VASCULAR RESPONSIVENESS IN THE UNSTRESSED BORDERLINE HYPERTENSIVE RAT

This study compares vascular responses of unstressed borderline hypert ensive rats (BHR) to age-matched Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Rings of thoracic aorta were mounted for isom etric force determinations in tissue baths. Cumulative additions of ph enylephrine (PE) or serotonin (5-HT) produced contractile responses in BHR aorta that were less than WKY but greater than SHR. Removing the endothelium increased force development to both agonists, with BHR sho wing the greatest increase. Based on EC(50) determinations, no differe nces in PE sensitivity existed between groups, although in response to 5-HT, rings from SKR were significantly more sensitive than WKY. Whil e the sensitivity of all rings increased with denuding, BHR showed a g reater relative increase in sensitivity than WKY or SHR (intact EC(50) divided by denuded EC(50)) to both PE and 5-HT. This suggests that th ere may be greater endothelium-derived relaxation in BHR than in WKY o r SHR. In rings precontracted with 1 mu M PE, the cumulative addition of acetylcholine resulted in greater relaxation in BHR than in SHR and WKY preparations. Thus, rings from unstressed BHR, which have only sl ightly higher blood pressures than WKY rats, demonstrate greater endot helium-mediated relaxation than WKY or SHR. Increased endothelium-deri ved relaxation may represent a possible mechanism for masking hyperten sion in BHR that is compromised as a result of exposure to a chronic s tress paradigm.