TRIIODO-L-THYRONINE ENHANCES TRH-INDUCED TSH RELEASE FROM PERIFUSED RAT PITUITARIES AND INTRACELLULAR CA2-CELLS( LEVELS FROM DISPERSED PITUITARY)

There is now increasing evidence that Ca2+ serves as the first messeng er for the prompt and non-genomic effects of 3,5,3' triiodo-L-thyronin e (T-3) in several tissues. We have previously shown that the first ph ase of thyroid stimulating hormone (TSH) release in response to thyrot ropin-releasing hormone (TRH) can be potentiated by messengers of hypo thalamic origin, by a Ca2+-dependent phenomenon involving the activati on of dihydropyridine-sensitive Ca2+ channels. By perifusing rat pitui tary fragments, we have investigated whether T-3 would modify TSH rele ase 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 th e genomic effect of T-3. We show that: (1) increasing concentrations o f T-3 (100 nM-10 mu M) in the perifused medium potentiates the TRH-ind uced TSH release in a dose-dependent manner; (2) the T-3 potentiation is not reproduced by diiodothyronine and T-3 does not potentiate the i ncrease of TSH release induced by a depolarizing concentration of KCl; (3) the protein synthesis inhibitor cycloheximide, does not significa ntly modify the effect of T-3; (4) addition of Co2+, nifedipine, verap amil, or omega-conotoxin in the medium, at a concentration which does not modify the TSH response to TRH, reverses the T-3 potentiation of t hat response. We also tested whether T-3 would change intracellular co ncentrations of Ca2+, by measuring [Ca2+](i) with fura-2 imaging on pr imary cultures of dispersed pituitary cells, either in basal condition s or after stimulation by TRH or/and T-3. Both substances induced a fa st increase of [Ca2+](i), with a peak at 15 s, followed by a subsequen t progressive decay with TRH and a rapid return with T-3. Our data sug gest that T-3 enhances TRH-induced TSH release by a protein synthesis- independent and Ca2+-dependent phenomenon, probably due to an increase in Ca2+ entry through the activation of dihydropyridine- and omega-co notoxin-sensitive Ca2+ channels. They also show that T-3 may acutely e nhance [Ca2+](i) in pituitary cells. These findings support the idea o f the occurrence of a prompt and stimulatory role of T-3 at the plasma membrane level in normal rat pituitary gland.