Bibliothek

Notizen: Abstract Batrachotoxinin-A 20-α-benzoate (BTX-B), an analog of the potent depolarizing agent batrachotoxin (BTX), was prepared by selective esterification of naturally occurring batrachotoxinin-A with benzoic acid. BTX-B depolarizes rat phrenic nerve-diaphragm preparations with a time course and concentration dependence virtually indistinguishable from that of BTX. A specific, saturable component of equilibrium binding of [3H]BTX-B to mouse cerebral cortex homogenates was measured, described by an equilibrium dissociation constant of 0.7 µM and a maximum number of binding sites of 90 pmol per gram of tissue (wet weight). Specific binding is inhibited by BTX and other BTX analogs, veratridine, and grayanotoxin but is unaffected by tetrodotoxin and cevine. Under conditions of this assay, neither crude Leiurus quinquestriatus scorpion venom nor purified sea anemone toxin have any effect on specific binding. The data support the conclusion that BTX-B interacts with a recognition site associated with voltage sensitive sodium channels which is identical to the recognition site for BTX.

Notizen: Abstract The binding of labeled batrachotoxinin-A benzoate (BTX-B) to voltage-sensitive sodium channels in broken membrane preparations of mouse cerebral cortex has been measured as a function of the pH. Specific binding is negligible at pH 〈6.0, maximum at pH 8.5, and decreases again at pH 9.0. A major component of nonspecific binding, however, increases linearly in the pH range 7.0-9.0. The pK a of batrachotoxinin-A, an analogue of BTX-B, was found by titrimetric methods to be ≥8.2. Analysis of the data shows that at least part of the pH dependence of BTX-B binding is due to the titration of a sodium channel residue(s) associated in some way with the BTX-B recognition site. The possible involvement of a histidine residue is suggested.

Notizen: Summary 1. Adenosine analogues inhibit calcium-dependent K+-evoked release of [3H]norepinephrine from guinea pig cerebral cortical and hippocampal vesicular preparations. Inhibition requires high concentrations (100µM) of the adenosine analogues and is abolished in the presence of high concentrations (2 mM) of calcium ions. The inhibitory effect of 2-chloroadenosine is blocked by theophylline. The structure activity profile (N 6-d-phenylisopropyladenosine ≥N 6-l-phenylisopropyladenosine ≥ 2-chloroadenosine 〉N 6-cyclohexyladenosine, adenosine 5′-cyclopropylcar-boxamide) is not that expected of either A1 (high-affinity) or A2 (low-affinity) adenosine receptors. 2. Calcium-dependent K+-evoked release of [3H]dopamine from guinea pig striatal vesicular preparations is inhibited by apomorphine. However, only 2-chloroadenoine causes an inhibition of K+-evoked release of [3H]dopamine. Other adenosine analogues such asd- andl-phenylisopropyladenosine and adenosine 5′-cyclopropylcar-boxamide cause a facilitation of K+-evoked release. The facilitation is abolished or reduced in the presence of high concentrations (2 mM) of calcium ions. The sites of action of adenosine analogues do not appear to have structural requirements identical to those expected of A1 (high-affinity) or A2 (low-affinity) adenosine receptors. 3. The results indicate that adenosine analogues can have either inhibitory or facilitory effects on K+-evoked release of catecholamines from central synaptic terminals.

