Inositol tetrakisphosphate isomers and elevation of cytosolic Ca2+ in vasopressin-stimulated insulin-secreting RINm5F cells.
Signal generation during the stimulation of insulin secretion by arginine vasopressin (AVP) was investigated in RINm5F cells. AVP (0.1 microM) caused a biphasic cytosolic Ca2+ ([Ca2+]i) rise, namely a rapid transient marked elevation after stimulation followed by a series of oscillations. In the absence of extracellular Ca2+, the sustained oscillations were abolished, while the initial [Ca2+]i transient was only partly decreased, indicating that the former are due to Ca2+ influx and the latter due mainly to mobilization from internal Ca2+ stores. AVP also evoked a transient depolarization of the average membrane potential. AVP-induced Ca2+ influx during the sustained phase, which was strictly dependent on receptor occupancy, was attenuated by membrane hyperpolarization with diazoxide. However, blockade of Ca2+ channels of the L- or T-type was ineffective. AVP stimulated production of diacylglycerol and inositol phosphates; for the latter both [3H] inositol labeling and mass determinations were performed. A transient increase in Ins(1,4,5)P3 was followed by a marked enhancement of Ins(1,3,4,5)P4 (8-fold) peaking at 15 s and gradually returning to basal values. Ins(1,3,4,6)P4 and Ins(3,4,5,6)P4 exhibited the most long-lasting augmentation (4- and 1.7-fold, respectively), and therefore correlated best with the period of sustained [Ca2+]i oscillations. InsP5 and InsP6 were not elevated. The effects of AVP, including the stimulation of insulin secretion from perifused cells, were obliterated by a V1 receptor antagonist. In conclusion, AVP induces protracted [Ca2+]i elevation in RINm5F cells which is associated with long-lasting increases in InsP4 isomers. The accumulation of InsP4 isomers reflects receptor occupancy and accelerated metabolism of the inositol phosphates. Activation of second messenger-operated Ca2+ channels is not necessarily implicated because of the attenuating effect of membrane hyperpolarization.
Li
G
G
Division de Biochimie Clinique, University of Geneva, Switzerland.
Pralong
W F
WF
Pittet
D
D
Mayr
G W
GW
Schlegel
W
W
Wollheim
C B
CB
D001127:Arginine Vasopressin / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects; D002413:Cations, Divalent; D002460:Cell Line / Q000187:drug effects; D002852:Chromatography, Ion Exchange; D003981:Diazoxide / Q000494:pharmacology; D004075:Diglycerides / Q000096:biosynthesis; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D007295:Inositol Phosphates / Q000378:metabolism*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007536:Isomerism; D008564:Membrane Potentials; D009532:Nickel / Q000494:pharmacology; D009543:Nifedipine / Q000494:pharmacology; D010766:Phosphorylation
D015220:Calcium Channels; D002413:Cations, Divalent; D004075:Diglycerides; D007295:Inositol Phosphates; D007328:Insulin; C081655:inositol-1,3,4,5-tetrakisphosphate; C081653:inositol-1,3,4,6-tetrakisphosphate; C081652:inositol-3,4,5,6-tetrakisphosphate; D001127:Arginine Vasopressin; D009532:Nickel; D015544:Inositol 1,4,5-Trisphosphate; C058097:inositol 1,3,4-trisphosphate; D009543:Nifedipine; D003981:Diazoxide