Norepinephrine uptake dependent upon apparent Mg++‐ATPase activity and proton transport in storage vesicles in axoplasm

Abstract

A Ca++-dependent secretion of norepinephrine ([3H]NE) was evoked in adrenergic nerve endings in rat heart ventricle slices incubated in a modified Krebs-HCO3 medium containing choline Cl as the replacement for NaCl (Ch+-Ca++). Exogenous ATP inhibited secretion and lithium ion, a known inhibitor of NE uptake dependent upon Mg++-ATPase activity in vesicles (but not ouabain) prevented the response to ATP (Bogdanski, 1983,1986). It was suggested that vesicles attached to the axolemma recaptured [3H]NE from the extracellular fluid. This report indicates that other known inhibitors of uptake in isolated vesicles also inhibited the response to ATP in the attached vesicles. Included were two inhibitors of Mg++-ATPase activity, N-ethylmaleimide (NEM) and dicyclohexylcarbodiimide (DCCD), and the proton transporters 2,4-dinitrophenol (2,4-DNP 1.0 mM) and chlorpromazine (CPZ). Potassium ionophores (valinomycin with 2,4-DNP 0.1 mM, and nigericin) and a pH neutralizing reagent for vesicles (NH3 from ammonium sulfate in solution) were also effective. The uptake inhibitors, except 2,4-DNP, could also increase the rate of depletion of stored NE and its deamination in nonsecreting nerve endings incubated in Krebs-HCO3 (KRB) medium. Valinomycin by itself stimulated uptake in the presence of ATP. It is suggested that mechanisms of uptake and retention of NE in isolated vesicles (symposium (1982) Fed. Proc. 41:2742-2780) apply to the axoplasmic vesicles as well. Thus, the activity of Mg++-ATPase drives proton transport to establish the electrochemical gradients of H+, which drive the transport of NE. A lowering of the gradients can mobilize amines and evoke secretion.(ABSTRACT TRUNCATED AT 250 WORDS)