SOMATOSTATIN INCREASES VOLTAGE-DEPENDENT POTASSIUM CURRENTS IN RAT SOMATOTROPHS

To study the modulatory effects of somatostatin on membrane K+ currents, whole cell voltage-clamp recordings were performed on identified rat somatotrophs in primary culture. In the presence of Co2+ (2mM) and tetrodotoxin (1-mu-M) in the bath solution to block Ca2+ and Na+ inward currents, two types of voltage-activated K+ currents were identified on the basis of their kinetics and pharmacology. First, a delayed rectifier K+ current (I(K)) had a threshold of -20 mV, did not decay during voltage steps lasting 300 ms, and was markedly attenuated by extracellular application of tetraethylammonium (TEA, 10 mM). Second, a transient outward K+ current (I(A)) was activated at -40 mV (from a holding potential of -80 mV) and persisted despite the presence of TEA. This I(A) was blocked by 4-aminopyridine (2 mM). Somatostatin (10 nM) increased I(K) by 75% and I(A) by 45% without obvious effects on steady-state voltage dependency of activation or inactivation, and these effects were reversible. This increase in K+ currents may contribute in part to the inhibitory effect of somatostatin on growth hormone release.