Phospholipid methylation in canine cardiac membranes. Relations to beta-adrenergic receptors and digitalis receptors.

Abstract

Phospholipid methylation of canine cardiac membranes was studied in vitro to determine its relationship to the regulation and function of beta-adrenergic and digitalis receptors. Cardiac membranes were prepared from the left ventricle of mongrel dogs. Phospholipid methylation was assayed by measuring [3H] methyl incorporation after incubation with methyl donor S-adenosyl-L-[methyl-3H] methione at 37 degrees C. The reaction of phospholipid methylation had an optimum pH around 9 and a linear time course of up to 60 min. The reaction was inhibited by the addition of S-adenosyl-L-homocysteine, a known antagonist of biological transmethylation. L-isoproterenol enhanced phospholipid methylation markedly, but ouabain had no effect. On the other hand, after phospholipid methylation in cardiac membranes was stimulated by pre-incubation with S-adenosyl-L-methionine for 60 min at 37 degrees C, [125I] iodohydroxybenzylpindolol binding to the membranes was increased. The number of the beta-adrenergic receptor binding sites (Bmax), calculated from Scatchard analysis in cardiac membranes of 3 dogs significantly increased. On the other hand, [3H] ouabain binding and Na+, K+, Mg2+-ATPase activity were not increased. The increase in [125I] iodohydroxybenzylpindolol binding was inhibited by the addition of S-adenosyl-L-homocysteine. These binding suggest that phospholipid methylation is closely related to beta-adrenergic receptors, but not digitalis receptors in cardiac membranes.