In vivo pressure measurement

Animals were anesthetized using a face mask with 2–4% isoflurane in air, and the delivery was adjusted in response to respiratory rate. Recovery animals underwent tail‐cuff plethysmography while under anesthetic to determine mean arterial pressure (Buñag and Butterfield 1982) prior to MRI. In animals undergoing terminal studies, the right internal carotid artery was exposed and a pressure transducer (Model SPR‐320, Millar Instruments, Houston, TX) was inserted and advanced to the aortic arch prior to MRI. Mean arterial pressure was derived from pressure traces recorded at this location. Tail‐cuff plethysmography measurements were taken during recovery studies for all 3 month WKY (n = 6), all 3 month SHR (n = 8), all 14 month WKY (n = 4), and some of the 14 month SHR (n = 4). Arterial pressure transducer measurements were obtained during terminal studies for some 14 month SHR (n = 5), all of the 21 month SHR (n = 8), and all of the 21 month WKY (n = 6). At each time‐point, we obtained measurements from a sample of animals from each animal group. We re‐sampled 6 of 14 WKYs, but only 1 of 25 SHRs due to poor survival rate. In the SHR group, 13 of 34 animals died before they could be studied, which may have biased our SHR group results at 21 month toward healthier animals. In the WKY group, 2 of 18 animals died without being studied.