Effect of the α2-adrenoceptor antagonist yohimbine on vascular regulation of the middle cerebral artery and the ophthalmic artery in healthy subjects

There is evidence that vascular beds distal to the ophthalmic artery (OA) show vasoconstriction in response to a step decrease in systemic blood pressure (BP). The mediators of this response are mostly unidentified. The aim of the current study was to test the hypothesis that α2-adrenoreceptors may contribute to the regulatory process in response to a decrease in BP. In this randomized, double-masked, placebo-controlled study 14 healthy male volunteers received either 22 mg yohimbine hydrochloride or placebo. Beat-to-beat BP was measured by analysis of arterial pressure waveform; blood flow velocities in the middle cerebral artery (MCA) and the OA were measured with Doppler ultrasound. Measurements were done before, during and after a step decrease in BP. The step decrease in BP was induced by bilateral thigh cuffs at a suprasystolic pressure followed by a rapid cuff deflation. After cuff deflation, BP returned to baseline after 7–8 pulse cycles (PC). Blood velocities in the MCA returned to baseline earlier (4 PC) than BP indicating peripheral vasodilatation. Blood velocities in the OA returned to baseline later (15–20 PC) indicating peripheral vasoconstriction. Yohimbine did not affect the blood velocity response in the MCA, but significantly shortened the time of OA blood velocities to return to baseline values (6–7 PC, p < 0.05). In conclusion, our results indicate that yohimbine did not alter the regulatory response in the MCA, but modified the response of vascular beds distal to the OA. This suggests that α2-adrenoceptors play a role in the vasoconstrictor response of the vasculatures distal to the OA.

Graphical AbstractTime course of the outcome variables in the ophthalmic artery placebo study day (OA, left panels) and yohimbine study day (OA, right panels) before and after thigh cuff release. (SBP = systolic blood pressure, DBP = diastolic blood pressure, PSV = peak systolic blood velocity, EDV = end diastolic blood velocity, AIS = index of resistance for systolic data, AID = index of resistance for diastolic data). Data are presented as means ± SEM (n = 14). The vertical line in each graph represents the time point when the cuff was released.Figure optionsDownload full-size imageDownload as PowerPoint slide