Search PubMed⌕ Search

Biomedical subjects

A F Moorman

Publications and source records attributed to A F Moorman.

At least 91 records · Page 5Linked to original sources
PubMed1995

The spatio-temporal control of the expression of glutamine synthetase in the liver is mediated by its 5'-enhancer.

In previous studies of the glutamine synthetase gene, the promoter and two enhancer elements, one in the upstream region and one within the first intron, were identified. To analyze the role of the far-upstream enhancer element in the regulation of the expression of the glutamine synthetase gene, two classes of transgenic mice were generated. In GSK mice, the basal promoter directs the expression of the chloramphenicol acetyltransferase reporter gene. In GSL mice reporter gene expression is driven, in addition, by the upstream regulatory region, including the far-upstream enhancer. Whereas chloramphenicol acetyltransferase expression was barely detectable in GSK mice, high levels were detected in GSL mice. By comparing chloramphenicol acetyltransferase expression with that of endogenous glutamine synthetase in GSL mice, three groups of organs were distinguished in which the effects of the upstream regulatory region on the expression of glutamine synthetase were quantitatively different. The chloramphenicol acetyltransferase mRNA in the GSL mice was shown to be localized in the pericentral hepatocytes of the liver. The developmental changes in chloramphenicol acetyltransferase enzyme activity in the liver were similar to those in endogenous glutamine synthetase. These results show that the upstream region is a major determinant for three characteristics of glutamine synthetase expression: its organ specificity, its pericentral expression pattern in the liver, and its developmental appearance in the liver.

Aging↗
PubMed1995

The immunohistochemical localization of the non-specific lipid transfer protein (sterol carrier protein-2) in rat small intestine enterocytes.

A 13 kDa protein was isolated from rabbit small intestine brush-border membrane vesicles that was postulated to be involved in intestinal phosphatidylcholine (PC) and cholesterol uptake. This protein has cholesterol and PC-transfer activity in vitro (Turnhofer, H. et al. (1991) Biochim. Biophys. Acta 1064, 275-286) and has a molecular mass and isoelectric point similar to that of the non-specific lipid transfer protein (nsL-TP, identical to sterol carrier protein-2). In addition, the first 28 N-terminal amino acid residues of the 13 kDa protein are nearly identical to nsL-TP from different species (Lipka, G. et al. (1995) J. Biol. Chem. 270, 5917-5925). In view of its possible role in intestinal lipid absorption, the localization of nsL-TP in rat small intestine was investigated using immunohistochemistry and immunoblotting. It is shown that nsLTP is predominantly localized in a subapical zone of the enterocyte but not in the brush-border membrane, thereby excluding a role in lipid uptake of this protein at the level of the plasma membrane. nsL-TP co-localized with the peroxisomal marker PMP70, underscoring earlier observations that nsL-TP is a peroxisomal protein. nsL-TP was found to be present along the entire length of the small intestine. The 58 kDa cross-reactive protein that was recently identified as a peroxisomal thiolase was shown to be present only in a small segment approximately halfway down the jejunum. The close apposition of the peroxisomes with the apical membrane and the discrete distribution of the 58 kDa protein may indicate that these organelles play a role in the intracellular processing of absorbed lipids.

Amino Acid Sequence↗
PubMed1995

The far-upstream enhancer of the carbamoyl-phosphate synthetase I gene is responsible for the tissue specificity and hormone inducibility of its expression.

The role of the proximal promoter and the far-upstream enhancer in the hepatocyte-specific and hormonal regulation of the carbamoyl-phosphate synthetase I (CPS) gene was investigated in transient transfection assays using primary rat hepatocytes, hepatoma cells, and fibroblasts. These experiments revealed that the activity of the promoter is comparable in all cells tested and is, therefore, not responsible for tissue-specific expression. The 5'-untranslated region of the mRNA is a major, non-tissue specific stimulator of expression in FTO-2B hepatoma cells, acting at the post-transcriptional level. A 469-base pair DNA fragment, 6 kilobase pairs upstream of the transcription start-site in the CPS gene, confers strong hormone-dependent tissue specific expression, both in combination with the CPS promoter and a minimized viral thymidine kinase promoter. Sequences similar to a cyclic AMP-responsive element and a glucocorticosteroid-responsive element were found in the isolated enhancer. Substitutional mutations in these sites strongly affected hormone-induced expression. Analysis of the interaction between the enhancer and parts of the CPS promoter revealed that, in addition to the TATA box, the GAG box, a motif similar to the GC box near the TATA motif, is instrumental in conferring the enhancer activity.

