CALCIUM-SENSITIVITY OF INOSITOL 1,4,5-TRISPHOSPHATE METABOLISM IN EXOCRINE CELLS FROM THE AVIAN SALT-GLAND

The generation of inositol phosphates upon muscarinic-receptor activation was studied in [H-3]inositol-loaded exocrine cells from the nasal salt glands of the duck Anas platyrhynchos, and the metabolism of different inositol phosphates in vitro was studied in tissue homogenates, with particular refe...

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Bibliographic Details
Published in:Biochemical journal 1992-03, Vol.282 (3), p.703-710
Main Authors: HILDEBRANDT, JP, SHUTTLEWORTH, TJ
Format: Article
Language:English
Subjects:
Analytical, structural and metabolic biochemistry
Anas platyrhynchos
Animals
Biochemistry & Molecular Biology
Biological and medical sciences
Calcium - pharmacology
Calcium - physiology
Ducks
Fundamental and applied biological sciences. Psychology
In Vitro Techniques
Inositol - pharmacokinetics
Inositol 1,4,5-Trisphosphate - metabolism
Inositol 1,4,5-Trisphosphate - pharmacokinetics
Inositol Phosphates - metabolism
Intermediary metabolites. Miscellaneous
Life Sciences & Biomedicine
Other biological molecules
Phosphotransferases (Alcohol Group Acceptor)
Phosphotransferases - physiology
Salt Gland - cytology
Salt Gland - metabolism
Science & Technology
Signal Transduction - physiology
Tritium
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Summary:The generation of inositol phosphates upon muscarinic-receptor activation was studied in [H-3]inositol-loaded exocrine cells from the nasal salt glands of the duck Anas platyrhynchos, and the metabolism of different inositol phosphates in vitro was studied in tissue homogenates, with particular reference to the possible interaction of changes in intracellular [Ca2+] ([Ca2+]i) with the metabolic processes. In intact cells, there was a rapid (within 15 s) generation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4, followed by an accumulation of their breakdown products, Ins(1,3,4)P3 and inositol bis- and mono-phosphates. Ca2+-sensitivity of the Ins(1,4,5)P3 3-kinase was demonstrated in tissue homogenates, with the rate of phosphorylation increasing 2-fold at free Ca2+ concentrations > 1-mu-M. However, addition of calmodulin or the presence of the calmodulin inhibitor W-7 (up to 100-mu-M) had no effect. 3-Kinase activity increased proportionally with the initial Ins(1,4,5)P3 concentration up to 1-mu-M, but a 10-fold higher substrate concentration produced only a doubling in the phosphorylation rate. Ins(1,3,4,5)P4 was dephosphorylated to Ins(1,3,4)P3, which accumulated in the homogenate assays as well as in intact cells. Depending on its concentration, Ins(1,3,4)P3 was phosphorylated [in part to Ins(1,3,4,6)P4] or dephosphorylated. To investigate the Ca2+-sensitivity of the 3-kinase in intact cells, excess quin2 was used to buffer the receptor-mediated transient changes in [Ca2+]i in [H-3]inositol-loaded cells. These experiments revealed that increasing [Ca2+]i, from < 100 to approx. 400 nM (i.e. within the physiological range) has no effect on the partitioning of Ins(1,4,5)P3 metabolism (phosphorylation versus dephosphorylation) and on the accumulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4. This indicates that activation of the 3-kinase by physiologically relevant Ca2+ concentrations may not play a major role in the generation of Ins(1,3,4,5)P4 signals upon receptor activation in these cells. The latter are mainly achieved by the receptor-mediated increase in Ins(1,4,5)P3 in the cell and its phosphorylation by the 3-kinase in a substrate-concentration-dependent manner.
ISSN:0264-6021
1470-8728
DOI:10.1042/bj2820703