Intravital Förster resonance energy transfer imaging reveals elevated [Ca²⁺]i and enhanced sympathetic tone in femoral arteries of angiotensin II‐infused hypertensive biosensor mice

Key points It is desirable to study altered artery function in hypertension in living animals, where factors influencing artery function are intact. We infused ‘biosensor’ mice chronically with angiotensin II to produce hypertension, and used intravital Förster resonance energy transfer microscopy to measure, simultaneously, [Ca 2+ ] i and artery diameter in vivo . Femoral arteries in hypertensive mice had increased basal (resting) [Ca 2+ ] i and were more constricted than femoral arteries in saline‐infused mice. These differences were abolished by blocking (1) all peripheral sympathetic nerve activity (SNA), or (2) vascular α 1 ‐adrenoceptors, but not by blocking vascular angiotensin II or arginine vasopressin receptors. Neither contractility nor structural changes were found in femoral arteries of angiotensin II‐infused mice. The results support previous suggestions that angiotensin II infusion raises blood pressure by acting on the CNS to increase SNA, which increases smooth muscle [Ca 2+ ] i . Infused angiotensin II does not act directly on arterial angiotensin II receptors. Abstract Artery narrowing in hypertension can only result from structural remodelling of the artery, or increased smooth muscle contraction. The latter may occur with, or without, increases in [Ca 2+ ] i . Here, we sought to measure, in living hypertensive mice, possible changes in artery dimensions and/or [Ca 2+ ] i , and to determine some of the mechanisms involved. Ca 2+ /calmodulin biosensor (Förster resonance energy transfer‐based) mice were made hypertensive by s.c. infusion of angiotensin II (Ang II, 400 ng kg −1 min −1 , 2–3 weeks). Intravital fluorescence microscopy was used to determine [Ca 2+ ] i and outer diameter of surgically exposed, intact femoral artery (FA) of anaesthetized mice. Active contractile FA ‘tone’ was calculated from the basal‐state diameter and the passive (i.e. Ca 2+ ‐free) diameter (PD). Compared to saline control, FAs of Ang II‐infused mice had (1) ∼21% higher active tone and (2) ∼78 n m higher smooth muscle [Ca 2+ ] i , but (3) the same PDs. The local Ang II receptor (AT 1 R) blocker losartan had negligible effect on tone or [Ca 2+ ] i in control FAs, but reduced the basal tone by ∼9% in Ang II FAs. Both i.v. hexamethonium and locally applied prazosin abolished the difference in FA tone and [Ca 2+ ] i , suggesting a dominant role of sympathetic nerve activity (SNA). Changes in diameter and [Ca 2+ ] i in response to locally applied phenylephrine, Ang II, arginine vasopressin, elevated [K + ] o and acetylcholine were not altered. In summary, FAs of living Ang II hypertensive mice have higher [Ca 2+ ] i, and are more constricted, due, primarily, to elevated SNA and some increased arterial AT 1 R activation. Evidence of altered artery reactivity or remodeling was not found.