Critical role for the α-1B adrenergic receptor at the sympathetic neuroeffector junction

The α-1 adrenergic receptors (α₁ARs) are critical in sympathetically mediated vasoconstriction. The specific role of each α₁AR subtype in regulating vasoconstriction remains highly controversial. Limited pharmacological studies suggest that differential α₁AR responses may be the result of differential activation of junctional versus extrajunctional receptors. We tested the hypothesis that the α_1BAR subtype is critical in mediating sympathetic junctional neurotransmission. We measured in vivo integrated cardiovascular responses to a hypotensive stimulus (induced via transient bilateral carotid occlusion [TBCO]) in α_1BAR knockout (KO) mice and their wild-type (WT) littermates. In WT mice, after dissection of the carotid arteries and denervation of aortic baroreceptor buffering nerves, TBCO produced significant pressor and positive inotropic effects. Both responses were markedly attenuated in α_1BAR KO mice (change systolic blood pressure 46±8 versus 11±2 mm Hg; percentage change in the end-systolic pressure-volume relationship [ESPVR] 36±7% versus 12±2%; WT versus KO; P<0.003). In vitro α₁AR mesenteric microvascular contractile responses to endogenous norepinephrine (NE; elicited by electrical field stimulation 10 Hz) was markedly depressed in α_1BAR KO mice compared with WT (12.4±1.7% versus 21.5±1.2%; P<0.001). In contrast, responses to exogenous NE were similar in α_1BAR KO and WT mice (22.4±7.3% versus 33.4±4.3%; NS). Collectively, these results demonstrate a critical role for the α_1BAR in baroreceptor-mediated adrenergic signaling at the vascular neuroeffector junction. Moreover, α_1BARs modulate inotropic responses to baroreceptor activation. The critical role for α_1BAR in neuroeffector regulation of vascular tone and myocardial contractility has profound clinical implications for designing therapies for orthostatic intolerance.