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Studies of receptor-mediated inhibition of 45Ca accumulation into synaptosomes.

Abstract：

:1. The effects of alpha 2-adrenoceptor and kappa-opiate receptor activation on 45Ca accumulation into rat cortical synaptosomes were examined. 2. Clonidine (1 microM) and U50488H (1 microM) significantly reduced 45Ca accumulation under both resting (5 mM K+) and depolarizing (15-30 mM K+) conditions. 3. The inhibitory effects of the agonists on 45Ca accumulation into synaptosomes were enhanced in the presence of the Na(+)-Ca2+ exchange inhibitor sodium orthovanadate (vanadate, 2 mM), and were not present in mitochondrial preparations. 4. When the agonists were used together, their inhibitory effects were not additive but were, in fact, attenuated. 5. In the presence of the alpha 2-adrenoceptor antagonist idazoxan (1 microM), the inhibitory effect of U50488H on 45Ca accumulation was enhanced. A similar increase in the inhibitory effectiveness of clonidine was observed in the presence of naloxone (20 microM). 6. When synaptosomes were pretreated with 3-isobutyl-1-methylxanthine (IBMX, 0.5 mM), dibutyrylcyclic AMP (db-cyclic AMP, 10 microM) or 8-bromo-cyclic AMP (8Br-cyclic AMP, 10 microM), the inhibitory effects of clonidine and U50488H were abolished, suggesting that a decrease in cyclic AMP production is part of the receptor-effector coupling mechanism of both receptor systems. 7. The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA, 0.05 microM) increased 45Ca accumulation but did not alter the inhibitory effects of clonidine or U50488H, thus showing that the effects of the agonists are not mediated by protein kinase C. 8. We conclude that alpha 2-adrenoceptor and Kappa-opiate receptor activation dramatically reduce 45Ca influx through Ca21 channels (e.g., by 50%), that there is a functional antagonism between the two receptor systems and that in both cases, the receptor effector mechanism involves a decrease in cyclic AMP production.

journal_name

Br J Pharmacol

journal_title

British journal of pharmacology

authors

Xiang JZ,Brammer MJ,Campbell IC

doi

10.1111/j.1476-5381.1990.tb12103.x

subject

Has Abstract

pub_date

1990-09-01 00:00:00

pages

140-4

issue

eissn

0007-1188

issn

1476-5381

journal_volume

101

pub_type

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Studies of receptor-mediated inhibition of 45Ca accumulation into synaptosomes.