A variety of cellular activities are often regulated by two opposing groups of extracellular messengers. In many tissues it seems plausible that cyclic AMP (CAMP) plays a role in the inhibitory cellular processes, which may be induced by a group of hormones, neurotransmitters, prostaglandins and many other extracellular messengers. On the other hand, Ca2+ has been proposed to play a role of crucial importance in the activation of cellular functions, although the precise mechanism of this transmembrane signaling has not yet been clarified. This chapter describes a new receptor function that is recently found in laboratory studies. This receptor function appears to be directly related to phosphatidylinositol turnover, which is induced by another group of extracellular messengers, such as acetylcholine (muscarinic), catecholamines, various peptide hormones, and many other biologically active substances, whose actions are not mediated by CAMP but are implicated in Ca2+ gating or movement. In a manner analogous to the CAMP-dependent signal pathway, a new species of protein kinase is involved as an essential ingredient in this receptor function. This protein kinase, referred to as “protein kinase C,” is found widely in mammalian tissues, and has been purified from rat brain to near homogeneity. The enzyme absolutely requires Ca2+ and membrane phospholipid for its activation. In addition, a small quantity of diacylglycerol derived from the receptor-linked cleavage of phosphatidylinositol dramatically increases the affinity of enzyme for Ca2+ as well as for phospholipid, and thereby initiates the activation of enzyme at less than micromolar concentrations of Ca2+.
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