Animals↗
PubMed1995

Differential connexin distribution accommodates cardiac function in different species.

Using immunohistochemical staining, the distribution of connexin40 (Cx40) and connexin43 (Cx43) was studied in rat, guinea pig, porcine, bovine and human hearts. These species display differences in the degree of morphological differentiation of the conduction system. This study was performed in the anticipation that comparison of the distributions of Cx40 and Cx43 in young and adult specimens may provide clues as to the physiological role of connexins in the heart. To a large extent, the distribution patterns of Cx40 and Cx43 are comparable between species. In neonates and adults, Cx43 was immunolocalized throughout the working myocardium, but in the conduction system Cx43 was detected only after birth. Cx40 was found to appear slightly earlier in development than Cx43 and to disappear when levels of Cx43 became more abundant. This time course was seen in working myocardium and in the ventricular conduction system. Together these data suggest that expression of Cx40 induces or facilitates expression of Cx43, while abundant expression of Cx43 in turn leads to suppression of Cx40 expression. The exceptions to this may represent blocks in this potential regulatory sequence. A second conclusion is that Cx40 and Cx43 containing gap junctions appear in the ventricular conduction system from distal to proximal and only after birth. This indicates that terminal differentiation of the conduction system occurs unexpectedly late in development.

Adult↗
PubMed1995

Dystrophin expression in the developing conduction system of the human heart.

Duchenne muscular dystrophy (DMD) is frequently associated with myocardial involvement. Dystrophin, the DMD protein, is found at the plasmamembrane of striated muscle fibers. Although dystrophin is missing in most or all muscle fibers of DMD patients, cardiac muscle is not as severely affected as skeletal muscle. Therefore it is of great importance to study the expression of dystrophin in normal cardiac muscle. We performed immunohistochemical studies and examined cardiac muscle of fetuses of 8 to 13 weeks of development on dystrophin expression. At these stages dystrophin is observed in the myocytes of the developing ventricular conduction system and in the atrial cardiomyocytes. Dystrophin was absent from the heart of a 12-week-old DMD fetus.

Antibodies, Monoclonal↗
PubMed1995

Capillary distribution in the ventricles of hearts with pulmonary atresia and intact ventricular septum.

BACKGROUND: Pulmonary atresia and intact ventricular septum (PA-IVS) can be complicated by the presence of a severely hypoplastic thick-walled right ventricle with or without ventriculo-coronary arterial communications. A variable amount of myocardial pathology has been described in these hearts, probably the result of ischemic conditions and a high pressure in the right ventricle. We studied whether the capillary network is still intact, allowing a sufficient perfusion of the myocardium, which will be important for the success of palliative surgery. METHODS AND RESULTS: We studied the distribution of capillaries in the myocardium of hearts with PA-IVS and compared the results with normal hearts. The capillaries were detected by immunohistochemistry using a monoclonal antibody (408) against endothelium. Remarkable abnormalities in capillary distribution were found in the right ventricle of hearts with PA-IVS and reflect the arrangement of the myocytes. Thus, disorganization of capillaries, which is found to be the most common pattern, always paralleled the myocardial disarray. A low density of capillaries is always found in areas with a low density of myocytes, ie, with hypertrophied myocytes, compact fibrotic tissue, or diffuse fibrosis. Disarray and other disturbances in orientation of capillaries and myocytes are present in hearts with PA-IVS, a hypoplastic right ventricle, and ventriculo-coronary arterial communications. These disturbances are more extensive when interruptions of the coronary arteries are also present. In hearts with PA-IVS and a hypoplastic right ventricle only, extensive regions with low capillary densities and severe myocyte pathology are observed. On the contrary, hearts with PA-IVS and a normal-size right ventricle show minor abnormalities in capillary and myocyte organization. CONCLUSIONS: In hearts with PA-IVS, various abnormal capillary distribution patterns are found. Our findings correlate well with clinical data that reported the best surgical results in hearts in which the major part of the myocardium showed a normal capillary distribution and myocyte morphology. This suggests that the capillary distribution may be an important parameter for the function of the heart. Because the distribution of the capillaries is found to be a good reflection of the arrangement of the myocytes, antibody 408 is also a useful tool in detecting abnormalities of the myocardium in a fast and easy way.

