Norepinephrine Inhibits Endocytosis In Insulin-secreting Cells

Abstract

Norepinephrine (NE) is a well-known physiological inhibitor of insulin secretion in pancreatic β cells. We investigated modulation of exocytosis and endocytosis by NE in INS 832/13 cells using whole-cell capacitance measurements. Exocytosis was stimulated by depolarizing pulses from −70mV to +10mV of variable duration and was followed by compensatory endocytosis. Inhibition of Ca2+-evoked exocytosis by NE was overcome by increasing Ca2+ influx, either by increasing the depolarizing pulse duration (up to 500ms) or by increasing the extracellular Ca2+ concentration up to 10mM. When stimulated by a short train of 500ms pulses in the presence of NE (5μM), robust exocytosis was observed but endocytosis was markedly inhibited. The NE inhibition of endocytosis was abolished by the α2-adrenergic receptor antagonist yohimbine (10μM) and was not affected by PTX-treatment (150ng/ml), demonstrating that NE inhibition of endocytosis is mediated via the α2-adrenergic receptor and not via Gi and/or Go proteins. When a synthetic peptide that mimicked the last 13 c-terminal amino acids of the Gzα subunit was dialyzed into the cells via the whole-cell patch pipette, NE inhibition of endocytosis was fully blocked, suggesting that Gz may be mediating the inhibition. Single vesicle recordings by cell attached capacitance measurements indicate that inhibition of endocytosis by NE is due to a decreased number of endocytic events without a significant change in endocytic vesicle size. Further analysis of fission pore kinetics revealed that NE selectively inhibited the rapid fission events. Our findings establish a novel action for NE and suggest the possibility that NE may modulate endocytosis in the central nervous system and elsewhere.