COREGULATION OF ADRENOCEPTORS AND THE LIPID ENVIRONMENT IN HEART-MUSCLE DURING REPEATED ADRENERGIC-STIMULATION

The purpose of this study was to examine changes in binding properties of alpha 1-adrenoceptors, beta-adrenoceptors and L-Ca channels in rat heart muscle in relation to changes in the lipid environment within the membrane, i.e. fatty acyl chain composition of membrane phospholipids, occuring during repeated adrenergic stimulation. The effect of daily administration of epinephrine for seven days upon the maximum number of binding sites of adrenoceptors and upon the fatty acyl chain composition of phosphatidylcholine and phosphatidylethanolamine was examined in 10 months old rats. Decrease in Bmax of adrenoceptors during repeated adrenergic stimulation coincided with remodeling of the membrane phospholipids, with replacement of 18:2n-6 by 20:4n-6 in phosphatidylcholine and by 22:6n-3 in phosphatidylethanolamine, respectively. The effect of repeated adrenergic stimulation was also examined in rats fed different dietary fats and oils, i.e. butter, corn oil or cod liver oil, in hearts with markedly different levels of 18:2n-6, 20:4n-6 and 22:6n-3. The binding properties of the adrenoceptors and L-Ca channels did not relate to the fatty acyl chain composition of bulk phospholipids, but the epinephrine induced decrease in Bmax of the receptors was always accompanied by replacement of 18:2n-6 by 20:4n-6 in phosphatidylcholine and by 22:6n-3 in phosphatidylethanolamine, regardless of the initial level of these fatty acyl chains in the phospholipids. It is concluded that adaption to repeated adrenergic stimulation may include coregulation of the lipid environment within the membrane and the binding properties of adrenoceptors, and possibly other functionally coupled proteins such as L-Ca channels, residing in the membrane.