Tubocurarine blocks a calcium-dependent potassium current in rat tumoral pituitary cells.
We investigated the effects of potassium channel inhibitors on electrical activity, membrane ionic currents, intracellular calcium concentration ([Ca2+]i) and hormone release in GH3/B6 cells (a line of pituitary origin). Patch-clamp recordings show a two-component after hyperpolarization (AHP) following each action potential (current clamp) or a two-component tail current (voltage-clamp). Both components can be blocked by inhibiting Ca2+ influx. Application of D-tubocurarine (dTc) (20-500 microM) reversibly suppressed the slowly decaying Ca2+-activated K+ tail current (I AHPs) in a concentration-dependent manner. On the other hand, low doses of tetraethylammonium ions (TEA+) only blocked the rapidly decaying voltage- and Ca2+-activated K+ tail current (I AHPf). Therefore, GH3/B6 cells exhibit at least two quite distinct Ca2+-dependent K+ currents, which differ in size, voltage- and Ca2+-sensitivity, kinetics and pharmacology. These two currents also play quite separate roles in shaping the action potential. d-tubocurarine increased spontaneous Ca2+ action potential firing, whereas TEA increased action potential duration. Thus, both agents stimulated Ca2+ entry. I AHPs is activated by a transient increase in [Ca2+]i such as a thyrotrophin releasing hormone-induced Ca2+ mobilization. All the K+ channel inhibitors we tested: TEA, apamin, dTC and charybdotoxin, stimulated prolactin and growth hormone release in GH3/B6 cells. Our results show that I AHPs is a good sensor for subplasmalemmal Ca2+ and that dTc is a good pharmacological tool for studying this current.
Laboratory of Neurophysiology, CNRS UMR 5543 University V. Segalen, Bordeaux, France.
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Mollard
D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D013006:Growth Hormone / Q000378:metabolism; D018408:Patch-Clamp Techniques; D010903:Pituitary Gland, Anterior / Q000502:physiology*; D010911:Pituitary Neoplasms; D026902:Potassium Channel Blockers; D011388:Prolactin / Q000378:metabolism; D051381:Rats; D019789:Tetraethylammonium / Q000494:pharmacology; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology; D014403:Tubocurarine / Q000494:pharmacology*; D014407:Tumor Cells, Cultured
D026902:Potassium Channel Blockers; D013973:Thyrotropin-Releasing Hormone; D019789:Tetraethylammonium; D011388:Prolactin; D013006:Growth Hormone; D002118:Calcium; D014403:Tubocurarine