title: Hyperpolarization of the membrane potential caused by somatostatin in dissociated human pituitary adenoma cells that secrete growth hormone.
abstract: Membrane electrical properties and the response to somatostatin were examined in dissociated human pituitary adenoma cells that secrete growth hormone (GH). Under current clamp condition with a patch electrode, the resting potential was -52.4 +/- 8.0 mV, and spontaneous action potentials were observed in 58% of the cells. Under voltage clamp condition an outward K+ current, a tetrodotoxin-sensitive Na+ current, and a Ca2+ current were observed. Cobalt ions suppressed the Ca2+ current. The threshold of Ca2+ current activation was about -60 mV. Somatostatin elicited a membrane hyperpolarization associated with increased membrane permeability in these cells. The reversal potential of somatostatin-induced hyperpolarization was -78.4 +/- 4.3 mV in 6 mM K+ medium and -97.2 +/- 6.4 mV in 3 mM K+ medium. These reversal potential values and a shift with the external K+ concentration indicated that membrane hyperpolarization was caused by increased permeability to K+. The hyperpolarized membrane potential induced by somatostatin was -63.6 +/- 5.9 mV in the standard medium. This level was subthreshold for Ca2+ and Na+ currents and was sufficient to inhibit spontaneous action potentials. Hormone secretion was significantly suppressed by somatostatin and cobalt ions. Therefore, we suggest that Ca2+ entering the cell through voltage-dependent channels are playing an important role for GH secretion and that somatostatin suppresses GH secretion by blocking Ca2+ currents. Finally, we discuss other possibilities for the inhibitory effect of somatostatin on GH secretion.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000236:Adenoma / Q000378:metabolism / Q000503:physiopathology*; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D002463:Cell Membrane Permeability / Q000187:drug effects; D013006:Growth Hormone / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D010911:Pituitary Neoplasms / Q000378:metabolism / Q000503:physiopathology*; D013004:Somatostatin / Q000494:pharmacology*
chemical_list: D013004:Somatostatin; D013006:Growth Hormone