Received for publication June 6, 2005.
Revised September 9, 2005.
Accepted for publication September 12, 2005.

HIV protease inhibitors: Suppression of insulin secretion by inhibition of voltage-dependent K⁺ currents and anion currents

Abstract

We have shown before that the HIV protease inhibitors ritonavir and nelfinavir, but not indinavir suppress insulin secretion from mouse pancreatic B-cells via reduction of the cytosolic free calcium concentration ([Ca²⁺]_c). This was not owing to an effect on ATP-dependent K⁺ channels (K_ATP channels) or L-type Ca²⁺ channels. The study was intended to elucidate the mechanisms by which distinct HIV protease inhibitors decrease [Ca²⁺]_c and thus evoke their adverse side effect on insulin release. Membrane potential and whole-cell currents were measured with the patch-clamp technique and [Ca²⁺]_c was determined with a fluorescence dye. Ritonavir and nelfinavir both inhibited the same component(s) of voltage-dependent K⁺ currents with a concomitant change in action potential waveform, while indinavir was ineffective. Comparison with other blockers of voltage-dependent K⁺ currents revealed that suppression of distinct non-inactivating current component(s) altered action potential waveform and decreased [Ca²⁺]_c similar to ritonavir and nelfinavir whereas blockage of inactivating component(s) was without effect. Complete inhibition of voltage-dependent K⁺ currents by 80 mM TEA⁺ drastically increased [Ca²⁺]_c demonstrating that voltage-dependent K⁺ channels are not the sole target of ritonavir and nelfinavir. Accordingly, the Ca²⁺-lowering effect of ritonavir was preserved in the presence of 80 mM TEA⁺. This effect was mimicked by the anion channel blocker DIDS. Consequentially, ritonavir and nelfinavir inhibited a DIDS-sensitive anion current in B-cells. We suggest that ritonavir and nelfinavir decrease insulin secretion by inhibition of voltage-dependent K⁺ channels and anion channels which are essential to provide counterion currents for Ca²⁺ influx across the plasma membrane.

Key words: cytosolic calcium concentration, insulin secretion, nelfinavir, ritonavir, voltage dependent potassium current, volume sensitive anion current