Mechanisms of action of calcium‐mobilizing agonists: some variations on a young theme

Abstract

It is now accepted that many hormones and neurotransmitters exert their effects through G protein-mediated activation of a phospholipase C, which breaks down phosphatidylinositol bisphosphate. This releases inositol trisphosphate, which mobilizes intracellular calcium, and diacylglycerol, which, in turn, activates protein kinase C. However, recent evidence indicates that other mechanisms are involved. In some cells, the increases in cytosolic calcium elicited within 1-2 s by high concentrations of agonists or at later times by low, physiological concentrations of agonists occur without any detectable changes in inositol phosphates and calcium mobilization, and result from the opening of plasma membrane channels that are permeable to Ca2+. This response appears to be mediated more directly by G proteins. These findings question the postulated roles of inositol phosphates and calcium mobilization in the stimulation of calcium influx. Measurements of the mass and fatty acid composition of the inositol phospholipids and of the diacylglycerol and phosphatidic acid generated by agonists in several cell types indicate that phosphatidylinositol bisphosphate is probably a minor source of these lipids. On the other hand, measurements of phosphatidylcholine, choline, and phosphocholine indicate that this phospholipid is a major source, and that its breakdown involves both phospholipase C and D. These findings indicate that phosphatidylcholine breakdown may be more important than phosphoinositide hydrolysis in the regulation of protein kinase C and perhaps other cell functions.

Cyclic adenosine monophosphate and diacylglycerol. Mutually inhibitory second messengers in cultured rat inner medullary collecting duct cells.

“…1). It has been suggested that production ofDAG, the endogenous activator ofprotein kinase C, may result not only from the hydrolysis of polyphosphoinositides but of other phospholipids (e.g., phosphatidylcholine) as well (30). It is important to note, therefore, that pretreatment with ClPheScAMP inhibits EGF-stimulated DAG production (Fig.…”

“…As activation of PLC results in hydrolysis of not only phosphatidylinositol bisphosphate but of phosphatidylinositol monophosphate and phosphatidylinositol as well (and perhaps other phospholipids, e.g., phosphatidylcholine) production of DAG, on a molar basis, is likely to exceed productionof IP3 (30). Therefore, inhibition of AC by concentrations of EGF below the threshold for detectionofIP3 production itself suggests that the inhibition is a consequence of DAG production.…”

Diacylglycerols induce both ion pumping in patch-clamped guard-cell protoplasts and opening of intact stomata.

“…Additional physiological studies linking second messengers to ion transport activities and assays of endogenous levels of InsP3 and diacylglycerol are required to further define the roles of these second messengers in guard cells. Diacylglycerol can be formed from phosphatidylinositol 4-phosphate, phosphatidylinositol, or phosphatidylcholine without concomitant production of InsP3, and temporal differences in levels of InsP3 and diacylglycerol have been noted (6). Steady-state stomatal aperture, which reflects a balance between opening and closing processes (15,28), may be determined by the relative quantities and actions of diacylglycerol and InsP3 present under a given environmental condition.…”

“…Diacylglycerol is produced endogenously via several biochemical pathways, including hydrolysis ofthe phosphatidylinositols and phosphatidylcholines (6). In animal systems, diacylglycerol activates a Ca2+-and phospholipid-dependent kinase, protein kinase C (PKC).…”

Modulation of the hepatic α1‐adrenoceptor responsiveness by colchicine: dissociation of free cytosolic Ca2+‐dependent and independent responses

“…The hepatic actions of the Ca2" mobilizing agonists have been intimately associated with their ability to increase the intracellular concentration of free cytosolic Ca2" and the subsequent activation of Ca2"-dependent processes (Williamson et al, 1985;Exton 1988). However, work showing that alagonists may activate hepatic functions under Ca2" loading conditions that precluded any further significant increase in the cytosolic free Ca2" concentration (Buxton et al, 1982;Garcia Sainz & Hernandez-Sotomayor, 1985;Saz et al, 1989), supported the operation of a free cytosolic Ca2+-independent branch of the signalling pathway.…”