The effects of L-thyroxine (T4; 0.2 microgram/g body weight) an hydrocortisone (HC 5 microgram/g body weight) on the M. masseter superficialis in developing rats were investigated. Administration of T4 and/or HC to rats for 5 consecutive days from day 5 after birth induced precocious increase in diameter of the muscle fibers, increase of SDH activity and differences of the activity in various fibers and precocious disappearance of fibers with EDTA-insensitive ATPase activity. The effects of these hormones on the muscle gradually decreased after the 10th postnatal day. In thyroidectomized-adrenalectomized rats. T4 increased the diameter and SDH-activity of the muscle fibers and induced differences in SDH activity between fibers, but HC did not. Thus HC may act permissively on the SDH activity and the diameter of the muscle fibers. On the other hand, muscle fibers with EDTA-insensitive ATPase activity did not disappear on treatment with T4 or HC alone, but did disappear on treatment with T4 plus HC. These results suggest that increases in the circulating levels of T4 and HC in suckling rats are involved in development of the M. masseter superficialis.

Adenosine Triphosphatases↗

PubMed1981

[The arteria stapedia and its branches in the hamster].

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Animals↗

PubMed1981

Postnatal development of masticatory organs in rats. I. consecutive changes in histochemical properties and diameter of muscle fibers of the M. masseter superficialis.

At birth, the diameter of muscle fibers in the M. masseter superficialis of rats has a narrow range. The diameters begin to increase rapidly from day 10 after birth, and on day 30 the diameters range between about 25 and 55 micrometers, and can be distinguished into 3 groups. SDH activity also increases in some fibers from day 10 and on day 15, 2 types of fibers can be distinguished on the basis of SDH activity, fibers of larger diameter having high activity. From day 30 to 60, 3 types of fibers can be distinguished on the basis of diameter and SDH activity. During this period, the diameter of fibers with the lowest SDH activity increases rapidly, the diameter of fibers with the highest SDH activity decrease transiently, and the diameter of fibers with intermediate SDH activity increases slightly. After day 60, the diameter of fibers with the highest SDH activity begins to increase again. Some fibers show high EDTA-insensitive ATPase activity for the first 15 days. This activity begins to decrease from day 15 after birth, and after day 25 it is no longer detectable in any fibers. No relation between SDH activity and EDTA-insensitive ATPase activity was found in serial sections of fibers. These results indicate that histochemical differences between muscle fibers of the M. masseter superficialis first appear during the late suckling period and develop gradually over a comparatively long period after weaning during use of the muscle for mastication.

Animals↗

PubMed1981

Postnatal development of masticatory organs in rats. III. Effects of mastication on the postnatal development of the M. masseter superficialis.

When suckling of rats was continued on days 17--25 after birth, the development of the M. masseter superficialis was inhibited: increase in the diameter and SDH activity of its muscle fibers was retarded. Conversely, forced weaning of rats on 17 after birth stimulated the development of the muscle. These results suggest that mastication after weaning is involved in the further postnatal development of the M. masseter superficialis in rats.

Animals↗

PubMed1980

Permissive role of L-thyroxine in induction of pancreatic amylase by cortisol in neonatal rats.

The effect of L-thyroxine (T4) on amylase activity in the developing rat pancreas has been investigated. Administration of T4 (0.2 microgram/g body wt) alone to intact rats on days 5-10 after birth did not induce pancreatic amylase but the enzyme was induced significantly by daily injection of cortisol (10 microgram/g body wt) alone into intact rats over the same period. In thyroidectomized, adrenalectomized rats pancreatic amylase was not induced by the injection of cortisol alone but it was induced by the administration of cortisol plus T4. Increase in enzyme activity was much less in thyroidectomized animals than in intact animals. These results suggested that T4 does not have a direct effect in increasing pancreatic amylase activity but plays a permissive role in increasing enzyme activity.

Adrenalectomy↗

PubMed1980