Very high affinity interaction of DPI 201-106 and BDF 8784 enantiomers with the phenylalkylamine-sensitive Ca2+-channel in Drosophila head membranes

Abstract

1. Piperazinylindoles (DPI 201-106, BDF 8784), drugs known to act on voltage-dependent Na+-channels, bind with very high affinity to a Ca2+-channel-associated phenylalkylamine receptor in Drosophila melanogaster head membranes. These compounds and (+)-tetrandrine, a naturally occurring Ca2+-antagonist, were the most selective inhibitors for phenylalkylamine-labelled Drosophila Ca2+-channels compared to mammalian L-type Ca2+-channels. 2. Replacement of the cyano group by a methyl group in (+)-DPI 201-106 ((+)-BDF 8784) increases the IC50 value for inhibition of phenylalkylamine labelling of Drosophila Ca2+-channels from 0.29 to 2.1 nM but decreases the IC50 value for inhibition of phenylalkylamine labelling of mammalian skeletal muscle Ca2+-channels from 3480 to 49 nM. 3. DPI 201-106 enantiomers completely block (at 0.1 μM) phenylalkylamine photolabelling of a 136 K polypeptide in Drosophila head membranes whereas 10 μM aconitine or lidocaine are without effect. 4. Assessment of the Ca2+-antagonist effects of the substituted DPI 201-106 enantiomers in K+-depolarized taenia strips from guinea-pig caecum yielded pA2 values of 6.33 ± 0.07 for (-)-BDF 8784 and 6.99 ± 0.17 for (+)-BDF 8784, respectively. 5. Piperazinylindoles, previously believed to act nonspecifically on voltage-dependent mammalian L-type Ca2+-channels, therefore have stereoselectivity for a novel binding site and chemical selectivity unrelated to local anaesthetic activity. 6. It is proposed that a very high affinity piperazinylindole-selective site is coupled to the phenylalkylamine receptor of Drosophila Ca2+-channels. These sites are still present on mammalian L-type Ca2+-channels but have lower affinity and/or are less tightly coupled to phenylalkylamine receptors on the α1-subunit.