Two types of voltage-dependent calcium current in rat somatotrophs are reduced by somatostatin.
1. Somatotrophs were obtained from rat pituitary glands after dissociation, separation and enrichment on a continuous gradient of bovine serum albumin at unit gravity. Somatotrophs were enriched up to 85% in the heavy fractions (F8 and F9). 2. After identification by reverse hemolytic plaque assay, patch-clamp recording in the whole-cell mode was performed on somatotrophs. 3. Under voltage-clamp conditions, two types of Ca2+ currents were recorded. From a holding potential of -70 mV, depolarizing voltage steps to potentials more positive than -50 mV activated a current which rapidly inactivated and which was very sensitive to Ni2+ but not to Cd2+. This current corresponds to T-type current. Depolarizing steps to potentials more positive than -30 mV from a holding potential of -40 mV triggered a current which slowly inactivated and which was very sensitive to Cd2+ but not to Ni2+. This current corresponds to L-type current. 4. Application of somatostatin to the bath solution (10 nM) markedly reduced the amplitudes of both T- and L-type currents. Somatostatin decreased the conductance of L-type current without modifying its time- and voltage-dependent inactivation but its activation was not affected. However, somatostatin decreased the conductance of T-type currents, and also accelerated its time-dependent inactivation. Half-inactivation voltage of T-type current was shifted from -52 to -63 mV by somatostatin but no change was obtained in the current activation curve. 5. All these modifications in Ca2+ currents were abolished by a pre-treatment of the cultures with pertussis toxin (100 ng/ml, for 10 h). This pre-treatment also blocked the inhibitory effect of somatostatin on high-K(+)-stimulated growth hormone release. 6. Our results show that somatostatin acts on somatotrophs by attenuating the voltage-dependent Ca2+ currents. These effects may contribute to a somatostatin-induced reduction in [Ca2+]i and the subsequent decline in growth hormone release.
Chen
C
C
INSERM U.-176, Bordeaux, France.
Zhang
J
J
Vincent
J D
JD
Israel
J M
JM
D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D004553:Electric Conductivity; D013006:Growth Hormone / Q000378:metabolism*; D006462:Hemolytic Plaque Technique; D008297:Male; D008564:Membrane Potentials; D009532:Nickel / Q000494:pharmacology; D037342:Pertussis Toxin; D010902:Pituitary Gland / Q000166:cytology / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013004:Somatostatin / Q000494:pharmacology*; D013997:Time Factors; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D010566:Virulence Factors, Bordetella; D002104:Cadmium; D013004:Somatostatin; D009532:Nickel; D013006:Growth Hormone; D037342:Pertussis Toxin; D011188:Potassium; D002118:Calcium