Endocrinology, Vol 125, 1889-1897, Copyright © 1989 by Endocrine Society

ARTICLES

Intracellular Ca2+ mobilization by thyrotropin, carbachol, and adenosine triphosphate in dog thyroid cells

CS Rani, WP Schilling and JB Field
Department of Medicine, Baylor College of Medicine, Houston, Texas 77030.

The effect of TSH, carbachol (CC), and ATP on intracellular calcium concentration ([Ca2+]i) in primary cultures of dog thyroid cells was examined using the fluorescent Ca2+ indicator fura-2. TSH caused an increase in [Ca2+]i at 37 C, but not 22 C, while it increased cAMP formation in these cells at both 22 and 37 C. CC and ATP increased [Ca2+]i at both 22 and 37 C. The CC-induced increase in [Ca2+]i was under muscarinic receptor control, and it was biphasic, with an initial spike followed by a sustained increase at a lower level. TSH and ATP were weaker agonists compared to CC, since maximal doses of TSH (100- 500 mU/ml) and ATP (100-500 microM) increased [Ca2+]i by 40-70% over basal levels, compared to a 2- to 4-fold increase in [Ca2+] induced by maximal doses of CC (10-50 microM). The TSH-induced increase in [Ca2+]i was transient, returning to basal levels within 1-2 min after application of the agonist. All three agents were able to transiently increase [Ca2+]i to be internal stores. In the presence of the inorganic Ca2+ channel blockers La3+, Ni2+, and Co2+, the peak [Ca2+]i change was little affected, while the persistent response to CC and ATP was blocked, indicating dependence of this phase on influx of Ca2+. Paradoxically, these channel blockers abolished the effect of TSH on [Ca2+]i. TSH stimulation of cAMP formation was also inhibited 80-90% by these blockers, but not in Ca2+-free/EGTA buffer. These results suggest that the Ca2+ channel blockers may have actions in addition to inhibition of Ca2+ entry in these cells. TMB-8 [8-(N,N- diethylamino)octyl-3,4,5-trimethoxybenzoate HCl] specifically blocked both the initial and sustained increase induced by CC, while having no effect on ATP or TSH-induced [Ca2+]i, suggesting that TMB-8 may not be a general antagonist of Ca2+ mobilization. Activators of protein kinase- C, such as phorbol esters or an analog of diacylglycerol, inhibited the [Ca2+]i rise induced by all the three agonists used, indicating a regulatory role of protein kinase-C activation on [Ca2+]i in these cells. In FRTL-5 cells, [Ca2+]i was also increased by TSH and ATP, but not by CC. ATP, however, was a more effective agonist than in dog thyroid cells, while TSH increased [Ca2+]i by a similar magnitude in both cell types. The results of the present study demonstrate that TSH, albeit of lesser potency than CC, increases [Ca2+]i by causing intracellular Ca2+ mobilization in cultured dog thyroid cells.(ABSTRACT TRUNCATED AT 400 WORDS)

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