Inositol 1,4,5-trisphosphate and calcium regulate the calcium channel function of the hepatic inositol 1,4,5-trisphosphate receptor.
The regulation of the inositol 1,4,5-trisphosphate (IP3) receptor in liver was analyzed using a novel superfusion method. Hepatic microsomes were loaded with 45Ca2+, and superfused at high flow rates to provide precise control over IP3 and Ca2+ concentrations ([Ca2+]) and to isolate 45Ca2+ release from reuptake. 45Ca2+ release was dependent on both [Ca2+] and IP3. The initial rate of 45Ca2+ release was a biphasic function of [Ca2+], increasing as [Ca2+] approached 3 microM but decreasing at higher concentrations, suggesting that the hepatic IP3 receptor is regulated by [Ca2+] at two sites, a high affinity potentiation site and a low affinity inhibitory site. The relationship between initial rates and IP3 concentration was steep (Hill coefficient of 3.4), suggesting that activation of the calcium channel requires binding of at least 3 IP3 molecules. IP3 concentrations above 10 microM produced rapid decay of release rates, suggesting receptor inactivation. Superfusion with 10 microM IP3 under conditions that minimize calcium release ([Ca2+] < 1 nM) inhibited 45Ca2+ release in response to subsequent stimulation (400 nM Ca2+). These data suggest sequential positive and negative regulation of the hepatic IP3 receptor by cytosolic calcium and by IP3, which may underlie hepatocellular propagation of regenerative, oscillatory calcium signals.
Dufour
J F
JF
Department of Cellular and Molecular Physiology, Tufts University School of Medicine, Boston, Massachusetts 02111, USA. JDUFOUR@OPAL.TUFTS.EDU
Arias
I M
IM
Turner
T J
TJ
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000502:physiology*; D008962:Models, Theoretical; D051381:Rats; D017207:Rats, Sprague-Dawley; D018160:Receptors, Cytoplasmic and Nuclear / Q000187:drug effects / Q000502:physiology*
D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium