Regulation of inositol 1,4,5-trisphosphate receptors in rat basophilic leukemia cells. II. Modulation of the receptor in permeabilized cells by the cytosolic compartment

Some P2 purinergic agonists increase cytosolic calcium but not inositol 1,4,5-trisphosphate in isolated rat hepatocytes

Based on the capacity to increase IP3 (inositol 1,4,5-trisphosphate), P2 purinergic agonists can be subdivided into two classes: ATP, ADP, UTP, 2deoxyATP, NAD and GTP significantly increased IP3 levels whereas ADP beta S, 2MeSATP, NADP, alpha, beta MeATP, beta, gamma MeATP and ATP alpha S had only a minor, non-significant effect. Irrespective of their potency to increase IP3, all agonists were full glycogenolytic agonists and they all increased cytosolic calcium. With ATP and NAD, IP3 increasing agonists, and 2MeSATP and ADP beta S, non-IP3 increasing agonists, we found that the initial calcium response appeared to be an 'all or none' phenomenon, small amounts of the agonists being either ineffective or equally effective as high amounts. The minimal amount of an agonist needed to initiate a calcium increase and to promote glycogenolysis was very similar. In the absence of extracellular calcium, both groups of purinergic agonists (tested with ATP and 2MeSATP) were equally able to release calcium from intracellular stores. Cells with emptied intracellular calcium stores rapidly took up extracellular calcium upon treatment with ATP or 2MeSATP, the latter being the most potent. It seems therefore that all nucleotides tested increased cytosolic calcium and activated phosphorylase in a very similar way but some nucleotides had no effect on the levels of IP3.

Two inositol 1,4,5-trisphosphate binding sites in rat basophilic leukemia cells: relationship between receptor occupancy and calcium release

Quantal calcium release is a novel paradigm for second messenger signal transduction which provides spatial and temporal control of calcium release from intracellular stores by inositol 1,4,5-trisphosphate (InsP3). We have proposed a mechanism to account for this phenomenon [Kindman, L. A., & Meyer, T. (1993) Biochemistry 32, 1270-1277], which hypothesized the existence of five channels, each with a different affinity for InsP3. As a direct test of this hypothesis, InsP3 binding to microsomes from RBL cells was examined under conditions similar to those used for calcium release. Scatchard analyses performed under a variety of conditions indicates the presence of high affinity (KD = 0.9 +/- 0.3 nM) and low affinity (KD = 47 +/- 5 nM) InsP3 binding sites. The low affinity sites are more prevalent, constituting 82 +/- 5% of the total. Both sites are identified in the presence and absence of MgATP. Moreover, both sites are selective for InsP3 over InsP4, through high concentrations of InsP4 displace InsP3 from each site (with inhibition constants of 16 and 267 nM InsP4, respectively). The relative abundance of the two InsP3 binding sites is Ca2+ dependent. An increase in Ca2+ from 0.1 to 0.5 microM results in the apparent conversion of a portion of the low affinity sites into high affinity sites into high affinity sites. Ca2+ (0.5 microM) also increased the KD of the low affinity InsP3 binding site. Given the presence of both high and low affinity InsP3 binding sites, two simple mathematical models describing both the kinetics of calcium release and quantal calcium release from RBL cells were developed.(ABSTRACT TRUNCATED AT 250 WORDS)