was localized along the cardiomyocyte membrane and/or Z lines in regular intervals, and later (25-28 weeks gestation) it was structurally well integrated; however, its network was incomplete. Only cardiomyocytes from a 1-year-old child revealed highly developed and integrated desmin lattices. However, vimentin was present in the mesenchymal tissue including fibroblasts and surrounding blood vessels. In part, some cardiomyocytes showed a weakly positive reaction with monoclonal antibody against vimentin in 9-14 weeks gestation. Vimentin-positive areas, however, were progressively diminished with increasing fetal age. Vimentin was present only in the connective tissue and coverings of the 1-year-old child's heart. Relative density of fluorescence of desmin was increased with increasing fetal age, whereas that of vimentin decreased.

CONCLUSIONS

These results indicate that there is a fetal age (or gestation)-dependent expression of IFs in human fetal heart: desmin increases with increasing fetal age, whereas vimentin decreases.

Alternative RNA splicing-generated cardiac troponin T isoform switching: a non-heart-restricted genetic programming synchronized in developing cardiac and skeletal muscles

An alternative RNA splicing-generated cardiac troponin T (cTnT) isoform switching has been found during both avian and mammalian heart development. The present study demonstrates that the mouse cTnT isoform switching is approximately 10 days earlier than that in rat during perinatal heart development. The mouse cTnT isoform switching is also approximately 10 days earlier than the transcriptionally regulated troponin I (TnI) isoform switching, a transition that showed similar timing during mouse and rat heart development. In addition, synchronized cTnT and TnI switches are observed during both rat and chicken heart development. More interestingly, the expression of cTnT in both mouse and rat neonatal skeletal muscles showed an isoform transition pattern synchronized with the cTnT isoform switching in the neonatal heart of the corresponding species. The results suggest that the alternative RNA splicing-generated cTnT isoform switching during development is (a) not a direct response to the functional changes in perinatal myocardium and (b) not a restricted regulation in the heart, but is controlled by a species-specific programming that is synchronized in developing cardiac and skeletal muscles.

Molecular cloning and characterization of human non-smooth muscle calponin

cDNA clones encoding a calponin isoform with 309 amino acids have been isolated from human heart. The deduced amino acid polypeptide (M(r) 33,697) showed a neutral isoelectric point of 7.1. The mRNA, expressed in cultured smooth muscle cells as well as in fibroblasts, vascular endothelial cells, and keratinocytes, contains a 3' untranslated region of 1.2 kilobases that includes an Alu repetitive sequence in the antisense direction. On the basis of the nucleotide sequence identity to an expressed sequence tag, HUM21ES93 [Cheng, J.-F., Boyartchuk, V., and Zhu, Y. (1994) Genomics 23, 75-84], the human neutral calponin gene is assigned to chromosome 21q11.1. The amino acid sequence indicates that this protein is the human equivalent of mouse calponin-h2 (94.8% identity) [Strasser, P., Gimona, M., Moessler, H., Herzog, M., and Small, J.V. (1993) FEBS Lett. 330, 13-18]. Three tandem repeats of 29 amino acids, a Vav-homologous region and an actin-binding sequence, originally identified in the basic calponin isoform, are conserved. There are two consensus phosphorylation sites for tyrosine kinase. An immunoreactive form of the neutral calponin appears to be localized with vinculin in the cell-to-cell junctions of cardiomyocytes. Mouse calponin-h2 is also expressed in both embryonic and adult heart. These results indicate that the human neutral calponin is a non-smooth muscle isoform, and may play a physiological role in cytoskeletal organization.

Cardiovascular lesions and skeletal myopathy in mice lacking desmin

In order to further our understanding of the biological role of desmin, the muscle-specific intermediate filament protein, a null mutation in the desmin gene was introduced into the germ line of mice. Despite the complete lack of desmin, these mice developed and reproduced. Since we show that skeletal, cardiac, and smooth muscles form in the Des-/- mice, it is reasonable to propose that desmin is not essential for myogenic commitment or for myoblast fusion or differentiation in vivo. However, morphological abnormalities were observed in the diaphragm of adult mice; these were demonstrated by disorganized, distended, and nonaligned fibers. The heart presented areas of hemorrhaging in which fibrosis and ischemia were observed. We have also shown that the absence of desmin produces specific defects in smooth muscles. In conclusion, our results have demonstrated that desmin is not required for the differentiation of skeletal, cardiac, and smooth muscles but is essential to strengthen and maintain the integrity of these tissues.

Induction of DNA synthesis and apoptosis in cardiac myocytes by E1A oncoprotein

Beginning during the second half of gestation, increasing numbers of cardiac myocytes withdraw from the cell cycle such that DNA synthesis is no longer detectable in these cells by neonatal day 17 in vivo. The mechanisms that exclude these and other terminally differentiated cells from the cell division cycle are poorly understood. To begin to explore the molecular basis of the barrier to G1/S progression in cardiac myocytes, we used adenoviruses to express wild-type and mutant E1A proteins in primary cultures from embryonic day 20 rats. While most of these cardiac myocytes are ordinarily refractory to DNA synthesis, even in the presence of serum growth factors, expression of wild-type E1A stimulates DNA synthesis in up to 94% or almost all successfully transduced cells. Rather than complete the cell cycle, however, these cells undergo apoptosis. Apoptosis is limited to those cells that engage in DNA synthesis, and the kinetics of the two processes suggest that DNA synthesis precedes apoptosis. Mutations in E1A that disable it from binding Rb and related pocket proteins have little effect on its ability to stimulate DNA synthesis in cardiac myocytes. In contrast, mutants that are defective in binding the cellular protein p300 stimulate DNA synthesis 2.4-4.1-fold less efficiently, even in the context of retained E1A pocket protein binding. In the absence of ElA pocket protein binding, the usual situation in the cell, loss of p300 binding severely decreases the ability of ElA to stimulate DNA synthesis. These results suggest that the barrier to G1/S progression in cardiac myocytes is mediated. at least in part, by the same molecules that gate the G1/S transition in actively cycling cells, and that p300 or related family members play an important role in this process.

Vaginal bleeding and early pregnancy outcome in an infertile population

OBJECTIVE

Our purpose was to determine the prognostic value of vaginal bleeding in early pregnancy outcome as well as to analyze the role of cardiac activity in predicting pregnancy viability in the presence of vaginal bleeding.

STUDY DESIGN

This was a cohort study of pregnancies obtained using either assisted reproductive technology (ART) or routine infertility treatment (RIT). Two hundred twenty-eight pregnant women were divided into two groups based on the presence or absence of vaginal bleeding. Successive measurements of beta-hCG levels were obtained every 2 days, starting on day 14 after ovulation or embryo transfer. All pregnancies underwent weekly transvaginal ultrasound (UTZ) examinations beginning on day 21. The occurrence of vaginal bleeding was monitored weekly.

RESULTS

Seventy of the 228 patients (31%) had bleeding in early pregnancy, resulting in 31 (44%) pregnancy losses. Only 22 pregnancy losses (14%) were observed in 158 patients who did not have bleeding (P < 0.001). The abortion rate for the bleeding versus nonbleeding groups was 35 and 9%, respectively (P < 0.001). Vaginal bleeding was associated with a higher abortion rate in pregnancies following RIT than ART (51 vs 8%; P < 0.001). Fetal cardiac activity was noted by vaginal ultrasound in 189 patients. In this subpopulation, bleeding was also associated with a higher abortion rate than that in the nonbleeding group (17 vs 4%; P < 0.001). However this higher incidence was observed only in pregnancies following RIT, not ART (28 vs 5%; P < 0.001).

