Cyclic GMP directly regulates a cation conductance in membranes of bovine rods by a cooperative mechanism

Cyclic GMP causes the release of endogenous Ca2+ from rod outer segments, whose plasma membrane has been made permeable, or from isolated discs. Approximately 11,000 Ca2+ ions are released per disc at saturating concentrations of cyclic GMP. The velocity and the amplitude of the release of Ca2+ are dependent on the concentration of cyclic GMP. The maximal rate of the Ca2+ efflux is approximately 7 X 10(4) Ca2+ ions s-1 rod-1. The Ca2+ release by cyclic GMP is independent of light. The activation of the efflux occurred within a narrow range of the cyclic GMP concentration (30-80 microM) and does not obey a simple Michaelis-Menten scheme. Instead, the kinetic analysis of the Ca2+ efflux suggests that a minimum number of 2 molecules of cyclic GMP activates the ion conductance in a cooperative fashion. The release of Ca2+ by cyclic GMP requires a gradient of Ca2+ ions across the disc membrane. If the endogenous Ca2+ gradient is dissipated by means of the ionophore A23187, the release of Ca2+ by cyclic GMP is abolished. Ca2+ is released by analogues of cyclic GMP which are either modified at the 8-carbon position of the imidazole ring or by the deaza-analogue of cyclic GMP. Congeners of cyclic GMP which are modified at the ribose, phosphodiester, or pyrimidine portion of the molecule are ineffective. The hydrolysis of cyclic GMP by the light-regulated phosphodiesterase of rod outer segments is not a necessary condition for the Ca2+ release because 8-bromo-cyclic GMP, a congener resistant to hydrolysis, is a more powerful activator of the release than cyclic GMP itself. Ca2+ release by cyclic GMP is inhibited by organic and inorganic blockers of Ca2+ channels. The l-stereoisomer of cis-diltiazem blocks the release of Ca2+ at micromolar concentrations, whereas the d-form is much less effective. These results suggest that disc membranes contain a cationic conductance which is permeable to Ca2+ ions and which is regulated through the cooperative binding of at least 2 molecules of cyclic GMP to regulatory sites of the transport protein. By this mechanism, subtle changes in the concentration of cyclic GMP could promote large changes in the flux of Ca2+ ions across the disc membrane.     

CCK-immunoreactive terminals form different types of synapses in the rat and monkey hippocampus

Electronmicroscopic immunocytochemical analysis of the types and patterns of synaptic contacts formed by cholecystokinin (CCK)-containing terminals in the CA1 and CA3 region of the rat and monkey hippocampus reveals numerous symmetric synaptic contacts on cell bodies and dendritic shafts of pyramidal cells in both species. In the monkey, however, CCK-immunoreactive terminals also form asymmetric synaptic contacts with dendritic spines, such contacts are absent or very rare in the rat hippocampus. The present finding in primate hippocampus provides evidence that the same neuropeptides can be found in both symmetric and asymmetric contacts and may be added to other evidence challenging the traditional concept that symmetric synapses mediate exclusively inhibitory and asymmetric exclusively excitatory transmission. Furthermore, although our comparative analysis confirms considerable similarities in the distribution of CCK-containing elements in primate and rodent hippocampus it also revealed a potentially important difference in synaptoarchitecture that should be taken into account in extrapolations from one species to the other.     

Insolubility of Cr2O3 in Bioaccessibility Tests Points to Requirement for a New Human Oral Reference Dose for Trivalent Chromium

Bioaccessibility measurements have the potential to improve the accuracy of risk assessments and reduce the potential costs of remediation when they reveal that the solubility of chemicals in a matrix (e.g., soil) differs markedly from that in the critical toxicity study (i.e., the key study from which a toxicological or toxicity reference value is derived). We aimed to apply this approach to a brownfield site contaminated with chromium, and found that the speciation was Cr^III, using a combination of alkaline digestion/diphenylcarbazide complexation and X-ray absorption near edge structure analysis. The bioaccessibility of Cr₂O₃, the compound on which a reference dose for Cr^III is based, was substantially lower (<0.1%) than that of the Cr^III in the soils, which was a maximum of 9%, giving relative bioaccessibility values of 13,000% in soil. This shows that the reference dose is based on essentially an insoluble compound, and thus we suggest that other compounds be considered for toxicity testing and derivation of reference dose. Two possibilities are CrCl₃·6H₂O and KCr(SO₄)₂·12H₂O, which have been used for derivation of ecological toxicity reference values and are soluble at a range of dosing levels in our bioaccessibility tests.