Regulation of the cerebellar inositol 1,4,5-trisphosphate receptor by univalent cations.
In the present study we investigated the effects of K and other univalent cations on [3H]InsP3 [[3H]Ins(1,4,5)P3] binding to sheep cerebellar microsomes. In equilibrium binding experiments performed over 4 s at pH 7.1 and 20 degrees C, the addition of K to the binding medium decreased the affinity and increased the total number of binding sites for InsP3 in a dose-dependent manner. At low InsP3 concentration (0.5 nM) these effects resulted in a biphasic dose-response curve, with maximal binding at about 75 mM K. In contrast, the dose-response curve calculated for InsP3 at the physiological concentration of 5 mM, was linear up to 200 mM K. Univalent inorganic cations stimulated [3H]InsP3 binding to various extents, with the following descending order of efficiency at 75 mM: Cs approximately Rb approximately K>Na>Li. The effect of K on InsP3R affinity was rapidly reversed upon cation removal. We were therefore also able to demonstrate that K increased Bmax (maximal specific binding) by pre-treating microsomes with K before measuring [3H]InsP3 binding in the absence of that cation. The increase in Bmax was reversible, but this reversal occurred less rapidly than the change in affinity. These results are consistent with a process by which K reversibly converted very low-affinity sites into sites with higher affinity, making them detectable in competitive binding experiments. They suggest that interconversion between these two affinity states constitutes the basis of a K-controlled regulatory mechanism for cerebellar InsP3R.
Coquil
Jean-François
JF
INSERM U442, Signalisation Cellulaire et Calcium, Université Paris Sud, Bât 443, F-91405 Orsay Cédex, France.
Blazquez
Samantha
S
Soave
Sabrina
S
Mauger
Jean-Pierre
JP
D000818:Animals; D002412:Cations / Q000378:metabolism*; D002531:Cerebellum / Q000737:chemistry*; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D007700:Kinetics; D008861:Microsomes / Q000378:metabolism; D011188:Potassium / Q000502:physiology; D012756:Sheep
D002412:Cations; D015544:Inositol 1,4,5-Trisphosphate; D011188:Potassium