C14-aminosteroid LNF 209, an agent with positive inotropic and antimuscarinic activity

LNF 209 is a cardioactive steroid containing at position C14 a protonized amino group. It was tested whether LNF 209 has antimuscarinic properties besides its cardiotonic effect, which was also quantified. In guinea pig left atria (3 Hz, 0.9 mM Ca2+, 5.4 mM K+), LNF 209 increased the force of contraction at concentrations > or = 1 microM, thus having a tenfold lower potency than ouabain. Correspondingly, cardiac glycoside binding sites in guinea pig cardiac membranes labeled with [3H]ouabain were occupied by LNF 209 (Ki = 1.7 microM) at concentrations tenfold higher compared with ouabain (Kd = 0.14 microM). The negative inotropic effect of the muscarinic agonist oxotremorine was antagonized by LNF 209, but not by ouabain; in the Schild plot, the data for LNF 209 could be connected by a line with a slope of unity and a pA2 = 7.5. The binding of [3H]N-methylscopolamine ([3H]NMS) to the M2-cholinoceptors in guinea pig cardiac membranes was inhibited by LNF 209 with a Ki = 0.5 microM; LNF reduced the affinity of [3H]NMS binding without affecting the number of receptor sites. In isolated segments of guinea pig ileum, LNF 209 antagonized the effect of oxotremorine with a pA2 = 7.4. It is concluded that LNF 209 is a competitive antagonist at M-cholinoceptors without preference for the cardiac M2-cholinoceptors. Whether the antimuscarinic property is of significance in species highly sensitive to digitalis remains to be established.