CONCLUSIONS

Although bleeding significantly decreased the chance of a normal pregnancy outcome, more than 50% of the pregnancies did progress to term. The presence of cardiac activity in this population as a sign of fetal viability offered a better pregnancy prognosis. However, the predictive value of fetal cardiac activity was reduced in the presence of vaginal bleeding in an infertile population treated with RIT.

Myocardial and cerebral oxygen delivery are not adversely affected by cocaine administration to early-gestation fetal sheep

OBJECTIVES

Cocaine administration to near-term pregnant sheep causes fetal hypoxemia, but oxygen delivery to the heart and brain are preserved because of increased blood flow. We hypothesized that cocaine administration during earlier fetal gestation impairs oxygen delivery to the heart and brain.

STUDY DESIGN

Ten pregnant ewes and fetuses at 0.7 term gestation underwent surgical instrumentation. After 48 hours of recovery fetal blood pressure, heart rate, cerebral and myocardial blood flow, and arterial oxygen content were determined before and during cocaine administration to the ewe.

RESULTS

Fetal hypoxemia was not noted in these animals. Fetal myocardial blood flow increased from 220 +/- 100 ml per 100 gm per minute to 349 +/- 183 ml per 100 gm per minute (p=0.03), and oxygen delivery increased from 16 +/- 5 ml of oxygen per 100 gm per minute to 22 +/- ml of oxygen per 100 gm per minute (p=0.02). Fetal cerebral blood flow and oxygen delivery remained unchanged.

CONCLUSION

Cerebral and myocardial oxygen delivery are unimpeded by maternal cocaine administration in 0.7 term gestation ovine fetuses.

Cardiac oxygenation by extracorporeal membrane oxygenation in exteriorized fetal lambs

OBJECTIVE

The purpose of this study was to determine the degree of cardiac oxygenation produced by different routes of extracorporeal membrane oxygenation in fetal lambs submerged in warm saline solution.

STUDY DESIGN

Seven fetal lambs ranging in age from 113 to 133 days of gestation were delivered by cesarean section and oxygenated with extracorporeal membrane oxygenation. To maintain the patency of the ductus arteriosus, prostaglandin E1 was continuously infused intravenously to the fetus. Initially the extracorporeal membrane oxygenation route was from the right atrium to the carotid artery. Then the extracorporeal membrane oxygenation route was changed to flow from the right atrium to the umbilical vein. The fetus was kept in a warm saline solution bath, and the fetal circulation was maintained. Extracorporeal membrane oxygenation flow ranged between 100 and 200 ml/min throughout the experiment. Simultaneous blood samples were taken during both types of extracorporeal membrane oxygenation from the following points in the fetal circulation: premembrane (least oxygenated blood leaving the fetus from the right atrium), postmembrane (oxygenated blood returning to the fetus), the carotid artery, and the left ventricle. The respiratory gases and pH of each sample were measured. Six fetuses received nonradioactive colored microspheres injected into the oxygenated blood returning to the fetus flow before returning to the fetuses during both types of extracorporeal membrane oxygenation. After the animals were killed, microspheres were counted in the myocardium separately taken from the right and left atria and the right and left ventricles to determine cardiac blood flow.

RESULTS

During right atrium to carotid artery extracorporeal membrane oxygenation, left ventricle PO2 remained low as postmembrane PO2 increased; these values were not significantly correlated (r = 0.234, p = 0.61). During right atrium to umbilical vein extracorporeal membrane oxygenation, left ventricle and postmembrane PO2 exhibited a significant positive correlation (r = 0.855, p = 0.014). When the extracorporeal membrane oxygenation route was switched from the right atrium to carotid artery to the right atrium to umbilical vein, there was a significant increase in left ventricle PO2 and a decrease in left ventricle PCO2, whereas the respiratory gases and pH remained unchanged at other sites in the circulation. Microsphere counts were consistently higher during right atrium to umbilical vein extracorporeal membrane oxygenation than during right atrium to carotid artery extracorporeal membrane oxygenation in all four samples from different parts of myocardium (p < 0.001 by paired t test).

CONCLUSION

More effective cardiac oxygenation is provided by right atrium to umbilical vein extracorporeal membrane oxygenation than by right atrium to carotid artery extracorporeal membrane oxygenation.

Developmental changes in beta-adrenergic modulation of L-type Ca2+ channels in embryonic mouse heart

In the adult mammalian myocardium, cellular Ca2+ entry is regulated by the sympathetic nervous system. L-type Ca2+ channel currents are markedly increased by beta-adrenergic (beta-A) agonists, which contribute to changes in pacing and contractile activity of the heart. In the developing mammalian heart, the regulation of Ca2+ entry by this enzyme cascade has not been clearly established, because changes in receptor density and coupling to downstream elements of the signaling cascade are known to occur during embryogenesis. In this study, we systematically investigated the regulation of L-type Ca2+ channel currents during development of the murine embryonic heart. We used conventional whole-cell and perforated-patch-clamp procedures to study modulation of L- type Ca2+ channel currents and to assay functional activity of distinct steps in the beta-A signaling cascade in murine embryonic myocytes at different stages of gestation. Our data indicate that the L-type Ca2+ channels in early-stage (day-11 to -13) myocytes are unresponsive to either isoproterenol or cAMP. L-type Ca2+ channels in late-stage (day-17 to -19) murine myocytes, however, exhibit responses to isoproterenol and cAMP similar to responses in adult cells, providing evidence that the beta-A cascade becomes functionally active during this period of embryonic development. We found that L-type Ca2+ channel activity in early-stage cells is increased by cell dialysis with the catalytic subunit of cAMP-dependent protein kinase (cA-PK) and that dialysis of early-stage cells with the holoenzyme of cA-PK restores functional responses to forskolin and cAMP, but not to isoproterenol. Our results provide strong evidence that a key factor in the early-stage insensitivity of L-type Ca2+ channels to cAMP is the absence, or low expression level, of the holoenzyme of cA-PK but that in addition, another element in the signaling cascade upstream from adenylate cyclase is expressed at a nonfunctional level or is uncoupled from the cascade and thus contributes to L-type Ca2+ channel insensitivity to beta-A agonists in early stages of the developing murine heart.

Cardiomyopathy in transgenic myf5 mice

To explore the compatibility of skeletal and cardiac programs of gene expression, transgenic mice that express a skeletal muscle myogenic regulator, bmyf5, in the heart were analyzed. These mice develop a severe cardiomyopathy and exhibit a significantly shorter life span than do their nontransgenic littermates. The transgene was expressed from day 7.5 post coitum forward, resulting in activation of skeletal muscle genes not normally seen in the myocardium. Cardiac pathology was not apparent at midgestation but was evident by day 2 of postnatal life, and by 42 days, hearts exhibited multifocal interstitial inflammation, fibrosis, cellular hypertrophy, and occasional myocyte degeneration. All four chambers of the heart were enlarged to varying degrees, with the atria demonstrating the most significant hypertrophy (>100% in 42-day-old mice). The transgene and several skeletal muscle-specific genes were expressed only in patchy areas of the heart in heterozygous mice. However, molecular markers of hypertrophy (such as alpha-skeletal actin and atrial myosin light chain- 1) were expressed with a wider distribution, suggesting that their induction was secondary to the expression of the transgene, In older (28-week-old) mice, lung weights were also significantly increased, consistent with congestive heart failure. The life span of bmyf5 mice was significantly shortened, with an average life span of 109 days, compared with at least a twofold longer life expectancy for nontransgenic littermates. Expression of the transgene was associated with an increase in Ca2+-stimulated myofibrillar ATPase in myofibrils obtained from the left ventricles of 42-day-old bmyf5 mice. Myocardial bmyf5 expression therefore induces a program of skeletal muscle gene expression that results in progressive cardiomyopathy that may be due to incompatibility of heart and skeletal muscle structural proteins.

