ber die Entwicklung der Arbeits- und Erregungsleitungsmuskulatur des Herzens von Ratte und Meerschweinchen: Histologische, histochemische und elektrophysiologische Untersuchungen

Inotropic effects of ryanodine and nicardipine on fetal, neonatal and adult guinea-pig myocardium

The effects of extracellular Ca2+ and inotropic agents on contractile force were examined in left atrial and right ventricular myocardia isolated from fetal (35-55 days after conception), neonatal (0-5 days after birth) and adult (30-90 days after birth) guinea-pigs. In both atrial and ventricular muscles, the contractile force increased with increasing extracellular calcium concentration ([Ca2+]o) and the sensitivity to [Ca2+]o was higher in the fetus than in the neonate and adult. Nicardipine almost abolished the contractile force in all groups examined. The sensitivity to nicardipine was similar among the three age groups and also between the two tissues. Ryanodine almost abolished the contractile force in atrial muscles from all age groups. In ventricular muscles it partially reduced the contractile force to the same extent in the neonate and adult while it slightly increased the contractile force in the fetus. In both atrial and ventricular muscles, isoproterenol shortened the relaxation time to the same extent in the neonate and adult, but little or not in the fetus. Taken together, our present results suggest that the sarcoplasmic reticulum function in regulating myocardial contraction is less developed in the fetal guinea-pig when compared with the neonate and adult, and that the atrial contraction is more dependent on Ca2+ release from the sarcoplasmic reticulum than the ventricular contraction is. Thus, it appears that the development of contractile function of the guinea-pig myocardium is mostly completed by birth.

Developmental changes in action potential properties of the guinea-pig myocardium

Developmental changes in action potential properties were examined in electrically driven (1 Hz) left atria and right ventricles from foetal, neonatal and adult guinea-pig hearts, using standard micro-electrode recording techniques. In both left atria and right ventricles, the overshoot, resting potential and maximum upstroke velocity of the action potential increased progressively with age until birth, and then remained almost unchanged. Action potential duration (APD) changed markedly with age during foetal and neonatal periods. In left atria, APD at 50% repolarization initially decreased until foetal day 50, and then increased until the adult period. In right ventricles, APD initially increased until approximately foetal day 45, then decreased for 5 days following birth, thereafter it increased again. In addition, after-hyperpolarization was observed only in left atria of younger foetuses. Thus we have demonstrated that in the guinea-pig myocardium developmental changes in action potential properties occur more extensively during the foetal period than during the postnatal period.

Developmental regulation of lipoprotein lipase in rats

To evaluate changes in lipoprotein lipase (LPL) expression during development, levels of LPL mRNA, protein, and enzyme activity were measured in heart, epididymal fat, kidney, and brain of rats, from late gestation through 24 mo. LPL mRNA, protein, and enzyme activity were low in fetal and neonatal hearts. LPL mRNA increased 11-fold by 60 days and remained at this level thereafter; LPL protein and enzyme activity increased 10-fold by weaning, before declining to low values by 3 mo. LPL mRNA levels, protein, and enzyme activity did not change in epididymal fat from 3 wk to 21 mo. In the kidney, LPL mRNA levels were high at the end of gestation but fluctuated during the first month. LPL protein and activity were low at day 1 and rose eightfold to peak values by day 7 before decreasing to low levels by weaning. LPL mRNA levels were relatively high in fetal brains and then fell 60% during the neonatal period. LPL protein peaked at day 7 before falling 95% by weaning. Thus LPL is under complex tissue-specific regulation involving transcriptional and posttranscriptional mechanisms.

Human cytochrome c oxidase during cardiac growth and development

Human heart mitochondrial cytochrome c oxidase specific content and specific activity was measured in five fetuses 15-21 weeks gestational age and in five patients whose age ranged from 6 days to 22 years. None had evidence of cardiac pathology. An increase in cytochrome c oxidase specific content and specific activity was observed in the fetal heart with increasing gestational age (0.13-0.38 nmol heme a/mg protein and 67-295 nmol O2 utilized/min/mg protein) and from the neonatal period (0.35 nmol heme a/mg protein and 140 nmol O2/min/mg protein) to adulthood (1.2 nmol heme a/mg protein and 1104 nmol O2/min/mg protein). A marked increase was observed postnatally between 4 and 19 months.

Intercalated discs of mammalian heart: a review of structure and function

Intercalated discs are exceptionally complex entities, and possess considerable functional significance in terms of the workings of the myocardium. Examination of different species and heart regions indicates that the original histological term has become out-moded; it is likely, however, that all such complexes will continue to fall under the generic heading of 'intercalated discs'. The membranes of the intercalated discs establish specific associations with a variety of intracellular and extracellular structures, as well as with numerous types of proteins and glycoproteins. Characterization of discs and their components has already brought together a large number of research disciplines, including microscopy, cytochemistry, morphometry, cell isolation and culture, cell fractionation, cryogenics, immunology, biochemistry, and electrophysiology. The continued dissection of substance and function of intercalated discs will depend on such interdisciplinary approaches. The intercalated disc component which continues to attract the greatest amount of interest is the so-called gap junction. All indications thus far point to a great deal of inherent lability in the architecture of the gap junction. There is thus considerable potential for the creation of artefact while preserving and observing gap junctions, and this problem will doubtless continue to hamper the understanding of their functions. A question of special interest concerns whether the gap junctions of intercalated discs are required for transfer of electrical excitation between cells, or maintain cell-to-cell adhesion, or in fact subserve both electrical and structural phenomena. Two schools of thought exist with respect to cell-to-cell coupling in the heart. One proposes that low-resistance junctions in the discs mediate electrical coupling, whereas the other supports the possibility of coupling across ordinary high-resistance membranes. Thus the intercalated discs continue to be a source of controversy, just as they have been since they were originally discovered in heart muscle over a century ago.

