Interaction between alpha 2- and beta-adrenergic receptors in rat cerebral cortical membranes: clonidine-induced reduction in agonist and antagonist affinity for beta-adrenergic receptors.
The interaction between alpha 2- and beta-adrenergic receptors was investigated in rat cerebral cortical membranes. Clonidine inhibition of [3H]dihydroalprenolol ([3H]DHA) binding resulted in biphasic competition curves with a mean Hill coefficient of 0.45. The addition of 1 microM yohimbine caused a rightward shift of the first portion of the clonidine inhibition curve. In the presence of 1 microM clonidine, the maximum concentration which did not inhibit [3H]DHA binding, inhibition curves of [3H]DHA binding by isoproterenol shifted to the right. A mean Hill coefficient increased from a control value of 0.63 to 0.76. Computer modeling analysis revealed that 1 microM clonidine decreased a beta-adrenergic high-affinity state from 28% to 13%. However, the addition of 1 microM yohimbine completely prevented the clonidine-induced reduction in the beta-adrenergic high-affinity state. In the presence of 200 microM GTP, the effect of clonidine was not observed. In addition, Kd and Bmax values for [3H]p-aminoclonidine ([3H]PAC) binding were not significantly changed by the addition of 100 nM isoproterenol, the maximum concentration which did not inhibit [3H]PAC binding. Moreover, isoproterenol inhibition of [3H]PAC binding resulted in steep competition curves with a mean Hill coefficient of 0.97. The addition of 1 microM alprenolol did not affect the isoproterenol inhibition curve. These data demonstrated that clonidine caused a decrease in agonist and antagonist affinity for beta-adrenergic receptors, while isoproterenol did not modulate the binding characteristics of alpha 2-adrenergic receptors. Furthermore, these results suggest that regulation between alpha 2- and beta-adrenergic receptors is not bidirectional, but is instead unidirectional from alpha 2-adrenergic receptors to beta-adrenergic receptors.
Nakamura
T
T
Department of Psychiatry, Mie University School of Medicine, Japan.
Tsujimura
R
R
Nomura
J
J
D000818:Animals; D001667:Binding, Competitive; D002540:Cerebral Cortex / Q000378:metabolism*; D003000:Clonidine / Q000031:analogs & derivatives / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000494:pharmacology*; D004082:Dihydroalprenolol / Q000037:antagonists & inhibitors; D006160:Guanosine Triphosphate / Q000494:pharmacology; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D011943:Receptors, Adrenergic, beta / Q000502:physiology*; D013566:Sympathomimetics / Q000378:metabolism*; D015016:Yohimbine / Q000494:pharmacology
D011942:Receptors, Adrenergic, alpha; D011943:Receptors, Adrenergic, beta; D013566:Sympathomimetics; D015016:Yohimbine; D004082:Dihydroalprenolol; C016986:apraclonidine; D006160:Guanosine Triphosphate; D007545:Isoproterenol; D003000:Clonidine