Expression of mRNAs for different types of IP3 receptors in rat kidneys

Mechanisms of vasopressin-induced intracellular Ca2+ oscillations in rat inner medullary collecting duct

Arginine vasopressin (AVP) causes increase in intracellular Ca(2+) concentration with an oscillatory pattern. Ca(2+) mobilization is required for AVP-stimulated apical exocytosis in inner medullary collecting duct (IMCD). The mechanistic basis of these Ca(2+) oscillations was investigated by confocal fluorescence microscopy and flash photolysis of caged molecules in perfused IMCD. Photorelease of caged cAMP and direct activation of ryanodine receptors (RyRs) by photorelease of caged cyclic ADP-ribose (cADPR) both mimicked the AVP-induced Ca(2+) oscillations. Preincubation of IMCD with 100 μM 8-bromo-cADPR (a competitive inhibitor of cADPR) delayed the onset and attenuated the magnitude of AVP-induced Ca(2+) oscillations. These observations indicate that the cADPR/RyR pathway is capable of supporting Ca(2+) oscillations and endogenous cADPR plays a major role in the AVP-induced Ca(2+) oscillations in IMCD. In contrast, photorelease of caged inositol 1,4,5-trisphosphate (IP(3)) induced Ca(2+) release but did not maintain sustained Ca(2+) oscillations. Removal of extracellular Ca(2+) halted ongoing AVP-mediated Ca(2+) oscillation, suggesting that it requires extracellular Ca(2+) entry. AVP-induced Ca(2+) oscillation was unaffected by nifedipine. Intracellular Ca(2+) store depletion induced by 20 μM thapsigargin in Ca(2+)-free medium triggered store-operated Ca(2+) entry (SOCE) in IMCD, which was attenuated by 1 μM GdCl(3) and 50 μM SKF-96365. After incubation of IMCD with 1 nM AVP in Ca(2+)-free medium, application of extracellular Ca(2+) also triggered Ca(2+) influx, which was sensitive to GdCl(3) and SKF-96365. In summary, our observations are consistent with the notion that AVP-induced Ca(2+) oscillations in IMCD are mediated by the interplay of Ca(2+) release from RyRs and a Ca(2+) influx mechanism involving nonselective cation channels that resembles SOCE.

Inhibition of type I and III IP(3)Rs by TGF-beta is associated with impaired calcium release in mesangial cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) mediate cytosolic free calcium concentration ([Ca(2+)](c)) signals in response to a variety of agonists that stimulate mesangial cell contraction and proliferation. In the present study, we demonstrate that mesangial cells express both type I and III IP(3)Rs and that these receptors occupy different cellular locations. Chronic treatment with transforming growth factor-beta1 (TGF-beta1; 10 ng/ml, 24 h) leads to downregulation of both type I and III IP(3)Rs as measured by immunoblot and confocal analysis. TGF-beta1 treatment does not affect IP(3) levels, and downregulation of type I IP(3)R is not due to enhanced degradation of the protein, as the half-life of type I IP(3)R is unchanged in the presence or absence of TGF-beta1. Functional effects of TGF-beta1-induced downregulation of the IP(3)Rs were evaluated by measuring [Ca(2+)](c) changes in response to epidermal growth factor (EGF) in intact cells and sensitivity of [Ca(2+)](c) release to IP(3) in permeabilized cells. TGF-beta1 pretreatment led to a significant decrease of [Ca(2+)](c) release induced by EGF in intact cells and by submaximal IP(3) (400 nm) in permeabilized cells. Total IP(3)-sensitive [Ca(2+)](c) stores were not changed, as assessed by stimulation with maximal doses of IP(3) (10.5 microm) and thapsigargin-mediated calcium release in permeabilized cells. We conclude that prolonged exposure to TGF-beta1 leads to downregulation of both type I and III IP(3)Rs in mesangial cells and this is associated with impaired sensitivity to IP(3).

Isoform diversity of the inositol trisphosphate receptor in cell types of mouse origin

Previous reports suggested the expression of four or five different Ins(1,4,5)P3 receptor [Ins(1,4,5)P3R] isoforms in mouse cells [Ross, Danoff, Schell, Snyder and Ullrich (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 4265-4269; De Smedt, Missiaen, Parys, Bootman, Mertens, Van Den Bosch and Casteels (1994) J. Biol. Chem. 269, 21691-21698]. To explore this diversity further, we have isolated and sequenced partial clones of two Ins(1,4,5)P3R mRNAs from the mouse embryonic C3H10T1/2 cell line. These clones showed between 94.2 and 94.9% sequence identity with the corresponding rat Ins(1,4,5)P3R-II and Ins(1,4,5)P3R-III isoforms. Based on these newly obtained sequences we have determined the relative expression of the different Ins(1,4,5)P3R mRNAs in cultured cells and in animal tissues of mouse origin by a ratio reverse transcriptase polymerase chain reaction (RT-PCR). Ins(1,4,5)P3R-I was very prominent in brain and cerebellum and Ins(1,4,5)P3R-II in epithelia such as kidney as well as in both cardiac and skeletal muscle. Ins(1,4,5)P3R-III was highly expressed in all cultured cell types and in tissues with high cell turnover, e.g. testis. The prominent expression of Ins(1,4,5)P3R-I and Ins(1,4,5)P3R-III in A7r5 and C3H10T1/2 cells respectively was confirmed by immunoblot analysis and was compatible with a lower threshold for Ins(1,4,5)P3-induced Ca2+ release in the former cell type. Screening of a large number of mouse cell lines and tissues revealed the presence of Ins(1,4,5)P3R-I as well as of the Ins(1,4,5)P3R-II and Ins(1,4,5)P3R-III isoforms which were identified in the present study, but in contrast with previous reports there was no evidence for more isoform diversity.