Adrenergic control in three artery segments of diminishing diameter in rabbit ear

The adrenergic neuroeffector mechanism has been assessed in three different-sized vessels of the rabbit ear, i.e., the central ear artery (CEA), which has an unstretched lumen diameter (ULD) of approximately 300 micron; a main side branch (MSB) that comes off the CEA immediately distal to the crossover position of the central vein (ULD approx 150 micron); and a terminal branch (TB) that originates from the MSB (ULD approximately 75 micron). The vessels receive adrenergic innervation limited to the adventitia as assessed by fluorescence histochemistry. Density of innervation, as indexed by neuronal uptake of tritiated norepinephrine (NE) and NE content expressed in terms of surface area of the vascular segment, appears greater in the larger vessel than in the small vessels. The neurogenic contractile response of isolated segments in relation to the maximum response to iota-NE was significantly greater in CEA than in the smaller vessels; this paralleled the neuronal density. Phentolamine (10(-6) M) blocked neurally evoked contractions of ear vessels at 2 Hz and depressed the contraction at least 90% at 4 and 8 Hz. All arterial segments constricted in response to histamine and NE with equal maximal effects. Sensitivity to NE appeared equal, whereas that to histamine appeared greater in the smaller vessels. Only the two smaller vessels possessed intrinsic tone. On the basis of the substantial innervation and neurogenic response of the vessels studied, (substantive) sympathetic control of vascular tone occurs in the three different-sized vessels of the rabbit ear, a fact that may be related to the temperature regulation role of that vascular bed.