Unloading of baroreceptors by carotid occlusion does not increase heart rate in fetal sheep

Unloading of baroreceptors by carotid occlusion does not increase heart rate in fetal sheep; Objectives: To test the hypothesis that in fetal sheep reduction of carotid sinus pressure by carotid occlusion increases heart rate. Study design: Fetal sheep (gestational age 121-132 days) were chronically instrumented with bilateral carotid occluders, catheters and electrodes (ECG) to measure systemic arterial and carotid sinus (CSP) blood pressures, and fetal heart rate. Results: Bilateral carotid occlusion (BCO) increased mean arterial blood pressure from 46+/-7 mmHg to 53+/-8 mmHg (mean+/-S.D.) while CSP decreased from 44+/-7 mmHg to 17+/-7 mmHg. Fetal heart rate fell during occlusion significantly from 186+/-34 bpm to 159+/-26 bpm (n=20 animals). Infusion of phenylephrine (8.5-20 microg min(-1)kg(-1)) or methoxamine (60-200 microg min(-1)kg(-1)) increased mean blood pressure from 44+/-6 to 61+/-9 mmHg, and fetal heart rate decreased from 186+/-30 to 132+/-31 bpm (n=12). BCO increased systemic arterial pressure further to 70+/-11 mmHg whereas carotid sinus pressure was reduced to 31+/-13 mmHg. However, average heart rate did not increase significantly (136+/-28 bpm). Conclusion: We conclude that in contrast to adult animals, in fetal sheep carotid occlusion with subsequent unloading of baroreceptors does not increase heart rate even when the baroreflex had been activated by arterial hypertension. It seems likely that stimulation of carotid chemoreceptors prevents the expected baroreceptor mediated heart-rate response.