Characterization of a hyperpolarization-activated cation current in rat pituitary cells

Whole cell patch-clamp techniques were used on clonal pituitary cells (GH3) and primary cultures of somatotrophs and lactotrophs to study currents that would be active at or below voltages for the threshold for action potential generation. When GH3 cells were held at -60 mV and pulsed to -120 mV, a slow-activating sustained inward current was observed (-16.5 +/- 1.5 pA in physiological baths, n = 72; approximately 1 s to half-maximal activation, voltage for 50% activation - 101 mV). The current was insensitive to bath application of 10 mM tetraethylammonium, 10 mM 4-aminopyridine, and 1 mM barium but was completely blocked by 3 mM cesium. The current was found to be a mixed cation current with a sodium permeability of 0.29 relative to potassium. These properties indicate that the current belongs to the hyperpolarization-activated cation current (Ih), or I(f), family of currents. However, the current was not altered by the addition of adenosine 3',5'-cyclic monophosphate (cAMP) to the pipette or forskolin to the bath. A similar but smaller current was observed in 15 of 16 somatotrophs but in only 1 of 9 lactotrophs. Application of cesium to spontaneously spiking GH3 cells or somatotrophs had no effect. However, cesium did block an inward holding current observed at -80 mV. These results demonstrate that the I(h) in pituitary cells does not serve as a pacemaking current but suggest that it may influence membrane potential responses when somatotrophs become hyperpolarized.