Endothelin-1-induced venous contraction is maintained in DOCA-salt hypertension; studies with receptor agonists.

Deoxycorticosterone acetate (DOCA) salt hypertension is associated with an endothelin-1 (ET-1)-dependent increase in arterial resistance and mean circulatory filling pressure. Contraction of endothelium-intact arteries and veins from sham and DOCA-salt hypertensive rats to agonists of the ETA (ET-1(1–31)) and ETB receptor (sarafotoxin 6c; S6c) was investigated in tissue baths as was expression of mRNA for ET-1 and mRNA and protein for the ETA and ETB receptor. ET-1(1–31) contracted aorta and vena cava from sham rats with a 30 fold lower potency than ET-1. Contraction was not altered by the ETB receptor antagonist BQ788 (100 nM) but was abolished by the ETA receptor antagonist ABT-627 (30 nM). In DOCA-salt thoracic aorta, maximum contraction to ET-1 and ET-1(1–31) was reduced (36.6±6.3 and 13.3±4.4% of sham response, respectively); aorta did not contract to S6c. In vena cava from DOCA-salt rats, contraction to ET-1 and ET-1(1–31) was not reduced compared to sham contraction; vena cava from sham and DOCA-salt rats contracted to S6c with a similar potency. Real time RT–PCR revealed that prepro ET-1 mRNA was increased 6.6±3.3 fold and 8.7±3.9 fold greater in DOCA-salt aorta and vena cava, respectively, compared to sham. Vena cava expressed a higher content of ETA and ETB receptor mRNA than aorta (P<0.05), but no differences were observed between sham and DOCA-salt tissues. ETA and ETB receptor protein was identified in all tissues. Immunoreactive ETA receptor, observed as a 65, 30 and 28 kDa bands, was expressed 400% greater in DOCA-salt aorta compared to sham, but was not altered in vena cava. Immunoreactive ETB receptor, observed as 120, 45 and 30 kDa bands, tended to be higher in vena cava compared to aorta, but was not different in sham and DOCA-salt vena cava. These results suggest that ETA receptor function is impaired in aorta but not vena cava of DOCA-salt rats. The ETB receptor was present in the aorta but, unlike in veins, does not mediate contraction directly. A sustained response to ET-1 in the venous circulation may contribute to the elevated blood pressure in the DOCA-salt model. Keywords: ET-1, ET-1(1–31), venous smooth muscle, mineralocorticoid hypertension Introduction The DOCA-salt model of hypertension is associated with elevated arterial levels of the peptide endothelin (ET-1) (Lariviere et al., 1993). ET-1 has been implicated in multiple cardiovascular functions and diseases, but recent evidence suggests a significant involvement in hypertension (Li et al., 1994; Verhagen et al., 1998; Matsumura et al., 1999; Schiffrin, 2001). The endothelins combine with two receptor subtypes to exert their physiological effect, the ETA and ETB receptors. The ETB receptor can be activated selectively by sarafotoxin 6c (S6c) and antagonized by BQ788. Several receptor antagonists, including ABT-627 and BQ610, are selective for the ETA receptor and recently the chymase-derived peptide ET-1(1–31) has been described as a relatively selective agonist of ETA receptors (Hanson et al., 1997; Nagata et al., 2000; Maguire et al., 2001). We are interested in changes in venous function that may contribute to the elevated blood pressure observed in the DOCA-salt model. We and others have demonstrated that endothelin receptor antagonists reduce total peripheral resistance in the DOCA-salt rat and have recently observed that endothelin receptor antagonists also reduce mean circulatory filling pressure, indicating a possible contribution of increased venous smooth muscle contractility to determination of blood pressure in the DOCA-salt model (Fink et al., 2000; Johnson et al., 2001). Notably, veins are more sensitive to contraction induced by ET-1 than are arteries. This increased sensitivity, observed as a lower threshold concentration of ET-1 and higher potency, has been observed by several groups; the contractile potency of ET-1 in veins has been reported as anywhere from 3–10 times more than in arteries (Cocks et al., 1989; D'orleans-Juste et al., 1989). Importantly, small unpressurized mesenteric veins from the DOCA-salt rat maintain their responsiveness to ET-1 whereas arteries from the same site have a depressed contraction to ET-1 (Nguyen et al., 1992; Fujita et al., 1995; Laurant & Berthelot, 1996; Zhao et al., 2000; Johnson et al., 2002). Thus, we currently test the hypothesis that veins maintain endothelin responsiveness in the face of overexpression of ET-1. We used large arteries and veins for two reasons. First, this was done to test the principle that the maintained responsiveness is occurring in large veins as it is in small veins. Second, arteries and veins of this size allow us to perform biochemical experiments to measure mRNA and protein for the ETA receptor and ETB receptor.