Serotonin-induced acute desensitization of serotonin2 receptors in human platelets via a mechanism involving protein kinase C.
Serotonin (5-HT)-induced changes in the levels of intracellular Ca++ were analyzed in human platelets, using the Ca+(+)-sensitive dye 1-(2-(5'-carboxyoxazol-2'-yl)-6-aminobenzofuran-5-oxy)-2-(2' -amino-5'- methylphenox)-ethane-N,N,N',N'-tetraacetic acid, pentaacetoxymethyl ester, to investigate the regulation of 5-HT2 receptor function. Serotonin mobilized intracellular Ca++ in a dose-dependent fashion from basal level of 98 +/- 2.7 and up to 211 +/- 5.8 nM with an EC50 value for 5-HT of 0.2 microM. Ketanserin, a 5-HT2 antagonist, reversed the 5-HT (10 microM)-induced Ca++ increase in a dose-dependent manner with an IC50 value of 2 nM. An initial treatment with 10 microM 5-HT abolished the response to a second treatment with 100 microM 5-HT, suggesting that 5-HT evoked an acute desensitization of 5-HT2 receptors in human platelets. Mezerein and phorbol 12-myristate 13-acetate, activators of protein kinase C, inhibited 5-HT-stimulated inositol monophosphate accumulation with IC50 values of 3 and 10 nM, respectively. Furthermore, a protein kinase C inhibitor 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride prevented the protein kinase C activator-induced inhibition against 5-HT-mediated inositol monophosphate accumulation. Mezerein also inhibited 5-HT (10 microM)-mediated Ca++ release with an IC50 value of 3 nM. 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride prevented the inhibition by mezerein of the 5-HT-stimulated Ca++ increase. Moreover, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride by itself enhanced the Ca++ spike induced by 100 microM 5-HT, the plateau phase induced by 10 microM 5-HT and the second response to 5-HT. These findings suggest that 5-HT2 receptor activation mobilizes intracellular Ca++ in human platelets and that this receptor may be desensitized acutely by a protein kinase C mediated feedback system.
Division of Mental Disorder Research, National Institute of Neuroscience, Tokyo, Japan.
A
A
Kagaya
M
M
Mikuni
I
I
Kusumi
H
H
Yamamoto
K
K
Takahashi
D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D000662:Amphetamines / Q000494:pharmacology; D001792:Blood Platelets / Q000187:drug effects* / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D006801:Humans; D007295:Inositol Phosphates / Q000378:metabolism; D007546:Isoquinolines / Q000494:pharmacology; D007650:Ketanserin / Q000494:pharmacology; D010879:Piperazines / Q000494:pharmacology; D011493:Protein Kinase C / Q000502:physiology*; D011985:Receptors, Serotonin / Q000187:drug effects*; D012701:Serotonin / Q000494:pharmacology*
D000662:Amphetamines; D007295:Inositol Phosphates; D007546:Isoquinolines; D010879:Piperazines; D011985:Receptors, Serotonin; D012701:Serotonin; D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D007650:Ketanserin; D011493:Protein Kinase C; C015952:4-iodo-2,5-dimethoxyphenylisopropylamine; D002118:Calcium