Possible mechanism of the regulation of smooth muscle relaxation by calcium ions

Kinetics of Ca2+ energy-dependent transport in sarcolemma and mitochondrion fractions of myometrium was studied. On the basis of the results obtained the mechanism of calcium control of smooth muscle relaxation was analysed. In terms of this mechanism kinetic curves of myometrium relaxation were calculated. It follows from their pattern that the mitochondria play the role of the main intercellular depo of Ca2+, while the calcium pump of the sarcolemma carries out fine regulation of this process making its contribution to relaxation at its later stage.     

Possible mechanism of entrapment of neuraminidase-treated lymphocytes in the liver

Neuraminidase-treated rat lymphocytes adhere strongly to rat hepatocytes in vitro. Binding between cells is due to stereo-specific interactions between a mammalian hepatic membrane lectin and galactosyl residues which are exposed on the lymphocyte surface after removal of sialic acid residues. The hepatic galactose specific lectin may play a role in the trapping of recirculating desialylated lymphocytes in the liver.     

Possible involvement of ceramide in the regulation of rat Leydig cell function

In the present study, a possible role of a ceramide-dependent pathway in the regulation of Leydig cell function was investigated. Intracellular ceramide levels were increased by: (a) adding ceramide analogs; (b) inhibiting ceramidase activity; and (c) adding sphingomyelinase (SMase). The cell-permeable ceramide analogs N-acetyl-, N-hexanoyl- and N-octanoylsphingosine (C2, C6 and C8) were used. As inhibitor of ceramidase activity 1S,2R-D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol (MAPP) was used. Sphingomyelinase from S. aureus origin was utilized. Leydig cells were cultured for 3 or 24 h with or without the different drugs (10 microM) and SMase (0.3 U/ml) in the presence or absence of hCG (10 ng/ml). Basal testosterone production was not modified under any of the experimental conditions. A decrease in hCG-stimulated testosterone production was observed at 3 and 24 h in all cases. The inactive analog (N-hexanoyl dihydrosphingosine) did not produce inhibition in hCG-stimulated testosterone production. TNFalpha and IL1beta, two possible inducers of sphingomyelin hydrolysis, produced similar effects on hCG-stimulated testosterone production. In experiments performed in the presence of C6, inhibition in hCG-stimulated cAMP production was observed. The inhibitory effect of ceramide was also observed in dbcAMP-stimulated cultures indicating that this pathway inhibits post-cAMP formation events. To study possible loci for the action of ceramide on the steroidogenic pathway, cells were incubated with C6 and MAPP in the presence of different testosterone precursors. The drugs inhibited testosterone produced from 22(R)-hydroxycholesterol (22R-OHChol), pregnenolone and 17alpha-hydroxyprogesterone (17OHP4) but not from androstenedione (Delta4). These results suggest that a ceramide-dependent pathway regulates hCG-stimulated Leydig cell steroidogenesis at the level of cAMP production and at post-cAMP events.     

Induction of phosphoenolpyruvate carboxykinase by hydrocortisone in rat liver and brain as a function of age

The activity and induction pattern of phosphoenolpyruvate carboxykinase (PEPCK) in the liver and brain of young (6-), adult (30-) and old (90-weeks) male rats were studied. The activity of this enzyme increases in both tissues until adulthood and decreases gradually thereafter. Further, the activity of PEPCK is higher in the liver than the brain. Adrenalectomy decreases significantly the activity of this enzyme in the liver of rats of all ages. However, this treatment inhibits brain PEPCK in young and adult rats. Administration of hydrocortisone to adrenalectomized rats increases PEPCK in both tissues of young and adult rats. However, the magnitude of induction is higher in the young, as compared to the adult, rats. This hormone-mediated induction of the enzyme is actinomycin D-sensitive.     

Synthesis of phosphatidylcholines in rat hepatocytes. Possible regulation by norepinephrine via an alpha-adrenergic mechanism

The effect of norepinephrine on phosphatidylcholine and phosphatidylethanolamine formation was investigated in short-term incubations with freshly isolated rat hepatocytes. In the presence of dl-propranolol, norepinephrine decreases the incorporation of [methyl-14C]choline into phosphatidylcholines in a dose-dependent manner. At a concentration of 50 microM, norepinephrine (plus 20 microM propranolol) inhibits the incorporation of [methyl-14C]choline over a wide range of choline concentrations (59% inhibition at 5 microM choline; 34% inhibition at 1 mM choline). Norepinephrine also decreases the incorporation rates of [1-14C]palmitic acid and [1-14C]oleic acid into phosphatidylcholines. The effect of norepinephrine is mediated through an alpha-adrenergic receptor. Norepinephrine (plus propranolol) does not decrease the uptake or phosphorylation rate of [methyl-14C]choline. Pulse-label and pulse-chase studies indicate that the conversion rate of phosphocholine to CDP-choline, catalyzed by CTP:phosphocholine cytidylyltransferase, is diminished by norepinephrine. In contrast with the inhibitory effect of norepinephrine on phosphatidylcholine synthesis, this hormone stimulates the formation of phosphatidylethanolamines from [1,2-14C]ethanolamine. This increased incorporation rate is apparent at ethanolamine concentrations above 25 microM. A combination of norepinephrine and propranolol decreases, however, the synthesis of phosphatidylcholines from [1,2-14C]ethanolamine. The results indicate that alpha-adrenergic regulation dissociates the synthesis of phosphatidylcholines from that of phosphatidylethanolamines.     

