Phorbol Ester - and Diacylglycerol - Mediated Desensitization of Cardiac / ft - Adrenergic Receptors

There are specific phorbol ester receptors on cardiac myocytes which may be identical with the calcium/phospholipid-dependent protein kinase (protein kinase C). Incubation of enzymatically dissociated rat cardiac myocytes with biologically active phorbol esters (such as 40phorbol-12,13-dibutyrate and 12-O-tetradecanoyl phorbol13-acetate) leads to a timeand concentration-dependent loss of jS-adrenergjc receptors detectable with the hydrophilic ligand [H]CGP-12177. This loss is attributable to a reduction in both maximal /S-receptor numbers and their affinities. The synthetic diacylglycerol, l-oleyl-2-acetyldiglycerol, which is known to activate protein kinase C, also induces desensitization of 0-receptors. Both phorbol diburyrate and 1-oleyl2-acetyldiglycerol have additive effects to isoproterenol, suggesting a separate site of action in promoting 0-receptor desensitization. The effects of phorbol diburyrate and l-oleyl-2-acetyldiglycerol are prevented by colchicine (but not its inactive analog, trimethylcolchicinic acid), indicating a microtubule dependence. The loss of membrane-bound /3-receptors after phorbol dibutyrateor l-oleyl-2-acetyldiglycerol preincubation is accompanied by an increase in /3receptors associated with a cytosol-derived vesicular fraction devoid of plasma membrane markers, a finding consistent with an intemalization process. These results suggest that protein kinase C activation by diacylglycerols derived from receptor-linked phosphoinositide hydrolysis may be a novel mechanism of cardiac /3-receptor desensitization. (Circ Res 57: 443-449, 1985)