Abstract
Continuing formation of inositol phosphates during stimulation of pancreatic β-cells by hormones and neurotransmitters requires the continued synthesis of the polyphosphoinositides phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5 bisphosphate (PIP2) from phosphatidylinositol (PI). In the present study we have investigated how this pathway and the activity of phosphoinositide-specific phospholipase C (PI-PLC) are regulated by carbamoylcholine (CCh), Ca2+, the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), GTPγS and NaF in 44-h [3H]inositol-labelled, dispersed and digitonin-permeabilized mouse pancreatic islet cells.
CCh stimulated not only PI-PLC (G-protein-mediated) but also, by an as yet unknown mechanism, significantly enhanced PI 4-kinase activity, estimated as the PIP:PI ratio, by 100%, and further increased the flux from PI to PIP and PIP2. GTPγS and NaF mimicked the effects of CCh on PI-PLC but had no effect on the levels of PIP and PIP2. TPA raised the PIP:PI ratio by 75%. In addition TPA counteracted the CCh stimulation of PI-PLC. There was no effect of 10−6 mol/l Ca2+ on the levels of PIP and PIP2. Experiments with quinacrine and adenosine confirmed that PI-PLC and PI 4-kinase could be regulated independently of each other. In conclusion, these data point to differential regulation of polyphosphoinositide synthesis and breakdown.
European Journal of Endocrinology 136 539–545
Transport of ascorbic acid and dehydroascorbic acid by pancreatic islet cells from neonatal rats