Possible growth hormone regulation of rat liver glutamine synthetase activity.

Hypophysectomy results in a marked decrease in glutamine synthetase activity of rat liver homogenates. The enzyme is affected to a lesser extent in the kidneys and is not influenced in the brain. Bovine growth hormone treatment of hypophysectomized rats elevates the diminished glutamine synthetase activity in liver and kidneys but has no effect on the brain enzyme. Adrenalectomy also results in decreased liver glutamine synthetase activity although less than the decline seen with hypophysectomy. Cortisol treatment has no effect on glutamine synthetase activity in hypophysectomized animals. Our results suggest that growth hormone is involved in the regulation of liver glutamine synthetase activity. This regulation may be important in the utilization of α-amino nitrogen from glucogenic amino acids associated with growth hormone enhanced glucose production.     

Possible involvement of manganese in the catalytic mechanism of bovine liver arginase.

1. Bovine liver arginase followed Michaelis-Menten kinetics in the pH range of 4.5-9.0. The variation of vi with pH implied that a basic group (pKa 8.7) functions at the catalytic site. 2. Treatment of the enzyme with N-ethylmaleimide showed that there are no critical sulfhydryl groups on the enzyme. 3. The less selective reagent, 3-bromopyruvate, caused biphasic inactivation which was unaffected by the presence of ornithine. 4. The data pointed against critical involvement of active site amino acid side chains in the catalytic sequence in arginase. 5. The observed pH-rate profile may reflect ionization of metal-bound water.     

Lipid composition of the rat liver nuclear matrix as affected by hydrocortisone

It has been found that the rat liver nuclear matrix contains a small amount of phospholipids (2%) and neutral lipids (1.6%). The injection of hydrocortisone increased the nuclear matrix phospholipid content and reduced its neutral lipid levels. A marked increase in sphingomyelin content observed was accompanied by the reduction in phosphatidylcholine level. The decrease in neutral lipid content took place mainly at the expense of a sharp lowering of triglyceride and cholesterol ether levels. The latter may turn into free cholesterol, thus increasing its content. The data obtained testify to steroid hormone influence on lipid metabolism in intranuclear structures. The observed changes in lipid composition may be related to specific hydrocortisone-induced activation of genome.     

Possible regulation of the growth of thyroid tissue by plasma iodide

Rats on iodine deficient diet for 6 months received propylthiouracil (PTU) (0.15%) during the last 2 months. At the end of this treatment, PTU was withdrawn and the rats were iodine refed. 48 hrs. before the iodine refeeding all rats were injected with 3H thymidine. The results showed that some prelabelled cells in the hyperplastic goitre preferentially disappeared during its involution and therefore are more sensitive to iodine.     

Possible regulation of pyruvate carboxylase fromThiobacillus novellus by hydroxypyruvate

Aspartate, glutamate, or dicarboxylic acids did not inhibit the activity of a highly purified but not homogeneous preparation of pyruvate carboxylase fromThiobacillus novellus. The only effective inhibitors were end-products of the reaction and to a lesser degree hydroxypyruvate. The latter has not been shown previously to regulate the enzyme's activity. Lineweaver-Burk plots revealed that it was uncompetitive with respect to acetyl CoA with a K_ii of 3.6 mM, and noncompetitive with respect to bicarbonate, magnesium ATP, and pyruvate with respective K_ii values of 7.1, 5.5, and 6.47 mM. The corresponding K_is values were 7.02, 5.4, and 4.25 mM. A mathematical model is presented that supports the findings.     

Induction of particulate and soluble isoenzymes of tyrosine aminotransferase by hydrocortisone in the liver of rats as a function of age.

The induction of soluble cytoplasmic (c-), and particulate mitochondrial (m-) and nuclear (n-) isoenzymes of tyrosine aminotransferase (TAT) by hydrocortisone in the liver of 6-, 35- and 76- week old rats was studied. In contrast to the earlier reports, both the particulate isoenzymes (m- &; n-TAT) are induced by hydrocortisone. This induction is actinomycin D sensitive. The degree and pattern of induction of the three isoenzymes of TAT vary with age. The possibility of separate regulatory mechanisms for the synthesis of the three isoenzymes is discussed.