Notizen: Summary 1. The potencies of caffeine and related methylxanthines as adenosine antagonists were assessed with respect to three apparent subtypes of adenosine receptors in rat brain preparations: (i) the A1-adenosine receptor which binds with a very high affinity the ligand [3H]cyclohexyladenosine (K D, 1 nM) in rat brain membranes; (ii) a ubiquitous low-affinity A2-adenosine receptor which activates cyclic AMP accumulation in rat brain slices—this A2-adenosine system exhibits an EC50 for 2-chloroadenosine of about 20µM; and (iii) a relatively high-affinity A2-adenosine receptor which activates adenylate cyclase in rat striatal membranes—this A2-adenosine system exhibits an EC50 for 2-chloroadenosine of about 0.5µM and is present in striatal but not in cerebral cortical membranes. 2. The rank order of potency for methylxanthines versus binding of 1 nM [3H]cyclohexyladenosine in membranes from eight rat brain regions is theophylline (IC50, 20–30µM) 〉 paraxanthine (IC50, 40–65µM) 〉 caffeine (IC50, 90–110µM) 〉 theobromine (IC50, 210–280µM). There thus appears to be little difference in A1-receptors in different brain regions in terms of interaction with these methylxanthines. 1-Methylxanthine is more potent than caffeine in rat cerebral cortical membranes, while 3-methylxanthine and 7-methylxanthine are less potent than caffeine. 3. The rank order of potency for methylxanthines versus activation of cyclic AMP accumulation by 50µM 2-chloroadenosine in rat striatal slices is theophylline (IC50, 60µM) 〉 paraxanthine (IC50, 90µM) 〉 caffeine (IC50, 120µM) » theobromine (IC50, 〉 1000µM). Similar potencies pertain in cerebral cortical slices. 4. The rank order of potency of methylxanthines versus activation of adenylate cyclase by 1µM 2-chloroadenosine in rat striatal membranes is theophylline (IC50, 20µM) 〉 paraxanthine (IC50, 40µM) 〉 caffeine (IC50, 80µM) » theobromine (IC50, 〉 1000µM). 5. Caffeine and other methylxanthines, thus, antagonize effectively both A1- and A2-adenosine receptors in brain perparations. Theobromine appears less effective versus A2-receptors than versus A1-receptors. Caffeine exhibits aK i value of about 50µM at the very high-affinity A1-binding sites, aK i value of about 30µM at the low-affinity A2-adenosine site in brain slices, and aK i value of about 27µM at the high-affinity A2-adenosine site in striatal membranes. The functional significance of antagonism of such adenosine receptors by caffeinein situ will depend both on the local levels of adenosine and on the affinity for adenosine for the receptor, since antagonism by xanthines is competitive in nature. In addition, the functional significance of xanthine action will depend on the degree of inhibition of adenosine input which is required to alter the output signal. For a stimulatory input to adenylate cyclase via an A2-adenosine receptor, profound antagonism by methylxanthines is probably required to alter the cyclic AMP-mediated output signal, while for inhibitory input to adenylate cyclase via an A1-adenosine receptor, presumably a lesser degree of antagonism by methylxanthines may be required to alter the cyclic AMP-mediated output signal.

Notizen: Summary 1. The calcium antagonists D-600 (1–10µM) and diltiazem (10–25µM) inhibit K+-evoked release of [3H]norepinephrine from guinea pig cerebral cortical vesicular preparations. The inhibition of release is partially reversed by increasing concentrations of calcium to 2 mM. Diltiazem at 100µM has no effect on K+-evoked release of [3H]norepinephrine at 0.15 mM calcium but does inhibit release at 2.0 mM calcium. 2. The calcium antagonist nifedipine and dantrolene, an agent purported to antagonize release of calcium from intracellular storage sites, have no effect on K+-evoked release of [3H]norepinephrine. 3. The calcium antagonists D-600 (1µM) and diltiazem (10µM) inhibit K+-evoked release of [3H]dopamine from guinea pig striatal vesicular preparations. Higher concentrations of drug, namely, 10µM for D-600 and 100µM for diltiazem, cause a potentiation rather than an inhibition of K+-evoked release. The potentiation is reduced in magnitude upon raising the extracellular calcium to 2.0 mM. Indeed, 10µM D-600 then inhibits K+-evoked release of [3H]dopamine. 4. The results indicate that putative calcium antagonists can have both inhibitory and facilitory effects on calcium-dependent K+-evoked release of catecholamines from central synaptic endings. Furthermore, certain peripheral calcium antagonists such as nifedipine and dantrolene may prove ineffective in central systems.

Notizen: Summary 1. Histrionicotoxin (HTX) at low concentrations of 5–10µM blocks the postsynaptic potential of the electroplax ofElectrophorus electricus. 2. At 100-fold higher concentrations, HTX blocks the directly evoked action potentials of the conducting membrane. 3. The pH dependence of the blockade by HTX at synaptic channels is different from that at the conducting membrane. At the synapse HTX is more potent at acid pH, while at the conducting membrane it is more potent at basic pH. 4. HTX at high concentrations antagonizes the effects of batrachotoxin, indicative of an effect on the batrachotoxin-sensitive sodium channels involved in action potential generation. 5. While the effects of HTX on the synaptic channels are concentration, time, and pH dependent, the effects on the channels of the conducting membrane are, in addition, use dependent, suggesting interactions of HTX with the activated forms of these channels.