Phosphatidvlinositol breakdown and Ca2+ -mobilizing receptors A universal feature of hormones and neurotransmitters that exert their physiological effects through changes in cytosolic Ca2+ ion concentration is their ability to influence the metabolism of PtdIns* (see Michell & Kirk, 1981, for references). Hormones and neurotransmitters of this group enhance the incorporation [32PIP, into PtdIns in their target cells and, wherever the appropriate experiments have been performed, this enhanced PtdIns labelling has been shown to be secondary to hormone-stimulated PtdIns degradation (Michell & Kirk, 198 1). Unlike the other physiological effects of such ligands, enhanced PtdIns breakdown in relatively insensitive to Ca2+ depletion and it is not evoked by the ionophore A23 187. These observations lead to the suggestion that the degradation of PtdIns is a direct consequence of receptor activation, which precedes, and may evoke, Ca2+-mobilization in the cytosol (Michell, 1975). We have sought to test this hypothesis in rat hepatocytes, where at least three Ca*+-mobilizing hormones (vasopressin, angiotensin and adrenaline acting through a, receptors) cause a breakdown of PtdIns that does not appear to be mediated by a rise in intracellular Ca2+ concentration (Kirk et al., 1977, 1978. 1979, 1980, 1981; Billah & Michell, 1979; Michell et al., 1979: Tolbert et al., 1980). A further indication that PtdIns breakdown may be involved in the mechanism of stimulus-response coupling at hepatic vasopressin receptors was provided by the observation that the concentration dependence of vasopressinstimulated PtdIns breakdown is very similar to the binding curve for 3H-Lyss]vasopressin at hepatocyte receptors (Fig. I) . In contrast, the curves describing the concentration dependence of hepatocyte Ca2+-mobilization and glycogen phosphorylase activation are displaced by about one and two orders of magnitude respectively to the left (Fig. I). We interpret these results to indicate that only a small proportion of the hepatic vasopressin receptors need be occupied to provoke maximum phosphorylase activation, whereas maximum Ca2+ release requires greater (but submaximal) receptor occupation. Thus there is a 'receptor reserve' for these two effects, but maximum PtdIns degradation is only provoked when all the vasopressin receptors are occupied (Kirk et al., 1981). It therefore seems that vasopressin-stimulated PtdIns degradation in hepatocytes is not a consequence of intracellular Ca2+-mobilization and it appears instead to be intimately coupled to receptor occupation. It thus fulfills the basic criteria expected of a reaction involved in stimulus-response coupling at hepatic vasopressin receptors, but our attempts to demonstrate the phenomenon in cell-free systems have so far failed. Furthermore, when hepatocytes are stimulated with vasopressin for 5min and then subjected to subcellular fractionation, PtdIns depletion is not confined to the plasma membrane (the site of hormone receptors), but is observed in fractions enriched in membranes from a variety of intracellular organelles (Kirk et al., I98 1).