A transient outward current dependent on external calcium in rat cerebellar granule cells

The outward potassium current of rat cerebellar granule cells in culture was studied with the whole-cell patch-clamp method. Two voltage-dependent components were identified: a slow current, resembling the classical delayed rectifier current, and a fast component, similar to anI A-type current. The slow current was insensitive to 4-aminopyridine and independent of external Ca2+, but significantly inhibited by 3mM tetraethylammonium. The fast current was depressed by external 4-aminopyridine, with an ED50=0.7mM, and it was abolished by removal of divalent cations from the external medium. The sensitivity of the transient outward current to different divalent cations was investigated by equimolar substitution of Ca2+, Mn2+ and Mg2+. In 2.8mM Mn2+, the transient potassium conductance was comparable to that in 2.8mM Ca2+, while in 2.8mM Mg2+ the transient component was drastically reduced, as in the absence of any divalent cations. However, when Ca2+ was present, Mg2+ up to 5mM had no effect. The transient current increased with increasing concentrations of external Ca2+, [Ca2+] o , and the maximum conductancevs. [Ca2+] o curve could be approximated by a one-site model. In addition, the current recorded with 5.5mM BAPTA in the intracellular solution was not different from that recorded in the absence of any Ca2+ buffer. These results suggest that divalent cations modulate the potassium channel interacting with a site on the external side of the cell membrane.