The Mechanism of Zinc Uptake in Isolated Islets of Langerhans

Abstract

The uptake of zinc65 (Zn) was determined in isolated pancreatic islets employing a dual isotope procedure with sucrose as an extracellular marker. Islets slowly accumulated Zn over long periods of time (24 h or longer) reaching three to five times the extracellular concentration by 70 min and a 30-fold differential over extracellular concentration by 24 h. Zn uptake at low concentrations (0.5–7 μM) demonstrated saturation kinetics with the Km and Vmax, 1.5 nM and 11.1 Fmol/islet/min, respectively. At higher Zn concentrations (10–3000 μM), a linear relationship between initial rates of uptake and Zn concentration was observed. Zn at a low concentration (2 μM) was countertransported by 2 mM ZnCI2 pretreatment and inhibited by the presence of cadmium (2–20 μM). Countertransportation and cadmium inhibition of Zn uptake was not found at a higher Zn concentration (44 μM). Zn uptake at either 2 or 44 μM was not inhibited by pretreatment and presence of 300 μM dinitrophenol. Lowering of temperature from 37 °C to 20 °C and 4 °C depressed Zn uptake at 44 μM by 65% and 82%, respectively. However, at 2 μM Zn, the uptake was equivalent at 20 °C and 37 °C and showed a decrease of 70% at 4 °C. Zn uptake at either 44 or 2 μM in short (1–70 min) or long (1–24 h) studies was unaffected by concentrations of glucose that stimulated insulin secretion. The results present the evidence that a facilitated mechanism of Zn transport predominates at low Zn concentrations and a diffusion mechanism of entry predominates at higher Zn concentrations.