Hormonal stimulation of phosphatidylinositol breakdown with particular reference to the hepatic effects of vasopressin.

Stimulated phosphatidylinositol breakdown, followed by compensatory resynthesis, is a response of a wide variety of cells to many hormones and neurotransmitters. Until recently, most work on this response employed secretory tissues, nervous tissues or smooth muscles, and the most intensively studied stimuli were muscarinic, cholinergic and a-adrenergic. As a tentative synthesis of all of these past studies, we have developed the view that there is a family of cell-surface receptors that all act upon their target cells to cause a rise in the cytosolic Ca2+ concentration. We suggested that phosphatidylinositol breakdown might be a reaction intrinsic to some unitary mechanism whereby these receptors bring about Ca2+ mobilization (Michell, 1975, 1979a,b,c; Michell et al., 1977; Jones et a[., 1979). In its most general interpretation, this hypothesis predicts that stimulation of phosphatidylinositol breakdown (and, secondarily, its resynthesis) should always accompany hormone-stimulated Ca2+ mobilization. Since the majority of studies of stimulated phosphatidylinositol metabolism have previously used tissue fragments, it seemed appropriate to test this prediction in isolated hepatocytes, a dispersed cell system that is amenable to many types of biochemical investigation. We have therefore demonstrated stimulation of phosphatidylinositol metabolism in isolated rat hepatocytes exposed to three glycogenolytic hormones whose actions appear to be mediated by Ca2+ ions, namely vasopressin, angiotensin and adrenaline acting through a-receptors (Kirk et al., 1977, 1978, 1979; Billah & Michell, 1978, 1979): the possibility that there might be such an a-adrenergic response was previously suggested by De Torrentegui & Berthet (1966). Studies with an a-adrenergic antagonist and an angiotensin antagonist have established that the three hormones act at three different receptors (Kirk et al., 1977; Billah & Michell, 1978, 1979). Glucagon, another hormone that is glycogenolytic but whose actions are mediated through cyclic AMP, had no effect on phosphatidylinositol metabolism in either liver slices or hepatocytes (De Torrentegui & Berthet, 1966; Kirk et af., 1977). The phosphatidylinositol responses of hepatocytes were initially detected as a stimulation of 32Pi incorporation into the phosphatidylinositol of cells incubated with these hormones in a physiological buffer. However, as with the responses of other cells to appropriate ligands (Jones et al., 1979), the initiating reaction in these responses appears to be phosphatidylinositol breakdown; this has been detected as a decrease in cellular phosphatidylinositol content or as a loss of 32P or 3H from the phosphatidylinositol of prelabelled cells (Billah & Michell, 1978, 1979; C. J. Kirk, unpublished work). Stimulation of phosphatidylinositol metabolism is rapid; stimulated labelling is clearly observed within 1-2min after addition of vasopressin or angiotensin (Billah & Michell, 1979; Kirk et af., 1977) and stimulated breakdown can readily be detected within 5min (C. J. Kirk, unpublished work). Although the glycogenolytic responses of rat hepatocytes to vasopressin, angiotensin and a-adrenergic stimuli all seem to involve Caz+, the phosphatidylinositol response appears to occur independently of hormone-induced changes in cytosolic Ca2+ concentration. Two observations lead to this conclusion: (a) phosphatidylinositol breakdown and labelling are both resistant, at least partially, to cellular Ca2+ deprivation ; (b) admission of CaZ+ into hepatocytes with the ionophore A23 187 brings about neither of these responses (Kirk et af., 1978; Billah & Michell, 1978, 1979). Kirk et al. (1979) used seven vasopressin-like peptides to investigate the ligand selectivity of the receptors involved in stimulation of hepatic phosphorylase and