The effect of divalent cations on IK(f) : a transient outward potassium current expressed in melanotrophs of the rat pituitary gland

• Main Author: Davidson, Jana-Lea
• Language: English
• Published: 2008
• Online Access: http://hdl.handle.net/2429/2950

Divalent cations are known to exert a charge screening effect on voltage-gated ion channels either through non-specific interactions with fixed negative charges on the cell membrane or via binding to negatively charged sites on or electrically close to the channel forming protein. In some instances, divalent cations bind directly to the gating apparatus of voltage-sensitive sodium and potassium channels thereby stabilizing the channels in a closed conformation. Most of the investigations into the effects of divalent cations on voltage-gated ion channels have concentrated on the Na + and delayed rectifier type K + channels. Although, there has been a recent explosion of information regarding the molecular structure of the transient outward potassium channel, few investigators have examined the actions of divalent cations on the behaviour of the transient outward potassium current (TOC). A transient outward potassium current, (IK(f)), has been characterized in melanotrophs, the major cell type found in the pars intermedia of the pituitary gland in rats (Kehl, 1989). I K(f) activates and inactivates rapidly. Cd2+ (5 mM) reduced the peak amplitude of IK(f) and increased the 50% rise time of this current (Kehl, 1989). The present study elaborated on these observations and examined the effects of varying the extracellular concentrations of Cd 2+ , Zn2+ , Ca2+ and Mg2+ on the behaviour of IK(f). Acutely dissociated melanotrophs were obtained from male Wistar rats and wholecell currents were recorded, using conventional patch clamp techniques, from cells maintained in culture for 1-14 hrs. Divalent cations shift the activation and inactivation curves and the gating kinetics of IK(f) right-ward along the voltage axis. The cations tested varied in their ability to shift the potential-sensitive parameters of IK(f) and ranked in the following order: Zn2+ ≥ Cd2+ > > Ca2+ > Mg2+ , in good agreement with previous observations of their effect on sodium channels. The mean control half-activation potential was -13.6 mV with a slope-factor of + 12.8 mV (n = 55) and the mean control half-inactivation potential was -54.7 mV with a slopefactor of -4.4 mV (n =50). The relationships between the shift of the half-activation potential and the divalent cation concentration indicated that the K m's for the half-maximal shift of the activation curve were 221 μM (Cd2+), 92 μM (Zn2 +) and 3.4 mM (Ca2+) and the maximal shifts of the activation curve were, respectively, +28 mV, +34 mV and + 15.6 mV. Mg2+ was far less potent than any of the other divalent cations examined. Shifts of the inactivation curve were equal to the shifts of the activation curve at each divalent cation concentration tested. The slope-factors of the activation and inactivation curves were not altered by the application of divalent cations. Removal of Ca2+ from the external media significantly increased the slope-factor for the activation curve. That is, zero Ca2+ resulted in a decrease in the equivalent charge transferred during the activation gating process. The prediction central to non-specific charge screening is that all divalent cations will be equally effective. The results reported here show that this is not the case for IK(f). It is proposed that a specific binding site for divalent cations exists on or electrically close to the channel protein. Divalent cations also slowed the rise time of I K(f) suggesting that they might stabilize the channel conducting this current in the closed conformation.