INTERACTION OF TRICYCLIC DRUG ANALOGS WITH SYNAPTIC PLASMA-MEMBRANES - STRUCTURE-MECHANISM RELATIONSHIPS IN INHIBITION OF NEURONAL NA+ K+-ATPASE ACTIVITY/

Perturbations of rat brain synaptic plasma membrane (SPM) bilayer stru cture and Na/K+-ATPase activity were correlated for drugs that are str ucturally related and exhibit similar toxicological side effects but b elong to different pharmacological classes. Na+/K+-ATPase IC50 values decrease linearly with increasing octanol/water partition coefficients (log-log plot) for a series of dimethylethylamine-containing drugs (i .e , chlorpromazine, amitriptyline, imipramine, doxepin, and diphenhyd ramine), emphasizing hydrophobicity in inhibition. However, nortriptyl ine and desipramine are 1.2 log units less hydrophobic than their N-me thylated parent drugs but more potent inhibitors. To investigate this, bilayer surface structure was examined by the binding of the fluoroph ore 1-anilinonaphthalene-8-sulfonic acid (ANS) to SPMs. The dissociati on constant and wavelength maximum of ANS are invariant with drug bind ing; however, the limiting fluorescence intensity of ANS (F(infinity)) is increased. Such data indicate that these cationic drugs bind to th e membrane surface, increasing the number but not the polarity of ANS binding sites by canceling charge at anionic phospholipid groups. More importantly, there is a close linear correlation between the concentr ations of drugs necessary to increase F(infinity) by 40% and the IC50 values, with full compensation for the N-demethylated drugs. This corr elation implies that drug-induced increases in SPM-bound ANS fluoresce nce are a better predictor of Na+/K+-ATPase inhibition than are octano l/water partition coefficients and that electrostatic interactions are also involved in inhibition. Furthermore, it points toward similar me chanisms of biomembrane surface interaction governing both inhibition and fluorescence change that are common to these drugs. K+-dependent p -nitrophenylphosphatase activity is inhibited with the same potency as Na+/K+-ATPase activity, indicating that inhibition may involve drug i nteraction near the K+ binding sites. Furthermore, chlorpromazine, dip henhydramine, and dimethylaminopropyl chloride alter K+-activation of K+-dependent p-nitrophenylphosphatase, progressing from noncompetitive through mixed to competitive inhibition as their hydrophobicity chang es, and these mechanisms are consistent with steric hindrance of K+ bi nding. In contrast to the ANS data, decreases in 1,6-diphenyl-1,3,5-he xatriene fluorescence anisotropy induced by these drugs do not correla te with Na+/K+-ATPase inhibition, and drug N-demethylation enhances in hibition without altering anisotropy; both findings indicate that Na+/ K+-ATPase activity is not predominantly influenced by changes in bulk fluidity. Taken together, these data suggest that electrostatic intera ctions at the biomembrane surface between the protonated amino group o f the drug and anionic groups on the enzyme and/or phospholipids near the K+ binding sites are crucial to inhibition and that drug hydrophob icity modulates the number and orientation of these interactions.