A Possible Role for Phosphatidylinositol Breakdown in Muscarinic Cholinergic Stimulus-Response Coupling

Abstract

A substantial body of evidence now favours the view that an increase in intracellular Ca2+ concentration, caused largely by a downhill flow of Ca2+ ions through a receptorcontrolled Ca2++-gating system at the plasma membrane, is the factor that couples activation of muscarinic cholinergic receptors to the initiation of a physiological response in target cells (Rubin, 1970, 1974; Hurwitz & Suria, 1971; Triggle, 1972; Chang & Triggle, 1973a; Douglas, 1974). Much of this evidence has come from studies of exocrine (e.g. pancreas and parotid gland) and endocrine (adrenal medulla) exocytotic secretory tissues and from analysis of the contractile responses of gastrointestinal smooth muscles, but even in tissues where the evidence is less complete there is at present no very strong indication of any other form of coupling between activation of this type of receptor and cell responses. The only muscarinic response that is particularly difficult to understand in these terms is the muscarinic cholinergic feedback-inhibition of acetylcholine release from nerve terminals (Muscholl, 1973 ; Szerb & Somogyi, 1973). In addition, there is evidence which indicates that some of the Ca2+ ions responsible for the triggering of the initial ‘phasic’ response of stimulated tissues are released from a membrane-bound pool at the plasma membrane (Chang & Triggle, 1973~; Triggle & Triggle, 1976; Petersen & Ueda, 1976). Studies with Ca2+antagonistic cations, both inorganic (e.g. Mn2+, La3+ and Tm3+) and organic [e.g. nifedipineormethoxyverapamil(D600); Fleckenstein etal., 19751, haveindicated that the sensitivities to inhibition of the phasic response and of the sustained tonic response are often similar, despite the fact that the phasic response is triggered by Ca2+ release from a membrane-bound pool and the tonic response by an influx of Ca2+ ions from the exterior (Triggle & Triggle, 1976; Ticku & Triggle, 1976). It has also been noted that the same Ca2+ antagonists are inhibitory to the entry of Ca2+ ions both through receptorcontrolled gates and through the potential-sensitive slow-respondingCa2+ gates of tissues such as squid axon, heart muscle and ileum muscle (Fleckenstein et al., 1975 ; Godfraind & Kaba, 1972; Ticku & Triggle, 1976). Electrophysiological evidence indicates that ion movements are not detected in muscarinically stimulated cells until about 100-200ms after the application of a stimulus and that the effect of receptor activation may persist for many seconds after removal of the stimulus (Bolton, 1975; Purves, 1976). It therefore seems probable that the initial rise in Ca2+ concentration within a muscarinically controlled cell is a relatively slow process, with kinetics more reminiscent of the enzyme-catalysed production of a signal such as CAMP than of a dramatic event of the type produced by the rapid opening of ion channels through conformational re-arrangement of a pore-forming protein (e.g. the nicotinic cholinergic receptor), and that closure of the Ca2+ gates is also a relatively slow process. Scheme 1 shows a speculative attempt to synthesize this information into a working model of a muscarinic receptor-sensitive Ca2+-gating system: this scheme is partially based on those considered before by Triggle (1972), Hurwitz & Suria (1971), Matthews (1974) and others. A cell surface Ca2+-binding site, which is in relatively slow equilibrium with the extracellular CaZ+ pool and which lies in or adjacent to a closed Ca2+ gate, is occupied by Ca2+ ion in the unstimulated cell. When a receptor is activated this triggers an enzyme reaction that somehow activates the gate mechanism and this bound Ca2+ ion is released and passes into the cell; the channel then remains open for the subsequent passage of a considerable number of additional Ca2+ ions. The opening of the gate and the release of the Ca2+ ion from its binding site are probably consequences