Crosstalk between the angiotensin and endothelin system in the cerebrovasculature after experimental induced subarachnoid hemorrhage

Under physiologic conditions, losartan showed a dose-dependent antagonistic effect to the endothelin-1 (ET-1)-mediated vasoconstriction. This reduced vasoconstriction was abolished after preincubation with an endothelin B₁ receptor (ET(B₁)-receptor) antagonist. Also, an increased ET(B₁)-receptor-dependent relaxation to sarafotoxin S6c (S6c; an ET(B₁)-receptor agonist) was detected by preincubation with losartan. Investigations after experimental induced subarachnoid hemorrhage (SAH) are still missing. Therefore, we analyzed losartan in a further pathological setup. Cerebral vasospasm was induced by a modified double hemorrhage model. Rats were sacrificed on day 3 and isometric force of basilar artery ring segments was measured. Parallel to physiological conditions, after SAH, the ET-1-induced vasoconstriction was decreased by preincubation with losartan. This reduced contraction has been abolished after preincubation with BQ-788, an ET(B₁)-receptor antagonist. In precontracted vessels, ET-1 induced a higher vasorelaxation under losartan and the endothelin A receptor (ET(A)-receptor) antagonist BQ-123. After SAH, losartan caused a modulatory effect on the ET(B₁)-receptor-dependent vasorelaxation. It further induced an upregulation of the NO pathway. Under losartan, the formerly known loss of the ET(B₁)-receptor vasomotor function was abolished and a significantly increased relaxation, accompanied with an enhanced sensitivity of the ET(B₁)-receptor, has been detected. Also, the dose-dependent antagonistic effect to the ET-1-induced contraction can be effected by angiotensin II type 1 receptor (AT₁-receptor) antagonism due to losartan directly via the ET(B₁)-receptor.