Crystal structures of the 1,4-dihydropyridine (1,4-DHP) calcium channel activators Bay K 8643 [methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(3-nitrophenyl)-pyridine-5-carboxylate], Bay O 8495 [methyl l,4-dihydro-2,6-dimethyl-3-nitro-4-(3-trifluoromethylphenyl)-pyridine-5-carboxylate], and Bay O 9507 [methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(4-nitrophenyl)-pyridine-5-carboxytate] were determined. The conformations of the 1,4-DHP rings of these activator analogues of Bay K 8644 [methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyridine-5-carboxylate] do not suggest that their activator properties are as strongly correlated with the degree of 1,4-DHP ring flattening as was indicated for members of the corresponding antagonist series. The solid state hydrogen bonding of the N(1)-H groups of the activators is not, unlike that of their antagonist counterparts, to acceptors that are directly in line with the donor. Rather, acceptor groups are positioned within _+ 60 degrees of the N(1)-H bond in the vertical plane of the 1,4-DHP ring. Previously determined structure-activity relationships have indicated the importance of this N(1)-H group to the activity of the 1,4-DHP antagonists. Based on these observations, a model is advanced to describe the 1,4-DHP binding site of the voltage-gated Ca 2÷ channel and its ability to accommodate both antagonist and activator ligands.

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To examine the status of ATP-sensitive K+ (K+ATP) channels and 1,4-dihydropyridine-sensitive Ca2+ (Ca2+DHP) channels during experimental cardiac failure, we have measured the radioligand binding properties of [3H]glyburide and [3H]PN 200 110, respectively, in tissue homogenates from the rat cardiac left ventricle, right ventricle, and brain 4 wk after myocardial infarction induced by left coronary artery ligation. The maximal values (Bmax) for [3H]glyburide and [3H]PN 200 110 binding were reduced by 39 and 40%, respectively, in the left ventricle, and these reductions showed a good correlation with the right ventricle-to-body weight ratio in heart-failure rats. The ligand binding affinities were not altered. In the hypertrophied right ventricle, Bmax values for both the ligands were not significantly different when data were normalized to DNA content or right ventricle weights but showed an apparent reduction when normalized to unit protein or tissue weight. Moderate reductions in channel densities were observed also in whole brain homogenates from heart failure rats. Assessment of muscarinic receptors, beta-adrenoceptors and alpha 1-adrenoceptors by [3H]quinuclidinyl benzilate, [3H]dihydroalprenolol, and [3H]prazosin showed reductions in left ventricular muscarinic and beta-adrenoceptor densities but not in alpha 1-adrenoceptor densities, consistent with earlier observations. It is suggested that these changes may in part contribute to the pathology of cardiac failure.

Radioligand binding affinities of four new muscarinic antagonists and six potential muscarinic agonists which possess the 2-alkyl-2-azabicyclo[2.2.1]heptane ring system have been determined in rat heart, rat brain, and m1- or m3-transfected CHO cell membrane preparations to examine the selectivity for subtypes of muscarinic receptor. The efficacies of the potential muscarinic agonists were determined by the ratio of binding affinities against [3H]QNB and [3H]Oxo-M. Four muscarinic antagonists which have the 2,2-diphenylpropionate side chain at either the C5 (5-endo or 5-exo) or the C6 (6-endo or 6-exo) positions did not discriminate between the subtypes of muscarinic receptors. The 2,2-diphenylpropionate 5-endo substituted compound was the most potent, showing affinities between 4.23 x 10(-10) and 1.18 x 10(-9) M in rat heart, rat brain, and m1- or m3-transfected CHO cell membrane preparations. The rank order of ester potency was 5-endo greater than 5-exo greater than 6-endo greater than 6-exo. A molecular modeling study based on the pharmacophore developed for azaprophen was used to account for the relative potency of these antagonists. Six potential muscarinic agonists which have acetoxy groups in the C5 or C6 position with an N-methyl or N-benzyl substituent did not discriminate subtypes of muscarinic receptors and had affinities between 6.63 x 10(-6) and 4.76 x 10(-5) M in rat heart, rat brain, and m1- or m3-transfected CHO cell membrane preparations. exo-2-Methyl-5-acetoxy-2-azabicyclo[2.2.1]heptane was the most efficacious partial agonist.

The structure of azaprophen, which was originally assigned by 1H NMR analysis, was confirmed by X-ray crystallography. A comparison of 13C NMR isotropic chemical shift data for azaprophen in the solid state and in CDCl3 and DMSO-d6 solution was used to correlate solution and solid-state conformation as determined by the X-ray data. The data suggested that the solid-state and solution conformation of azaprophen were similar. The observed solid-state structure was also compared to low-energy conformations identified by molecular-mechanics calculations. A comparison of azaprophen and atropine radioligand binding in guinea pig ileum, rat heart, rat brain, and in CHO cells expressing transfected m1 and m3 receptors was conducted. Azaprophen is more active than atropine in all preparations except the m3 receptor expressed in CHO cells. However, like atropine, it does not provide major discrimination among the muscarinic receptor subtypes.