Effects of secretagogues on [32P]phosphatidylinositol 4,5-bisphosphate metabolism in the exocrine pancreas

Experiments were carried out to assess the effects of secretagogues on the polyphosphoinositides phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] on preparations of exocrine pancreas in vitro. Carbachol and caerulein provoked a rapid (less than 1 min) breakdown of 15-20% of [32P]PtdIns(4,5)P2 in isolated pancreatic acini, but did not affect [32P]PtdIns4P. In contrast, the Ca2+ ionophore ionomycin had no immediate effect on the levels of either inositide but caused a parallel fall in both lipids after 5-10 min. A similar decrease in [32P]PtdIns(4,5)P2 due to carbachol was obtained with isolated acini and isolated cells, despite the fact that the secretory response of isolated cells was considerably less than that of isolated acini. Loss of [32P]PtdIns(4,5)P2 elicited by carbachol or caerulein was unaffected either by the addition of EGTA in excess of extracellular Ca2+ or when a protocol was employed that eliminated caerulein-induced intracellular Ca2+-release. These results suggest that agonist-induced PtdIns(4,5)P2 breakdown in the exocrine pancreas may be an early step in the stimulus-response coupling pathway and also suggest that this breakdown is not dependent on Ca2+-mobilization.

Platelet activating factor (PAF-acether) promotes an early degradation of phosphatidylinositol-4,5-biphosphate in rabbit platelets

32P-Labelled washed rabbit platelets were incubated with 0.6 nM platelet activating factor (PAF-acether), giving a full aggregation and release response within 30-60 s. The major phospholipid changes observed under these conditions were: (1) An increased labelling of phosphatidic acid (PA) within 10 s and of phosphatidylinositol (MPI) at 30 s, reflecting the activation of the MPI cycle via the cytosolic phospholipase C; (2) an enhancement of phosphatidylinositol-4-phosphate (DPI) and phosphatidylinositol-4,5-bisphosphate (TPI) labelling at later incubation times; (3) an early degradation of TPI with a counterbalancing formation of DPI. The latter changes suggest a receptor-mediated stimulation of TPI-phosphomonoesterase, the role of which in the mechanism of platelet activation is discussed.

Stimulation of phosphatidylinositol 4,5-bisphosphate hydrolysis in hepatocytes by vasopressin

Hepatocyte phosphatidylinositol 4,5-bisphosphate (4,5-P2), phosphatidylinositol 4-phosphate (4-P), and phosphatidylinositol were labeled with 3H when rats were injected intraperitoneally with 200 microCi of [2-3H] myo-inositol 18 h previously. Phosphatidylinositol 4,5-P2 and phosphatidylinositol 4-P accounted for 0.84 +/- 0.06 and 7.48 +/- 0.36%, respectively, of the total [3H] myo-inositol containing phospholipids. The breakdown of phosphatidylinositol 4,5-P2 was stimulated transiently (maximum effect seen at 15 s) and in a Ca2+-dependent manner by 10(-8) M vasopressin. Phosphatidylinositol 4-P breakdown was enhanced to a smaller, but longer, extent by vasopressin, whereas no changes in phosphatidylinositol were detected up to 120 s. Subcellular fractionation studies also showed no preferential breakdown of phosphatidylinositol in plasma membranes at 5-20 min. Only doses of vasopressin (10(-8) and 10(-7) M) in excess of those producing maximum effects on phosphorylase activation and Ca2+ efflux (10(-9) M) were effective at stimulating phosphatidylinositol 4,5-P2 breakdown. It is concluded that phosphatidylinositol 4,5-P2 breakdown induced by vasopressin in rat hepatocytes is not responsible for the mobilization of Ca2+ which leads to the activation of phosphorylase. On the contrary, it is Ca2+-dependent and appears to require the occupation of more receptors than are required for Ca2+ mobilization and phosphorylase activation.

Thrombin-induced phosphodiesteratic cleavage of phosphatidylinositol bisphosphate in human platelets

The addition of thrombin to human platelets prelabeled with 32Pi led to significant loss of radioactivity in phosphatidylinositol 4,5-bisphosphate within 5 s, followed by recovery or even increase by 2 min. Loss of label from phosphatidylinositol phosphate was much less marked. Stimulated loss of label from phosphatidylinositol was not seen, while labeled phosphatidate increased severalfold. The principal labeled water-soluble phosphates observed, in addition to 32Pi and [32P] ATP, co-migrated with inositol diphosphate and inositol triphosphate. This suggests that a pool of polyphosphoinositides is constantly undergoing phosphodiesteratic cleavage and resynthesis. Thrombin addition led to rapid increase in radioactivity in inositol triphosphate, but not in inositol diphosphate. We conclude that this early consequence of the thrombin-platelet interaction is the result of an increase in the phosphodiesteratic cleavage of phosphatidylinositol bisphosphate.

Rapid decrease of phosphatidylinositol 4,5-bisphosphate in thrombin-stimulated platelets

The addition of thrombin to horse platelets prelabeled with 32P induces a rapid decrease of the radioactivity from phosphatidylinositol 4,5-bisphosphate. Maximum loss of the radioactivity from phosphatidylinositol 4,5-bisphosphate occurs with 10 s of stimulation and is followed by an increased incorporation of 32P into this lipid. The stimulation of phosphatidylinositol 4,5-bisphosphate loss by thrombin is concentration-dependent. The ionophore A23187, which mobilizes Ca2+, is ineffective in inducing the degradation of phosphatidylinositol 4,5-bisphosphate. Measurements of polyphosphoinositides by phosphorus estimation show that, 10 s after thrombin stimulation, there is a decrease of 15-20% of the total phosphatidylinositol 4,5-bisphosphate without any significant change in phosphatidylinositol 4-monophosphate. It appears that thrombin causes a rapid and transient degradation of phosphatidylinositol 4,5-bisphosphate and that this effect might be related to the initiation of platelet activation.

