INS and pancreatic insulinoma: A further study examined the in vitro effect of (pGlu)apelin-13 on insulin secretion from rat insulinoma INS-1 cells, revealing that (pGlu)apelin-13 (10-1000 nmol/l) had no insulinotropic effects at lower glucose concentrations and inhibited insulin secretion by up to 40% at 10 mmol glucose.60 It was determined that this inhibitory effect was due to the PI3-kinase activation of phosphodiesterase 3.60