CARDIOVASCULAR-RESPONSE TO STRESS - BAROREFLEX RESETTING AND HEMODYNAMICS

Borderline hypertensive rats (BHR) were used to test the hypothesis th at baroreflex resetting prevents a fall in blood pressure (BP) when ca rdiac output (GO) is reduced during air-jet stress. Eight-week-old BHR were instrumented with flow probes around the ascending aorta for mea suring CO, femoral and jugular catheters were inserted for measurement of arterial pressure and infusion of drugs, and sinoaortic barorecept ors were either denervated (SAD) or left intact. Alternating bolus inj ections of phenylephrine and sodium nitroprusside were given at baseli ne and during air-jet stress to assess the baroreflex. Air-jet stress immediately shifted the midpoint of the baroreflex curve for heart rat e (HR) to a higher BP level. When metoprolol was administered during a ir-jet stress, HR was reduced and CO reverted to prestress levels, but the stress-induced presser response was not changed. In SAD BHR, air- jet stress caused an elevation of BP that was not different from intac t rats. Administration of metoprolol to SAD rats during air-jet stress resulted in a further elevation rather than a reduction in BP. We con clude that the sinoaortic cardiac baroreflex is reset during air-jet s tress and that it integrates reflex changes in BP during stress. The a rterial baroreflex is not, however, necessary for the initiation or ma intenance of the presser response during stress, nor does it prevent a fall in BP when CO is compromised during stress.