Capillaries↗
PubMed1995

Isolation and characterization of the rat gene for carbamoylphosphate synthetase I.

Carbamoylphosphate synthetase I (CbmPS) is first expressed in rat hepatocytes shortly before birth. After birth, expression of CbmPS gradually becomes confined to the hepatocytes surrounding the portal veins. To obtain insight into the spatiotemporal regulation of its expression, the rat CbmPS gene was isolated and characterized. The gene is 110 kb in length and contains 38 exons. The basal promoter comprises the first 161 nucleotides upstream of the transcription-initiation site. Determination of the state of methylation of the 5' portion of the gene identified a CCGG sequence at -6.3 kb that is selectively demethylated in adult tissues which express CbmPS. This site remains methylated before birth, however, despite recruitment of all hepatocytes for CbmPS synthesis, indicating that its demethylation is a consequence of rather than a condition for expression of CbmPS. Transient expression assays revealed that the region surrounding the CCGG site at 6.3 kb functions as an enhancer. In FTO-2B hepatoma cells and Rat-1 fibroblasts, this enhancer is constitutively active when tested in front of the basal viral thymidine kinase promoter. When tested in front of the basal CbmPS promoter in hepatoma cells, however, the activity of this enhancer is dependent on the presence of glucocorticoids. In Rat-1 fibroblasts, the presence of both glucocorticoids and cyclic AMP is required for full activity, suggesting that the hepatocyte-specific expression of CbmPS is related to tissue-specific differences in the sensitivity to cyclic AMP. Matrix-attachment regions (MAR) are present upstream and downstream of the CbmPS gene. The downstream MAR defines the 3' boundary of the gene. The upstream MAR is located midway between the basal promoter and the enhancer, and may function as a hinge point to facilitate the positioning of the enhancer in the vicinity of the basal promoter.

Animals↗
PubMed1995

Formation of the tricuspid valve in the human heart.

BACKGROUND: Some of the problems concerning the origin of the inlet component of the definitive right ventricle were resolved in a previous study in which we showed it to be derived exclusively from the embryonic right ventricle. Questions remain, however, concerning the relative contributions of endocardial cushion tissue and myocardium to the definitive valvar apparatus guarding the right atrioventricular orifice and the origin of the valvar leaflets. METHODS AND RESULTS: The formation of the tricuspid valve was studied by scanning electron microscopic and immunohistochemical techniques. Concurrent with the development of the right atrioventricular connection, a myocardial ridge forms at the boundary between the atrioventricular canal and the embryonic right ventricle. It grows to become a myocardial gully that funnels atrial blood beneath the lesser curvature of the initial heart tube toward the middle of the right ventricle. Fenestrations in the floor of the gully create an additional inferior opening in the funnel, transforming its initial anterior rim into the septomarginal trabeculation. The septum formed by the fusion of the endocardial ridges of the outflow tract becomes myocardialized in its inferior portion to form, in part, the outlet septum and, in part, the supraventricular crest. The smooth atrial surface of the tricuspid valvar leaflets develops from endocardial cushion tissue. The leaflets become freely movable, however, only after delamination of the tension apparatus within the myocardium. The inferior and septal leaflets derive from the gully and the ventricular septum, their delamination being a single, continuous process. The antero-superior leaflet forms by delamination from the developing supraventricular crest. CONCLUSIONS: The leaflets of the tricuspid valve develop equally from the endocardial cushion tissues and the myocardium. The myocardium contributing to the valve comes from two sources, the tricuspid gully complex and the developing supraventricular crest. These findings facilitate the understanding of several congenital malformations.

Antibodies, Monoclonal↗
PubMed1995

New method for the accurate characterization of single human skeletal muscle fibres demonstrates a relation between mATPase and MyHC expression in pure and hybrid fibre types.

