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(Received for publication, September 17, 1996, and in revised form, November 8, 1996)
From the Department of Cellular and Molecular Physiology, Tufts
University School of Medicine, Boston, Massachusetts 02111
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 µM 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 µM produced rapid decay of
release rates, suggesting receptor inactivation. Superfusion with 10 µM 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.
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