Purification and cardiovascular activity of [Met¹, Met⁵]-bradykinin from the plasma of a sturgeon (Acipenseriformes)

The sturgeons (Order Acipenseriformes) are extant representatives of a group of primitive Actinopterygian (ray-finned) fish that probably shared a common ancestor with present-day teleosts. Incubation of heat-denatured plasma from a sturgeon (a hybrid of the shovelnosed sturgeon Scaphirhynchus platorynchus and the pallid sturgeon Scaphirhynchus albus) with either trypsin or porcine pancreatic kallikrein generated bradykinin-like immunoreactivity. The primary structure of sturgeon bradykinin was established as Met-Pro-Pro-Gly-Met-Ser-Pro-Phe-Arg. This amino acid sequence contains two amino acid substitutions (Arg¹ → Met and Phe⁵ → Met) compared with mammalian bradykinin. Bolus injections of synthetic sturgeon bradykinin in doses as low as 1 pmol/kg into the dorsal aorta of unanesthetized sturgeon resulted in an immediate and significant fall in arterial blood pressure with a maximum depressor response at 300 pmol/kg. Thus, the cardiovascular response of the sturgeon to bradykinin resembles more closely the response of mammals rather than the predominantly pressor response seen in teleost fish. Sturgeon bradykinin produced a strong and concentration-dependent (EC₅₀ = 4.7 ± 0.7 x 10^-10 M) relaxation of rings of vascular tissue from the sturgeon ventral aorta that had been pre- contracted with acetylcholine. The data indicate that sturgeon tissues are particularly responsive to native bradykinin and suggest that the kallikrein- kinin system may have evolved before the appearance of the neopterygians (gars, bowfin and teleosts).