Poor vasoconstrictor response to adrenergic stimulation in the arteriovenous anastomoses present in the carotid vascular bed of young Yorkshire pigs.

Studies using electromagnetic flowmeters and radioactive labeled microspheres (15 +/- 1 micron) have revealed that about 80% of the total common carotid artery blood flow in young domestic pigs (12-14 weeks) is shunted through arteriovenous anastomoses (AVAs). To investigate the influence of the sympathetic nervous system we measured the distribution of carotid blood flow before and after cervical sympathetic nerve stimulation (2-16 Hz), intracarotid infusions of noradrenaline (0.025-0.2 micrograms X kg-1 X min-1) or intracarotid bolus injections of noradrenaline (0.3 and 1.0 micrograms X kg-1) and clonidine (1 and 5 micrograms X kg-1). The first two interventions did not exert any effect, but bolus injections of noradrenaline and clonidine, achieving high local concentrations temporarily, caused a short-lasting decrease in AVA-flow, which was attenuated by phentolamine (1 mg X kg-1). It is concluded that the sympathetic nervous system in the carotid vascular bed of young Yorkshire pigs is poorly developed.