Effects of high extracellular calcium and strontium on inositol polyphosphates in bovine parathyroid cells

Expression and functional assessment of an alternatively spliced extracellular Ca2+-sensing receptor in growth plate chondrocytes

The extracellular Ca(2+)-sensing receptor (CaR) plays an essential role in mineral homeostasis. Studies to generate CaR-knockout (CaR(-/-)) mice indicate that insertion of a neomycin cassette into exon 5 of the mouse CaR gene blocks the expression of full-length CaRs. This strategy, however, allows for the expression of alternatively spliced CaRs missing exon 5 [(Exon5(-))CaRs]. These experiments addressed whether growth plate chondrocytes (GPCs) from CaR(-/-) mice express (Exon5(-))CaRs and whether these receptors activate signaling. RT-PCR and immunocytochemistry confirmed the expression of (Exon5(-))CaR in growth plates from CaR(-/-) mice. In Chinese hamster ovary or human embryonic kidney-293 cells, recombinant human (Exon5(-))CaRs failed to activate phospholipase C likely due to their inability to reach the cell surface as assessed by intact-cell ELISA and immunocytochemistry. Human (Exon5(-))CaRs, however, trafficked normally to the cell surface when overexpressed in wild-type or CaR(-/-) GPCs. Immunocytochemistry of growth plate sections and cultured GPCs from CaR(-/-) mice showed easily detectable cell-membrane expression of endogenous CaRs (presumably (Exon5(-))CaRs), suggesting that trafficking of this receptor form to the membrane can occur in GPCs. In GPCs from CaR(-/-) mice, high extracellular [Ca(2+)] ([Ca(2+)](e)) increased inositol phosphate production with a potency comparable with that of wild-type GPCs. Raising [Ca(2+)](e) also promoted the differentiation of CaR(-/-) GPCs as indicated by changes in proteoglycan accumulation, mineral deposition, and matrix gene expression. Taken together, our data support the idea that expression of (Exon5(-))CaRs may compensate for the loss of full-length CaRs and be responsible for sensing changes in [Ca(2+)](e) in GPCs in CaR(-/-) mice.

Parathyroid cells express dihydropyridine-sensitive cation currents and L-type calcium channel subunits

Parathyroid cells express Ca2+ -conducting currents that are activated by raising the extracellular Ca2+ concentration ([Ca2+]o). We investigated the sensitivity of these currents to dihydropyridines, the expression of voltage-dependent Ca(2+) channel (VDCC) subunits, and the effects of dihydropyridines on the intracellular free [Ca2+] ([Ca2+]i) and secretion in these cells. Dihydropyridine channel antagonists dose dependently suppressed Ca2+ -conducting currents, and agonists partially reversed the inhibitory effects of the antagonists in these cells. From a bovine parathyroid cDNA library, we isolated cDNA fragments encoding parts of an alpha(1S)- and a beta(3)-subunit of L-type Ca(2+) channels. The alpha(1S)-subunit cDNA from the parathyroid represents an alternatively spliced variant lacking exon 29 of the corresponding gene. Northern blot analysis and immunocytochemistry confirmed the presence of transcripts and proteins for alpha(1)- and beta(3)-subunits in the parathyroid gland. The addition of dihydropyridines had no significant effects on high [Ca2+]o-induced changes in [Ca2+]i and parathyroid hormone (PTH) release. Thus our studies indicate that parathyroid cells express alternatively spliced L-type Ca2+ channel subunits, which do not modulate acute intracellular Ca2+ responses or changes in PTH release.

Strontium ranelate increases cartilage matrix formation

Based on previous studies showing that strontium ranelate (S12911) modulates bone loss in osteoporosis, it could be hypothesized that this drug also is effective on cartilage degradation in osteoarthritis (OA). This was investigated in vitro on normal and OA human chondrocytes treated or not treated with interleukin-1beta (IL-1beta). This model mimics, in vitro, the imbalance between chondroformation and chondroresorption processes observed in vivo in OA cartilage. Chondrocytes were isolated from cartilage by enzymatic digestion and cultured for 24-72 h with 10(-4)-10(-3) M strontium ranelate, 10(-3) M calcium ranelate, or 2 x 10(-3) M SrCl2 with or without IL-1beta or insulin-like growth factor I (IGF-I). Stromelysin activity and stromelysin quantitation were assayed by spectrofluorometry and enzyme amplified sensitivity immunoassay (EASIA), respectively. Proteoglycans (PG) were quantified using a radioimmunoassay. Newly synthesized glycosaminoglycans (GAGs) were quantified by labeled sulfate (Na2(35)SO4) incorporation. This method allowed the PG size after exclusion chromatography to be determined. Strontium ranelate, calcium ranelate, and SrCl2 did not modify stromelysin synthesis even in the presence of IL-1beta. Calcium ranelate induced stromelysin activation whereas strontium compounds were ineffective. Strontium ranelate and SrCl2 both strongly stimulated PG production suggesting an ionic effect of strontium independent of the organic moiety. Moreover, 10(-3) M strontium ranelate increased the stimulatory effect of IGF-I (10(-9) M) on PG synthesis but did not reverse the inhibitory effect of IL-1beta. Strontium ranelate strongly stimulates human cartilage matrix formation in vitro by a direct ionic effect without stimulating the chondroresorption processes. This finding provides a preclinical basis for in vivo testing of strontium ranelate in OA.

