Suppression of hepatic HMG-CoA reductase activity by beta-muricholic acid in mice fed a diet containing cholesterol and cholic acid.

A diet containing cholesterol and cholic acid (SID) is known to induce the formation of cholesterol fatty liver as well as cholesterol gallstones. The activity of HMG-CoA reductase, one of the key enzymes for cholesterol synthesis in the liver, is significantly lowered by addition of beta-muricholic acid to SID. The prevention of fatty liver formation by beta-muricholic acid was accompanied by the suppression of HMG-CoA reductase activity.

Free fatty acids in gallstones in mice fed a diet containing cholic acid.

In mice, combined addition of 1% cholesterol and 0.5% cholic acid to a diet induced cholesterol gallstones within 40 days as a result of the supersaturation of cholesterol in the bile, as has been reported. The major component of the gallstone was cholesterol, which was measured by HPLC. In this study, however, single addition of 1% cholic acid to a diet, which did not decrease cholesterol solubilizing capacity in bile, contributed to gallstone formation in mice within 50 days. The gallstones thus formed contained a large amount of palmitic acid. In the hepatic bile of this animal, palmitic acid was also detected; however, no solid material was observed by light and polarized-light microscopes. Free fatty acids such as palmitic acid seem to be dissolved in a complex micelle composed of bile acids and lecithin. This probably causes gallstone formation by reducing cholesterol solubilizing capacity in bile.

Effect of organic and inorganic mercury compounds on the growth of incisor and tibia in rats.

The pharmacological effects of methyl mercury chloride (MMC) and mercuric chloride (HgCl2) (2-16 mg/Kg per day subcutaneously, for 6 d) upon the growth were studied in the incisors and proximal tibiae of immature rats histologically. Lead acetate was used as a time marker. 1. Mercury compounds slightly affected the body weight gains of the rats but apparently inhibited the longitudinal growth of proximal tibia and the effect increased with higher dosages. 2. Mercury compounds definitely inhibited not only the longitudinal growth (incisor growth) but also the appositional growth (dentin formation) of incisal dentin. 3. The inhibitory effect on the growth was ranked as follows: bone growth greater than dentin formation greater than incisor growth. 4. The actions of MMC on the growth of incisors and proximal tibiae appeared gradually and the response was biphasic; stimulatory and then inhibitory. The inhibitory effect appeared even after the injection was discontinued and appeared more extremely than during the injections. 5. In HgCl2 groups the inhibitory effect on the growth appeared rapidly. The effect increased with higher dosages and became stronger as the injections were repeated. However, this effect was weakened promptly after the injection was discontinued. 6. The repeated injections of mercury compounds hardly affected the level of serum calcium but disturbed the calcification of incisal dentin. From the above-mentioned results a possible mechanism was discussed. It is suggestive that MMC acts directly upon a cell and is transported into it. Once MMC was introduced into a cell it is slowly demethylated to inorganic Hg and acts as the demethylated mercury. When accumulated mercury is slight in volume, it stimulates and then inhibits the cell function with increasing mercury. However HgCl2 binds directly with an effector cell membrane in loose fashion. This may cause the ready reversibility of the effect.

Structural and functional adaptation of the pulpal nerve fibers in the rat incisor.

Special fine structure of the pulpal nerve fibers adapted to the continuously growing incisor was studied on 20 rats (25-d-old-males) electron microscopically and was compared with that of the molars of limited growth. The incisal pulp was innervated by 2 kinds of the nerve fibers derived from the cervical sympathetic and the trigeminal ganglia: polyaxonal unmyelinated and monoaxonal myelinated. In the apical pulp, however, the pulpal nerve fibers were unmyelinated, showing 3 different profiles in the fine structure: 1. polyaxonal, 2. polyaxonal with dial plate-like profile and 3. monoaxonal. The monoaxonal unmyelinated fiber reflected the transverse profile of the unmyelinated part of the myelinated axon derived from the trigeminal ganglion. It can be concluded that the pulpal nerve fibers derived from the trigeminal ganglion maintain the immatured nature in the apical proliferating pulp to adapt to the continuous growth of the incisor.

Experimental morphological studies on the functional role of the pulpal nerves in dentinogenesis.

The role of the pulpal nerves in the dentinogenesis of the rat dentition was studied experimentally by means of a time marker of lead acetate and by electron microscope after the individual dissection of the trigeminal and cervical sympathetic nerves or both. Following trigeminectomy, the longitudinal growth of the denervated incisor decreased. Contrary to this, following cervical sympathectomy, the longitudinal growth of the denervated incisor and molar increased 10-5%, and the collagen fibers increased predominantly in the pulpal tissue. The sympathetic nerves seem to play an important part in controlling the differentiation of the odontoblasts, inhibiting the maturation of the growth center of the apical pulp in the continuously developing teeth.

