2 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. © 1990 The Physiological Society