Nonintracellular, cell-associated O-methylation of isoproterenol in the isolated rabbit thoracic aorta

The present study examines the subcellular site of catecholamine O-methylation in extraneuronal tissue. S-Adenosyl-l-methionine, a methyl donor that does not diffuse across biological membranes, was used to assess the participation of plasma membrane bound catechol-O-methyltransferase vs. cytoplasmic catechol-O-methyltransferase in the catecholamine O-methylating process. Segments of rabbit thoracic aorta incubated with [methyl-3H]-S-adenosyl-l-methionine and isoproterenol generate [3H]methoxy-isoproterenol. The formation of [3H]methoxy-isoproterenol from [methyl-3H]-S-adenosyl-l-methionine was proportional to the isoproterenol concentrations in the range of 0.1 to 1.0 microM. There was a marked preference for the O-methylation of the (+)- rather than the (-)-isomer of isoproterenol. The O-methylation of isoproterenol in the presence of [methyl-3H]-S-adenosyl-l-methionine was stimulated as much as 8-fold by the removal of calcium ions from the incubation solutions. In contrast, the O-methylation of (+)-[3H]isoproterenol by endogenous, intracellular S-adenosyl-l-methionine was only slightly inhibited by the removal of calcium ions from incubation solutions. The formation of [3H]methoxy-isoproterenol from [methyl-3H]-S-adenosyl-l-methionine and isoproterenol was not inhibited by pretreatment of tissues with phenoxybenzamine (32 microM) or treatment with metanephrine (27 mumol 1(-1] or deoxycorticosterone acetate (27 microM), i.e., drug treatments that inhibit the extraneuronal uptake and O-methylation of [3H]-isoproterenol by endogenous intracellular S-adenosyl-l-methionine. The results of this study provide evidence for a nonintracellular, cell-associated site of O-methylation of isoproterenol in the rabbit aorta.