Regulation of extracellular calcium-activated cation currents by cAMP in parathyroid cells

Parathyroid cells express Ca2+-sensing receptors that couple changes in the extracellular Ca2+ concentration ([Ca2+]o) to increases in the intracellular free Ca2+ concentration ([Ca2+]i) and to the suppression of parathyroid hormone secretion. Using whole cell patch clamping, we previously identified voltage-independent Ca2+-conducting currents in bovine parathyroid cells that increased with rising [Ca2+]o and were blocked by Cd2+ and nifedipine. Because cAMP-dependent phosphorylation regulates dihydropyridine-sensitive Ca2+ channels in other systems, we tested whether cAMP modulates these currents. At 0.7 mM Ca2+, nonselective Ca2+-conducting currents were suppressed by 30-50% when the recording pipette was perfused with cAMP. High-[Ca2+]o-induced increases in membrane currents were also abrogated. The effects of cAMP were reversible and dose dependent (3 x 10(-9) to 3 x 10(-3) M) and required ATP in the pipette solution. Perfusion of the cell interior with the catalytic subunit of protein kinase A mimicked the effects of cAMP, as did perfusion of the bath with the adenylate cyclase activator forskolin. These findings support the idea that cAMP-dependent phosphorylation suppresses high-[Ca2+]o-induced cation currents and may play a role in regulating ion fluxes in parathyroid cells.

Coupling of calcium receptors to inositol phosphate and cyclic AMP generation in mammalian cells and Xenopus laevis oocytes and immunodetection of receptor protein by region-specific antipeptide antisera

Ca2+ and other divalent cations modulate parathyroid hormone secretion by interacting with cell-surface Ca2+-sensing receptors (CaRs). We assessed the ability of these receptors to couple to Ca2+ mobilization, inositol phosphate (InsP) accumulation, and cyclic AMP production in different expression systems. In Xenopus laevis oocytes injected with bovine parathyroid CaR cRNA, the addition of extracellular cations to 1.5 mM Ca2+, 5.5 mM Mg2+, or 10 microM Gd3+ significantly increased 45Ca efflux (p < 0.01). InsP accumulation also increased dramatically when adding these cations to human embryonic kidney (HEK) 293 cells stably transfected with wild-type bovine parathyroid CaR cDNA. Raising the extracellular [Ca2+] ([Ca2+]o) from 0.1 to > 1.4 mM in oocytes and to > 1.0 mM in HEK 293 cells stimulated significant increments in 45Ca efflux and InsP accumulation, respectively (p < 0.05). In contrast, Ca2+ and Mg2+ increased InsPs to a lesser extent in COS 7 cells transiently transfected with CaR cDNA. In HEK 293 cells stably expressing CaR cDNA, there were significant reductions in cAMP content when adding high Ca2+, Mg2+, Gd3+, or the CaR modulator NPS R-467. Three region-specific anti-CaR peptide antisera immunoblotted bands of approximately 140 and 155 kDa in membranes from CaR-transfected HEK 293 cells and bovine parathyroid tissue. Immunocytochemistry demonstrated strong cell-surface staining in CaR-transfected HEK 293 cells and parathyroid tissue, which was absent when antisera were preabsorbed with CaR peptides. These results indicate that the activation of the recombinant CaR by extracellular Ca2+ can couple negatively to adenylate cyclase but positively to phospholipase C (PLC), the latter at physiological [Ca2+]o.

