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KMID : 0371919910040010172
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Journal of Wonju College of Medicine
1991 Volume.4 No. 1 p.172 ~ p.178
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Effects of Monovalent Cations on the$Adrenergic Receptor-Adenylyl Cyclase System in Rat Adipocytic Membranes
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Abstract
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It has been known that the activity of adenylyl Cyclase can be modified by the various ions of the assay medium. These effects vary with both species and tissues. In the present experiment, we studied the effects of monovalent cations such Li´. K+ and Na* on the isoproterenol sodium fluoride- or forskolin-stimulated adenylyl cyclase activities of rat adipocytes in an effort to determine the sites of action of these monovalent cations. Adenylyl Cyclase activities were determined in the presence of 0 to 80 mM of LiCl, KCl or NaCl using adipocytic plasma membrane fractions prepared from Sprague-Dawley rats weighing 150-200 g.
Na´ had little effects on isoproterenol-, sodium fluoride- or forskolin-stimulated adenylyl Cyclase activities. However, K* and Li´ showed rather complex effects. Both K´ and Li+ inhibited sodium fluoride- or forskolin- stimulated adenylyl Cyclase similarly in proportion to concentrations of these cations in. the assay mixtures, suggesting that these effects were primarily on the catalytic subunit of adenylyl Cyclase system. In isoproterenol-stimulated adenylyl cyclase activity determinations, both Li* and K+ produced biphasic effects. Thus, these enzyme activities were inhibited by low concentrations of Li´ or K´, and around 5 to 20 mM of salts the activities were at the nadir. At salt concentrations greater than 20 mM the enzyme activities began to rise and at 80 mM they were slightly greater than or about the same as that in the absence of the salt. This suggests that these stimulatory effects at high Li´ or K´ concentrations may be caused by their actions on the R-adrenergic receptor molecules. At all concentrations of , GTP used the enzyme activities were slightly elevated by 80 mM of these salts, but the typical biphasic curve shape shown by GTP were maintained, suggesting that, rather than the coupling of the receptors with G proteins, the affinity of ,8-adrenergic receptors for the ligand might be slightly elevated by high concetrations of these salts.
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