Phoneutria nigriventer toxin Tx3-1 blocks A-type K+ currents controlling Ca2+ oscillation frequency in GH(3) cells

GH(3) cells present spontaneous Ca2+ action potentials and oscillations of intracellular Ca2+, which can be modified by altering the activity of K+ Or Ca2+ channels. We took advantage of this spontaneous activity to screen fo r effects of a purified toxin (Tx3-1) from the venom of Phoneutria nigriven ter on ion channels. We report that Tx3-1 increases the frequency of Ca2+ o scillations, as do two blockers of potassium channels, 4-aminopyridine and charybdotoxin. Whole-cell patch clamp experiments show that Tx3-1 reversibl y inhibits the A-type K+ current (I-A but does not block other K+ currents delayed-rectifying, inward-rectifying, and large-conductance Ca2+-sensitive ) or Ca2+ channels CT and L type in these cells. In addition, we describe t he sequence of a full cDNA clone of Tx3-1, which shows that Tx3-1 has no ho mology to other known blockers of K+ channels and gives insights into the p rocessing of this neurotoxin. We conclude that Tx3-1 is a selective inhibit or of I,, which can be used to probe the role of this channel in the contro l of cellular function. Based on the effect of Tx3-1, we suggest that I, is an important determinant of the frequency of Ca2+ oscillations in unstimul ated GH(3) cells.