SHR Y chromosome enhances the nocturnal blood pressure in socially interacting rats

Our objective was to test the hypothesis that nocturnal mean arterial press ure (MAP), heart rate (HR), and activity would be increased in 1) colony ov er individually caged rats and 2) the spontaneously hypertensive rat (SHR) Y chromosome strain (SHR/y colony) compared with Wistar-Kyoto (WKY) rats. M AP, HR, and activity were monitored using radiotelemetry. The nocturnal MAP rise expressed as the percentage change in MAP from light to dark was incr eased (P< 0.05) in the SHR/y colony. The SHR Y chromosome increased MAP in both the colony and caged groups compared with WKY (P< 0.001). The SHR/y co lony animals spent 23% of a 24-h period at a MAP >120 mmHg, whereas the WKY colony animals spent 2% of a 24-h period in this range. The MAP of the SHR /y colony on clonidine was reduced (P< 0.001) to WKY baseline values. Activ ity but not HR was increased (P< 0.01) in the WKY and SHR/y colonies compar ed with caged animals. In conclusion, colony housing and the SHR Y chromoso me increased MAP compared with individually caged housing.