Parathyroid Ca(2+)-conducting currents are modulated by muscarinic receptor agonists and antagonists

Parathyroid cells express Ca(2+)-conducting cation currents, which are activated by raising the extracellular Ca2+ concentration ([Ca2+]o) and blocked by dihydropyridines. We found that acetylcholine (ACh) inhibited these currents in a reversible, dose-dependent manner (50% inhibitory concentration approximately equal to 10(-8) M). The inhibitory effects could be mimicked by the agonist (+)-muscarine. The effects of ACh were blunted by the antagonist atropine and reversed by removing ATP from the pipette solution (+)-Muscarine enhanced the adenosine 3',5'-cyclic monophosphate (cAMP) production by 30% but had no effect on inositol phosphate accumulation in parathyroid cells. Oligonucleotide primers, based on sequences of known muscarinic receptors (M1-M5), were used in reverse transcriptase-polymerase chain reaction (RT-PCR) to amplify receptor cDNA from parathyroid poly (A)+ RNA. RT-PCR products displayed > 90% nucleotide sequence identity to human M2- and M4-receptor cDNAs. Expression of M2-receptor protein was further confirmed by immunoblotting and immunocytochemistry. Thus parathyroid cells express muscarinic receptors of M2 and possibly M4 subtypes. These receptors may couple to dihydropyridine-sensitive, cation-selective currents through the activation of adenylate cyclase and ATP-dependent pathways in these cells.

Regulation of Ca(2+)-conducting currents in parathyroid cells by extracellular Ca(2+) and channel blockers

High extracellular Ca2+ concentrations ([Ca2+]o) produce sustained intracellular Ca2+ responses in parathyroid cells that correlate with suppression of parathyroid hormone release. Using whole cell patch clamping, we identified two types of Ca(2+)-conducting currents in these cells. Type 1 currents were enhanced by raising [Ca2+]o and blocked by Cd2+ and nifedipine, whereas type 2 currents were resistant to blockade by these agents. Both types of membrane currents were cation nonselective, voltage independent over a broad range of membrane potentials, and blocked by the trivalent ions La3+ and Gd3+ (> 98%). Cd2+, La3+, and Gd3+ had biphasic effects on membrane conductance (Gm). At submicromolar concentrations, these ions increased Gm, whereas at higher concentrations they reduced Gm. In contrast to ionic channel blockers, nifedipine had only an inhibitory effect on the Ca(2+)-conducting currents that were sensitive to changes in [Ca2+]o (dose inhibiting 50% of maximal response = approximately 3-10 x 10(-8) M). Microfluorimetric ratio-imaging analysis of single parathyroid cells loaded with fura 2 showed that Gd3+ inhibited sustained intracellular Ca2+ responses to high [Ca2+]o. These findings suggest that the Ca(2+)-conducting currents identified in these studies may play a role in regulating intracellular Ca2+ responses in this system.

Thapsigargin stimulates intracellular calcium mobilization and inhibits parathyroid hormone release

Ca2+ and other divalent cations like Sr2+, Ba2+, and Mg2+ stimulate rapid and sustained increases in intracellular Ca2+ ([Ca2+]i) and 1,4,5-inositol trisphosphate (1,4,5-InsP3) presumably by interacting with recently identified parathyroid cell membrane Ca2+ receptors. We used thapsigargin (THAPS), an inhibitor of the microsomal Ca(2+)-ATPase, to deplete InsP3-sensitive intracellular Ca2+ stores to determine whether sustained increases in [Ca2+]i due to divalent cations require intact cytosolic Ca2+ pools. In Fura 2-loaded parathyroid cells, THAPS produced a gradual increase in [Ca2+]i which reached a steady-state level by 2-3 minutes. The effect of THAPS (3 x 10(-6) M) was substantial with [Ca2+]i, rising from 281 +/- 27 nM at 0.5 mM Ca2+ to a peak value of 684 +/- 30 nM (p < 0.0001). The addition of Sr2+ to cells at 0.5 mM extracellular Ca2+ induced an immediate 2- to 3-fold increase in [Ca2+]i which stabilized at a [Ca2+]i above baseline for > or = 10 minutes. THAPS (3 x 10(-6) M) pretreatment for > or = 5 minutes blocked this sustained-phase increment in [Ca2+]i due to Sr2+. In the absence of extracellular Ca2+, there was a slight but nonsignificant effect of THAPS on [Ca2+]i. Incubation of cells with THAPS did not change the levels of 3H-inositol phosphates (InsP3, InsP2, and InsP1) or alter Sr(2+)-induced accumulation of InsP3, InsP2, and InsP1.(ABSTRACT TRUNCATED AT 250 WORDS)