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±2fmol·mg^-1 of protein for the Ad.CMV.ET-1 group compared with 39±6fmol·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 mRNA encoding the ET_B sub-type 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 sub-type 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.