Differentiation of Purkinje fibres and ordinary ventricular and atrial myocytes in the bovine heart: an immuno- and enzyme histochemical study

The differentiation of Purkinje fibres and ordinary ventricular and atrial myocytes in bovine hearts was studied with specific antibodies against M-line proteins (MM-creatine kinase and myomesin) and with enzyme histochemistry (succinate dehydrogenase and mitochondrial glycerol-3-phosphate dehydrogenase). MM-creatine kinase was detected at an earlier stage in Purkinje fibres and atrial myocytes than in ordinary ventricular myocytes. The findings are in agreement with previous ultrastructural observations that an earlier appearance of a dense M-band occurs in Purkinje fibres than in ordinary ventricular myocytes. Myomesin was detected in all three cell types even at early foetal stages, in accordance with suggestions that it is an integral component of the myofibrillar structure. The activity of succinate dehydrogenase gradually increased in both ordinary ventricular and atrial myocytes, while the activity of mitochondrial glycerol-3-phosphate dehydrogenase was high at different stages of early foetal development in the two tissues, finally becoming low in the adult stage. The activity of succinate dehydrogenase and mitochondrial glycerol-3-phosphate dehydrogenase seemed to remain unchanged in the Purkinje fibres from early to late foetal stages. The present study shows that the Purkinje fibres are already different from ordinary ventricular myocytes at early foetal stages and that the two cell types differentiate in different ways. It is concluded that there are also developmental differences between ordinary ventricular and atrial myocytes.

The development of the Purkinje fibre system in the bovine fetal heart

In the bovine fetal heart, subendocardial bundles of cells could be distinguished from the main myocardial mass. Their morphological characteristics suggest that they represent bundles of Purkinje fibres. An intense fluorescence after incubation in antisera against the intermediate filament protein skeletin also supports this suggestion. Further, the bundles exhibited different histochemical reaction from the main myocardial mass. During development the histochemical pattern changed. Bundle cells in mitosis were observed. With increasing fetal age, binucleate cells were seen progressively more frequently. Our observations indicate that the Purkinje fibres differentiate along a line separate from the ordinary myocardial cells and that they acquire their adult characteristics gradually.

Pre- and post-natal development of lipoprotein lipase and hepatic triglyceride hydrolase activity in rat tissues

The ontogenic development of lipoprotein lipase and liver triglyceride hydrolase was studied in the rat. The enzyme activity measured in extrahepatic tissues fulfilled the criteria of lipoprotein lipase from the onset of measurable activity, i.e. it was inhibited by protamine and 1 M NaCl and showed requirement for serum and heparin for optimal activity. In the liver, measurable amounts of triglyceride hydrolase, active at pH 8.6 were detected 6 days prior to birth. However, till the fourth postnatal day about 50% of this activity was inhibited by NaCl and its sensitivity towards protamine was also higher than that of the enzyme in adult liver. Three patterns of development of enzymic activity were observed in extrahepatic tissues. In the lung, the lipoprotein activity reached the adult values one day prior to birth, while in the kidney only 30% of adult activity were found at birth. A linear increase of enzyme activity was observed in the heart; only 25% of adult activity were detected at birth and 100% were reached only 20 days after birth. The increase in lipoprotein lipase activity in the heart was accompanied by morphological differentiation of cardiocytes and by a progressive development of the capillary bed, which might be related to the pattern of development of enzyme activity in this organ. Adipose tissue lipoprotein lipase activity in inguinal fat fell from values 15 times than adult values between the 4th and 40th postnatal days. The enzyme activity in epididymal fat increased steeply between day 10 and 40, at which time it exceeded the adult values very considerably. These findings indicate that the regulation of the development of lipoprotein lipase activity in extrahepatic tissues is governed by local factors, which can differ even in the same type of tissue, as exemplified by the difference between inguinal and epididymal fat.

Differentiation and aging of the intestinal mucosa of the rat. I. Morphological, enzyme-histochemical and disc electrophoretic aspects of the prenatal and postnatal differentiation of the intestinal mucosa

The large and small intestines of fetal, newborn, juvenile and adult rats were investigated at various points by means of enzyme histochemistry, disc electrophoretic separations of enzymes and electron microscopy. The results obtained permit division of the morphological and enzymatic differentiation of the rat intestinal epithelium into four periods of development. 1. The period of prenatal development from the 19th to the 21st fetal day with a continuous increase of the enzyme activities in the enterocytes of the entire intestinal mucosa. 2. The period of postnatal development from the first day of life until the end of the second week with enzyme activity remaining roughly the same compared with the 21st day of life. 3. The period of adaptation from the beginning of the third week of life up to the middle of the fourth week of life. During this phase the transition into the stage of adult maturity occurs both morphologically and enzymatically. 4. The stage of adult maturity, which is fully developed in the middle of the fourth week of life.

The presence of transverse and axial tubules in the ventricular myocardium of embryonic and neonatal guinea pigs

Developing transverse (T) tubules are found in embryonic guinea pig ventricular myocardium after approximately eight weeks of gestation. By the time of birth (nine weeks total gestation); longitudinally-oriented axial tubules connected to the T tubules also have formed, and the majority of cells closely resemble those of the adult. The form taken by the developing T and axial tubules suggests that they are generated in a manner similar to that for T tubules in chick and rat skeletal muscle, namely by repeated formation of caveolae.