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Effects of Growth Hormone‐Releasing Peptide‐2 (GHRP‐2) on Membrane Ca 2+ Permeability in Cultured Ovine Somatotrophs
Author(s) -
Chen Chen,
Clarke Iain J.
Publication year - 1995
Publication title -
journal of neuroendocrinology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.062
H-Index - 116
eISSN - 1365-2826
pISSN - 0953-8194
DOI - 10.1111/j.1365-2826.1995.tb00745.x
Subject(s) - chemistry , somatotropic cell , endocrinology , medicine , tetraethylammonium , voltage clamp , nifedipine , channel blocker , intracellular , patch clamp , biophysics , membrane potential , receptor , calcium , biology , biochemistry , pituitary gland , hormone , potassium , organic chemistry
The newly synthesized GH‐releasing peptide, GHRP‐2 (D‐Ala‐D‐βNal‐Ala‐Trp‐D‐Phe‐Lys‐NH 2 ), 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 + , Ca 2+ ) 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 Ca 2+ channel blocker, Co 2+ (0.5 mM). Ca 2+ 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) Ca 2+ currents. The L type current was abolished by addition of nifedipine (3 μM) 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 Ca 2+ currents in response to GHRP‐2. The voltage‐dependent inactivation curve for T type Ca 2+ current was shifted towards a less negative level by the peptide. Intracellular free Ca 2+ concentration ([Ca 2+ ]i) in somatotroph‐enriched populations was specifically increased by GHRP‐2 but this effect was totally abolished by blockade of membrane Ca 2+ channels. These data show that GHRP‐2 causes an influx of Ca 2+ leading to an increase in [Ca 2+ ]i in ovine somatotrophs. The Ca 2+ currents were both L type and T type with a shift in the inactivation curve of the latter by the releasing peptide.