SUBUNIT-DEPENDENT INTERACTION OF THE GENERAL ANESTHETIC ETOMIDATE WITH THE GAMMA-AMINOBUTYRIC-ACID TYPE-A RECEPTOR

1 The GABA modulating and GABA-mimetic actions of the general anaesthe tic etomidate were examined in voltage-clamp recordings performed on X enopus laevis oocytes induced, by cRNA injection, to express human rec ombinant gamma-aminobutyric acid(A) (GABA(A)) receptor subunits. 2 Cur rents mediated by recombinant receptors with the ternary subunit compo sition alpha(x) beta(y) gamma(2L) (where x=1,2,3 or 6 and y=1 or 2), i n response to GABA applied at the appropriate EC(10), were enhanced by etomidate in a manner that was dependent upon the identity of both th e alpha and beta subunit isoforms. 3 For the beta(2)-subunit containin g receptors tested, the EC(50) for the potentiation of GABA-evoked cur rents by etomidate (range 0.6 to 1.2 mu M) was little affected by the nature of the alpha subunit present within the hetero-oligomeric compl ex. However, replacement of the beta(2) by the beta(1) subunit produce d a 9-12 fold increase in the etomidate EC(50) (6 to 11 mu M) for all alpha-isoforms tested. 4 For alpha(1), alpha(2) and alpha(6), but not alpha(3)-subunit containing receptors, the maximal potentiation of GAB A-evoked currents by etomidate was greater for beta(2)- than for beta( 1)-subunit containing receptors. This was most clearly exemplified by receptors composed of alpha(6) beta(1) gamma(2L) compared to alpha(6) beta(2) gamma(2L) subunits, where a maximally effective concentration of etomidate potentiated currents evoked by GABA at EC(50) to 28+/-2% and 169+/-4% of the maximal GABA response, respectively. 5 For alpha(1 ) subunit-containing receptors, the potency and maximal potentiating e ffect of either pentobarbitone or propofol was essentially unaffected by the beta subunit isoform contained within the receptor complex. The potency of the anaesthetic neurosteroid 5 alpha-pregnan-3 alpha-ol-20 -one was marginally higher for beta(1) rather than the beta(2) subunit -containing receptor, although its maximal effect was similar at the t wo receptor isoforms. 6 The GABA-mimetic action of etomidate was suppo rted by beta(2)- but not beta(1)-subunit containing receptors, whereas that of pentobarbitone or propofol was evident with either beta isofo rm. For beta(2)-subunit containing receptors, both the agonist EC(50) and the maximal current produced by etomidate were additionally influe nced by the alpha isoform. 7 It is concluded that the subtype of beta- subunit influences the potency with which etomidate potentiates GABA-e voked currents and that the beta isoform is a crucial determinant of t he GABA-mimetic activity of this compound. The nature of the alpha-sub unit also impacts upon the maximal potentiation and activation that th e compound may elicit. Such pronounced influences may aid the identifi cation of the site that recognises etomidate. More generally, these re sults provide a clear example of structural specificity in anaesthetic action.