monensin and 2-aminoisobutyric-acid

The action of the polyether antibiotic monensin on the release of gamma-[3H]amino-n-butyric acid [( 3H]GABA) from mouse brain synaptosomes is characterized. Monensin enhances the release of this amino acid transmitter in a dose-dependent manner and does not modify the efflux of the nontransmitter amino acid alpha-[3H]aminoisobutyrate. The absence of external Ca2+ fails to prevent the stimulatory effect of monensin on [3H]GABA release. Furthermore, monensin is less effective in stimulating [3H]GABA release in the presence of Ca2+. The releasing response to monensin is absolutely dependent on external Na+. The blockade of voltage-sensitive Na+ or Ca2+ channels does not modify monensin-induced release of the transmitter. Also, the blockade of the GABA uptake pathway fails to prevent the stimulatory effect of monensin on [3H]GABA release. Although monensin markedly increases Na+ permeability in synaptosomes, these data indicate that the Ca2+-independent monensin-stimulated transmitter release is not mediated by the Na+-dependent uptake pathway. It is concluded that the entrance of Na+ through monensin molecules inserted in the presynaptic membrane might be sufficient to initiate the intraterminal molecular events underlying transmitter release.

Topics: Aminobutyrates; Aminoisobutyric Acids; Animals; Calcium; gamma-Aminobutyric Acid; In Vitro Techniques; Mice; Mice, Inbred Strains; Monensin; Nerve Endings; Osmolar Concentration; Sodium; Synaptosomes; Tetrodotoxin; Verapamil; Veratrine

Basolateral plasma membranes were prepared from rat parotid gland after centrifugation in a self-orienting Percoll gradient. K+-dependent phosphatase [Na+ + K+)-ATPase), a marker enzyme for basolateral membranes, was enriched 10-fold from tissue homogenates. Using this preparation, the transport of alpha-aminoisobutyrate was studied. The uptake of alpha-aminoisobutyrate was Na+-dependent, osmotically sensitive, and temperature-dependent. In the presence of a Na+ gradient between the extra- and intravesicular solutions, vesicles showed an 'overshoot' accumulation of alpha-aminoisobutyrate. Sodium-dependent alpha-aminoisobutyrate uptake was saturable, exhibiting an apparent Km of 1.28 +/- 0.35 mM and Vmax of 780 +/- 170 pmol/min per mg protein. alpha-Aminoisobutyrate transport was inhibited considerably by monensin, but incubating with ouabain was without effect. These results suggest that basolateral membrane vesicles, which possess an active amino acid transport system (system A), can be prepared from the rat parotid gland.

Topics: Aminoisobutyric Acids; Animals; Basement Membrane; Cell Membrane; Centrifugation, Density Gradient; Kinetics; Male; Monensin; Osmolar Concentration; Ouabain; Parotid Gland; Rats; Rats, Inbred Strains; Sodium