Receptor-mediated net breakdown of phosphatidylinositol 4,5-bisphosphate in parotid acinar cells

The metabolism of phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] in rat parotid acinar cells was investigated, particularly with regard to the effects of receptor-active agonists. Stimulation of cholinergic-muscarinic receptors with methacholine provoked a rapid disappearance of 40--50% of [32P]PtdIns(4,5)P2, but had no effect on PtdIns4P. Adrenaline, acting on alpha-adrenoceptors, and Substance P also stimulated net loss of PtdIns(4,5)P2. The beta-adrenoceptor agonist, isoprenaline, and the Ca2+ ionophore, ionomycin, failed to affect labelled PtdIns(4,5)P2 or PtdIns4P. By chelation of extracellular Ca2+ with excess EGTA, and by an experimental protocol that eliminates cellular Ca2+ release, it was demonstrated that the agonist-induced decrease in PtdIns(4,5)P2 is independent of both Ca2+ influx and Ca2+ release. These results may suggest that net PtdIns(4,5)P2 breakdown is an early event in the stimulus-response pathway of the parotid acinar cell and could be directly involved in the mechanism of agonist-induced Ca2+ release from the plasma membrane.

Metabolism of phospholipids in peripheral nerve from rats with chronic streptozotocin-induced diabetes: increased turnover of phosphatidylinositol-4,5-bisphosphate

The effect of chronic streptozotocin-induced diabetes on phospholipid metabolism in rat sciatic nerve in vitro was investigated. In normal nerve incubated for 2 h in Krebs-Ringer-bicarbonate buffer containing [32P]orthophosphate, radioactivity was primarily incorporated into phosphatidylinositol-4,5-bisphosphate and phosphatidylcholine. Smaller amounts were present in phosphatidylinositol-4-phosphate, phosphatidylinositol, and phosphatidic acid. As compared to controls, phosphatidylinositol-4,5-bisphosphate in nerves from animals made diabetic 2, 10, and 20 weeks earlier accounted for 30-46% more of the isotope, expressed as a percentage, incorporated into all phospholipids. In contrast, the proportion of radioactivity in phosphatidylcholine decreased by 10-25%. When the results were expressed as the quantity of phosphorus incorporated into phospholipid, only phosphatidylinositol-4,5-bisphosphate displayed a change. The amount of isotope which entered this lipid increased 60% and 67% for 2- and 10-week diabetic animals, respectively. Increased phosphatidylinositol-4,5-bisphosphate labeling was observed when epineurial-free preparations were used or when the composition of the incubation medium was varied. Sciatic and caudal nerve conduction velocities were decreased after 10 and 20 weeks but were unchanged after 2 weeks. We conclude that an increase in the turnover of phosphatidylinositol-4,5-bisphosphate in sciatic nerve from streptozotocin-diabetic rats appears relatively early and persists throughout the course of the disease. This metabolic alteration may be related to a primary defect responsible for the accompanying deficient peripheral nerve function.

Requirement for calcium ions in acetylcholine-stimulated phosphodiesteratic cleavage of phosphatidyl-myo-inositol 4,5-bisphosphate in rabbit iris smooth muscle

1. The mechanism of acetylcholine-stimulated breakdown of phosphatidyl-myo-inositol 4,5-bisphosphate and its dependence on extracellular Ca(2+) was investigated in the rabbit iris smooth muscle. 2. Acetylcholine (50mum) increased the breakdown of phosphatidylinositol bisphosphate in [(3)H]inositol-labelled muscle by 28% and the labelling of phosphatidylinositol by 24% of that of the control. Under the same experimental conditions there was a 33 and 48% increase in the production of (3)H-labelled inositol trisphosphate and inositol monophosphate respectively. Similarly carbamoylcholine and ionophore A23187 increased the production of these water-soluble inositol phosphates. Little change was observed in the (3)H radioactivity of inositol bisphosphate. 3. Both inositol trisphosphatase and inositol monophosphatase were demonstrated in subcellular fractions of this tissue and the specific activity of the former was severalfold higher than that of the latter. 4. The acetylcholine-stimulated production of inositol trisphosphate and inositol monophosphate was inhibited by atropine (20mum), but not tubocurarine (100mum); and it was abolished by depletion of extracellular Ca(2+) with EGTA, but restored on addition of low concentrations of Ca(2+) (20mum). 5. Calcium-antagonistic agents, such as verapamil (20mum), dibenamine (20mum) or La(3+) (2mm), also abolished the production of the water-soluble inositol phosphates in response to acetylcholine. 6. Release of inositol trisphosphate from exogenous phosphatidylinositol bisphosphate by iris muscle microsomal fraction (;microsomes') was stimulated by 43% in the presence of 50mum-Ca(2+). 7. The results indicate that increased Ca(2+) influx into the iris smooth muscle by acetylcholine and ionophore A23187 markedly activates phosphatidylinositol bisphosphate phosphodiesterase and subsequently increases the production of inositol trisphosphate and its hydrolytic product inositol monophosphate. The marked increase observed in the production of inositol monophosphate could also result from Ca(2+) activation of phosphatidylinositol phosphodiesterase. However, there was no concomitant decrease in the (3)H radioactivity of this phospholipid.