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  • 1
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract— An adenylate cyclase present in the caudate nucleus of the rat brain, which is selectively stimulated by low concentrations of dopamine and which is believed to mediate dopaminergic synaptic transmission has been characterized with respect to interactions with guanyl nucleotides and magnesium ions. The addition of GTP (10−5 M) or GMPPNP (10−5 M) did not alter the linear increase in cyclic AMP as a function of time in the presence and absence of dopamine. GTP did, however, inhibit basal adenylate cyclase activity in homogenates of the caudate but the dopamine-stimulated enzyme activity was not affected, thus increasing the percent stimulation produced by dopamine. This ability of GTP to increase responsiveness to dopamine was not dependent on the level of added Mg2+ in the assay medium. Conversely, GMPPNP (10−5 M) markedly lowered the basal as well as the dopamine stimulated activity as a function of magnesium in the caudate. These results suggest an important role for GTP and Mg2+ in the regulation of caudate adenylate cyclase and its activation by dopamine. In homogenates of the cortex the most dramatic effect of GTP and GMPPNP was on the potentiation of the norepinephrine effect. The guanyl nucleotides studied in the cortex did not have the inhibitory effect on basal enzyme activity as observed in homogenates of the caudate.Interactions between Mg2+ and MgATP2- and the dopamine-sensitive adenylate cyclase from the rat caudate nucleus have also been studied. The data are consistent with a bireactant sequential mechanism where Mg2+ is a required activator and free ATP4 is not a significant inhibitor of the reaction. The data show that dopamine lowered the apparent Km for free Mg2+ about 2-fold without significantly changing the Vmax. GTP and GMPPNP did not alter the apparent Km for Mg2+ but did lower the Vmax both in the presence and absence of dopamine. GMPPNP was more potent than GTP in producing these effects in the striatal enzyme. The data presented herein implicate an important role for guanyl nucleotides and Mg2+ in the regulation of brain adenylate cyclase and its activation by various neurotransmitters. Furthermore, the results also suggest that although the role for guanyl nucleotides may be equally important in various areas of the brain, the mechanisms of action of guanyl nucleotides in these areas may differ.
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