Effects of growth hormone-releasing peptide-2 (GHRP-2) on membrane Ca2+ permeability in cultured ovine somatotrophs.
The newly synthesized GH-releasing peptide, GHRP-2 (D-Ala-D-beta Nal-Ala-Trp-D-Phe-Lys-NH2), was studied in somatotroph-enriched populations of ovine pituitary cells in primary culture. Nystatin-perforated whole-cell recordings were made on identified somatotrophs after 4-14 days of culture. Using a standard bath solution (containing Na+, Ca2+) and an electrode solution containing K+ in current-clamp recordings, GHRP-2 (10 nM) depolarized the membrane potential of the cells triggering a burst of action potentials. Voltage-clamp recordings indicated that GHRP-2 produced a slowly inactivated inward current with a slight reduction in outward current. The inward current was blocked by the Ca2+ channel blocker, Co2+ (0.5 mM). Ca2+ currents were then isolated using tetraethylammonium bath solution and an electrode solution containing Cs+. Ovine somatotrophs possess transient (T type) and long lasting (L type) Ca2+ currents. The L type current was abolished by addition of nifedipine (3 microM) to the bath solution and T type current was isolated on this basis. Current-voltage relationships indicated an increase in both T and L type Ca2+ currents in response to GHRP-2. The voltage-dependent inactivation curve for T type Ca2+ current was shifted towards a less negative level by the peptide. Intracellular free Ca2+ concentration ([Ca2+]i) in somatotroph-enriched populations was specifically increased by GHRP-2 but this effect was totally abolished by blockade of membrane Ca2+ channels. These data show that GHRP-2 causes an influx of Ca2+ leading to an increase in [Ca2+]i in ovine somatotrophs.(ABSTRACT TRUNCATED AT 250 WORDS)
Chen
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Prince Henry's Institute of Medical Research, Clayton, Victoria, Australia.
Clarke
I J
IJ
D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002463:Cell Membrane Permeability / Q000502:physiology*; D002478:Cells, Cultured; D008564:Membrane Potentials / Q000187:drug effects; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000494:pharmacology*; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects*; D012756:Sheep
D009842:Oligopeptides; C091874:growth hormone-releasing peptide-2; D002118:Calcium