The effect of the thyroid hormone(T4) on the internalization of beta-adrenergic receptors in isolated hepatocytes was examined. Uptake of [3H]-dihydroalprenolol(DHA) by the cells, which were prepared from untreated-rats, was increased by the time-dependent manner in vitro. The amount of incorporated-[3H]-DHA was decreased by further incubation in the presence of 10(-5) M of d, 1-isoproterenol. However, about 30% of incorporated-[3H]-DHA remained in the cells by this treatment. The amount of this [3H]-DHA remaining in the cells was lowered by pretreatment of the cells with methylamine, monodansylcadaverine, d, 1-isoproterenol or d, 1-propranolol. Then the amount of this [3H]-DHA was recognized as an internalized-receptor-hormone(or -DHA) complex. The total uptake(internalized- + membrane bound-) of [3H]-DHA by the cells was not significantly different between the thyroidectomized (TX + Saline) and T4-treated-thyroidectomized(TX + T4) rats. However, the rate of disappearance of incorporated-[3H]-DHA was greater in the TX + T4 animals compared to that in the TX + Saline animals. In the cells obtained from the TX + T4 animals, internalized-[3H]-DHA was significantly higher than that of the TX + Saline animals. The number of membrane associated beta adrenergic receptors was significantly lower in the TX + T4 group. These results suggest that the thyroid hormone induces an acceleration of internalization of beta adrenergic receptors in hepatocytes.

Alprenolol↗

PubMed1983

Guanosine 5'-triphosphate modulation of S-adenosyl-L-methionine-mediated methylation of phosphatidylethanolamine in rat liver plasma membrane.

Effect of guanosine 5'-triphosphate(GTP) on the S-adenosyl-L-methionine-mediated methylation of phosphatidylethanolamine was examined using rat liver plasma membranes. Methyltransferase I, which catalyzes methylation of phosphatidylethanolamine to phosphatidyl-N-mono-methylethanolamine was inhibited by GTP, whereas methyltransferase II, which transfers methyl groups from S-adenosyl-L-methionine to produce phosphatidyl-N,N-dimethylethanolamine or phosphatidyl-choline was stimulated by GTP. d,l-isoproterenol stimulated methyl-transferase II activity slightly. This stimulation was greatly augmented by GTP. d,l-isoproterenol inhibited methyltransferase I and this inhibition was enhanced by GTP. The results indicate that GTP has a regulatory role in the methylation of phospholipids in the plasma membrane through inactivation of methyltransferase I and activation of methyltransferase II by binding to these enzymes.

Animals↗

PubMed1983

Evidence for the existence of protein inhibitors for S-adenosyl-L-methionine-mediated methylation of phosphatidylethanolamine in rat liver cytosol.

The effect of rat liver cytosol on phosphatidylcholine biosynthesis via the N-methylation of phosphatidylethanolamine has been studied. The purified rat liver plasma membrane was used as an enzyme source. The rat liver 105,000 x g supernatant(cytosol) contained two different inhibitors for S-adenosyl-L-methionine-mediated methylation of phosphatidylethanolamine to phosphatidyl-N-mono-methylethanolamine. The inhibitors were inactivated by pretreatment with trypsin or heating at 96 degrees C, but were not inactivated by RNase. The inhibitors did not inhibit the synthesis of phosphatidylcholine from phosphatidyl-N-mono-methylethanolamine. The results indicate that two different protein inhibitors for methylation are present in rat liver cytosol fraction and that the inhibition by these proteins may have a role for the regulation of phosphatidylcholine synthesis in the plasma membrane.

Animals↗

PubMed1983

[Color vision and ophthalmological findings in a family with congenital tritanopia].

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Adolescent↗

PubMed1983

Thyrotropin secretion in patients with hyperparathyroidism or hypoparathyroidism: effect of serum calcium on thyrotropin release.

The influence of changes in the serum calcium concentration on TSH secretion was evaluated in patients with primary hyperparathyroidism and idiopathic hypoparathyroidism and in normal subjects. Serum calcium concentrations were 12.7 +/- 0.8, 9.0 +/- 0.4, and 5.7 +/- 0.5 mg/100 ml in hyperparathyroid, normal, and hypoparathyroid subjects, respectively, and were significantly different from each other. Serum T3 and T4 concentrations were comparable among the three groups. The basal serum TSH concentration was highest in hypoparathyroid, lowest in hyperparathyroid, and intermediate in normal subjects. However, all values were within normal limits and were not significantly different from each other. TRH-stimulated TSH secretion was significantly greater in hypoparathyroid patients and significantly less in hyperparathyroid patients than in normal subjects, respectively. The TSH response to TRH was normalized when the serum calcium concentration was normalized by parathyroidectomy in a hyperparathyroid patient or by 1 alpha-hydroxyvitamin D3 administration in a hypoparathyroid patient. To further clarify the mechanism responsible for the modified TSH response to TRH in the hypercalcemic state, rats were made chronically hypercalcemic by the administration of 1 alpha-hydroxyvitamin D3 (0.2 micrograms/100 g BW, ip, for 10 days) and 3% calcium chloride in drinking water. The pituitary TSH content of hypercalcemic rats was significantly greater than that of control rats. The results suggest that decreased TSH secretion produced by chronic hypercalcemia is due to diminished TSH release, rather than to decreased pituitary TSH reserve.

Adolescent↗

PubMed1983