Endothelin receptor antagonists reduce the pressor effects of angiotensin II into the periaqueductal gray area of rats

ETA endothelin receptors are involved in the ouabain-induced haemodynamic effects in the periaqueductal gray area of rats

The aim of the present study was to asses the effects on blood pressure and vascular resistances elicited by microinjections of ouabain (OUA) within the periaqueductal gray area (PAG). We also tested whether peripheral vascular responses caused by exogenous intra-PAG ouabain involve activation of the PAG-endothelin system. In normotensive Sprague-Dawley rats the basal mean arterial blood pressure (MABP) was 114 +/- 3 mmHg. This was significantly increased by OUA (3 micro g, 122 +/- 2 mmHg, p < 0.05; and 6 micro g, 139 +/- 3 mmHg, p < 0.01) microinjected into the PAG area. Increases in MABP were associated with increases in total peripheral resistances (TPR), organ vascular resistances, and with reduced blood flow of almost all the organs tested: kidneys, skeletal muscle, skin, stomach, spleen, testes and intestine. Cardiac output did not change. Changes in the above vascular parameters induced by OUA were significantly (p < 0.01) reduced by intra-PAG microinjections of FR139317 (a selective ETA receptor antagonist, 5 nmol), SB209670 (a non-selective ETA/ETB receptor antagonist, 3 nmol), but not by BQ 788 (a selective ETB receptor antagonist, 5 nmol). In conclusion, OUA into the PAG area of normotensive rats caused significant changes in peripheral vascular parameters that are reduced by ETA receptor antagonists. These results indicate that PAG-ET-1 system via an action on ETA receptors is involved in the OUA effects.

Chronic peripheral ouabain treatment affects the brain endothelin system of rats

BACKGROUND

Chronically administered ouabain increases the content of ouabain in several brain areas that are closely related to central cardiovascular regulation. However, the pattern of central changes induced by chronic infusion of ouabain is not completely understood.

OBJECTIVES

To investigate whether chronic peripheral ouabain treatment affects the brain endothelin system.

METHODS

By enzyme immunoassay, reverse transcription-polymerase chain reaction and autoradiography, we assessed brain endothelin and endothelin receptors contents following chronic (4 weeks) subcutaneous treatment of normotensive Sprague-Dawley rats with ouabain (OUA) (14 microg/kg per day). We also investigated the involvement of central endothelin receptors in increased sympathetic activity and hypertension induced by chronic OUA. Sympathetic activity was indirectly evaluated by recording changes in renal vascular resistance (RVR).

RESULTS

Brains of Sprague-Dawley rats collected following OUA treatment showed increased levels of the endothelin-1, and a decrease in ET(A) receptor mRNA and receptor levels. These, following chronic treatment with the submaximal dose of 14 microg/kg per day OUA, were a three-fold augmentation (P < 0.05) of the endothelin-1 peptide and a 38% decrease in ET(A) receptor mRNA. In addition, dose-dependent increases in RVR and in the basal mean arterial blood pressure (MABP) were found when compared with the vehicle (saline)-treated blood pressure (i.e. RVR, 14 microg/kg per day OUA, +161 +/- 15%). Total renal blood flows were consequently decreased (14 microg/kg per day OUA, P < 0.01). Interestingly, increases in RVR and MABP elicited by the submaximal dose of 14 microg/kg per day chronic OUA were significantly (P < 0.01) reduced by intra-periaqueductal gray (PAG) microinjections of FR139317 (selective ET(A) receptor antagonist, 5 nmol) and SB209670 (ET(A)/ET(B) non-selective antagonist, 3 nmol), but not by BQ 788 (selective ET(B) receptor antagonist, 5 nmol).

CONCLUSIONS

These data indicate that chronic treatment with OUA increases central endogenous synthesis/release of endothelin. This contributes to the peripheral OUA actions. As an example, an antagonism at the PAG endothelin receptors, mainly of the ET(A) type, reduced the effects of chronic OUA on both MABP and RVR.