Voltage-dependent potassium currents in ovine somatotrophs and their function in growth hormone secretion.
Sheep somatotroph-enriched cultures were obtained by means of enzyme dissociation and Percoll gradient separation. Nystatin-perforated-whole-cell recordings were performed on post-recording-identified somatotrophs after 4-14 days in vitro. Using Ca(2+)-free, tetrodotoxin-containing (1 microM) bath solution and K+ electrode solution, three types of voltage-dependent K+ currents were recorded as inward rectifying, outward transient and outward delayed rectifying K+ currents. The inward rectifying K+ current was very small at physiological extracellular K+ concentrations (5 mM) and enhanced by increasing the K+ concentration in the bath to 55 mM; it was blocked by tetraethylammonium (2 mM) but not by 4-aminopyridine (5 mM). A transient outward K+ current appeared at -50 mV and was selectively diminished by 4-aminopyridine (2 or 4 mM). A delayed rectifying outward K+ current was observed when the membrane potential was depolarized to -20 mV and was blocked by tetraethylammonium (2 mM) but not 4-aminopyridine (4 mM). Application of 4-aminopyridine but not tetraethylammonium (up to 5 mM) depolarized the cell membrane potential recorded under current clamp conditions and triggered action potentials when the bath solution contained Ca2+ (2 mM) but not tetrodotoxin. The intracellular Ca2+ concentration was increased by 4-aminopyridine as was growth hormone release. Therefore, the 4-aminopyridine-sensitive transient outward K+ current appears to be important in the determining the resting potential of ovine somatotrophs and plays a major role in regulating basal intracellular Ca2+ concentration and growth hormone secretion.
Chen
C
C
Prince Henry's Institute of Medical Research, Clayton, Australia.
Heyward
P
P
Zhang
J
J
Wu
D
D
Clarke
I J
IJ
D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002469:Cell Separation; D002478:Cells, Cultured; D004594:Electrophysiology; D013006:Growth Hormone / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects; D009994:Osmolar Concentration; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000378:metabolism* / Q000502:physiology*; D011188:Potassium / Q000502:physiology*; D026902:Potassium Channel Blockers; D012756:Sheep; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
D026902:Potassium Channel Blockers; D013757:Tetraethylammonium Compounds; D019789:Tetraethylammonium; D013006:Growth Hormone; D015761:4-Aminopyridine; D011188:Potassium; D002118:Calcium