Molecular cloning and expression of murine vascular endothelial-cadherin in early stage development of cardiovascular system

An early step in the formation of the extraembryonic and intraembryonic vasculature is endothelial cell differentiation and organization in blood islands and vascular structures. This involves the expression and function of specific adhesive molecules at cell-to-cell junctions. Previous work showed that endothelial cells express a cell-specific cadherin (vascular endothelial [VE]-cadherin, or 7B4/cadherin-5) that is organized at cell-to-cell contacts in cultured cells and is able to promote intercellular adhesion. In this study, we investigated whether VE-cadherin could be involved in early cardiovascular development in the mouse embryo. We first cloned and sequenced the mouse VE-cadherin cDNA. At the protein level, murine VE-cadherin presented 75% identity (90%, considering conservative amino acid substitutions) with the human homologue. Transfection of murine VE-cadherin cDNA in L cells induced Ca(++)-dependent cell-to-cell aggregation and reduced cell detachment from monolayers. In situ hybridization of adult tissues showed that the murine molecule is specifically expressed by endothelial cells. In mouse embryos, VE-cadherin transcripts were detected at the very earliest stages of vascular development (E7.5) in mesodermal cells of the yolk sac mesenchyme. At E9.5, expression of VE-cadherin was restricted to the peripheral cell layer of blood islands that gives rise to endothelial cells. Hematopoietic cells in the center of blood islands were not labeled. At later embryonic stages, VE-cadherin transcripts were detected in vascular structures of all organs examined, eg, in the ventricle of the heart, the inner cell lining of the atrium and the dorsal aorta, in intersomitic vessels, and in the capillaries of the developing brain. A comparison with flk-1 expression during brain angiogenesis revealed that brain capillaries expressed relatively low amounts of VE-cadherin. In the adult brain, the level of VE-cadherin transcript was further reduced. By immunohistochemistry, murine VE-cadherin protein was detected at cell-to-cell junctions of endothelial cells. Overall, these data demonstrate that VE-cadherin is an early, constitutive, and specific marker of endothelial cells. This distinguishes this molecule from other cadherins and suggests that its expression is associated with the early assembly of vascular structures.

Influence of graft innervation on neovascularization of embryonic heart tissue grafted in oculo

We have shown that sympathetic denervation increases subendocardial capillarity during left ventricular hypertrophy. To determine the direct effects of sympathetic innervation on development of the coronary microvasculature in the absence of hemodynamic load, we grafted avascular fetal rat atrial or ventricular tissue into the anterior eye chamber of host rats which had undergone unilateral superior cervical gangliectomies. Innervation to the contralateral eye chamber remained intact. The grafts were harvested 14 or 35 days later, and volume densities of blood vessels, myocytes, and extracellular matrix were determined using standard point-counting techniques on low-power electron micrographs. Graft perfusion and metabolism were assessed simultaneously with thallium-201 and 2-[14C]deoxy-D-glucose uptake, respectively. Innervation did not significantly alter the vascular volume densities or cellular composition of atrial or ventricular tissue compared with noninnervated tissue after either 14 or 35 days in oculo. Similarly, innervation did not significantly alter graft perfusion or metabolism. We conclude that sympathetic innervation does not directly influence the growth of the microvasculature or myocardial metabolism in hemodynamically unloaded, developing heart tissue.

Inotropic effect of increasing concentration of Ca2+ in the fetal rat heart with retinoic acid-induced malformations

Cardiac malformations (pulmonary trunk stenosis, ventricular septal defect, and double outlet right ventricle) were induced by the administration of two doses of retinoic acid (RA) to Wistar rats on d 13 of pregnancy. Contractile performance of the isolated perfused rat heart and its inotropic response to Ca2+ (0.6-10.0 mmol.L-1) was studied in 20-d-old fetuses. The body weight of RA-exposed fetuses was significantly lower compared with controls. RA negatively influenced the contractile parameters of the fetal rat heart. The most pronounced effect was, except at a Ca2+ concentration of 2.5 mmol.L-1, observed at developed force at all other concentrations. Simultaneously, the sensitivity to Ca2+, expressed as the Ca2+ concentration at which 30% of maximum was attained, ws significantly lower in RA-exposed hearts. This implies that the malformed heart is more dependent on the extracellular sources of Ca2+.

Collagen production and replication by cardiac fibroblasts is enhanced in response to diverse classes of growth factors

The tissue distribution and cellular effects of platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), transforming growth factor beta-1 (TGF beta 1) and insulin-like growth factor 1 (IGF-1) suggest a potential role for these factors in cardiovascular matrix deposition. The objective of this study was to assess the capacity of these growth factors to promote cardiac fibroblast collagen production and replication in vitro which will lead to studies identifying their role in vivo during cardiac development and disease. Fibroblasts were isolated from fetal rat hearts by explant culture, and their response to growth factors was assessed with respect to fibroblast replication and collagen synthesis. Fibroblast replication was stimulated by PDGF and by bFGF.IGF-1 and TGF beta 1 had no effect on fibroblast replication. Collagen production was stimulated by all of the growth factors tested in order of potency TGF beta 1 > PDGF, IGF > bFGF. None of the growth factors affected the proportion of newly synthesized collagen rapidly degraded. We have shown that TGF beta 1, PDGF, bFGF and IGF-1 are all capable of increasing collagen deposition by cardiac fibroblasts by either stimulating fibroblast replication or collagen synthesis or both. The sensitivity of cardiac fibroblasts to these factors is consistent with their playing a role in the rapid changes in cardiac collagen deposition seen during development and disease.

Molecular cloning of human cardiac troponin T isoforms: expression in developing and failing heart

Troponin T, which links the troponin complex to tropomyosin, is found as multiple isoforms in the hearts of many animal species. Changes in isoform composition have been correlated with variation in myofilament sensitivity to calcium. In order to determine the origin of diversity of the cardiac troponin T (cTnT) isoforms indicated by existing protein data, we have determined the sequences and patterns of expression of mRNAs encoding troponin T in fetal and adult heart and those present in adult heart in end-stage failure. Three main regions of alternative splicing within the cTnT coding region were identified using reverse-transcriptase polymerase chain reaction (RT-PCR). Alternatively spliced RNAs are developmentally regulated and some of the fetal forms are expressed in adult failing heart. The molecular structure of the spliced regions was determined from cloned cDNAs and RT-PCR products. In the 5' region of the mRNA, isoforms are generated by the inclusion or exclusion of 15-, 3- and 27-nucleotide (nt) sequences and by the inclusion or exclusion of a separate 3-nt sequence. In the 3' region of the mRNA, alternative splicing involves a 9-nt sequence which can be present in full, in part or not at all. A further splicing site was identified in the central region involving a 234-nt sequence and resulting in rare but detectable mRNAs. This work demonstrates the complexity of cTnT RNA composition in human heart and provides the information necessary to address the function of cTnT isoforms in contraction.

