Blockade of Ba2+ current through human alpha 1E channels by two steroid analogs, (+)-ACN and (+)-ECN

Previous work suggests that different neuroactive steroids may exhibit some selectivity in their blocking effects on different high-voltage activated (HVA) Ca2+ currents. At least some of these effects appear to involve direc t blocking actions on Ca2+ channels. Thus, direct investigation of the effe cts of various steroids on cloned Ca2+ channel variants may lead to the dev elopment of potent and selective small-molecular weight Ca2+ channel blocke rs. Here we examine the effects of two steroids on a cloned human alpha 1E Ca2+ channel both with and without a beta 3 subunit, when expressed in HEK2 93 cells. One compound, (+)-ACN, has been previously shown to block N-, Q-, and R-subtypes of HVA current without affecting L- and P-type current. The second compound, (+)-ECN, weakly blocks total HVA current in hippocampal n eurons. (+)-ECN differs from(+)-ACN in lacking effects on GABA receptors, b ut shares with (+)-ACN an ability to partially inhibit T current in DRG neu rons (Todorovic, S.M., Prakriya, M., Nakashima, Y.M. et al., 1998. Enantios elective blockade of T-type Ca2+ current in adult rat sensory neurons by a steroid lacking GABA-mimetic activity. Mel. Pharmacol. 54, 918-927). (+)-AC N can block 100% of Ba2+ current in HEK cells arising either from the alpha 1E subunit (IC(50)similar to 10 mu M) or the alpha 1E beta 3 combination ( IC(50)similar to 5 mu M), while (+)-ECN maximally blocks only about 80% of the alpha 1E (10 mu M) or alpha 1E beta 3 (16 mu M) current. Blockade by ()-ACN exhibits several differences from blockade by (+)-ECN. (+)-ACN increa ses the apparent rate of onset of inactivation, particularly for the alpha 1E variant, slows recovery from inactivation, and more profoundly shifts th e voltage-dependence of current availability for both a1E and alpha 1E beta 3 variants than does (+)-ECN. Although the complexity of the normal inacti vation kinetics of alpha 1E variants makes interpretation of the (+)-ACN-in duced kinetic alterations difficult, the results suggest that the two stero ids are to some extent acting by distinct mechanisms, and perhaps at differ ent sites. (C) 1999 Elsevier Science Ltd. All rights reserved.