ETB receptor blockade potentiates the pressor response to big endothelin-1 but not big endothelin-2 in the anesthetized rabbit

The precursor of endothelin-1, big endothelin-1, is considered to be a more reliable marker of systemic production of vasoactive peptide. However, it is largely unclear whether ETB receptor-dependent clearance and endothelium-derived relaxing factors affect the precursor in a similar manner to mature ET-1. These ETB-dependent modulations of big ET-1 and big ET-2 presser properties were therefore studied in the anesthetized rabbit. When injected into the left cardiac ventricle, ET-1 and ET-2 (0.01 to 1 nmol/kg) each induced biphasic responses (a depressor followed by a presser response), whereas big ET-1 and big ET-2 (0.1 to 3 nmol/kg) caused only protracted presser responses. The highest dose of big ET-1 caused significantly greater responses than ET-1, ET-2, or big ET-2. A selective ETA receptor antagonist, BQ-123 (1 mg/kg), markedly reduced presser responses to all 4 peptides, whereas blockade of ETB receptors with BQ-788 (0.25 mg/kg) sharply potentiated the responses to ET-1, ET-2, and big ET-1, but not to big ET-2. Indomethacin (10 mg/kg) sharply potentiated the presser response to ET-1 (1 nmol/kg), but not big ET-1, at all time points. In control animals, ET-1, but not big ET-1, also triggered an indomethacin-sensitive increase in circulating prostacyclin. Finally, systemically administered big ET-1, but not big ET-2, induced a phosphoramidon-sensitive increase in plasma IR-ET. Our results suggest a significant limiting role of ETB receptors on presser responses to big ET-1. In contrast, the same receptor entities do not modulate the hemodynamic properties of the ET-2 precursor, given that, unlike big ET-1, it is poorly converted in the pulmonary or systemic circulation in anesthetized rabbits.