Adenosine 3',5'-cyclic-monophosphate-dependent regulation of alpha 1-adrenergic receptor number in rabbit aortic smooth muscle cells.

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Abstract
The purpose of this study was to determine whether a cyclic adenosine 3',5'-monophosphate-dependent process can be involved in the regulation of vascular smooth muscle alpha 1-adrenergic receptor responsiveness. Experiments were performed in cultured rabbit aortic smooth muscle cells which were characterized previously according to alpha-adrenergic receptor-binding characteristics and receptor-coupled norepinephrine-stimulated 45Ca++ efflux. The addition of dibutyryl-cyclic adenosine monophosphate to the cell culture medium for 24 hours resulted in a concentration-related decrease in maximal [3H]prazosin-binding capacity (41 +/- 4% decrease with 1 mM dibutyryl-cyclic adenosine monophosphate) without an effect on [3H]prazosin-binding affinity. Prostaglandin E1 (10 microM) and forskolin (10 microM) caused similar decreases in maximal [3H]prazosin-binding capacity, whereas butyrate (1 mM) and dibutyryl-guanosine-3',5' cyclic-monophosphate (1 mM) had no effect. Dibutyryl-cyclic adenosine monophosphate (1 mM) caused significant potentiation of the decrease in [3H]prazosin-binding caused by a submaximal (10 nM) but not a maximal (10 microM) concentration of norepinephrine, suggesting that cyclic adenosine monophosphate may act at a distal step in common with norepinephrine to reduce alpha-adrenergic receptor number. Despite the approximately 41% reduction in alpha-adrenergic receptor number following 24-hour incubation of cells with dibutyryl-cyclic adenosine monophosphate, maximal norepinephrine-stimulated 45Ca++ efflux was not reduced, consistent with the markedly nonlinear relationship between alpha-adrenergic receptor occupancy and maximal norepinephrine-stimulated 45Ca++ efflux in this cell system. These data provide evidence for a novel mechanism by which hormones or drugs which increase cyclic adenosine monophosphate levels can modulate alpha-adrenergic responsiveness in vascular smooth muscle.