The effects of two fractions (II, containing anti-insect toxins, and III, containing eight anti-mammal toxins) isolated from the venom of the Old World scorpion Buthus martensii (Karsch) on Na+ currents of rat anterior pituitary cells (GH3/B6 cells) were investigated using the whole-cell configuration of the patch clamp technique. Fraction II induced a temporary, and fraction III a permanent increase of the Na+ current amplitude. Application of each of the venom fractions resulted in a flattening of the curve relating steady state Na+ inactivation to membrane potential. In addition, the two fractions had specific effects. Fraction II shifted the voltage dependence of Na+ current activation by -42 mV, and the voltage dependence of Na+ inactivation by -25 mV in the absence of a conditioning depolarizing pre-pulse. Slowing of Na+ inactivation was most prominent at negative membrane potentials, resulting in a steady Na+ inward current at the holding potential of -80 mV. Fraction III induced a pronounced slowing of Na+ inactivation leading to an increase of peak Na+ currents and to incomplete steady state Na+ inactivation even at positive membrane potentials.
