Zinc-induced changes in ionic currents of clonal rat pancreatic β-cells: activation of ATP-sensitive K+ channels

摘要

class="enumerated" style="list-style-type:decimal">The effects of zinc (Zn²⁺) on excitability and ionic conductances were analysed on RINm5F insulinoma cells under whole-cell and outside-out patch-clamp recording conditions.We found that extracellular application of 10-20 μM Zn²⁺ induced a reversible abolition of Ca²⁺ action potential firing, which was accompanied by an hyperpolarisation of the resting membrane potential.Higher concentrations of Zn²⁺, in the tens to hundreds micromolar range, induced a reversible reduction of voltage-gated Ca²⁺ and, to a lesser extent, K⁺ currents. Low-voltage-activated Ca²⁺ currents were more sensitive to Zn²⁺ block than high voltage-activated Ca²⁺ currents.The Zn²⁺-induced hyperpolarisation arose from a dose-dependent increase in a voltage-independent K⁺ conductance that was pharmacologically identified as an ATP-sensitive K⁺ (KATP) conductance. The effect was rapid in onset, readily reversible, voltage independent, and related to intracellular ATP concentration. In the presence of 1 mM intracellular ATP, half-maximal activation of KATP channels was obtained with extracellular application of 1.7 μM Zn²⁺.Single channel analysis revealed that extracellular Zn²⁺ increased the KATP channel open-state probability with no change in the single channel conductance.Our data support the hypothesis that Zn²⁺ binding to KATP protein subunits results in an activation of the channels, therefore regulating the resting membrane potential and decreasing the excitability of RINm5F cells. Taken together, our results suggest that Zn²⁺ can influence insulin secretion in pancreatic β-cells through a negative feedback loop, involving both KATP and voltage-gated conductances.