Lynne M. Jones

The Possible Involvement of Phosphatidylinositol Breakdown in the Mechanism of Stimulus-Response Coupling at Receptors which Control Cell-Surface Calcium Gates

Although the general contributions of phospholipids to membrane structure and function are now becoming reasonably well understood, there are still relatively few situations in which one can confidently point to a correlation between a specific metabolic or structural characteristic of an individual lipid and a specific membrane function. However, one of the few very clear indications that individual phospholipids can display highly individual behaviour was provided many years ago by Hokin and Hokin (1) when they discovered that in the pancreas acetylcholine and pancreozymin greatly stimulated the incorporation of 32Pi into phosphatidylinositol (PI), a quantitatively minor anionic membrane phospholipid. Since that time a similar ‘phosphatidylinositol response’ has been observed in cells exposed to some, but not all, of the extracellular stimuli that interact with receptors on cell surfaces. Despite this, there is still no single accepted view on the function of this response in the overall pattern of events brought about by cell stimulation. We have recently developed a hypothesis which proposes that the PI response is intimately involved in the functioning of a variety of cell-surface receptor systems which control cell-surface permeability to Ca2+ ions (2–5). This paper will review the evidence relating to this hypothesis and will speculate briefly on how PI breakdown might be implicated in the control of cell surface Ca2+ gates.

Receptor occupancy dose--response curve suggests that phosphatidyl-inositol breakdown may be intrinsic to the mechanism of the muscarinic cholinergic receptor.

Formation of CAMP has been characterised as an enzyme-catalysed reaction involved in the amplification of the chemical signals which are detected by various types of cell surface receptors [ 1 ] , but little progress has been made towards identification of analogous reactions linked to receptors which do not exert their effects through control of adenylate cyclase. In some cases such reactions may not exist (e.g. the nicotinic cholinergic receptor? [2]) but in others it seems more likely that they have simply eluded identification by the experiments done so far. In most investigations of receptor mechanisms emphasis has been placed on responses which are provoked maximally by low (‘physiological’) concentrations of agonists. We would like to suggest that it might be more appropriate to concentrate on those responses which occur at higher, so-called ‘unphysiological’, agonist concentrations. This is because with many agonists (particularly small molecules such as neurotransmitters) it is necessary for only a small proportion of the cell’s large population of functionally equivalent receptors to interact with agonist in order to provoke a maximal ‘physiological’ response such as secretion or contraction. This is thought to be because activation of only this small fraction of the total receptor population is sufficient to raise the intracellular concentration of a second messenger such as cyclic AMP or Ca*+ enough to stimulate the responsive enzyme(s) fully. High concentrations of agonists are, though, needed to bring about a maximum increase in the second messenger concentration. Thus, if the reaction which causes the change in concentration of a second messenger (for example, adenylate

Effects of alkylating antagonists on the stimulated turnover of phosphatidylinositol produced by a variety of calcium-mobilising receptor systems

We have investigated the effects of two halogenoalkylamine drugs, dibenamine and phenoxybenzamine, on the stimulated phosphatidylinositol turnover that is produced by neurotransmitters and hormones which interact with receptors to bring about an increase in cell surface Ca2+ permeability. The phosphatidylinositol responses we have investigated were those evoked by muscarinic cholinergic stimuli (parotid gland and pancreas), by α-adrenergic stimuli (parotid gland, vas deferens smooth muscle), by pancreozymin or caerulein (pancreas), by phytohaemagglutinin (lymphocytes) and by either 5-hydroxytryptamine or elevation of the extracellular K+ concentration (ileum smooth muscle). Phenoxybenzamine inhibited the muscarinic cholinergic, α-adrenergic, 5-hydroxytryptamine and high K+ responses, but not the responses to phytohaemagglutinin and to pancreozymin (or caerulein). Dibenamine was less effective than phenoxybenzamine in inhibiting the α-adrenergic response and the high K+ response, and it did not inhibit the responses to muscarinic cholinergic stimuli, to 5-hydroxytryptamine or to the polypeptides. N,N-dimethyl-2-bromo-2-phenylethylamine (DMPEA) inhibited the α-adrenergic response, but not the response to muscarinic cholinergic stimulation. The specificity of DMPEA for the α-adrenergic response agrees with its postulated site of action at the noradrenaline-binding site of this receptor system, whereas dibenamine and phenoxybenzamine are less specific drugs which inhibit a variety of the ‘physiological’ responses of cells, including those to muscarinic cholinergic, H1-histaminergic, α-adrenergic and 5-hydroxytryptamine stimuli. Previously, we suggested that dibenamine and phenoxybenzamine might show a constant pattern of effects on the phosphatidylinositol responses evoked through different receptors, phenoxybenzamine being inhibitory and dibenamine without effect [Jafferji & Michell (1976) Biochem. J. 160, 163–169]. However, this pattern has not been sustained throughout the present study of a larger range of Ca2+-mobilising stimuli.