The erg-like potassium current in rat lactotrophs

1. The ether-a-go-go-related gene (erg)-like K+ current in rat lactotrophs from primary culture was characterized and compared with that in clonal rat pituitary cells (GH(3)/B-6). The class III antiarrhythmic E-4031 known to block specifically erg K+ channels was used to isolate the erg-like current as the E-4031-sensitive current, The experiments were performed in 150 mM K+ external solution using the patch-clamp technique. 2. The erg-like K+ current elicited with hyperpolarizing pulses negative to -100 mV consisted of a fast and a pronounced slowly deactivating current c omponent. The contribution of the slow component to the total current ampli tude was potential dependent and varied from cell to cell. At -100 mV it ra nged from 50 to 85% and at -140 mV from 21 to 45%. 3. The potential-dependent channel availability curves determined with 2s p repulses were fitted with the sum of two Boltzmann functions. The function related to the slowly deactivating component of the erg-like current was sh ifted by more than 40 mV to more negative membrane potentials compared with that of the fast component. 4. In contrast to that of native lactotrophs studied under identical condit ions, the erg-like K+ current of GH(3)/B-6, cells was characterized by a pr edominant fast deactivating current component, with similar kinetic and ste ady-state properties to the fast deactivating current component of native l actotrophs. 5. Thyrotrophin-releasing hormone reduced the erg -like current in native l actotrophs via an intracellular signal cascade which seemed to involve a pa thway independent from protein kinase A and protein kinase C. 6. RT-PCR studies on cytoplasm from single lactotrophs revealed the presenc e of mRNA of the rat homologue of the human ether-a-go-go-related gene HERG (r-erg1) as well as mRNA of the two other cloned r-erg cDNAs (r-erg2 and r -erg3) in different combinations. In GH(3)/B-6 cells, only the transcripts of r-erg1 and r-erg2 were, found.