Effect of Phenylephrine on Cerebrovascular Regulation: A Translational Perspective

Abstract

Background: Phenylephrine is an alpha 1-adrenergic receptor (α1R) agonist. Evidence indicates activation of α1Rs can initiate both vasoconstrictor and dilator signaling. How phenylephrine affects cerebrovascular regulation remains unclear.

Methods: A retrospective analysis of data examining cerebral perfusion and blood pressure during systemic phenylephrine infusion in humans and swine was completed. Follow-up experiments examining cerebral hemodynamics during intracarotid arterial infusion of phenylephrine in anesthetized swine were performed. Ex vivo experiments were conducted on isolated porcine cerebral arteries.

Results: Systemic phenylephrine infusion increased indices of cerebrovascular resistance in both humans (P=0.0423) and swine (P<0.0001) but did not decrease perfusion. Intracarotid phenylephrine infusion did not alter cerebrovascular resistance, but increased perfusion in control conditions (P=0.0045), whereas resistance increased (P≤0.0155) without altered perfusion during NOS (nitric oxide synthase) inhibition conditions. α1Rs were detected on both extraluminal and intraluminal aspects of cerebral arteries, reflecting a population of vascular smooth muscle and endothelial α1Rs, respectively. Extraluminal phenylephrine caused vasoconstriction whereas intraluminal phenylephrine elicited an endothelium-dependent NO-mediated dilation. NOS inhibition enhanced phenylephrine-induced vasoconstriction in third-order branch of the middle cerebral artery, but not the first-order or second-order pial arteries (P=0.0267), and this corresponded with an increased ratio of phosphorylated to total endothelial NOS protein content in third-order versus first-order and second-order arteries (P≤0.0022). Phenylephrine-induced constriction was greatest in first-order arteries (P=0.0419), and this corresponded with increased perivascular adrenergic innervation and α1R protein content in first-order versus second-order and third-order arteries (P≤0.0054).

Conclusions: Neither systemic nor intracarotid phenylephrine infusion compromised cerebral perfusion, possibly related to increased endothelial NO signaling and reduced α1R density in downstream pial arteries.

Keywords: cerebrovascular; endothelium; nitric oxide; phenylephrine; α1‐adrenergic.

Figure 1. In vivo and ex vivo cerebrovascular responses to α1R agonism.

A, CVCi during saline infusion (N=10) and PE infusion under control (N=10) and NOS inhibition (N=8). B, %∆ in diameter during a concentration response curve to PE in intact arteries (control; filled circles; N=4) and arteries with their endothelium removed (denuded; open circles; N=4). C, %∆ in diameter in response to PE delivered intraluminally in intact arteries (control; N=5) and arteries with their endothelium removed (denuded; N=5). D, %∆ in diameter in response to PE delivered intraluminally in intact arteries under control (N=23) NOS inhibition conditions (N=23). E, %∆Diameter during a concentration response curve to NA in intact arteries under control (filled circles; N=7) and β‐blockade conditions (open circles; N=7). F, %∆ in diameter in response to NA delivered either extra‐ (N=4) or intraluminally (N=4) both under β‐blockade conditions. G, Z‐projection images of α1Rs both extra‐ and intraluminally, reflecting VSM α1R and EC α1R surfaces, respectively—observed similarly in N=6. Intraluminal imaging was repeated on arteries with their endothelium removed (denuded) representing a negative control (scale bar=50 μM). Individual dots reflect individual data points. Data in (B) and (E) are presented as mean±SD. Data were analyzed using independent and paired t tests as well as using a mixed design repeated measures ANOVA. ##P<0.01 vs baseline; *P<0.05; ****P<0.0001 vs control; ^§ P<0.05 vs extraluminal. α1R indicates alpha 1‐adrenergic receptor; AU, arbitrary unit; CVCi, cerebrovascular conductance index; EC, endothelial cell; NA, noradrenaline; NOS, nitric oxide synthase; PE, phenylephrine; and VSM, vascular smooth muscle.

Figure 2. Branch order differences in eNOS and α‐adrenergic control.

A, Branch order differences in 1A (filled circles), 2A (filled squares), and 3A (filled triangles) pial arteries in response to intraluminal PE (e⁻⁷M) under control and NOS inhibition conditions. B, %Vasoconstriction during a concentration response curve to PE in 1A (filled circles), 2A (filled squares), and 3A (filled triangles) pial arteries (N=6–7/branch order). %Vasoconstriction during a concentration response curve to PE in (C) 1A (circles), (D) 2A (squares) and (E) 3A (triangles) in intact pial arteries under control (filled; N=6–7/branch order) and NOS inhibition (filled; N=6–7/branch order) conditions. F, α1R protein content in 1A, 2A, and 3A whole‐artery lysates normalized to β‐actin and expressed relative to middle cerebral artery protein content within each gel (N=27) with representative Western blot data. G, Ratio of phosphorylated:total eNOS protein content in 1A, 2A, and 3A whole‐artery lysates (N=27) with representative Western blot data. H, %Vascular area (fluorescence within the segment) stained with tyrosine hydroxylase in 1A, 2A, and 3A arteries (N=24), indicative of sympathetic‐adrenergic perivascular innervation with representative Z‐projection images (scale bar=50 μM). Individual dots reflect individual data points. Data in (A–E) are presented as mean±SD. Data were analyzed using mixed design and 1‐way repeated measures ANOVA. *P<0.05 vs 1A; **P<0.01 vs 1A; ^§§ P<0.01 vs 2A. α1R indicates alpha 1‐adrenergic receptor; AU, arbitrary unit; eNOS, endothelial nitric oxide synthase; NOS inh., nitric oxide synthase inhibition; PE, phenylephrine; ph‐eNOS, phosphorylated endothelial nitric oxide synthase; 1A, first‐order branch of the middle cerebral artery; 2A, second‐order branch of the middle cerebral artery; and 3A, third‐order branch of the middle cerebral artery.