Direct interference of HIV protease inhibitors with pancreatic β-cell function

The aim of the present study was to evaluate whether HIV protease inhibitors directly interfere with stimulus-secretion coupling in pancreatic P-cells. Insulin secretion was determined by a radioimmunoassay (RIA), cytosolic free Ca2+ concentration ([Ca2+](c)) with the fluorescence dye fura-2 and whole-cell membrane currents with the patch-clamp technique. Glucose-induced insulin secretion was inhibited in a concentration-dependent manner by ritonavir and nelfinavir but not by indinavir. Ritonavir and nelfinavir lowered [Ca2+](c) in the presence of a stimulatory glucose concentration whereas indinavir again had no effect. Ritonavir and nelfinavir completely inhibited the effect of tolbutamide, which normally increases [Ca2+](c) by blocking K-ATP channels. This observation points to an action of both drugs on K-ATP channels or a step distal to these channels in stimulus-secretion coupling. Ritonavir was used to further evaluate the direct effects of HIV protease inhibitors on beta-cell ion channel currents. Unexpectedly, ritonavir inhibited neither the whole-cell K-ATP current nor the whole-cell L-type Ca2+ current. Tolbutamide almost completely suppressed the K-ATP current in the presence of ritonavir excluding that ritonavir alters the tolbutamide sensitivity of the K-ATP channel. Ritonavir increased the length and decreased the frequency of glucose-induced action potentials. This effect can be attributed to inhibition of voltage-dependent K+ currents. Intracellular stores seem not to be involved in the ritonavir-induced lowering of [Ca2+](c). In conclusion, different HIV protease inhibitors surprisingly reveal distinct effects on insulin secretion. Ritonavir inhibits insulin secretion by lowering [Ca2+](c) but this effect is evidently independent of the opening o K-ATP channels or the closure of voltage-dependent Ca2+ channels, which are commonly considered to play a key role in stimulus-secretion coupling.