Enhancement of 45Ca2+ binding to acidic lipids by barbiturates, diphenylhydantoin, and ethanol

Effect of pentobarbital on the contraction and calcium movements in cat cerebral and peripheral arteries

Pentobarbital (PB) induced concentration-dependent vasodilation in cylindrical segments of cat cerebral and femoral arteries precontracted with 75 mM K+ or 10(-5) M serotonin (5-HT). PB (10(-4) or 10(-3) M, 10 min preincubations) caused concentration-dependent inhibition of the contraction produced by both agents. The exposure of the vessels to a Ca2+-free medium annulled the response elicited by K+ in both kinds of arteries. In this medium, the contraction induced by 5-HT in cerebral arteries was abolished, whereas in femoral ones it was significantly reduced. This residual response was abolished by PB. The intracellular 45Ca2+ uptake (La3+ method) elicited by K+ in both kinds of arteries and by 5-HT in cerebral ones was reduced in a concentration-dependent manner by PB (10(-4)-10(-3) M). The spontaneous 45Ca2+ efflux from these arteries and that induced by La3+ (10 mM) were unaffected by PB (10(-3) M). These results indicate that the vasodepressor effects induced by PB are essentially due to an interference of the barbiturate with the Ca2+ entry into the vascular smooth muscle cell in both kinds of arteries and with the intracellular Ca2+ movements in femoral ones. It does not seem to exist a clear selectivity of PB for brain or peripheral arteries for antagonizing the responses caused by K+ or 5-HT.

Barbiturates block divalent cation action potentials in leech nociceptive cells

Phenobarbital (PNB), pentobarbital (PTB) and methohexital (MTX) decreased the maximum rate of depolarization Vmax and duration of divalent cation action potentials elicited in leech nociceptive neurons in Na+-free solutions containing the K+-channel blocker TEA, without significantly affecting resting membrane potential or conductance. The block of the divalent cation action potentials was reversible and dose-dependent, ED50 for inhibition of Vmax being 560 microM for MTX, 800 microM for PTB and 3000 microM for PNB. This order of potency correlated well with the ratio of unchanged/charged form of the drugs at physiological pH suggesting that in leech, as in other preparations, the non-ionized form was the active one. In Na+-containing Ringer, the 3 barbiturates depolarized and decreased membrane resistance in the lateral nociceptive cells, but not the medial nociceptive cells. These results provide additional information regarding the newly described pharmacological differences among closely related neurons. These membrane actions may be related to some of the excitatory properties described for other barbiturates in invertebrate and mammalian preparations.

Involvement of a GABAergic mechanism in the pharmacologic action of phenytoin

In vivo interactions between phenytoin (PHT) and baclofen (a GABAb receptor agonist) or PHT and progabide (a GABAa receptor agonist) were investigated by the rotorod minimal neurotoxicity test and maximal electroshock seizure (MES) test. The combination of PHT and baclofen produced an additive effect by the rotorod test, whereas the combination of PHT and progabide elicited a supra-additive (synergistic) effect. The median minimal neurotoxic dose of baclofen augmented the anti-MES activity of PHT. The combination of PHT and progabide induced a supra-additive effect by the MES test. These results imply that GABAa receptors are involved in both the minimal neurotoxicity and anti-MES activity of PHT.

Interaction of Ca2+ with cardiolipin-containing liposomes and its inhibition by adriamycin

The interaction of cardiolipin-containing, unilamellar liposomes with Ca2+ was assessed by flow dialysis in the presence of 2-100 microM 45Ca2+, using vesicles formed from phosphatidylcholine (PC) and from PC and cardiolipin in mole ratios from 16:1 to 1:1. Control (PC only) vesicles bound no detectable Ca2+. In contrast, Ca2+ binding to cardiolipin-containing vesicles was substantial and dependent on vesicle concentration. Scatchard plots for the binding were concave upward. Resolution of the data, assuming the presence of two independent classes of binding sites, indicated a high-affinity site with apparent KD = 5.57 +/- 0.48 microM (S.D.) and a second site with KD in the millimolar range. Interaction of cardiolipin-containing liposomes with Ca2+ was insensitive to monovalent cations (Na+, K+, Rb+), but was inhibited by ruthenium red much greater than La3+ greater than Mn2+ greater than Mg2+. Progressive increases in the PC: cardiolipin ratio markedly increased the apparent KD for Ca2+ at the high-affinity site. Stoichiometry of Ca2+ binding at the site passed through a maximum at a PC: cardiolipin ratio of 4:1. The potent antineoplastic agent adriamycin also inhibited the interaction of Ca2+ with cardiolipin-containing liposomes in a dose-dependent manner; effects were detected at 10 microM antibiotic. Unlike PC, adriamycin altered the stoichiometry of the high-affinity interaction but not the apparent KD. Adriamycin effects increased with pH in the range of the pKA of its amino group. These results suggest that inhibition by adriamycin may result from a mechanism other than simple competition for the charged head group of cardiolipin.

Effect of phenytoin on intracellular calcium and intracellular protein changes during pentylenetetrazole-induced bursting activity in snail neurons

Effects of phenytoin (PHT) on the intracellular calcium and intracellular protein changes during pentylenetetrazole (PTZ)-induced bursting activity in the neurons of the Japanese land snail Euhadra peliomphala were examined. In the examination with a computer controlled electron probe X-ray microanalyzer, PHT clearly inhibited the intracellular calcium shift induced by PTZ as well as the calcium binding state change near the cell membrane. PHT also clearly inhibited the intracellular protein changes induced by PTZ. PHT, however, did not show any change in the transmembrane ionic currents such as the sodium current, calcium current and potassium current. These findings suggest that one of the main sites of anticonvulsant action of PHT is pathologically changed intracellular calcium movement and intracellular protein changes during seizure discharge.