The type II inositol 1,4,5-trisphosphate receptor can trigger Ca2+ waves in rat hepatocytes.
OBJECTIVE
Ca2+ regulates cell functions through signaling patterns such as Ca2+ oscillations and Ca2+ waves. The type I inositol 1,4,5-trisphosphate receptor is thought to support Ca2+ oscillations, whereas the type III inositol 1,4,5-trisphosphate receptor is thought to initiate Ca2+ waves. The role of the type II inositol 1,4,5-trisphosphate receptor is less clear, because it behaves like the type III inositol 1,4,5-trisphosphate receptor at the single-channel level but can support Ca2+ oscillations in intact cells. Because the type II inositol 1,4,5-trisphosphate receptor is the predominant isoform in liver, we examined whether this isoform can trigger Ca2+ waves in hepatocytes.


METHODS
The expression and distribution of inositol 1,4,5-trisphosphate receptor isoforms was examined in rat liver by immunoblot and confocal immunofluorescence. The effects of inositol 1,4,5-trisphosphate on Ca2+ signaling were examined in isolated rat hepatocyte couplets by using flash photolysis and time-lapse confocal microscopy.


RESULTS
The type II inositol 1,4,5-trisphosphate receptor was concentrated near the canalicular pole in hepatocytes, whereas the type I inositol 1,4,5-trisphosphate receptor was found elsewhere. Stimulation of hepatocytes with vasopressin or directly with inositol 1,4,5-trisphosphate induced Ca2+ waves that began in the canalicular region and then spread to the rest of the cell. Inositol 1,4,5-Trisphosphate-induced Ca2+ signals also increased more rapidly in the canalicular region. Hepatocytes did not express the ryanodine receptor, and cyclic adenosine diphosphate-ribose had no effect on Ca2+ signaling in these cells.


CONCLUSIONS
The type II inositol 1,4,5-trisphosphate receptor establishes a pericanalicular trigger zone from which Ca2+ waves originate in hepatocytes.
Hirata
Keiji
K
Department of Medicine, Yale University School of Medicine, New Haven, Connecticut 06520-8019, USA.
Pusl
Thomas
T
O'Neill
Allison F
AF
Dranoff
Jonathan A
JA
Nathanson
Michael H
MH
D000246:Adenosine Diphosphate Ribose / Q000031:analogs & derivatives* / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D036563:Cyclic ADP-Ribose; D015870:Gene Expression / Q000502:physiology; D006490:Hemostatics / Q000494:pharmacology; D022781:Hepatocytes / Q000378:metabolism*; D007091:Image Processing, Computer-Assisted; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007536:Isomerism; D008297:Male; D018613:Microscopy, Confocal; D051381:Rats; D017207:Rats, Sprague-Dawley; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D015290:Second Messenger Systems / Q000187:drug effects / Q000502:physiology; D014667:Vasopressins / Q000494:pharmacology
D015220:Calcium Channels; D006490:Hemostatics; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D014667:Vasopressins; D036563:Cyclic ADP-Ribose; D000246:Adenosine Diphosphate Ribose; D002118:Calcium