Triiodo-L-thyronine enhances TRH-induced TSH release from perifused rat pituitaries and intracellular Ca2+ levels from dispersed pituitary cells.
There is now increasing evidence that Ca2+ serves as the first messenger for the prompt and non-genomic effects of 3,5,3' triiodo-L-thyronine (T3) in several tissues. We have previously shown that the first phase of thyroid stimulating hormone (TSH) release in response to thyrotropin-releasing hormone (TRH) can be potentiated by messengers of hypothalamic origin, by a Ca(2+)-dependent phenomenon involving the activation of dihydropyridine-sensitive Ca2+ channels. By perifusing rat pituitary fragments, we have investigated whether T3 would modify TSH release when the hormone is applied for a short time (i.e. 30 min) before a 6 min pulse of physiological concentration of TRH, thus excluding the genomic effect of T3. We show that: (1) increasing concentrations of T3 (100 nM-10 microM) in the perifused medium potentiates the TRH-induced TSH release in a dose-dependent manner; (2) the T3 potentiation is not reproduced by diiodothyronine and T3 does not potentiate the increase if TSH release induced by a depolarizing concentration of KCl; (3) the protein synthesis inhibitor cycloheximide, does not significantly modify the effect of T3; (4) addition of Co2+, nifedipine, verapamil, or omega-conotoxin in the medium, at a concentration which does not modify the TSH response to TRH, reverses the T3 potentiation of that response. We also tested whether T3 would change intracellular concentrations of Ca2+, by measuring [Ca2+]i with fura-2 imaging on primary cultures of dispersed pituitary cells, either in basal conditions or after stimulation by TRH or/and T3. Both substances induced a fast increase of [Ca2+]i, with a peak at 15 s, followed by a subsequent progressive decay with TRH and a rapid return with T3. Our data suggest that T3 enhances TRH-induced TSH release by a protein synthesis-independent and Ca(2+)-dependent phenomenon, probably due to an increase in Ca2+ entry through the activation of dihydropyridine- and omega-conotoxin-sensitive Ca2+ channels. They also show that T3 may acutely enhance [Ca2+]i in pituitary cells. These findings support the idea of the occurrence of a prompt and stimulatory role of T3 at the plasma membrane level in normal rat pituitary gland.
Roussel
J P
JP
Laboratoire de Neurobiologie Endocrinologique, URA 1197 CNRS, Université de Montpellier 2., France.
Grazzini
E
E
Zumbihl
R
R
Rodriguez
E
E
Astier
H
H
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D008297:Male; D009543:Nifedipine / Q000494:pharmacology; D010902:Pituitary Gland / Q000166:cytology / Q000187:drug effects*; D011863:Radioimmunoassay; D051381:Rats; D017208:Rats, Wistar; D013972:Thyrotropin / Q000494:pharmacology*; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*; D013997:Time Factors; D014284:Triiodothyronine / Q000494:pharmacology*; D014700:Verapamil / Q000494:pharmacology
D014284:Triiodothyronine; D013973:Thyrotropin-Releasing Hormone; D013972:Thyrotropin; D014700:Verapamil; D009543:Nifedipine; D002118:Calcium