Effects of emopamil on postischemic blood flow and neuronal damage in rat brain

Differential inhibition of neuronal calcium entry and [3H]-D-aspartate release by the quaternary derivatives of verapamil and emopamil

1. Verapamil and emopamil are structurally related phenylalkylamine calcium channel/5-HT2 receptor antagonists that differ in their anti-ischaemic properties in experimental studies. The quaternary ammonium derivatives of these compounds were prepared and tested in assays of neuronal voltage-sensitive calcium channel (VSCC) function to determine whether the compounds act at intra- or extracellular sites. 2. The compounds were tested in K(+)-evoked: (1) rat brain synaptosomal 45Ca2+ influx, (2) release of [3H]-D-aspartate from rat hippocampal brain slices and (3) increase of intracellular calcium in rat cortical neurones in primary culture. 3. Verapamil, emopamil and the emopamil quaternary derivative caused concentration-dependent and comparable (IC50 values approximately 30 microM) inhibition of synaptosomal 45Ca2+ influx and [3H]-D-aspartate release. The verapamil quaternary derivative was considerably less active in these assays (IC50 > 300 microM). 4. The evoked increase of intracellular calcium in cortical neurones was inhibited with the following rank order of potency (IC50 value, microM): emopamil (3.6) > verapamil (17) > emopamil quaternary derivative (38) > verapamil quaternary derivative (200). 5. The results suggest that verapamil and emopamil inhibit nerve terminal VSCC function (synaptosomal 45Ca2+ influx and [3H]-D-aspartate release) by acting at distinct intracellular and extracellular sites, respectively. Verapamil and emopamil may inhibit cell body VSCC function (evoked increase of intracellular calcium in neocortical neurones) by acting at both intracellular and extracellular sites. 6. The different 'sidedness' of action of emopamil and verapamil on nerve terminal VSCC function and/or the preferential inhibition of cell body VSCC function by emopamil may at least partially explain the relatively greater neuroprotective efficacy of emopamil in experimental models of ischaemia.

Mechanisms of drug actions against neuronal damage caused by ischemia--an overview

1. Oxygen and energy deficits induces a cascade of pathological processes which lead to neuronal dysfunction and cell death. 2. The pathogenesis of ischemia-induced neuronal damage includes a disturbed calcium homeostasis, an excessive release of EAA and an enhanced formation of free oxygen radicals. 3. Calcium antagonists inhibit Ca2+ influx into the neuronal cell via VSCC. 4. Glutamate antagonists reduce intracellular Ca2+ concentration by inactivation of NMDA receptor-associated calcium channels (NMDA antagonists) or AMPA/quisqualate receptor-linked sodium channels (non-NMDA antagonists). 5. Furthermore, oxygen radical scavengers can avoid neuronal damage. 6. Agonists of the adenosinergic and serotonergic transmitter systems contribute to neuroprotection by hyperpolarization of the neuronal membrane due to an increase of K+ permeability.

Novel sites for phenylalkylamines: characterisation of a sodium-sensitive drug receptor with (-)-[3H]emopamil

(-)-Emopamil ((S)-emopamil, (2S)-2-isopropyl-5-(methylphenethylamino)- 2-phenylvaleronitrile hydrochloride) is a Ca(2+)-antagonistic phenylalkylamine which also blocks serotonin (5-HT2) receptors and has antiischemic properties. The (-)-[3H]emopamil tissue distribution profile of specific binding is in striking contrast to that observed for (+)-[3H]PN 200-110 or (-)-[3H]desmethoxyverapamil: (-)-[3H]emopamil labels membrane fractions from guinea-pig liver much greater than adrenal gland greater than kidney approximately lung approximately ductus deferens approximately brain approximately skeletal muscle. Binding to liver membrane was saturable (KD = 12.8 nM, Bmax = 35 pmol/mg of protein), stereoselective, reversible (K-1 = 0.22 min-1 at 25 degrees C) and inhibited by tetraethylammonium (IC50: 1.8 mM) greater than Li+ (IC50: 12.5 mM) approximately Na+ (IC50: 13.6 mM) and [NH4+] (IC50: 79.3 mM) but not by Rb+, Cs+ or K+. The high-affinity liver membrane binding sites have a pharmacological profile that is distinct from the phenylalkylamine receptor domain of the voltage-dependent L-type Ca2+ channel. Similar sites exist in brain and other tissues, albeit with a lower density. Amiodarone, butoprozine and amiloride derivatives bind with high affinity whereas 1,4-dihydropyridines do not interact at all. It is suggested that the novel phenylalkylamine site is linked to a sodium-dependent carrier or transport system.