Mechanism of extracellular Ca²⁺ receptor‐stimulated hormone release from sheep thyroid parafollicular cells

Abstract

1. Expression of receptors to extracellular calcium enables parafollicular cells of the thyroid gland (PF cells) to release calcitonin (CT) and serotonin (5_HT) in response to increased external Ca¥. Recently, a calcium-sensing receptor (CaR), similar to the G protein-coupled receptor for external Ca¥ cloned from parathyroid gland, was shown to be expressed in PF cells. Using a highly purified preparation of sheep PF cells, we have examined the electrical and biochemical processes coupling CaR activation to hormone release. cation is Ca¥, indicating that influx via the cation conductance is another source of the increases in [Ca¥]é. 6. Extracellular Ca¥ stimulates 5_HT release with an ECÛÑ of 1·5 mÒ. Nimodipine blocks 90% of the Cafi-induced 5_HT release, while other inhibitors of voltage-gated calcium channels had no effect. These data support an important role for L-type Ca¥ channels in CaR-induced hormone secretion. Although earlier studies indicate that high [Ca¥]ï induces release of Ca¥ from intracellular stores, thapsigargin-induced depletion of these stores did not affect secretion from these cells, indicating that Ca¥ influx is necessary and sufficient for the Cafi-induced 5_HT secretion. 7. Inhibition of protein kinase C (PKC) using chelerythrine, staurosporine, or calphostin C inhibited Cafi-induced 5_HT release by 50% while phorbol ester-induced 5_HT secretion was completely inhibited. Thus, PKC is an important component of the pathway linking CaR activation to hormone release. However, another as yet unknown second messenger also contributes to this pathway. 8. We tested the contribution of two different phospholipases to the CaR responses to determine the source of the PKC activator diacylglycerol (DAG). Selective inhibition of phosphatidylinositol-specific phospholipase C (PI-PLC) with U73122 had no effect on the response to elevated [Ca¥]ï. However, pretreatment with D609, a selective inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC), inhibited Ca¥-induced 5_HT release to 50% of control indicating that phosphatidylcholine is a likely source of DAG in the response of PF cells to elevated [Ca¥]ï.

Ca2+-Evoked Serotonin Secretion by Parafollicular Cells: Roles in Signal Transduction of Phosphatidylinositol 3′-kinase, and the γ and ζ Isoforms of Protein Kinase C

“…PKC, however, appears to play an important role in the secretory process. Phorbol esters stimulate 5-HT secretion, and secretion in response to phorbol esters is abolished by phorbol ester downregulation of PKC (Tamir et al, 1990;McGehee et al, 1997). Nevertheless, despite the ability of phorbol ester downregulation of PKC to block phorbol ester-induced secretion, downregulation only slightly inhibits 5-HT secretion when it is evoked by 1[Ca 2ϩ ] e .…”

“…The CaR is G-protein-coupled (Herbert and Brown, 1995;Ruat et al, 1996) and has been demonstrated to be linked to Gi and Gq (Tamir et al, 1996). After the activation of the CaR by 1[Ca 2ϩ ] e , PF cells depolarize (as a result of an increase in a nonselective plasma membrane cation conductance) (McGehee et al, 1997), secretory vesicles acidify (Tamir et al, 1994b), and 5-HT is secreted (Tamir et al, 1990(Tamir et al, , 1994bMcGehee et al, 1997).…”

Effect of dietary components on renal inorganic phosphate (Pi) excretion induced by a Pi-depleted diet

“…Much has been learned from studies of lingual nutrient detection due to identification of the G protein family, which couples with nutrient-sensitive G protein coupled receptors located in the taste buds on the tongue and in the intestine (26,27). The CaR, a G protein-coupled receptor, is best known and understood for its control of PTH synthesis and secretion in response to changes in Ca, Mg, Sr, and La (17,18). CaRs are distributed along most of the gastrointestinal tract, from the stomach to the large intestine (23,28).…”

“…CaRs are distributed along most of the gastrointestinal tract, from the stomach to the large intestine (23,28). The physiologic role of CaRs is the maintenance of constant blood Ca 2 + levels (1.1-1.3 mM) through continuous adjustments of PTH release from the parathyroid chief cells, which are highly sensitive to the slightest changes in extracellular Ca ion concentrations (17,18). In the present study, we showed that Mg, La, or Sr, which are CaR agonists, can partially substitute for dietary Ca.…”

“…To examine the effects of various metal ions as substitutes for Ca, we investigated the effect of Mg, La, and Sr, which are Ca sensing receptor (CaR) agonists, on the Pi depletion signal (17,18). In this experiment, food intake did not differ among the five groups (data not shown).…”