Cardiac enzyme activities in fetal and adult pregnant and nonpregnant sheep exposed to high-altitude hypoxemia

We measured pyruvate kinase (PK), citrate synthase (CS), and lactate dehydrogenase (LDH) activities in the right and left ventricles of fetal, maternal, and nonpregnant adult sheep exposed to high altitude (3,820 m) for 112 days and compared them with control groups of animals kept at sea level. Enzymes were assayed by the spectrophotometric appearance of reaction products specific to each enzyme, and activity was expressed as micromoles per minute per gram of wet weight of tissue. In control sheep, CS activity was significantly higher in both ventricles of the pregnant and nonpregnant adult compared with the fetus. However, LDH and PK activities were only higher in the left ventricle of the nonpregnant adult compared with the fetus. Long-term hypoxemia significantly increased LDH activities in fetal (57 and 53%), pregnant adult (29 and 27%), and non-pregnant adult (25 and 24%) right and left ventricles, respectively. CS activities also increased in fetal (90 and 97%), pregnant adult (43 and 39%), and nonpregnant adult (46 and 48%) right and left ventricles, respectively. However, PK activity was not affected by altitude in any group of animals. In the fetal heart, which uses lactate as its primary metabolic fuel, these enzyme changes may help enhance aerobic energy production during hypoxemia. In the adult heart, which relies on free fatty acids as well as glucose for energy production, the significance of these enzyme changes is less clear.

Position independent expression and developmental regulation is directed by the beta myosin heavy chain gene's 5' upstream region in transgenic mice

Transgenic mice generated with constructs containing 5.6 kb of the beta myosin heavy chain (MyHC) gene's 5' flanking region linked to the cat reporter gene express the transgene at high levels. In all 47 lines analyzed, tissue-specific accumulation of chloramphenicol acetyltransferase was found at levels proportional to the number of integrated transgene copies. Deletion constructs containing only 0.6 kb of 5' upstream region showed position effects in transgenic mice and did not demonstrate copy number dependence although transgene expression remained muscle-specific. The 5.6 kb 5' upstream region conferred appropriate developmental control of the transgene to the cardiac compartment and directs copy number dependent and position independent expression. Lines generated with a construct in which three proximal cis-acting elements were mutated showed reduced levels of transgene expression, but all maintained their position independence and copy number dependence, suggesting the presence of distinct regulatory mechanisms.

Increased natriuretic peptides in fetal hearts of diabetic rats

The main purpose of these studies was to determine whether diabetic pregnancy altered maternal and fetal atrial natriuretic peptide (ANP). Diabetes was induced in rats by intravenous injection of 40 mg streptozotocin/kg on day 2 of gestation. Immunoreactive ANP in plasma, amniotic fluid and hearts on day 20 of gestation was measured by radioimmunoassay; fetal cardiac natriuretic peptides (ANP, proANP and BNP) were separated by reverse-phase high pressure liquid chromatography. Diabetes caused an increase in fetal plasma insulin, placental weight, amniotic fluid volume, the ratio of the fetal heart to body weight, maternal and fetal plasma ANP, fetal cardiac ANP and fetal cardiac BNP. It is suggested that the maternal diabetes-induced increase in fetal ANP might be related to fetal myocardial hypertrophy and could contribute to hydramnios.

Intermediate filaments in cardiac myogenesis: nestin in the developing mouse heart

By using immunohistology combined with immunoblotting, cell culture, and RT-PCR, we show that the intermediate filament protein nestin is transiently expressed in the midembryonic mouse heart. Monoclonal antibody (MAb) Rat-401, known to react with nestin in neural and skeletal muscle cells, was also found to react with ventricular and atrial cells throughout the mouse heart from embryonic day 9 (E9) through E10.5. Both before (E8.5) and after (E11-adult) this brief period, staining with Rat-401 was absent from atrial and ventricular myocytes. To evaluate the specificity of staining with MAb Rat-401 in the heart, we used immunoblotting, cell culture, and RT-PCR to verify that the authentic nestin protein and mRNA were expressed in cardiomyocytes of the E10 mouse. Nestin expression is the first molecular marker for this distinct midembryonic period of heart development.

Developmental- and tissue-specific DNA methylation patterns and expression of rat gamma-glutamyltransferase

In order to investigate the mechanism controlling gamma-glutamyltransferase (GGT) gene expression in adult and fetal rat tissues, as well as in H5-6 and Fao hepatoma cells, the relationship between DNA methylation and GGT expression was studied. Southern blot analysis of genomic DNA digested with MspI and HpaII showed that highly methylated DNA patterns observed especially in adult liver, heart and Fao cells as well as in fetal kidney, pancreas, lung and heart correlate well with low GGT activity determined in these tissues. In contrast, hypomethylated GGT DNA was found in fetal liver, adult kidney and lung, the tissues highly expressing GGT activity. So, observed developmental- and tissue-specific changes in the GGT gene methylation seem to play an important role in the control of GGT gene transcription.

Alterations in fetal and placental deoxyribonucleic acid synthesis rates after chronic fetal placental embolization

OBJECTIVE

Fetal growth and development are closely related to normal placental growth and function. We performed a study to determine the effect of a 10-day period of fetal hypoxemia induced by umbilical-placental hypoperfusion on tissue deoxyribonucleic acid synthesis rates in the 0.84 to 0.91 of gestation ovine fetus and placenta.

STUDY DESIGN

Daily fetal placental embolization was performed in four chronically catheterized sheep fetuses until fetal arterial oxygen content decreased by approximately 30% compared with preembolization values. Five control fetuses received vehicle only. On experimental day 10, the deoxyribonucleic acid synthesis rate was determined by injecting tritiated thymidine (1 mCi/kg) intravenously approximately 8 hours before the end of the study.

RESULTS

Fetal arterial oxygen decreased from 3.2 +/- 0.1 (SEM) mmol/L preembolization to 2.2 +/- 0.2 mmol/L on day 10 (p < 0.001) and remained unchanged in controls. On day 10 deoxyribonucleic acid synthesis rates were significantly reduced in embolized fetuses compared with controls, by 38% in cotyledons (83.0 +/- 15.1 vs 133.7 +/- 9.9 disintegrations/min/micrograms deoxyribonucleic acid, p < 0.05), 28% in the left ventricular wall (36.8 +/- 3.7 vs 51.0 +/- 4.7 disintegrations/min/micrograms deoxyribonucleic acid, p < 0.05), and 45% in the quadriceps muscle (15.4 +/- 4.0 vs 28.1 +/- 3.0 disintegrations/min/micrograms deoxyribonucleic acid, p < 0.05). Tritiated thymidine autoradiography demonstrated that cotyledonary deoxyribonucleic acid synthesis occurred exclusively in the fetal trophoblasts cells.

CONCLUSION

We concluded that a reduction in cotyledonary, quadriceps muscle, and left ventricular myocardium deoxyribonucleic acid synthesis rates are the earliest adaptive mechanisms of fetal growth associated with development of umbilical-placental insufficiency. We speculate that alteration in the myocardial deoxyribonucleic acid synthesis rate could be a major contributing factor in the deterioration of fetal myocardial function associated with increased placental vascular resistance.

Human cardiac troponin T: cloning and expression of new isoforms in the normal and failing heart

Troponin T, like many myofibrillar proteins, exists as multiple isoforms encoded by distinct genes or generated by splicing of the same primary RNA transcript. We have previously cloned the first human cardiac troponin T (cTnT) cDNA and showed the differential expression of cTnT in cardiac and skeletal muscle during ontogenic development. In this work we located the human cTnT gene by means of fluorescent in situ hybridization to 1q32 and, by sequencing thirteen cDNAs isolated from a human fetal heart cDNA library, identified three new isoforms resulting from specific combinations of three variable regions in human cTnT cDNA. The first variable region is a 30-bp box located at the 5' end of the cDNA, which can be excised either totally or only from the first 3 bp onwards; the second is a codon which can be completely excised; and the third is a 9-bp box in the 3' half of the cDNA, which can also be excised either totally or only from the first 3 bp. The existence of the corresponding RNAs in fetal and adult ventricles was confirmed by RNase protection studies. No accumulation of the fetal isoforms was found in failing ventricles compared with controls.