In the present study we have developed a method which, by combining histochemical, immunohistochemical, electrophoretic and immunoblotting analyses on a single fibre, enables a sensitive characterization of human skeletal muscle fibres dissected from freeze-dried biopsy samples. For histochemical (and immunohistochemical) analysis fibre fragments (500 microns) of individual fibres were mounted in an embedding medium to allow cryostat sections of normalized thickness to be reproducibly obtained. The specificity of the myofibrillar Ca2+ ATPase (mATPase) staining profiles in gelatin-embedded single fibre sections was tested by immunohistochemical reactions with anti-myosin heavy chain (MyHC) monoclonal antibodies specific to human MyHC I, IIA, IIB and IIA + IIB and by gel electrophoresis. The combined methodologies demonstrated the specificity of the mATPase staining patterns which correlated to the expression of distinct MyHC isoforms. In addition the results provide evidence that many fibres co-expressed different MyHC isoforms in variable relative amounts, forming a continuum. Staining intensities for mATPase, converted into optical density values by image analysis revealed that a relationship between mATPase and MyHC expression holds for hybrid fibres even when displaying one MyHC type with overwhelming dominance. The results also revealed that three MyHC isoforms I, IIA and IIB can be co-expressed on a single muscle fibre. In such a case mATPase alone, with the current protocols, does not allow an accurate characterization of the specific MyHC-based fibre type(s). Although some hybrid fibres may have displayed a non-uniform expression of myosins along their lengths, most fibres from the IIA/B group (type) remained very stable with respect to the relative amounts of the MyHCs expressed. Finally, a second slow MyHC isoform was recognized on immunoblots of a mixed muscle sample.

Animals↗
PubMed1995

Molecular aspects of myocardial differentiation.

The embryonic heart can pump blood in a single direction without one-way valves. With the development of molecular cell markers specific for contraction and relaxation, functional aspects of myocardial differentiation have been addressed through the use of in situ hybridization. In this study, we report how expression of the cardiac sarcoplasmic reticulum calcium-adenosine triphosphatase (SERCA2) and phospholamban (PLB) in the rat may partly explain why the embryonic atrium and ventricle function essentially as they do in the adult. SERCA2 is expressed in a craniocaudal gradient from as early as 10 embryonic days (ED) of development. PLB is first expressed at 12 ED but in a gradient essentially opposite to that seen for SERCA2. This spatial pattern of expression is maintained throughout much of fetal development. The spatial distribution of skeletal alpha-actin in the developing human heart indicates that alpha-actin isoform gradients or switching are not important in the establishment of unidirectional blood flow in the absence of valves, but it may serve as a marker for cardiac maturation.

Actins↗
PubMed1995

Patterns of expression of sarcoplasmic reticulum Ca(2+)-ATPase and phospholamban mRNAs during rat heart development.

This study reports the clonal analysis and sequence of rat phospholamban (PLB) cDNA clones and the temporal appearance and patterns of distribution of the mRNAs encoding sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA2) and PLB in the developing rat heart determined by in situ hybridization. Both proteins play a critical role in the contraction-relaxation cycle of the heart. SERCA2 mRNA is already abundantly present in the first stage studied, in the cardiogenic plate of the 9-day-old presomite embryo, before the occurrence of the first contractions. This very early expression makes it an excellent marker for the study of early heart development. Subsequently, SERCA2 mRNA becomes expressed in a craniocaudal gradient, being highest at the venous pole and decreasing in concentration toward the arterial pole of the heart. PLB mRNA can be detected in hearts from 12 days of development onward in a virtually opposite gradient. In essence, these patterns do not change during further development. PLB mRNA levels remain highest in the ventricle and outflow tract, whereas SERCA2 mRNA prevails in the inflow tract and atrium, although the difference between atrium and ventricle becomes less pronounced. These observations are compatible with a model in which the upstream part of the heart (inflow tract and atrium) would have a greater capacity to clear calcium and hence would have a longer duration of the diastole than the downstream compartments (atrioventricular canal, ventricle, and outflow tract), similar to the observed pattern of contraction of the embryonic heart. The sinoatrial and atrioventricular nodes do not reveal an expression pattern of SERCA2 and PLB mRNA that allows one to distinguish them from the surrounding atrial working myocardium. However, the ventricular part of the conduction system, comprising atrioventricular bundle and bundle branches, are almost devoid of SERCA2 mRNA.

Adenosine Triphosphatases↗
PubMed1995

Heterogeneous expression of cholesterol 7 alpha-hydroxylase and sterol 27-hydroxylase genes in the rat liver lobulus.

