Norepinephrine stimulation of phospholipid methylation in rat cortical synaptosomes: fact or artifact?

Increased methylation of chloroform extractable products in brain membrane preparations after propranolol injection to the rat

1. One and four hours after acute or chronic (8 days) intraperitoneal injection of propranolol to rat at doses up to 25 mg/kg, a beta-adrenergic receptor blockage occurred in brain homogenates, as monitored by [3H]dihydroalprenolol binding. 2. A dose-dependent increased methylation of chloroform extractable products was observed (acute treatment: 18% increase, chronic treatment: 136% increase for the dose of 25 mg/kg). 3. The implication of beta-adrenergic receptors in this effect was studied and this revealed that: (a) acute intraperitoneal injection of the less active propranolol stereoisomer (+) did not produce any effect; (b) the chronic intraperitoneal injection of salbutamol, a beta-adrenergic receptor agonist, induced a decreased methylation of chloroform extractable products (62% decrease for the dose of 2.5 mg/kg). 4. It is concluded that injection of beta-adrenergic antagonist to the rat increases basal methyltransferase activity in the brain. This induces a decreased sensitivity of the enzyme activity to noradrenaline.

Alteration of membrane phospholipid methylation by adenosine analogs does not affect T lymphocyte activation

Membrane phospholipid methylation has been described during activation of various immune cells. Moreover recent data indicated modulation of immune cells functions by adenosine. As S-Adenosyl-methionine and S-Adenosyl-homocysteine are adenosine analogs and modulators of transmethylation reactions, the effects of SAH and SAM were investigated on membrane phospholipid methylation and lymphocyte activation. SAM (10(-5) M) was shown to induce the membrane phospholipid methylation as assessed by the 3H-methyl-incorporation in membrane extract. This effect was inhibited by SAH. In contrast SAM and SAH did not affect the phytohemagglutinin-induced proliferative response of peripheral blood mononuclear cells. SAH neither modified the early internalization of membrane CD3 antigens nor did it prevent the late expression of HLA-DR antigens on lymphocytes activated by phytohemagglutinin. These results indicate that in vitro alteration of phospholipid methylation does not affect subsequent steps of human T lymphocyte activation and proliferation.

Increased methylation of chloroform extractable products and CTP: cholinephosphate cytidylyltransferase in brain membrane preparations from triethyltin-intoxicated rats

Rats were chronically intoxicated with triethyltin in the drinking water (0.002%) for a period of 15 days. Starting with day 5 of the intoxication period a decrease in the body weight was observed and, in parallel, the development of a cerebral oedema could be followed by measuring white matter density. At the same time, an increase of phosphatidylethanolamine-N-methyltransferase and cholinephosphate cytidylyltransferase activities was noted. This increase might be a compensatory mechanism for counteracting the membrane damages induced by triethyltin.

Comparison of basal and noradrenaline stimulated methylation of chloroform-extractable products in synaptosomal preparations from the rat brain

Noradrenaline addition increases (S-adenosyl-L-methionine)-methylation of chloroform extractable products in the rat brain synaptosomal preparation. The characteristics of this methylation (MgCl2 dependence, S-adenosyl-L-methionine concentration dependence) are different from those of phosphatidylethanolamine methylase. The greatest increase was induced by noradrenaline in the 10(-4) M concentration range; preincubation of noradrenaline with the membranes, before S-adenosyl-L-methionine addition, was necessary. The TLC analysis of chloroform extractable products showed that: methylation of phosphatidylethanolamine derivatives was increased and the production of another unknown product (corresponding to that described by Wazer et al., Life Sci. 32, 2535, 1983) was more enhanced.

Phospholipid methylase activity, [3H]S-adenosyl-L-homocysteine binding, and S-adenosyl-L-methionine and S-adenosyl-L-homocysteine levels in rat brain during maturation

The changes in activity of phospholipid methyltransferase I and [3H]S-adenosyl-L-homocysteine ([3H]SAH) binding were determined in cortical membrane preparations from newborn rats and rats 1, 2, and 8 months old. The activity of phospholipid methyltransferase I and the [3H]SAH binding were significantly greater (respectively, +30 and +40%) in newborn rats than in 1-, 2-, and 8-month-old rats. The methylated products at days 1 and 30 were identical. These changes in methyltransferase activity may be correlated with variations in concentration of S-adenosyl-L-methionine (SAM) and SAH. The endogenous SAM level was higher and the SAH level was lower in newborn compared with adult rats. These data suggested that the processes of methylation were favored in newborn rats. The modifications observed after treatment with L-homocysteine reinforced this hypothesis.

Beta-adrenergic coupled phospholipid methylation. A possible role in withdrawal from chronic ethanol

Phospholipid methylation was studied in rat brain cortex preparations. Adrenergic agonists stimulated methylation in a dose-dependent manner. The effect was stereospecific and the order of potency of agonists was isoproterenol greater than norepinephrine greater than or equal to epinephrine. The stimulation could be blocked by propranolol. It was concluded that adrenergic stimulation of phospholipid methylation in these preparations involved a beta-adrenergic receptor and that this response was dependent upon an intact membrane environment. Neither adenosine nor histamine stimulated methylation. In fact, histamine appeared to inhibit methylation. Cleavage of phosphatidylcholine to lyso-phosphatidylcholine occurred in the presence of either adrenergic agonists or histamine, indicating an involvement of phospholipase A2. Norepinephrine-sensitive methylation in cortex homogenates from rats withdrawn from chronic ethanol administration was double that of controls by 72 hr after the final ethanol dose. Furthermore, basal methylation exhibited a decreasing trend during this period.

Activity of cyclic AMP phosphodiesterase and methyltransferases in leukocyte membranes from allergic patients

Cyclic AMP metabolism and methylation of phospholipids are central events which occur at the membrane level. Since a dysfunction of cell membranes seems to characterize some allergic diseases, we investigated cyclic AMP phosphodiesterase and methyltransferase activities in leukocyte membrane fractions obtained from healthy volunteers and from allergic patients. The allergic group presented a significantly decreased methyltransferase activity compared with a control group, whereas cyclic AMP phosphodiesterase and noradrenaline (NA)-stimulated methyltransferase were found to be increased with respect to the control group. A significant correlation has been found between cyclic AMP phosphodiesterase and NA-stimulated methyltransferase with both control and allergic subjects, which suggests close relationships between these two enzymes within the cell membrane.