Elevated systemic levels of endothelin-1 and blood pressure correlate with blunted constrictor responses and downregulation of endothelin_A, but not endothelin_B, receptors in an animal model of hypertension

Hypertension is a major risk factor in the development of cardiovascular disease. Adenovirus gene transfer of endothelin-1 (Ad.CMV.ET-1) in rats produced significant (5-fold) increases in plasma ET-1 and systemic blood pressure (46%) 4 days after viral administration, compared with β-galactosidase (Ad.CMV.β-gal) injected as control. The density (B_max) of the ET receptor ET_A measured in aortas was reduced significantly by more than 50% to 17±2 fmol·mg^-1 of protein for the Ad.CMV.ET-1 group compared with 39±6 fmol·mg^-1 of protein for the control. There was no change in the density of the smaller population of the ET_B sub-type. In agreement, the ratio of ET_A mRNA to cyclophilin mRNA (a housekeeping gene) measured by Northern analysis was reduced in Ad.CMV.ET-1 rats compared with controls. The ratio of m RNA encoding the ET_B subtype did not change. ET-1 vasoconstriction was significantly reduced (P < 0.05) in aortas from Ad.CMV.ET-1-treated rats [pD₂ = 8.67±0.14 (where pD₂ is -log₁₀EC₅₀); n = 11] versus the control (pD₂ = 9.11±0.06; n = 14) but there was no significant difference in the potency of two other vasoconstrictors tested (noradrenaline and Arg-vasopressin), indicating this was a specific effect on ET receptors. There was no change in the affinity of ET-1 binding to either receptor subtype in the experimental group compared with the control, demonstrating that the attenuation in the constrictor response is the result of the reduced density of receptors rather than a change in affinity. The results show that ET_A (but not ET_B) receptors are modulated in this experimental model of hypertension and provide further evidence for selective blockade of the ET_A receptor as a therapeutic strategy.