Using Fiber‐Optic Confocal Microscopy for In Vivo Imaging of Brain and Kidney Vasculature and Haemodynamics

The brainstem and kidneys are regarded as being key players in the development and maintenance of hypertension. This is associated with pathology of their vascular structure and function. Repeated in vivo imaging of vasculature is challenging due to invasiveness and the limited physical accessibility of most tissues. We have assessed the possibility of using Cell Vizio, a small and flexible fiber‐optic confocal laser endomicroscope, to overcome these challenges and visualize brainstem and renal vasculature in vivo before and during an acute increase in arterial blood pressure (BP). METHODS All work was performed in accordance to Home Office and University of Bristol regulations. Normotensive Wistar rats (≥300g) were anaesthetised with isoflurane and mechanically ventilated. To allow visualization of the vascular system, an i.v. bolus injection of FITC‐dextran (2μg) and FITC‐stained autologous red blood cells (RBC) were administered. Animals were challenged with a bolus injection of phenylephrine (PE; 5–25 μg/ml) whilst recording femoral BP and carotid or renal blood flow (CBF, RBF) during imaging of the ventral brainstem or left kidney cortex (peritubular capillaries, diameter 10–30 μm), respectively. Images were acquired using CellVizio at 12–30 frames per second and analysed using the CellVizio Image Cell software. A two‐tailed, paired Students’ T‐test was used to analyse baseline vs. PE. RESULTS In Wistar rats (n=7) visualisation of both ventral brainstem and kidney vasculature was successful although renal images tended to be compromised by respiratory related movement. PE led to an increase in systemic BP (Δ+31±17mmHg, p=<0.05), however CBF and carotid vascular resistance (BP/CBF) did not change significantly. RBF decreased (Δ−5.2±1ml/min, p<0.001) and renal vascular resistance (BP/RBF) increased (Δ+87±17%, p<0.001). CellVizio imaging of brainstem vessels showed no change in vessel diameter but an increase in RBC velocity (Δ+100±70μm/sec). In contrast, the vessels in the kidney showed significant vasoconstriction with PE (from 25±6 to 11±4μm, p<0.05), with a decrease in RBC velocity (Δ−340±138μm/sec). CONCLUSION CellVizio made it possible to image the vasculature of both the ventral brainstem and the kidney before and during an acute increase in BP. However, CellVizio has significant limitations. The table below outlines some advantages and disadvantages, at least for imaging hemodynamics in hard to access blood vessels. Further optimisation is required in order to achieve the full potential of the system and overcome movement and analysis challenges. Support or Funding Information Supported by grants from the British Heart FoundationADVANTAGES DISADVANTAGESGood optical resolution ‐ 2–5μm Frame rate – although the range is 12–200 fps, increasing frame rate compromises the imaging window in the y axis onlyAccessibility to tissue – small probe size and flexible cable connection to laser scanning unit Prerequisite to make contact with imaged tissue – causes damage to the tissue, distortion of blood vesselsPotential for longitudinal studies Inability to do z stacks