Molecular pharmacology
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Molecular pharmacology · Oct 1993
Comparative StudySpongistatin 1, a highly cytotoxic, sponge-derived, marine natural product that inhibits mitosis, microtubule assembly, and the binding of vinblastine to tubulin.
A highly cytotoxic macrocyclic lactone polyether has been isolated from a Spongia species and named spongistatin 1. With L1210 murine leukemia cells an IC50 value for cell proliferation of 20 pM was obtained, and an increase in the mitotic index concordant with the decrease in cell number was observed. Kangaroo rat kidney PtK1 cells were examined by indirect immunofluorescence with a spongistatin 1 concentration that caused 50% reduction in cellular protein (0.3 nM) and with a 10-fold higher concentration. ⋯ Spongistatin 1 had no effect on the binding of colchicine to tubulin, but it was a potent inhibitor of the binding of vinblastine and GTP to tubulin. In initial experiments with 5 microM tubulin and 5 microM vinblastine, spongistatin 1 and dolastatin 10 both had IC50 values of 2 microM, whereas halichondrin B had an IC50 value of 5 microM. Spongistatin 1 thus represents a new member of the group of complex natural products that inhibit mitosis by binding in the Vinca alkaloid domain of tubulin.
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Molecular pharmacology · Apr 1992
gamma-Aminobutyric acid A or C receptor? gamma-Aminobutyric acid rho 1 receptor RNA induces bicuculline-, barbiturate-, and benzodiazepine-insensitive gamma-aminobutyric acid responses in Xenopus oocytes.
Xenopus oocyte expression of the recently cloned gamma-aminobutyric acid (GABA) rho 1 receptor subunit cDNA yields a pharmacologic profile characteristic of the GABAc responses described by Johnston [Benzodiazepine/GABA Receptors and Chloride Channels. Receptor Biochemistry and Methodology (R. W. ⋯ Sci. USA 88:4318-4322 (1991)]. A rationale for defining GABA rho 1 as forming a GABAc receptor is discussed.
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Molecular pharmacology · Feb 1992
Quinoxaline derivatives: structure-activity relationships and physiological implications of inhibition of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor-mediated currents and synaptic potentials.
The inhibitory potencies at excitatory amino acid (EAA) receptors of 11 quinoxaline derivatives were evaluated in two-electrode voltage-clamp recordings of Xenopus oocytes injected with rat cortex mRNA. Currents activated by kainate or (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) in Xenopus oocytes were inhibited competitively by all the quinoxaline derivatives, with apparent Ki values ranging from 0.27 to 300 microM against kainate and from 0.25 to 137 microM against AMPA. An excellent correlation was observed between inhibitory potencies of the quinoxaline derivatives against kainate and AMPA currents, in support of the contention that in this preparation these two agonists act at a single site. ⋯ Similarly, in slices superfused with nominally Mg(2+)-free medium, in which the EPSFP is amplified due to a relief of the Mg2+ block of NMDA receptors, IC50 values correlated closely with the Ki values against glycine/NMDA obtained in oocyte experiments but were 60-fold higher. This comparison of results from the two experimental systems lends further support to the argument that hippocampal synaptic transmission is mediated postsynaptically by kainate/AMPA-type and NMDA/glycine-type EAA receptors that are pharmacologically indistinguishable from those expressed in mRNA-injected Xenopus oocytes. Furthermore, it suggests that EAA receptors in situ may be nearly saturated by high local concentrations of the endogenous ligands, a condition that would contribute substantially to the apparent non-NMDA receptor selectivity of certain quinoxaline derivatives.
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Molecular pharmacology · Dec 1991
Muscarinic receptors in canine colonic circular smooth muscle. I. Coexistence of M2 and M3 subtypes.
The parasympathetic neurotransmitter acetylcholine, acting postsynaptically at the smooth muscle muscarinic receptor, is a principle determinant of colonic motility. In order to elucidate the receptor signal-transduction events responsible for muscarinic receptor-induced contraction of colonic circular smooth muscle, we present here and in the accompanying work studies designed to characterize the muscarinic receptors present in colon and to determine their biochemical coupling. Muscarinic receptor subtypes in canine colonic circular smooth muscle were characterized using radioligand binding techniques. ⋯ Agonist competition studies with a known ratio of M2 and M3 receptors, obtained by mixing pure M2 and M3 populations, predict the result obtained in colon. cDNA probes specific for each of the muscarinic receptors m1 through m4 were hybridized to colon RNA in a Northern blot analysis. Only m2 and m3 probes hybridized to colon RNA, suggesting the presence of both M2 and M3 receptors. Our data demonstrate that the colon circular smooth muscle contains muscarinic receptors of both the M2 and M3 subtypes, which may be coupled to disparate signal transduction pathways important in the physiological actions of acetylcholine in this tissue.
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Molecular pharmacology · Nov 1991
Frequency and voltage-dependent inhibition of type IIA Na+ channels, expressed in a mammalian cell line, by local anesthetic, antiarrhythmic, and anticonvulsant drugs.
This study examined the actions of phenytoin, carbamazepine, lidocaine, and verapamil on rat brain type IIA Na+ channels functionally expressed in mammalian cells, using the whole-cell voltage-clamp recording technique. The drugs blocked Na+ currents in both a tonic and use-dependent manner. Tonic block was more pronounced at depolarized holding potentials and reduced at hyperpolarized membrane potentials, reflecting an overall negative shift in the relationship between membrane potential and steady state inactivation. ⋯ The results indicate that type IIA Na+ channels expressed in a mammalian cell line retain the complex pharmacological properties characteristic of native Na+ channels. These channels are likely to be an important site of the anticonvulsant action of phenytoin and carbamazepine. Lidocaine and verapamil, drugs with well characterized effects on peripheral Na+ and Ca2+ channels, are also effective blockers of these brain Na+ channels.