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  • 1
    ISSN: 1432-1912
    Keywords: Px purinoceptors ; Calcium ; Magnesium ; Zinc ; Rat vas deferens ; α,β-methyleneATP binding
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract In this study we have examined the effect of metal cations (as their chloride salts) on the binding of [3H]α,β-methylene ATP ([3H]αβmeATP) to rat vas deferens membranes using a vacuum filtration receptor binding assay. Whereas NaCl and KCl (0.01 and 30 mM) did not affect total binding of 1 nM [3H]αβmeATP, several divalent and trivalent cation salts markedly increased binding. The trivalent cation salts, FeCl3 and AlCl3 (0.1 to 100 μM), produced the greatest increases in total binding of [3H]αβmeATP, however, their effects were most probably due to precipitation of the radioligand. In contrast, several divalent cations, at concentrations between 1 μM and 1–10 mM, increased total binding of [3H]αβmeATP to rat vas deferens by between 87% and 215% while having no effect on either filter binding or non specific binding. The following pEC50 values for potentiating binding of the radioligand were obtained: ZnCl2 (5.44), MnCl2 (4.52), CaCl2 (4.17), CoCl2 (4.06), MgCl2 (3.67) and BaCl2 (3.10). Both EDTA and EGTA (0.01–1 mM) inhibited the binding of the radioligand. The effects of ZnCl2, CaCl2 and MgCl2 were examined in saturation studies. In the absence of added divalent cations, [3H]αβmeATP labelled both high (pKd = 9.15) and low (pKd = 7.06) affinity binding sites. The affinity of the radioligand for its high affinity sites was increased by 3 mM CaCl2 (pKd = 9.56) and by 30 μM ZnCl2 (pKd = 9.46) but not by 3 mM MgCl2. The Bmax of the low affinity site for [3H]αβmeATP was increased (approximately 4 fold) by both 3 mM MgCl2 and 30 μM ZnCl2 but not by 3 mM CaCl2. The selective effect of CaCl2 on the high affinity binding sites enabled these sites to be labelled in the presence of 3 mM CaCl2 using a low concentration of [3H]αβmeATP (1 nM); the sites exhibited the binding characteristics expected of the P2x purinoceptor. The selective effect of MgCl2 on the low affinity binding sites enabled these sites to be labelled in the presence of 3 mM MgCl2 and using a high concentration of [3H]αβmeATP (100 nM). A comparison of the binding characteristics of the high and low affinity sites for [3H]αβmeATP revealed several other differences, in addition to their cation selectivity. First, the adenine analogues ADP, αβmeATP and adenosine tetraphosphate possessed between 13 and 62 fold higher affinity for the high affinity [3H]αβmeATP binding sites than for the low affinity binding sites. Secondly, GTP-γ-S and pyrophosphate were selective ligands for the low affinity [3H]αβmeATP binding sites possessing approximately 43 and 1995 fold, respectively, higher pIC50 values at the low affinity sites than at the high affinity sites. Finally, treatment of the membranes with 0.01–1 mM N-ethyl maleimide increased low affinity binding of the radioligand while not affecting binding to the high affinity sites. The binding characteristics of the low affinity sites suggest that they do not equate with functional P2x purinoceptors; their identity remains to be determined. There was evidence for heterogeneity of both the high and low affinity sites for [3H]αβmeATP since competition curves to several nucleotide and polyphosphate compounds displayed Hill slopes less than unity. In conclusion the present study has demonstrated that cations have a marked effect on the binding of [3H]αβmeATP in rat vas deferens. Of particular interest was the ability of CaCl2 to increase the affinity of the radioligand for its high affinity sites enabling these sites to be selectively labelled, while the ability of MgCl2 to increase the Bmax of the low affinity binding sites enabled these sites to be selectively labelled.
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