Dual effects of thyrotrophin-releasing hormone (TRH) on K+ conductance in frog pituitary melanotrophs. TRH-induced alpha-melanocyte-stimulating hormone release is not mediated through voltage-sensitive K+ channels.
Modulation of the activity of K+ channels by TRH and the possible involvement of this modulation in TRH-induced release of alpha-MSH were studied in cultured frog melanotrophs, using patch-clamp and perifusion techniques. Pars intermedia cells were enzymatically dispersed and cultured in Leibovitz medium. In order to test the viability of cultured cells, the amount of alpha-MSH released into the medium was measured by radioimmunoassay every day for 1 week of culture. The total amount of alpha-MSH released during the first 4 days of culture was 8.6 times higher than the intracellular content of alpha-MSH on day 1. Melanotrophs were identified by an indirect immunofluorescence technique using a specific antiserum to alpha-MSH. Recordings obtained in whole-cell, cell-attached and excised patch-clamp configurations showed that TRH induced a transient polarization concomitant with an increase in the probability of opening of Ca2+-activated K+ channels. This transient response was followed by a depolarization accompanied by an enhanced frequency of action potential discharge. TRH also induced a decrease in voltage-dependent K+ conductance. Application of tetraethylammonium, a K+ channel blocker, depolarized the cells and increased the basal secretory level without noticeable changes in TRH-evoked alpha-MSH release. These results demonstrate that the neuropeptide TRH both stimulates Ca2+-sensitive K+ channels and inhibits voltage-dependent K+ current in pituitary melanotrophs. Our data indicate that TRH-induced secretion of alpha-MSH is not a direct consequence of the lowering of K+ conductance. It thus appears that basal and TRH-induced alpha-MSH release occur through distinct pathways; the spontaneous release of alpha-MSH is probably linked to membrane potential, while modulation of the electrical activity is not directly involved in TRH-induced activation of the secretory process.
Louiset
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Groupe de Recherche en Endocrinologie Moléculaire, CNRS URA 650, Université de Rouen, Mont-Saint-Aignan, France.
Cazin
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Lamacz
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Tonon
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MC
Vaudry
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D000818:Animals; D004553:Electric Conductivity; D004594:Electrophysiology; D005455:Fluorescent Antibody Technique; D007150:Immunohistochemistry; D009074:Melanocyte-Stimulating Hormones / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*; D015221:Potassium Channels / Q000378:metabolism*; D011895:Rana ridibunda; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*; D013997:Time Factors
D015221:Potassium Channels; D013757:Tetraethylammonium Compounds; D013973:Thyrotropin-Releasing Hormone; D019789:Tetraethylammonium; D009074:Melanocyte-Stimulating Hormones; D011188:Potassium