1. 1. The constrictor response of isolated, perfused rabbit carotid artery to intra- (IN) and extra-(EX) luminal norepinephrine (NE, 5 × 10-3 to 10-1 mM) stimulation was studied. 2. 2. Maximal resistance to perfusion (R) was calculated and found to be equal for both IN and EX NE stimulation. 3. 3. However, ED50 for EX NE was increased compared to that for IN NE. 4. 4. In order to analyze the phasic components of the response to NE, the change in R during the first 30 sec of the NE response (initial ΔR) and the ΔR occurring during the remainder of the NE response (ΔR + 30) were determined and values for IN and EX NE were compared. 5. 5. The initial ΔR was greater for IN NE while ΔR + 30 was greater for EX NE. 6. 6. The IN NE response and the EX NE response were additive, but both completely blocked by phentolamine administered to either the luminal or adventitial side of the vessel. 7. 7. These data suggest that smooth muscle at the luminal and adventitial sides of rabbit carotid artery have different sensitivities to NE and respond to NE with different phasic characteristics.
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