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Mechanism of extracellular Ca 2+ receptor‐stimulated hormone release from sheep thyroid parafollicular cells
Author(s) -
McGehee Daniel S.,
Aldersberg Mella,
Liu KouPeing,
Hsuing Shuchi,
Heath Mark J. S.,
Tamir Hadassah
Publication year - 1997
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1997.031bl.x
Subject(s) - extracellular , chemistry , depolarization , biophysics , calcium , endocrinology , medicine , receptor , membrane potential , parathyroid hormone , intracellular , voltage dependent calcium channel , patch clamp , biochemistry , biology , organic chemistry
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 2+ . Recently, a calcium‐sensing receptor (CaR), similar to the G protein‐coupled receptor for external Ca 2+ 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. 2 Whole‐cell recordings in the permeabilized‐patch configuration show that elevated extracellular Ca 2+ concentration ([Ca 2+ ] o ) depolarizes these cells and induces oscillations in membrane potential. In voltage clamp, high [Ca 2+ ] o activates a cation conductance that underlies the depolarization. This conductance is cation selective, with a reversal potential near −25 mV indicating poor ion selectivity. 3 The CaR expressed in these cells is activated by other multivalent cations with a rank order potency of Gd 3+ > Ba 2+ > Ca 2+ ≫ Mg 2+ . The insensitivity of these cells to high external Mg 2+ contrasts with the reported sensitivity of the cloned CaR from parathyroid. 4 Elevation of [Ca 2+ ] o also stimulates increases in intracellular Ca 2+ concentration ([Ca 2+ ] o ) and this effect is largely inhibited by the Ca 2+ channel blocker nimodipine, indicating that L‐type voltage‐gated Ca 2+ channels contribute to the response to elevated [Ca 2+ ] o . 5 Elevated [Ca 2+ ] o induces an inward current under conditions where the only permeant external cation is Ca 2+ , indicating that influx via the cation conductance is another source of the increases in [Ca 2+ ] i . 6 Extracellular Ca 2+ stimulates 5‐HT release with an EC 50 of 1.5 m m . Nimodipine blocks 90% of the Ca 2+ ‐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 2+ channels in CaR‐induced hormone secretion. Although earlier studies indicate that high [Ca 2+ ] o induces release of Ca 2+ from intracellular stores, thapsigargin‐induced depletion of these stores did not affect secretion from these cells, indicating that Ca 2+ influx is necessary and sufficient for the Ca 2+ ‐induced 5‐HT secretion. 7 Inhibition of protein kinase C (PKC) using chelerythrine, staurosporine, or calphostin C inhibited Ca 2+ ‐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 2+ ] o . However, pretreatment with D609, a selective inhibitor of phosphatidylcholine‐specific phospholipase C (PC‐PLC), inhibited Ca 2+ ‐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 2+ ] o .