THE METABOLISM OF ENDOTHELIN-1 AND BIG ENDOTHELIN-1 BY THE ISOLATED-PERFUSED KIDNEY OF THE RABBIT

The activity of human big endothelin-I (bET-1) or endothelin-1 (ET-1) was investigated in the isolated perfused kidney of the rabbit. In some experiments the effluent superfused a rabbit jugular vein (RbJV) or rat colon(RC), and bolus doses of bET-1 or ET-1 were administered either directly over the tissue (OT) or through the kidney (TK). When injected OT, the contractile responses of the RbJV to ET-1 were greater-than-or-equal-to 100-200 times more than those to bET-1. However, at least 10 times the ET-1 dose given OT was needed TK to produce an equivalent contraction of the RbJV, showing that ET-1 was being inactivated or removed by the kidney. Injections of bET-1 TK produced increases in perfusion pressure of a magnitude 1/25th of those of ET-1. In some experiments administration of bET-1 TK was associated with the release into the perfusate of an ET-1-like factor that contracted the RbJV, but not the RC (used to detect any angiotensin II release), at doses that had little effect when administered OT, suggesting that bET-1 was being activated by the kidney. Phosphoramidon (10 muM) infused TK blocked (92 +/- 1% inhibition) the renal responses to bET-1 and reduced the overflow of ET-1-like material onto the tissues without affecting ET-1-induced renal vasoconstriction. Incubation of bET-1 with rabbit renal cortical microsomes (100 mug protein) resulted in the generation of ET-1-like activity, as assessed by bioassay, which was inhibited by phosphoramidon (10 muM). Reverse-phase high-performance liquid chromatography showed that renal cortical microsomes converted bET-1 to ET-1 and that this conversion was inhibited by phosphoramidon. The kidney may therefore represent a potential site of bET conversion in vivo and may serve to limit, by inactivation, the vasoconstrictor effects of ET-1.