Molecular basis of human cardiac troponin T isoforms expressed in the developing, adult, and failing heart

Cardiac troponin T (cTnT), a protein essential for calcium-regulated myofibrillar ATPase activity, is expressed in the human heart as four isoforms (cTnT1 through cTnT4, numbered in the order of decreasing molecular size). The expression of these isoforms at the protein level has previously been found by us to differ in the normal and failing adult and fetal human heart. In the present study, we have cloned and sequenced four full-length cDNAs corresponding to the four native cTnT protein isoforms and have expressed these cDNAs in an in vitro transcription and translation system. The cDNAs differ by the variable inclusion of a 15- and a 30-nt exon in the 5' half of the coding region. These cDNAs yielded proteins that comigrate with the native isoforms, cTnT1 through cTnT4. Polyclonal antisera, raised against a synthetic peptide corresponding to the 10-residue peptide encoded by the 30-nt exon, reacted with the two human isoforms largest in molecular size (cTnT1 and cTnT2) and the two largest cTnT isoforms of the rabbit and rat. The isoforms cTnT1 and cTnT2, containing either both peptides encoded by the 30- and 15-nt exons or the peptide encoded by the 30-nt exon alone, are expressed in the fetal heart, with cTnT2 being expressed at a very low level. cTnT4, lacking both of these sequences, is expressed in the fetal heart and is reexpressed in the failing adult heart, whereas cTnT3, containing the 5-residue peptide, is the dominant isoform in the adult heart.(ABSTRACT TRUNCATED AT 250 WORDS)

Protection of the chronic hypoxic immature rat heart during global ischemia

The benefit of cardioplegic cardiac arrest for the protection of immature myocardium is controversial. We therefore investigated the efficacy of (1) topical hypothermia alone, (2) slow cooling by coronary perfusion hypothermia, and (3) cardioplegic cardiac arrest for the protection of isolated immature rats hearts (28 days) during 8 hours of global ischemia at 10 degrees C. The study was conducted in hearts from rats that were kept hypoxemic by lifelong exposure to simulated high altitude. Left ventricular function, endothelial function, the metabolic status, and the extent of myocardial injury were all assessed. Topical hypothermia provided superior protection in hypoxic hearts, with recovery of the maximum developed left ventricular pressure by 70.6% +/- 18.0% (mean +/- standard deviation) of its preischemic value (p < 0.01 versus slow cooling and versus cardioplegic protection). The same pattern of recovery was observed among control hearts. The degree of recovery of endothelial function after sole topical hypothermia measured 54% +/- 36% in hypoxic hearts and 62% +/- 37% in control hearts, but was not recordable in any of the other groups. Creatine kinase leakage and the myocardial high-energy content did not differ significantly among any of the groups. Rapid cooling by topical hypothermia alone provides superior protection in chronic hypoxic, immature rat hearts versus the protection conferred by slow cooling. St. Thomas' Hospital cardioplegic solution II does not afford additional protection. Endothelial injury caused by cold asanguineous perfusates, including cardioplegia, interferes with the recovery of vascular function, which, in turn, may limit mechanical function.

Temporally and spatially restricted expression of apolipoprotein J in the developing heart defines discrete stages of valve morphogenesis

During cardiac valve morphogenesis, a series of interactions between the mesodermal-derived myocardium and the overlying endothelium lead to condensed leaflet structure formation. At the atrioventricular (AV) canal, endocardial cells are transformed by specialized underlying myocardial cells into endocardial cushions, and then remodeled into mitral and tricuspid valves. Aortic and pulmonary valves develop by a similar mechanism in the primitive outflow tract. Few genes exhibit restricted spatiotemporal expression in these critical embryonic structures, thus limiting the clues to the sequence of molecular events necessary for valvulogenesis. Apolipoprotein J (ApoJ), a secreted glycoprotein expressed in a variety of cell types at tissue interfaces, exhibits a highly restricted and dynamic expression pattern in the developing heart. ApoJ transcripts were detected in mice at day 9.0 of gestation in the wall of the developing truncus arteriosus. By day 10, intense signal occurred in a thin layer of myocardial cells adjacent to developing endocardial cushions of both atrioventricular canal and truncus arteriosus. No apoJ mRNA was present in the overlying endocardial cushions until day 13.5 when prevalvular condensation begins. Intense expression occurred in the stromal connective tissue throughout leaflet formation. The highly restricted spatiotemporal expression pattern of apoJ in the developing heart implicates its role in the morphogenesis of the AV canal and outflow tract into cardiac valves.

Cloning and expression of fibroblast growth factor receptor-1 isoforms in the mouse heart: evidence for isoform switching during heart development

Basic (b) fibroblast growth factor (FGF) mediates various biological responses including mitogenesis and angiogenesis by binding to specific cell surface receptors of the tyrosine kinase family. The bFGF receptor-1 FGFR1) exists in short and long isoforms due to alternate RNA splicing. Minor alterations in the amino acid sequence have also led to reports of different FGFR1 isoforms in different tissues even in the same species. In the absence of any sequence for heart FGFR1 and accumulating evidence for a role of bFGF in heart growth and differentiation, we cloned FGFR1 from embryonic mouse hearts. Reverse transcriptase-polymerase chain reaction (RT-PCR) was used to generate full-length short (2259 base pairs) and long (2526 base pairs) forms of FGFR1 cDNAs which generated 86 and 102 kDa proteins, respectively, following in vitro translation. Embryonic mouse heart FGFR1 differed by seven amino acids from the reported sequence for mouse neuroepithelial FGFR1 and appeared more similar to human placental FGFR1. A single FGFR1 transcript of approximately 4.3 kb was seen in RNA isolated from embryonic as well as adult mouse hearts. There was a decrease (approximately 8.5-fold) in FGFR1 RNA levels in the adult. The majority of FGFR1 transcripts in the adult as well as embryonic heart contained exon IIIc (FGFR1-IIIc) which is associated with isoforms that display the highest affinity for bFGF. However, the relative ratio of short versus long FGFR1 RNA expression was 0.5 in the embryonic heart compared to 5.9 in the adult heart. These results indicate that: (i) structurally distinct short and long FGFR1 isoform RNAs are expressed in the embryonic and adult heart; (ii) FGFR1-IIIc is the major form of receptor expressed in the embryonic as well as adult heart; (iii) the transition from the embryo to the adult stage is associated with a decrease but not absence of FGFR1 RNA expression; and (iv) long FGFR1-isoforms are more abundant in the embryo while short FGFR1 isoforms predominate in the adult.

A catalogue of genes in the cardiovascular system as identified by expressed sequence tags

The heart, which is composed of all the cellular components of the circulatory system, is a representative organ for obtaining genes expressed in the cardiovascular system in normal and disease states. We used partial sequences of cDNA clones, or expressed sequence tags, to identify and tag genes expressed in this organ. More than 3500 partial sequences representing > 3000 cDNA clones have been obtained from either the 5' or 3' end of inserts derived from human heart cDNA libraries. Of 3132 cDNA clones analyzed by sequence similarity searching against the GenBank/EMBL data bases, 1485 (47.4%) were found to represent additional, previously undiscovered genes, whereas 267 clones were matched to human brain expressed sequence tags. Clones matching to known genes were catalogued according to their putative structural and cellular functions. cDNA probes from reverse-transcribed mRNAs of fetal and adult hearts were used to study differential expression of selected clones in cardiac development. Cataloguing genes expressed in the heart may provide insight into the genes involved in health and cardiovascular disease.