We investigated the lobular localization and molecular level of expression of cholesterol 7 alpha-hydroxylase and sterol 27-hydroxylase, two key enzymes in bile acid synthesis, in isolated periportal and pericentral hepatocytes and by in situ hybridization of rat liver. Enzyme activity, mRNA, and gene transcription of cholesterol 7 alpha-hydroxylase were predominant in pericentral hepatocytes of control rats, being 7.9-, 9.9-, and 4.4-fold higher than in periportal hepatocytes, respectively. Similar localization was found for sterol 27-hydroxylase: 2.9-, 2.5-, and 1.7-fold higher enzyme activity, mRNA, and gene transcription, respectively, was found in pericentral hepatocytes. Interruption of the enterohepatic circulation with colestid resulted in upregulation of these parameters for both enzymes, as a consequence of stimulated gene expression mainly in the periportal zone. In contrast, mRNA levels and gene transcription of 3-hydroxy-3-methylglutaryl CoA reductase showed opposite lobular distribution. Selective periportal expression for the latter was enhanced, but remained local, after colestid treatment. In situ hybridization showed unambiguously that cholesterol 7 alpha-hydroxylase mRNA is localized exclusively in the pericentral zone and that sterol 27-hydroxylase mRNA is expressed preferentially in the pericentral region, though less pronounced. Administration of colestid led to expression of both genes within a larger area of the liver lobulus. In conclusion, we suggest that cholesterol 7 alpha-hydroxylase and sterol 27-hydroxylase are coordinately regulated by the bile acid gradient over the lobulus, resulting in predominant expression in the pericentral zone. Opposite lobular localization of cholesterol and bile acid synthesis provides an alternative view to interregulation of these metabolic pathways.

Animals↗
PubMed1995

The mATPase histochemical profile of rat type IIX fibres: correlation with myosin heavy chain immunolabelling.

In the present study we report a novel histochemical method which, by sequential pre-incubations in alkaline and acidic media, selectively differentiates muscle fibres expressing myosin heavy chain IIX, on the basis of a specific profile for myofibrillar actomyosin ATPase (mATPase) activity. The enzyme reactions were tested for specificity by means of anti-myosin heavy chain monoclonal antibodies, which were characterized on Western blots of muscle homogenates. Enzyme histochemical reactions with the traditional pH buffers were compared to those of the new method and, in conjunction with the immunoreactions, used to confirm the relationship between MyHC expression and the distinct profiles for mATPase. Immunohistochemical reactions demonstrated that the new method only differentiates those fibres expressing myosin heavy chain IIX. The method revealed a continuum in which the intermediate staining intensities corresponded to hybrid fibres expressing myosin heavy chain IIX in combination with either the IIA or IIB forms. Quantitative histochemistry and immunohistochemistry (by image analysis), used to examine the relationship between staining intensities for mATPase and amounts of myosin heavy chain IIX expression, revealed that the new method discriminates well between hybrid fibres expressing variable amounts of the IIX isoform (r2 = 0.93).

Animals↗
PubMed1994

Hepatic glutaminase mRNA is confined to part of the urea cycle domain in the adult rodent liver lobule.

This in situ hybridization study describes the developmental appearance of the lobular distribution of the mRNA encoding hepatic glutaminase in normal rat liver. Glutaminase has been proposed to provide the urea cycle with ammonia [Häussinger and Gerok (1983) Eur. J. Biochem. 133, 269-275]. Hence, the (developmental) pattern of expression of the mRNA would be expected to be closely linked to that of the urea cycle enzymes. From embryonic day 20 onward, hepatic glutaminase mRNA can be detected along the entire porto-central axis, with predominant expression in the portal area. In the adult phenotype, which is acquired at the end of the first postnatal week, glutaminase mRNA is no longer present along the entire porto-central distance but has become confined to a relatively small periportal domain in which the expression decreases in a porto-central direction. Thus, in contrast to the large periportal domain, in which the urea cycle enzymes are expressed, the glutaminase mRNA-expressing domain is much smaller and not contiguous with the glutamine synthase mRNA-expressing pericentral domain, leaving a midlobular area that is devoid of glutaminase mRNA. A similar pattern of distribution was found in adult mouse liver. The significance of these observations is that, within the liver lobules, there is an area in which glutaminase is not expressed and, hence, glutamine can not be the substrate for urea synthesis.

Animals↗
PubMed1994

Vascular branching pattern and zonation of gene expression in the mammalian liver. A comparative study in rat, mouse, cynomolgus monkey, and pig.

