Acute hypertension after nitric oxide synthase inhibition is mediated primarily by increased endothelin...

To determine the quantitative roles played by the different vasoconstrictor systems in the acute pressor response in conscious rats before and after inhibition of nitric oxide synthase with Nw-nitro-L-arginine methyl ester (L-NAME).

In conscious male Sprague-Dawley rats, previously instrumented with aortic and venous catheters, the contributions of the different systems were assessed by maximal cumulative pharmacological blockade of alpha 1-adrenoceptors (1 mg/kg prazosin intraperitoneally), AT1 receptors (30 mg/kg losartan intraperitoneally) and V1/V2 receptors (10 mg/kg [beta-mercapto-beta, beta-cyclopenta-methylenepropionyl1, O-Et-Tyr2-Val4-Arg8]-vasopressin per min intravenously). In addition, the contribution of endothelin-1-induced vasoconstriction in response to 100 mg/kg L-NAME intraperitoneally to the hypertension was assessed by administering 5-100 mg/kg ETA/ETB receptor antagonist PD 145 065 intravenously under three different conditions: as the last step of a series of antagonists in the cumulative pharmacological blockade after having induced the L-NAME pressor response; alone before L-NAME treatment; and alone after the full development of the L-NAME pressor response. A separate group of rats was treated acutely with 30 mg/kg losartan intraperitoneally or pretreated for 3 days with 30 mg/kg angiotensin I converting enzyme inhibitor enalapril via drinking water alone or in combination with 1% salt was used to assess the role of the renin-angiotensin system in the L-NAME-induced hypertension.

Short-term administration of the combined ETA/ETB receptor antagonist PD 145065 did not change the arterial pressure under control conditions. Inhibition of the renin-angiotensin system, alpha 1-adrenoceptors or vasopressin receptors alone or in combination did not alter the magnitude of the L-NAME pressor response. In contrast, our results show that both treatments before and during acute nitric oxide synthase blockade hypertension using the ETA/ETB receptor antagonist PD 145065 abolished almost completely (approximately 85%) the pressor response.

These studies indicate that the predominant mechanism of hypertension, at least in the acute phase, after acute nitric oxide synthase blockade with L-NAME is associated with a marked increase in ETA/ETB receptor activation rather than with increases in alpha 1, AT1 and V1/V2 receptor activation. It remains to be determined whether endothelin participates also in the chronic phase of nitric oxide-deficient hypertension.