The subtypes of endothelin receptors that mediate the effects of endothelin-1 (ET-1) on mean arterial pressure (MAP), heart rate (HR), mean circulatory filling pressure (MCFP), arterial resistance (RA), cardiac output (CO) and venous resistance (RV) were characterized in 9 groups of pentobarbitone-anaesthetized rats via the injection of ET-1 in the absence and presence of bosentan (Ro 47-0203, ETA- and ETB-receptor antagonist), PD 142893 (ETA- and ETB-receptor antagonist) or FR 139317 (ETA-receptor antagonist), as well as injection of the ETB-receptor agonist, IRL 1620. 2. Cumulative i.v. bolus injections of ET-1 or IRL 1620 (0.5, 1 and 2 nmol kg-1) dose-dependently increased MAP (ET: by 22, 34 and 44; IRL: 8, 17 and 28 mmHg), RA (ET: 62, 108 and 162; IRL: 51, 63 and 86% over baseline), RV (ET: 70, 132 and 179; IRL: 81, 89 and 98% over baseline) and MCFP (ET: 1.1, 1.8 and 1.9; IRL: 0.9, 1.0 and 1.2 mmHg) and reduced CO (ET: -18, -35 and -44; IRL: -24; -26; -25% below baseline). Equimolar doses of ET-1 and IRL 1620 caused similar initial transient depressor responses. Saline did not modify any haemodynamic variables in the time-control group. 3. Bosentan (10 mg kg-1, i.v.) inhibited ET-induced increases in MAP, RV, RA and MCFP and decrease in CO. PD 142893 (22 mg kg-1, i.v.) abolished ET-induced changes on MAP, RV, RA and CO, but did not alter effects on MCFP. Bosentan alone did not cause haemodynamic changes, but PD 142893 alone elevated MCFP (0.9 +/- 0.3 mmHg at 1 h after injection) and did not alter other variables. Both antagonists abolished the initial depressor effects of ET-1. 4. FR 139317 (1 mg kg-1, i.v.) partially inhibited the increases in MAP, RV, RA and MCFP and decreases in CO elicited by ET-1, but did not alter the transient depressor response of ET-1. 5. The results show that both ETA- and ETB-receptors mediate the arterial and venous constrictor effects of ET-1. Bosentan is more efficacious than PD 142893 in inhibiting the venous effects of ET-1.
