Abstract
Calcium-specific ionophores are used widely to stimulate Ca2+-dependent secretion from cells on the assumption that permeabilization of the cell membranes to Ca2+ ions leads to a rise in concentration of cytosolic Ca2+ ([Ca2+]i), which in turn serves as a signal for secretion. In this way, events that precede mobilization of Ca2+ ions via receptor stimulation are bypassed. One such event is thought to be the rapid hydrolysis of membrane inositol phospholipids to form inositol phosphates and diacylglycerol. Accordingly, rat leukemic basophil (2H3) cells can be stimulated to secrete histamine either with the ionophores or by aggregation of receptors for IgE in the plasma membrane. We find, however, that ionophore A23187 stimulates secretion of histamine only at concentrations (200-1000 nM) that stimulate hydrolysis of membrane inositol phospholipids. The extent of hydrolysis of inositol phospholipids was dependent on the concentration of ionophore and the presence of external Ca2+ ions and correlated with the magnitude of the secretory response. A similar correlation between secretion and hydrolysis of inositol phospholipids was observed in response to the Ca2+-specific ionophore, ionomycin. Although this hydrolysis (possibly a consequence of elevated [Ca2+]i) was less extensive than that induced by aggregation of receptors, it may govern the secretory response to A23187. The studies revealed one paradox. The rise in [Ca2+]i depended on intracellular ATP levels, when either an ionophore or antigen was used as a stimulant irrespective of whether hydrolysis of inositol phospholipids was stimulated or not. The concept of how the ionophores act, therefore, requires critical reevaluation.
Highlights
The Actionsof Ca2+ Ionophores on Rat Basophil(i2cH3) Cells Are Dependent on Cellular ATP andHydrolysis of Inositol Phospholipids
Calcium-specific ionophoresare used widely to stim- The hydrolysis is associated with a rise in cytosolic Ca2+
Inositol 1,4,5-trisphosphate [8].Release of Ca2+from intrahowever, that ionophore A23187 stimulates secretion cellular stores by the trisphosphate [9,10,11,12,13,14,15] is thought to proof histamineonly at concentrations(200-1000 nM) vide one signal for secretion
Summary
From the Laboraton, of Chemical Pharmacology, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892. Rat leukemic antigen-stimulated 2H3 cells [5, 6] This is in contrast to basophil (2H3) cells can be stimulated to secrete his- other secretory cells in which the rise in [Ca2+Ii1is closely tamine either with the inophoreosr by aggregation of correlated with the increases in the intracellular levels of receptors for IgE in the plasma membrane. The rise in [Ca2+Ii depletion of intracellular ATP should suppress the formation depended on intracellular ATPlevels, when either an of phosphatidylinositol bisphosphate and, as a consequence, ionophore or antigen waussed as a stimulant irrespec- the formation of inositol 1,4,5-trisphosphateinstimulated tive of whether hydrolysis of inositolphospholipids cells. The cells can be stimulated to secrete histamine by aggregation of these receptors with covalently linked oliof the Ca2+ionophores, we undertook a systematicevaluation of the stimulatory and secretory events incells stimulated by either antigen or the Ca2+ionophores, as to the effects of ATP depletion on these events
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