Developmental and gestational changes of phosphoethanolamine and taurine in rat brain, striated and smooth muscle

Concentrations of taurine and phosphoethanolamine in rat smooth (intestinal and uterine), skeletal and cardiac muscle, and brain have been determined, using high-performance liquid chromatography (HPLC), to examine possible interrelationships in their tissue content. Concentrations were determined in fetal and neonatal samples, as well as in adult tissue, to investigate whether phosphoethanolamine and taurine levels are influenced by developmental state. The effect of gestational state was also studied. A marked decrease in cerebral phosphoethanolamine concentration during development was found together with a concomitant decrease in striated muscle but not in the two smooth muscles studied. A rise in uterine phosphoethanolamine during the early postpartum period confirmed previous NMR data. This occurred only in the uterus, suggesting it is specific to the process of involution within the myometrium. Taurine concentrations showed no consistent pattern of change with postnatal development. In adult animals, the highest levels of taurine were found in cardiac muscle. Pregnancy was associated with a fall in taurine concentration in all tissues, suggesting an influence of steroid hormones. As taurine is cotransported with Na+ in many systems it may be related to the increased water retention seen in pregnancy. It is concluded that marked changes in phosphoethanolamine and taurine levels occur during development and gestation, but that the changes are not interdependent, i.e., the changes are tissue specific.

Mitochondrial gene expression during bovine cardiac growth and development

The expression of both mitochondrial and nuclear genes encoding enzymes involved in electron transport and oxidative phosphorylation was examined in bovine cardiac tissue during early growth, development and aging. The steady state level of mRNAs for mitochondrial genes including ATPase 6. COXII and cyt b increased 2.5-4-fold relative to early fetal levels in late fetal and young adult tissues and showed a marked decline (30-50%) in older adult tissues. Similar results were found with the nuclear genes, COXVB and ATP-beta synthase showing coordinate regulation of the two genomes. An increase in mtDNA copy number correlated with the increase in transcript level. Enzyme activity levels for NADH dehydrogenase and cytochrome c oxidase showed a similar trend, albeit of lesser magnitude. These activity levels contrasted with the activity level of an entirely nuclear-encoded mitochondrial enzyme, citrate synthase, which increased not only throughout development but in the older adult tissue. This study indicates that there is a pattern of increasing mitochondrial and nuclear gene expression for OXPHOS enzymes in developing cardiac tissue and decreasing OXPHOS gene expression in the aging heart.

Distribution of beta-1 integrin in the developing rat heart

Cell-cell and cell-matrix interactions play critical roles in various developmental processes including differentiation, proliferation, and migration. Members of the integrin family of cell surface components are important mediators of these cell-extracellular matrix (ECM) contacts or interactions. The ECM provides signals to individual cells essential for development and differentiation and plays essential roles in establishing and maintaining the complex structure of the vertebrate heart. Integrins provide a fundamental link for transduction of developmental signals to cells. Integrin expression by cardiac myocytes is altered during neonatal heart development and disease; however, little is known regarding the spatial and temporal patterns of integrin expression during embryonic and fetal heart development. Essential to understanding the role of integrins in the organization of the heart, the present studies have localized beta-1 integrin protein and mRNA in fetal and neonatal rat hearts. Beta-1 integrin is predominantly found in regions of remodeling (trabeculae) in the early heart (10-13 days of gestation). Later in development (15 days of gestation onward), beta-1 integrin is abundant in regions containing an elaborate ECM, such as the valves. These studies further support the hypothesis that the expressions of integrins and ECM are coordinately regulated in the developing heart.

Gene expression of atrial natriuretic factor in ovine fetal heart during development

OBJECTIVE

The present study quantified the abundance of atrial natriuretic factor (ANF) messenger RNA (mRNA) and determined the developmental pattern of ANF gene expression in the four cardiac chambers of the ovine fetus during the last two-thirds of gestation.

METHODS

Twenty-one fetuses from 13 time-dated pregnant ewes at gestational ages of 60-145 days were used for this study. Total RNA from fetal atria and ventricles was extracted and ANF mRNA was analyzed by Northern blotting. The ANF mRNA signal was quantified by light densitometry. The abundance of ANF mRNA in the cardiac chambers across gestational ages was analyzed by linear regression analysis and one-way analysis of variance.

RESULTS

Atrial natriuretic factor mRNA was much more abundant in the atria than in the ventricles of all fetuses at each gestational age studied. Atrial ANF mRNA levels were lowest in the younger fetuses at 60 days and increased with advancing gestation. Ventricular ANF mRNA levels were highest in fetuses at 60 days and decreased to almost nondetectable levels near term. No difference in ANF mRNA abundance was noted between the right and left atria or the right and left ventricles at each gestational age.

CONCLUSION

A developmental pattern of ANF gene expression is demonstrated in the ovine fetal heart during the last two-thirds of gestation. This pattern shows that atrial ANF mRNA abundance increases while ventricular abundance decreases as the fetus matures. Expression of the ANF gene in the fetal period may be regulated developmentally or induced by cardiovascular changes in utero.

[The development of cardiac function and its relation to metabolism during the perinatal period]

Several aspects of heart development are reviewed such as development of excitability and electrical and mechanical activity of the heart. Special emphasis is made upon metabolic changes during heart development and its possible consequences on electrical and mechanical activity.

Developmental changes in atrial natriuretic factor content and localization of its messenger ribonucleic acid in ovine fetal heart

OBJECTIVES

The current study investigated the developmental changes in atrial natriuretic factor peptide content and messenger ribonucleic acid localization in the atria and ventricles of the ovine fetus throughout the second half of gestation.

STUDY DESIGN

Ovine fetuses from 67 to 146 days' gestation (term 147 days) and newborn lambs were used for the study. Tissue atrial natriuretic factor contents were determined by radioimmunoassay, and atrial natriuretic factor messenger ribonucleic acid distribution was determined by in situ hybridization.

RESULTS

In fetal atria, atrial natriuretic factor peptide levels were much greater than those in the ventricles. The levels in the atria increased with advancing gestation from 70 to 140 days, reflecting an increase in weight of the atrial chambers. A similar trend was not observed in the ventricles. In the atria, atrial natriuretic factor peptide content (per unit protein) reached high levels at 100 to 110 days' gestation; this was associated with an increase in level of atrial natriuretic factor gene expression. In the ventricles, atrial natriuretic factor peptide content and gene expression were very low throughout the second half of gestation, except for a peak in content that occurred at 100 days. Atrial natriuretic factor messenger ribonucleic acid abundance was much greater in the atria than in the ventricles in fetuses from 90 to 130 days' gestation. The distribution of atrial natriuretic factor messenger ribonucleic acid was homogeneous throughout the thickness of the atria and ventricles of the fetal heart.

CONCLUSION

During the second half of gestation in the ovine fetus, the expression of atrial natriuretic factor messenger ribonucleic acid in the atria and ventricles paralleled the appearance of the peptide.

Heart-specific targeting of firefly luciferase by the myosin light chain-2 promoter and developmental regulation in transgenic mice

Based on hybridization studies indicating constitutive expression levels of the endogenous myosin light chain-2 (MLC-2) gene in embryonic, fetal, and adult myocardium, a model system for selective targeting of genes to the heart of transgenic mice has been developed. A 2.1-kb DNA fragment of the 5' flanking region of the rat cardiac MLC-2 gene was fused to the firefly luciferase reporter gene and introduced into fertilized mouse oocytes. In four independent transgenic mouse lines, the expression of the MLC-2-luciferase fusion gene was found exclusively in heart muscle. In contrast to the endogenous MLC-2 gene, no luciferase activity was detectable in slow-twitch skeletal muscle or any other tissue of transgenic mice. This result suggests that the 2.1-kb DNA fragment of the 5' flanking region of the cardiac MLC-2 gene contains the regulatory elements required for selective gene expression in cardiac myocytes in vivo. In contrast to the endogenous steady-state MLC-2 expression during development, transgenic luciferase activity was 10-fold higher during embryogenesis, when formation of the ventricular loop and septum takes place. The enhanced luciferase activity in early heart development may suggest a growth-dependent control mechanism, involving either transcriptional or posttranscriptional regulation. In conclusion, this model system with the 2.1-kb ventricle-specific MLC-2 promoter sequence should facilitate the overexpression of gene products in the developing and mature heart muscle and further elucidate molecular mechanisms of myocardial diseases such as cardiomyopathies.

