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

1.The effects of zinc (Zn2+) on excitability and ionic conductances were an alysed on RINm5F insulinoma cells under whole-cell and outside-out patch-cl amp recording conditions. 2.We found that extracellular application of 10-20 muM Zn2+ induced a rever sible abolition of Ca2+ action potential firing, which was accompanied by a n hyperpolarisation of the resting membrane potential. 3. Higher concentrations of Zn2+, in the tens to hundreds micromolar range, induced a reversible reduction of voltage-gated Ca2+ and, to a lesser exte nt, K+ currents. Low-voltage-activated,ed Ca2+ currents were more sensitive to Zn2+ block than high voltage-activated Ca2+ currents. 4. The Zn2+-induced hyperpolarisation arose from a dose-dependent increase in a voltage-independent K+ conductance that was pharmacologically identifi ed as an ATP-sensitive K+ (K-ATP) conductance. The effect was rapid in onse t, readily reversible, voltage independent, and related to intracellular AT P concentration. In the presence of 1 mM intracellular ATP, half-maximal ac tivation of K-ATP channels was obtained with extracellular application of 1 .7 muM Zn25. Single channel analysis revealed that extracellular Zn2+ increased the K -ATP channel open state probability with no change in the single channel co nductance. 6. Our data support the hypothesis that Zn2+ binding to K-ATP protein subun its results in an activation of the channels, therefore regulating the rest ing membrane potential and decreasing the excitability of RINm5F cells. Tak en together, our results suggest that Zn2+ can influence insulin secretion in pancreatic beta -cells through a negative feedback loop, involving both K-ATP and voltage-gated conductances.