DIVALENT CATION MODULATION OF A-TYPE POTASSIUM CHANNELS IN ACUTELY DISSOCIATED CENTRAL NEURONS FROM WIDE-TYPE AND MUTANT DROSOPHILA

Abstract

Drosophila mutants provide an ideal model to study channel-type specificity of ion channel regulation in situ. In this study, the effects of divalent cations on voltage-gated K⁺ currents were investigated in acutely dissociated central neurons of Drosophila third instar larvae using the whole-cell patch-clamp recording. Our data showed that micromolar Cd²⁺ enhanced the peak inactivating current (I_A) without affecting the delayed component (I_K). The same results were obtained in Ca²⁺-free external solution, and from slo¹ mutation, which eliminates transient Ca²⁺-activated K⁺ current. Micromolar Cd²⁺and Zn²⁺, and millimolar Ca²⁺and Mg²⁺ all shifted the steady-state inactivation curve of I_A without affecting the voltage-dependence of I_A activation, whereas millimolar Cd²⁺markedly affected both the activation and steady-state inactivation curves for I_A. Divalent cations affected I_A with different potency; the sequence was: Zn²⁺ > Cd²⁺ > Ca²⁺ > Mg²⁺. The modulation of I_A by Cd²⁺ was partially inhibited in Sh^M, a null Shaker (one of I_A-encoding genes) mutation. Taken together, the channel-type specificity, the asymmetric effects on I_A activation and inactivation kinetics, and the diverse potency of divalent cations all strongly support the idea that physiological divalent cations modulate A-type K⁺ channels through specific binding to extracellular sites of the channels.

Keywords:

• I_A
• A-type K⁺ channels
• Cd²⁺
• Zn²⁺
• Ca²⁺
• and Mg²⁺ ions
• slo¹
• Sh^M mutations
• larval neurons
• whole-cell patch-clamp