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Permissive Role of Calcium in α1‐Adrenergic Stimulation of Pineal Phosphatidylinositol Phosphodiesterase (Phospholipase C) Activity
Author(s) -
Ho Anthony K.,
Chik Constance L.,
Klein David C.
Publication year - 1988
Publication title -
journal of pineal research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.881
H-Index - 131
eISSN - 1600-079X
pISSN - 0742-3098
DOI - 10.1111/j.1600-079x.1988.tb00798.x
Subject(s) - medicine , pinealocyte , phospholipase c , endocrinology , phosphatidylinositol , phosphodiesterase , stimulation , prazosin , inositol trisphosphate , phenoxybenzamine , chemistry , inositol , egta , phospholipase , biology , calcium , receptor , biochemistry , melatonin , pineal gland , antagonist , signal transduction , propranolol , enzyme
Activation of α1‐adrenergic receptors increases [Ca +2 ]i and phosphatidylinositol phosphodiesterase (phospholipase C) activity in the pinealocyte. In this report the receptor involved in the stimulation of phospholipase C activity was further characterized, and the role of Ca 2+ in this effect was investigated in some detail. Phospholipase C activity was estimated by measuring the production of [ 3 H]inositol phosphates by [ 3 H]inositol‐labelled dispersed pinealocytes in suspension culture. Norepinephrine stimulated [ 3 H]inositol monophosphate production severalfold; this was blocked by α1‐adrenergic antagonists, including prazosin, WB 4101, and phenoxybenzamine, but by neither an α2‐ nor a β‐adrenergic antagonist, confirming that an α1‐adrenoceptor is involved in the regulation of phosphatidylinositol hydrolysis. Treatment with the Ca 2+ chelator, EGTA, or with inorganic Ca 2+ blockers, including Co 2+ , Mn 2+ , and La 3+ , reduced the norepinephrine‐stimulated response, suggesting that the α1‐adrenergic stimulation of phospholipase C activity is Ca 2+ dependent. However, phospholipase C activity was not increased by elevating intracellular Ca 2+ with either the Ca 2+ ionophore A23187 or with depolarizing concentrations of K + . These results indicate that although Ca 2+ is necessary for α1‐adrenergic stimulation of phospholipase C activity, an increase in [Ca 2+ ]i alone is not sufficient to stimulate the activity of this enzyme, and that effects which A23187 and depolarizing concentrations of K + have on pineal function probably do not involve stimulation of phospholipase C activity.