Effect of Neuraminidase on Diltiazem-Mediated Alteration of Nitrendipine Binding in the Hog Coronary Artery

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Abstract
The dissociation constant (Kd) and the maximum number of binding sites (Bmax) of 3H-nitrendipine (3H-NTD) were not altered by neuraminidase (NUase) treatment. The Kd and Bmax values were approximately 0.2 nM and 70 fmoles/mg protein, respectively. The influences of diltiazem on 3H-NTD bindings differed between NUase treated and untreated preparations. Diltiazem increased 3H-NTD (100 pM) binding dose-dependently with a 40% increase at about 10-6 M in the untreated preparation. After treatment with NUase, a low dose of diltiazem decreased 3H-NTD (100 pM) binding, and the greatest effect was observed at 10-9 M with a 50% decrease, while at a high dose of diltiazem (10-6 M), 80% increase of the binding was observed. Scatchard plot analysis indicated that a high dose of diltiazem (10-6 M) increased only the affinity of 3H-NTD to the binding site in both the NUase-treated and untreated preparations. In the untreated preparation, a low dose of diltiazem (10-9 M) had no effect on 3H-NTD binding, but in the NUase-treated preparation, diltiazem (10-9 M) showed opposing effects, namely, an increase and decrease of the binding at low and high concentrations of 3H-NTD, respectively. No straight line was obtained when a hill plot analysis of the effect of diltiazem on 3H-NTD binding was made. These results suggested that diltiazem allosterically regulates dihydropyridine derivative binding, and in this regulation, the concentration ratios between diltiazem and the dihydropyridine derivative is the most important factor. Furthermore, it is suggested that the sialic acid of the proteoglycan is involved in these interactions, especially in the effect of diltiazem on dihydropyridine binding.

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