BACKGROUND: A significant part of the liver volume consists of regions in which hepatocytes are in close contact with large branches of the afferent (portal vein) or efferent (hepatic vein) vessels. As most studies have addressed zonation of gene expression around the parenchymal branches of the portal and hepatic vein only, the patterns of gene expression in hepatocytes surrounding larger vessels are largely unknown. METHODS: For that reason, we studied the patterns of expression of the mRNAs and proteins of the pericentral marker enzymes glutamine synthase, ornithine aminotransferase, and glutamate dehydrogenase and the periportal marker enzymes phosphoenolpyruvate carboxykinase and carbamoylphosphate synthase in the rat liver, in relation to the branching pattern of the afferent and efferent hepatic veins with immuno and hybridocytochemical techniques. These patterns of expression were compared with those seen in mouse, monkey, and pig liver. RESULTS: The distribution patterns of the genes studied appear to reflect the "intensity" of the pericentral and periportal environment, glutamine synthase and phosphoenolypyruvate carboxykinase requiring the most pronounced environment, respectively. The patterns of gene expression around the large branches of the portal and hepatic vein were found to be related to the parenchymal branches in the neighbourhood of these large blood vessels. Only the cells of the limiting plate retain their periportal and pericentral phenotype for those marker enzymes that do not require a pronounced periportal or pericentral environment to be expressed. GS-negative areas in the pericentral limiting plate appear to correlate with a local absence of draining central veins, and become more frequent and extensive around the larger branches of the hepatic vein. CONCLUSIONS: The similarity of the observed patterns of gene expression of the genes studied in mouse, rat, monkey, pig, and man suggests that they reflect a general feature of gene expression in the mammalian liver. A comparison of mouse, rat, pig, and human liver suggests that the presence of glutamine synthase-negative areas reflects the branching order of the efferent hepatic blood vessel.

Animals↗
PubMed1994

Restriction of lactase gene expression along the proximal-to-distal axis of rat small intestine occurs during postnatal development.

BACKGROUND/AIMS: Developmental changes of lactase activity along the proximal-to-distal axis of the small intestine are poorly understood. A study of delineate lactase gene expression at the cellular level was undertaken. METHODS: The topographical regulation of lactase was studied in conjunction with sucrase-isomaltase in proximal, middle, and distal segments of 0-, 7-, 14-, 16-, 18-, 21-, and 28-day-old and adult rats using in sity hybridization, immunohistochemistry, and ribonuclease protection assays. RESULTS: From 0 to 16 days, lactase messenger RNA (mRNA) and protein were abundant along the total length of the small intestine. However, at weaning, lactase mRNA and protein were no longer detectable in the terminal ileum. After 28 days, zones of reduced lactase expression were found in the duodenum and terminal ileum. These zones demonstrated expression of lactase protein in scattered enterocytes along the villus (patchy expression). In contrast, sucrase-isomaltase was first detected at 16 days, with patchy expression along the total small intestine; at 21 days it was abundant. CONCLUSIONS: Concordant changes in both lactase mRNA and protein detection during development suggest that the horizontal gradient of lactase enzyme expression is dependent on lactase mRNA abundance. Furthermore, zones of patchy lactase expression appear around weaning and flank the area of high lactase expression in the midintestine. Patchy expression is also found for sucrase-isomaltase before weaning.

Animals↗
PubMed1994

The expression of liver-specific genes within rat embryonic hepatocytes is a discontinuous process.

The onset of transcription and mRNA accumulation of two liver-specific genes, carbamoylphosphate synthase (CPS) and phosphoenolpyruvate carboxykinase (PEPCK) in individual embryonic rat hepatocytes was investigated with in situ hybridization. In vitro CPS and PEPCK mRNAs can be induced prematurely in monolayer cultures of embryonic rat hepatocytes by glucocorticosteroids and cyclic AMP, i.e. the hormones that also regulate the expression of these genes in vivo. Upon exposure to hormones the cultures showed an interhepatocyte heterogeneity in CPS and PEPCK mRNA content. The pattern of accumulation of nuclear CPS mRNA-precursors indicates that this heterogeneity is generated by intercellular differences in the timing of the onset of transcription. However, under induced steady-state conditions the heterogeneity in the hepatocyte population persisted. The degree of heterogeneity is inversely related to the half life of the gene product (i.e. higher for PEPCK than for CPS and higher for mRNAs than for the respective proteins) and to the concentrations of inducing hormones. Accordingly, the interhepatocyte heterogeneity was most pronounced for the nuclear CPS mRNA-precursor. In contrast, no intercellular differences in the rate of degradation of the mRNAs were seen. These observations reveal that although all hepatocytes can and do express the genes, transcription of a gene in a particular cell is a discontinuous process.

1-Methyl-3-isobutylxanthine↗