Bioassay-directed isolation of hypotensive alkaloid from Holarrhena pubescens

Holarrhena pubescens belongs to the family Apocynaceae, commonly known as „kurchi“ is highly reputed in traditional medicine as a remedy for amoebic dysentery and other intestinal ailment. Bioassay-directed fractionation [1] of the ethanolic extract of Holarrhena pubescens resulted in the isolation of steroidal alkaloids i.e. holamide and pubscinine.

Holamide showed a three proton doublet at 1.45 (J=6.56Hz) and two AB doubles at 3.17 and 3.00 each for on proton (J=12.06Hz) in the 1H NMR spectrum suggested that it belongs to conanine series of alkaloid (a class of compounds with the steroid nucleus and a five member heterocyclic ring with nitrogen). In contrast pubscinine showed one methyl at 1.28 while the doublet is missing, a three proton singlet was observed at 2.28 due to a vinylic methyl indicated a double bond in the 18,20– epimino ring of the conanine series of alkaloids.

In anaesthetized rats, the holamide and pubscinine caused a fall in blood pressure in a dose-dependent manner. Pretreatment of animals with atropine completely abolished the hypotensive response of acetycholine; whereas hypotensive effect of holamide and pubscinine were not modified by atropine [1]. Similarly, acetylcholine produced contractile effect in guinea-pig ileum, which was antagonized by atropine, however both holamide and pubscinine failed to produced any stimulant response on guinea-pig ileum. These data indicate that the steroidal alkaloids i.e. holamide and pubscinine from Holarrhena pubescens mediated hypotensive response through a mechanism different to that of acetycholine.

Reference: [1] Aftab, K. et al. (1996) Adv. Exp. Med. Biol. 404:429–442.