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 (T-3) 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 Ca2+-dependent phenomenon involving the activation of dihydropyridine-sensitive Ca2+ channels. By perifusing rat pituitary fragments, we have investigated whether T-3 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 T-3. We show that: (1) increasing concentrations of T-3 (100 nM-10 mu M) in the perifused medium potentiates the TRH-induced TSH release in a dose-dependent manner; (2) the T-3 potentiation is not reproduced by diiodothyronine and T-3 does not potentiate the increase of TSH release induced by a depolarizing concentration of KCl; (3) the protein synthesis inhibitor cycloheximide, does not significantly modify the effect of T-3; (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 T-3 potentiation of that response. We also tested whether T-3 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 T-3. 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 T-3. Our data suggest 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-conotoxin-sensitive Ca2+ channels. They also show that T-3 may acutely enhance [Ca2+](i) in pituitary cells. These findings support the idea of the occurrence of a prompt and stimulatory role of T-3 at the plasma membrane level in normal rat pituitary gland.