On the reduced retention of extravesicular noradrenaline in rabbit atria produced by potassium

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

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)

Vesicular site of action of lithium ion in choline-calcium stimulated adrenergic nerve endings of rat heart

Reports from this laboratory have suggested that the secretion of norepinephrine (NE) in slices of ventricle from the rat heart, incubated in a medium of choline and Ca2+ (Ch+-Ca2+) deprived of Na+, is mediated by the outward transport of NE (blocked by cocaine) from synaptic vesicles fused or attached to the axolemma. Lithium (and reserpine), block the Mg2+-ATP-stimulated uptake of NE by isolated synaptic vesicles (Slotkin, Seidler, Whitmore, Salvaggio, and Lau, 1978). Hence, a hypothesis to be tested was that lithium ions would increase secretion of NE stimulated by Ch+-Ca2+. By contrast, amines mobilized by lithium ions (or reserpine) in control nerves would be deaminated. Experiments showed that lithium-Krebs increased the excretion of [3H]deaminated metabolites of [3H]NE. A much smaller quantity of [3H]NE was released by a process that was independent of Ca2+ and weakly inhibited by cocaine. When combined with Ch+Ca2+, lithium ions, in concentrations that were known to block uptake in isolated vesicles, induced a Ca2+-dependent secretion of [3H]amines. A small quantity of [3H]deaminated metabolites was excreted. Both processes were strongly inhibited by both cocaine and desipramine. Both reserpine (Bogdanski, 1982) and lithium ions blocked the inhibitory effect of exogenous ATP (2 mM) on secretion induced by Ch+-Ca2+. This effect indicated a proximity of vesicles to the axolemma. The effects of lithium ions could be explained by the hypothesis if lithium prevented storage of [3H]NE in vesicles by blocking the Mg2+ + ATP-dependent amine pump in vesicle membranes.

Release of 3H-noradrenaline from the rat vas deferens under various in vitro conditions

The release of 3H-(-)-noradrenaline (NA) from rat vas deferens in vitro was examined under various experimental conditions. It was found that in normal and reserpinized vas deferens the release of NA evoked by (+)-amphetamine (5 X 10(-6) M) or low external Na+ (26 mM) was antagonized by imipramine methiodide and desipramine, inhibitors of the NA uptake, but was not dependent on the presence of Ca2+ in the medium and was not antagonized by the potent local anaesthetic agent bethoxycaine. The release evoked by veratridine in reserpinized tissue was antagonized by the uptake inhibitors but was in normal tissue only partially inhibited in presence of Ca2+ but almost completely in absence of Ca2+. The release by high K+ (117 mM)+low Na+ (26 mM) in normal tissue was dependent on the presence of Ca2+ and was antagonized by the muscarinic agonists carbacholine and metacholine and by high concentrations of desipramine. In the reserpinized vasa the corresponding release was not dependent on Ca2+ and was not antagonized by the muscarinic agents but was inhibited by high concentrations of desipramine.