Molecular cloning and developmental expression of human cardiac troponin T

We have isolated a full-size cDNA coding for cardiac troponin T (cTnT) from a human adult heart library, using a slow skeletal TnT probe. This cDNA detected a 1.2 kb mRNA in fetal and post-natal human heart, the amount of which increased during ontogenic development. Interestingly, a similar transcript was coexpressed in fetal skeletal muscle, together with the 0.9 kb slow skeletal muscle mRNA, and its expression was down-regulated during further development.

Development of the collagen network of the human fetal myocardium: an immunohistochemical study

We have studied the development of the collagen network within the ventricular myocardium of sixteen human fetuses of gestational age 12 weeks to 22 weeks. Using the technique of indirect immunohistochemistry with antibodies raised against collagen Types I and III, we have demonstrated that the 12 week old heart has a rudimentary endomysial collagen structure which coexpresses both collagen types. Perimysial structures evolve with the onset of the second trimester and are collagen III positive before expressing collagen Type I. No differences in collagen deposition were detected between the right and left ventricular free walls, but collagen content of the interventricular septum appeared relatively high and expressed in thick highly organised fibrils. Transventricular gradients of collagen distribution were seen for both collagen isoforms which persisted with increased age. Intraventricular differences in collagen deposition were marked due to the insertion of the atrioventricular valves and tension apparatus. These findings suggest that the collagen network of the fetal myocardium is composed of copolymer fibrils, possibly rich in collagen type III which evolve at a time when the mechanical efficiency of the fetal heart must improve to keep pace with the escalating demands of a rapidly growing body.

Expression and accumulation of interstitial collagen in the neonatal rat heart

Significant physiological changes occur in the heart following birth including increased arterial blood pressure and heart rate. Concurrently, biochemical and structural alterations are evident in the neonatal heart in response to these dynamic physiological properties. Prominent among these is the elaborate development of the cardiac extracellular matrix, composed primarily of interstitial collagen. The collagenous fibers, together with other matrix components, form an elastic, stress-tolerant network which functions in the dissipation of force throughout the heart wall. The present studies have used biochemical and molecular techniques to show the temporal and spatial patterns of interstitial collagen accumulation and expression during late fetal and neonatal development of the rat heart. The use of biochemical and particularly molecular methodologies allows the analysis of the expression of matrix components at a resolution previously not attained by structural studies alone. These data show relative increases in interstitial collagen immediately following birth as well as spatial differences in collagen mRNAs within the heart. The data presented provide further evidence for a role of mechanical stimulation in the regulation of collagen gene expression during this period of heart development.

Beta-blockers and thimidine-3H captation in the human foetal heart

In the present research the effects that the beta-blockers may express on the development of foetal myocardium have been studied "in vivo" and "in vitro". On the basis of previous researches (7), the beta-blocker effects have been observed by utilizing low doses in comparison with the therapeutic levels. Wistar rats affected by a surgically elicited renal hypertension and "in vitro" cultivated human heart buds, represent the two models utilized for our research. In the first model the low dose of beta-blocker, able to reduce the plasmatic renin activity (PRA) but unable to determine normal conditions of blood pressure, evokes an eccentric dilation of ventricular room and severe alterations of the myofilament pattern. Also in human heart buds cultivated in presence of beta-blocker at low levels of concentration in the cultural media, deep changes in the structural and ultrastructural pictures of the myocardium can be observed; the major changes consist in a particular and constant accumulation of glycogen in the cytoplasm of the myoblasts whose organules are pushed to the periphery of the cells. At electron microscopic level, disorders affecting the myofilaments and their genesis are present, while the intercellular junctions seem to be blocked in their ability to transfer into functional contacts; in fact, they appear only as mechanical junctions. Utilizing an autoradiographic technique the above described alterations seem to derive from a different degree of captation of thimidine-methyl-3H that appears to be higher in the specimens exposed to high dose of beta-blocker. Finally, all the results obtained seem to indicate the negative influence that the studied drugs can express on the histogenesis and the metabolism of myocardial cells.

The relationship between fetal arterial oxygen saturation and heart and skeletal muscle myoglobin concentrations in the ovine fetus

Myoglobin, a hemoprotein found in abundance in the muscle of postnatal animals, increases in concentration in response to hypoxia, thereby protecting tissue from damage. Fetuses exposed to intrauterine hypoxemia are also susceptible to organ damage, but the response of fetal muscle myoglobin to hypoxemia is unknown. To study whether fetal muscle myoglobin concentrations are elevated following intrauterine hypoxemia, we exposed eight chronically catheterized late gestation sheep to a wide range of fetal oxygen levels over 15 to 30 days and correlated the level of fetal oxygenation with heart and skeletal muscle myoglobin concentrations measured at sacrifice. A lower level of fetal oxygenation, expressed as the integrated area under the arterial saturation (SaO2)-time curve, was associated with greater myocardial myoglobin concentration (r = 0.90; P < 0.01). This relationship was not observed for skeletal muscle (r = 0.43; P = ns). A lower level of fetal oxygenation was associated with lower myoglobin:iron (w/w) ratio in skeletal muscle (r = 0.71; P < 0.03), implying less incorporation of iron into myoglobin. A similar relationship was not apparent for cardiac tissue. The higher myocardial myoglobin concentrations found in the more hypoxic fetuses were consistent with previous observations in postnatal animals. This likely represents an intracellular compensatory mechanism for sustaining short-term mitochondrial oxygen delivery in a critical organ with a high rate of oxygen consumption. The lack of myoglobin responsiveness to hypoxia in fetal skeletal muscle may be due to its much lower oxygen consumption rate and activity level.

The role of dicarboxylic anion transport in the slower Ca2+ uptake in fetal cardiac sarcoplasmic reticulum

Sarcoplasmic reticulum- (SR-)mediated Ca2+ transport is slower in the fetal heart compared with the adult. Virtually all previous studies of cardiac SR Ca2+ transport were performed in the presence of oxalate, a dicarboxylic anion that is cotransported with Ca2+ in skeletal muscle SR. If anion transport is developmentally regulated in cardiac SR, this could explain, in part, the previously reported results. The purposes of this study were to establish the presence of an SR dicarboxylic anion transport process in the rabbit heart and to determine if the perinatal changes in SR Ca2+ transport occur in a dicarboxylic anion-dependent and/or independent manner. In isolated fetal and adult rabbit cardiac SR membranes, we measured Ca2+ ATPase rates and 45Ca2+ uptake in the presence of the dicarboxylic anions maleate and succinate compared with the zwitterionic buffer PIPES, to which cardiac SR is essentially impermeable. We also measured 14C-succinate uptake by fetal and adult SR membranes. Anion-independent Ca2+ ATPase activity and net 45Ca2+ uptake were significantly lower in the fetal SR membranes than in the adult. Maleate and succinate increased the Ca2+ ATPase rates in the fetal and adult SR, but the effect was significantly greater in the adult. Maleate and succinate stimulated earlier attainment of maximal net Ca2+ uptake in the fetal and adult SR, suggesting that these dicarboxylic anions stimulated the rate of Ca2+ accumulation. Maleate and succinate significantly increased the maximal net Ca2+ uptake in the adult SR, but not in the fetus. The percentage of stimulation of Ca2+ uptake by maleate and succinate was similar in the fetal and adult SR.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of dystrophin gene transcripts during mouse embryogenesis