A calcitonin-like action of prostaglandin E1.

The pharmacological effects of PGE1 (6 and 9 days, 2-1,250 micrograms/kg per day subcutaneously) upon the growth and the bone resorption of mammals were studied using the proximal tibia and upper incisor of immature rats along with lead acetate as a time marker, and upon the serum calcium and inorganic phosphorus levels. The following results were obtained. 1. PGE1 hardly affected the body weight or the weight of organs of the rats but apparently inhibited the longitudinal growth of proximal tibia in a dose related manner. 2. PGE1 clearly inhibited not only the longitudinal growth (incisor growth) but also the appositional growth (dentin formation) of incisal dentin. 3. The grade of the inhibitory effect on the growth was in the order of bone growth greater than dentin formation greater than incisor growth. 4. The occurrence of osteoporosis due to a low calcium diet was inhibited by the simultaneous administration of PGE1, the mechanism being considered to be mainly due to the inhibitory effect on the bone resorption. 5. PGE1 lowered the level of serum calcium and the lowering effect was not observed in the thyro-parathyroidectomized rat. From the facts that the above effects were exactly the same as those of calcitonin (1), the possibility that the subcutaneous injection of PGE1 may induce a calcitonin-like action, a part of which may dependent on the calcitonin secretion is suggested.

Action of parathyroid hormone, with special reference to its anabolic effect on different kinds of tissues in rats (I).

The effect of parathyroid hormone (PTH) on the spongiosa of the proximal tibia and the alveolar bone in immature rats was studied using a time marker by the injection of lead acetate. 1) When intact rats were fed on a low calcium diet, the promotion of the apposition of the alveolar bone and the longitudinal formation as well as the resorption of the spongiosa were observed, but no change was detected in the serum calcium level. The resorption was more remarkable than the formation. 2) By parathyroidectomy (PTX) or thyroparathyroidectomy (TPTX) the effect described in 1) disappeared completely, but there subsequently occurred a fall of the serum calcium level and a marked inhibition of the formation and resorption. The decrease in the appositional formation was stronger than in the longitudinal formation. 3) When PTH was injected into the rats having undergone PTX or TPTX, all of the effects in 2) were reversed with the recovery of the apposition being at an extremely high rate. All the foregoing results indicate that PTH has an evident anabolic action in addition to the action of increasing the bone resorption and that the sensitivity to PTH is stronger in the periosteal bone than in the spongiosa.

Action of parathyroid hormone, with special reference to its anabolic effect on different kinds of tissues in rats (II).

The effect of parathyroid hormone (PTH) on the formation of the incisor in the rats was investigated using a time marker by the injection of lead acetate. 1) When intact immature rats were fed on a low calcium diet, the rate of the longitudinal and appositional formation of the incisal dentin was increased. 2) Both formations were inhibited by parathyroidectomy (PTX) or thyro-parathyroidectomy (TPTX), the inhibition being stronger in the appositional than in the longitudinal formation. The mechanism generating the characteristic responses (thin dentin and irregularity of dentin formation) which appeared after the operations was made clear. 3) All of the effects in 2) were reversed by the injection of PTH. However, the restoration in the appositional formation was faster in appearance and higher in rate than in the longitudinal formation. The results of 1) to 3) indicate that PTH has an anabolic effect on the hard tissues. 4) The sensitivity of the tissues to PTH varies according to their embryological origin.

Action of parathyroid hormone, with special reference to its anabolic effect on different kinds of tissues in rats (III).

The effect of parathyroid hormone (PTH) on mineralization and matrix formation was studied on the incisal dentin of immature rats by using a time marker as well as the histochemical method. 1. When the hypersecretion of PTH was caused by a low calcium diet, mineralization and matrix formation were both accelerated. 2. Both mineralization and matrix formation were clearly inhibited by parathyroidectomy or thyro-parathyroidectomy which brought about a widening of the predentin (dentinoid) as a result of the conspicuous inhibition of mineralized matrix formation. The maturation of the matrix seemed to be inhibited also. 3. Demineralization was lower and slower in the labial than in the lingual dentin. 4. The inhibitory effect disappeared totally by the PTH injection, but the restoration of matrix formation was faster compared with that of mineralization. 5. The increase or decrease in mineralization did not necessarily occur in parallel with that in matrix formation. 6. Acid mucopolysaccharide formation in the dentin depended clearly on the quantity of PTH.