No abstract

The uncatalyzed reactions of phthalaldehydic acid with a number of alcohols, thiols, amines, amides, sulfonamides, ureas, carbamates, and acid anhydrides have been studied. The unusually high reactivity of phthalaldehydic acid is demonstrated and evidence is presented to show that the free acid and most of its derivatives exist in the phthalide form.

Initial velocity of uptake of dopamine (DA) has been measured in rat striatal synaptosomes as a function of both [DA] and [Na]. Carrier mediated uptake is totally dependent on external sodium. The data were fitted to a rapid equilibrium model which has been found in previous studies to fit, with appropriate simplification, uptake data for glutamate, GABA, and choline in several brain regions under varying conditions. This model also gives a good fit to the dopamine data. The minimal best fit simplification of this model allows for DA uptake along with two sodium ions and predicts that apparent maximal velocity of uptake should increase with [Na], while the Michaelis-Menten constant should decrease. The minimal best fit model for DA, and a number of kinetic parameters which quantitate the model, are compared to those for the GABA, glutamate, and choline transporters. The results are consistent with a symmetrical, rapid equilibrium model, which has been presented previously for other neurotransmitters and precursors (18). This model offers a unifying basis for understanding the sodium and membrane potential dependence of neurotransmitter transport and the possible participation of transporters in depolarization induced release throughout the CNS.

Frog sciatic nerves were incubated for 24 hours in either glycine, aspartic acid, glutamic acid, lysine, leucine, y-aminobutyric acid, glutamine, or pentanedioic acid (all labeled with C14), and the rates of release of these compounds were monitored under resting conditions and during stimulation. Upon stimulation, the rate or release of glutamic acid increased an average of 200% above the resting rate. This extra release is highly specific with regard to molecular size and structure, since of the compounds tested only glutamic acid gave significant increases in rates of release during stimulation. Ouabain (0.1 mM) had no effect on the rate of release; however, sodium azide (0.2 mM or 1.0 mM) completely eliminated the extra release during excitation, indicating that the increased permeability to glutamic acid is energy-dependent. Competition experiments show that the extra release of glutamic acid can be eliminated with 10 mM concentrations of non-isotopic choline.The hypothesis is advanced that glutamic acid is actively extruded by a highly specific carrier mechanism.

We have studied the effects of prolonged incubation in low [K+] medium (approximately 0.3 mM) on both K+ and amino acid transport in Chinese hamster ovary (CHO) cells. When incubated in low [K+] medium, CHO cells redressed partially the loss of intracellular K+ after 12 h. After 24 h of incubation, both the activity of Na+-K+-ATPase in crude homogenates, and the transport capacity (Vmax) for ouabain-sensitive (i.e., active) K+ influx approximately doubled. The magnitude of the ouabain-insensitive (i.e., passive) K+ influx decreased by 50%. Thus the regulatory response involves an apparent increase in Na+-K+ pump and a decrease in K+ leak. The transport capacity for the nonmetabolized amino acid, alpha-aminoisobutyric acid (alpha-AIB), also increased after 24 h in low [K+] medium. The Vmax for the Na+-dependent (i.e., active) alpha-AIB influx increased by about 150%, and the magnitude of the Na+-independent influx increased by 20-40%. These changes in alpha-AIB transport result in a twofold greater capacity to accumulate this amino acid. Thus the regulation of K+ and alpha-AIB transport systems appears to be linked and possible mechanisms of this linkage are discussed.