Stereospecific regulation of [ 3H]inositol monophosphate accumulation by calcium channel drugs from all three main chemical classes

Abstract

Depolarization of [ 3H]inositol-prelabelled rat cortical slices through the elevation of extracellular K + levels leads to increased accumulation of [ 3H]inositol phosphates. In the presence of 18 mM K +, Ca 2+ channel activators selectively stimulated the formation of [ 3H]inositol monophosphate ([ 3H]IP1) whereas Ca 2+ channel blockers were inhibitory. Blockade of the Na + channel by 1 μM tetrodotoxin had no effect but chelation of extracellular Ca 2+ abolished the response. The enantiomers of the benzoxadiazol 1,4-dihydropyridine 202-791 showed opposite stereospecific regulation of [ 3H]IP1 formation: (+)-(S)-202-791 stimulated (252%; ED 50: 88 nM), whereas (-)-(R)-202-791 inhibited (65% inhibition, ED 50: 602 nM). The (-) enantiomer of Bay K 8644 was a potent [ 3H]IP1 stimulator (258%; ED 50: 82 nM). While (+)-Bay K 8644 was inactive in the presence of 18 mM K +, it completely inhibited the (-)-Bay K 8644-induced stimulation with a K i of 103 nM. Representatives of the other two main classes of Ca 2+ channel blockers (phenylalkylamines and benzothiazepines) inhibited K + depolarization-induced and (-)-Bay K 8644 enhanced [ 3H]IP1 formation in a dose-dependent, stereospecific manner. The results show that Ca 2+ channel blockers are efficient modulators of depolarization-induced and Ca 2+ channel activator-induced [ 3H]inositol monophosphate formation in brain, and demonstrate the functional coupling of three distinct drug receptor sites on neuronal Ca 2+ channels.