The spatial and temporal expression of the dystrophin gene has been examined during mouse embryogenesis, using in situ hybridization on tissue sections with a probe from the 5' end of the dystrophin coding sequence. In striated muscle, dystrophin transcripts are detectable from about 9 d in the heart and slightly later in skeletal muscle. However, there is an important difference between the two types of muscle: the heart is already functional as a contractile organ before the appearance of dystrophin transcripts, whereas this is not the case in skeletal muscle, where dystrophin and myosin heavy chain transcripts are first detectable at the same time. In the heart, dystrophin transcripts accumulate initially in the outflow tract and, at later stages, in both the atria and ventricles. In skeletal muscle, the gene is expressed in all myocytes irrespective of fiber type. In smooth muscle dystrophin transcripts are first detectable from 11 d post coitum in blood vessels, and subsequently in lung bronchi and in the digestive tract. The other major tissue where the dystrophin gene is expressed is the brain, where transcripts are clearly detectable in the cerebellum from 13 d. High-level expression of the gene is also seen in particular regions of the forebrain involved in the regulation of circadian rhythms, the endocrine system, and olfactory function, not previously identified in this context. The findings are discussed in the context of the pathology of Duchenne muscular dystrophy.

Effect of caffeine on expression of cardiac myosin heavy chain gene in adult hypothyroid and fetal rats

Changes in cardiac myosin heavy chain (MHC) gene expression and isozyme transitions have been shown to be caused by developmental changes, hemodynamic overload, or the activity of various hormones. In this study, to examine whether caffeine, which has teratogenic effects on the fetal cardiovascular system, causes the distribution of cardiac MHC phenotype and, if so, to evaluate the mechanisms of the distribution of cardiac MHC phenotype by caffeine, we examined the effects of caffeine, theophylline, and cAMP on the cardiac MHC isoform transitions at the gene and protein levels using hypothyroid adult rats. Furthermore, we examined the expression of alpha- and beta-MHC gene in cardiac muscles of fetuses whose dams had received caffeine. The results showed that caffeine, theophylline, and cAMP caused accumulations of alpha-MHC mRNA and MHC isozyme V1. Furthermore, in the fetal hearts, it was recognized that caffeine induced an accumulation of alpha-MHC gene expression, as was also seen in the dams. However, this effect of caffeine on the heart was stronger in the fetus than in the dam. Intracellular cAMP concentration was increased by the administration of caffeine, theophylline, or cAMP, and the levels showed a positive correlation with those of alpha-MHC mRNA. These results suggest that the induction of alpha-MHC mRNA expression by the administration of caffeine may be induced by an increase in intracellular cAMP concentration.

tek, a novel tyrosine kinase gene located on mouse chromosome 4, is expressed in endothelial cells and their presumptive precursors

A search for protein tyrosine kinases expressed during murine cardiogenesis resulted in the isolation of a novel tyrosine kinase, designated tek, which maps to mouse chromosome 4 between the brown and pmv-23 loci. The deduced amino acid sequence of tek predicts that it encodes a putative receptor tyrosine kinase that contains a 21 amino acid kinase insert and which is most closely related in its catalytic domains to FGFR1 and the product of the ret proto-oncogene. In situ hybridization analysis of adult tissues, as well as sectioned and whole-mount embryos, showed that tek is specifically expressed in the endocardium, the leptomeninges and the endothelial lining of the vasculature from the earliest stages of their development. Moreover, examination of the morphology of tek-expressing cells, and staging of tek expression relative to that of the endothelial cell marker von Willebrand factor, revealed that tek is expressed prior to von Willebrand factor and appears to mark the embryonic progenitors of mature endothelial cells. tek encodes a novel putative receptor tyrosine kinase that may be critically involved in the determination and/or maintenance of cells of the endothelial lineage.

Myocardial cholinergic signaling changes with age

We examined the linkage of cholinergic receptors to the phosphoinositide signaling pathway to elucidate one facet of the autonomic response mechanism in fetal and adult sheep. Cholinergic stimulation with carbachol increases the production of 3H-inositol mono-, bis-, and trisphosphates in a time- and concentration-dependent manner in both fetal and adult myocardium. However, the maximal stimulation of inositol polyphosphates above basal activity was much greater in fetal (120 +/- 11%) than in adult (20 +/- 7%) myocardium (mean +/- SEM). Saturation binding analysis of myocardial muscarinic receptors using 3H-N-methylscopolamine revealed significantly higher receptor concentration in fetal (240 +/- 25 fmol/mg protein) than in adult (78 +/- 15 fmol/mg protein) myocardium (mean +/- SEM). Binding competition studies revealed a pattern of selectivity-atropine less than 4-diphenylacetoxy-N-methylpiperidine methiodide less than pirenzepine less than or equal to (4-hydroxy-2-butynyl)-1-trimethylammonium m-chlorocarbanilate chloride less than or equal to 11-2[[2-[(diethylamino)-methyl]-1-piperidinyl]acetyl]-5, 11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepine-6-one 116-compatible with the presence of muscarinic receptor (MR)2, MR3, and/or MR5 subtypes. Receptor subtype determination by Northern blot analysis revealed mRNA specific for the MR2 subtype in both fetal and adult myocardium, although expression was greater in fetal heart. We conclude that decreases in MR2 subtype protein and mRNA levels parallel the age-related decrease in carbachol-stimulated PLC activity. Our studies demonstrate differences between fetal and adult myocardium in the concentration of muscarinic cholinergic receptors and their linkage to a putative calcium mobilizing signaling pathway and suggest that this pathway may play a different role in the fetus than in the adult.(ABSTRACT TRUNCATED AT 250 WORDS)

Mutually exclusive action of cationic veratridine and cevadine at an intracellular site of the cardiac sodium channel

Veratridine modification of Na current was examined in single dissociated ventricular myocytes from late-fetal rats by applying pulses to -30 mV for 50 ms every 2 or 5 s from a holding potential of -100 mV (20 degrees C) and measuring amplitude, Itail, and time constant, tau tail, of the post-repolarization inward tail current induced by the alkaloid. Increasing the pH of a 30 microM veratridine superfusate from 7.3 to 8.3 (which increases the fraction of uncharged veratridine molecules from 0.5 to 5% while decreasing that of protonated molecules from 99.5 to 95%) increased Itail by a factor of 2.5 +/- 0.5 (mean +/- SEM; n = 3). Switching from 100 microM veratridine superfusate at pH 7.3 to 10 microM at pH 8.3 did not affect the size of Itail (n = 4). Intracellular (pipette) application of 100 microM veratridine at pH 7.3 or 8.3 produced small Itail's suggesting transmembrane loss of alkaloid. If this was compensated for by simultaneous extracellular application of 100 microM veratridine at a pH identical to intracellular pH, Itail (measured relative to the maximum amplitude induced by a combination of 100 microM veratridine and 1 microM BDF 9145 in the same cell) at pHi 7.3 did not significantly differ from that at pHi 8.3 (84 +/- 4 vs. 70 +/- 6%; n = 3 each). Results from six control cells and five cells subjected to extra- and/or intracellularly increased viscosity by the addition of 0.5 or 1 molal sucrose showed that increasing intracellular viscosity 1.6- and 2.5-fold increased tau tail 1.5- and 2.3-fold, respectively, while a selective 2.5-fold increase of extracellular viscosity did not significantly affect tau tail.(ABSTRACT TRUNCATED AT 250 WORDS)