(-)-DISCRETAMINE, A SELECTIVE ALPHA(1D)-ADRENOCEPTOR ANTAGONIST, ISOLATED FROM FISSISTIGMA-GLAUCESCENS

1 The selectivity of (-)-discretamine for alpha(1)-adrenoceptor subtypes was investigated by use of functional and binding studies in rat vas deferens, spleen and aorta, and in cultured DDT(1)MF-2 and A10 cells. 2 In prostatic portions of rat vas deferens, the competitive antagonists (-)-discretamine, 5-methyl-urapidil (5-MU) and prazosin inhibited contractions to noradrenaline (NA) with pA(2) values of 6.21, 8.71 and 9.27, respectively. The irreversible antagonist, chloroethylclonidine (CEC, 100 mu M) failed to affect contractions to NA while nifedipine (1 mu M) blocked them almost completely. 3 In rat spleen, the competitive antagonists (-)-discretamine, 5-MU and prazosin inhibited contractions to phenylephrine with pA(2) values of 6.44, 7.19 and 9.45, respectively. CEC (100 mu M) significantly reduced the maximum contraction to phenylephrine while nifedipine (1 mu M) did not affect it. 4 In rat aorta, the competitive antagonists (-)-discretamine, 5-MU and prazosin inhibited contractions to NA with pA(2) values of 7.60, 8.00 and 9.40, respectively. CEC also antagonized the contractions to NA in a competitive manner with a pA(2) value of 6.10. 5 The specific binding of [H-3]-prazosin to DDT(1)MF-2 and A10 cells was concentration-dependent and saturated at 3-5 nM with K-D values of 0.24 +/- 0.02 and 0.20 +/- 0.02 nM, respectively. (-)-Discretamine, 5-MU, CEC and prazosin inhibited specific [H-3]-prazosin binding to DDT(1)MF-2 and A10 cells in a concentration-dependent manner with IC50, values of 390.8 +/- 20.6, 43.6 +/- 3.9, 200.0 +/- 30.0 and 0.8 +/- 0.1 nM, respectively in DDT(1)MF-2 cells, and 25.0 +/- 3.2, 8.6 +/- 1.4, 1000.0 +/- 30.8 and 0.52 +/- 0.03 nM, respectively in A10 cells. 6 Pretreatment of A10 cells with CEC (10 mu M) for 30 min and then washed out thoroughly, reduced specific [H-3]-prazosin binding by 30%. The CEC-insensitive [H-3]-prazosin binding was inhibited by (-)-discretamine with an IC50 value of 7.0 +/- 0.3 nM. 7 5-MU (100 nM), CEC (1 mu M) and prazosin (10 nM) markedly inhibited NA (3 mu M)-induced [H-3]-inositol monophosphate formation in DDT(1)MF-2 and A10 cells, while (-)-discretamine (100 nM) inhibited NA-induced [H-3]-inositol monophosphate formation only in A10 cells. 8 In conclusion, (-)-discretamine is a selective alpha(1D)-adrenoceptor antagonist in vascular smooth muscle. Its selectivity among various alpha(1)-adrenoceptor subtypes is alpha(1A):alpha(1B):alpha(1D) = 0.04:0.07:1.0.