The canine internal carotid system: regional differences in reactivity to nerve stimulation, noradrenaline and 5-hydroxytryptamine.

Reactivity of the canine internal carotid system to field electrical stimulation, noradrenaline and 5-hydroxytryptamine was determined isometrically using 4-mm cylindrical rings from the cervical and cavernous portions and middle cerebral artery. Electrical stimulation at supramaximal voltage (0.5-4 Hz, 0.2 ms duration) produced a frequency-dependent contraction that was similar in cervical and cavernous portions and lower in the middle cerebral artery. This response was blocked by tetrodotoxin (10(-6) M) or phentolamine (10(-6) M). Noradrenaline (10(-9) - 3 x 10(-4) M) produced concentration-dependent contractions and the sensitivity was: cervical portion = cavernous portion greater than middle cerebral artery, and this effect was blocked by phentolamine (10(-6) M). 5-hydroxytryptamine (10(-9) - 10(-5) M) caused concentration-dependent contractions and the sensitivity was: cervical portion less than cavernous portion less than middle cerebral artery. This response was depressed by lysergic acid diethylamide (10(-6) M). The results indicate that reactivity of the internal carotid system: (a) to alpha adrenergic activation by endogenous and exogenous noradrenaline is lower in the distal segment (middle cerebral artery) than in the proximal segments (cervical and cavernous internal carotid artery) and (b) to tryptaminergic activation increases as it courses toward the brain. Therefore, functional characteristics could be related to embryological origin which the segments of this vascular system are derived from.