Effect of episodic eucapnic and hypocapnic hypoxia on systemic blood pressure in hypertension-prone rats.

Repetitive episodic (18-24 s twice per minute) hypocapnic hypoxia (HH) administered chronically (7 h/day, 35 days) to Sprague-Dawley or Wistar-Kyoto rats results in a sustained increase in daytime blood pressure (BP). We examined acute and chronic BP response to episodic HH and eucapnic hypoxia (EH) in borderline hypertensive rats [first generation (F1) cross between spontaneously hypertensive and Wistar-Kyoto rats]. We hypothesized that episodic HH and EH would create a greater increase in acute and chronic BP in this breed of rat than in previously studied strains. We also examined neural mechanisms by which BP changes from hypoxia are induced. BP and heart rate were examined acutely in nine F1 rats during baseline, HH, EH, EH with prazosin, and EH with prazosin and atropine. Five groups of male F1 rats were studied after 35-day exposure to the following: Unhandled (n = 8): no treatment; Sham (n = 10): episodic compressed air; HH (n = 14): daily episodic hypoxia (2.7%); EH1 (n = 12); hypoxia 2.9%, CO2 8.4%; and EH2 (n = 11): hypoxia 2.8% and CO2 10.5%. Under acute conditions, HH caused a 34.2-mmHg and EH a 77.9-mmHg increase in mean BP. Prazosin partially blocked the increase in BP. Under chronic conditions, HH caused a 10.3-mmHg increase in daytime mean BP, whereas EH caused a fall in mean BP of 16.6 and 9.3 mmHg in the two separately studied groups. In the F1 rat, acute EH causes an elevation of BP but chronic EH causes a fall in BP. The acute response to EH is not predictive of what occurs after chronic exposure in the hypertension-prone F-1 rat.