Decrease in neuronal uptake of noradrenaline simply explains the supersensitivity after sympathectomy in the rat iris dilator.

The effect of superior cervical ganglionectomy or long term treatment with guanethidine on the mechanical response of dilator muscle of the rat iris to several stimulants was examined. The dose-response curve for noradrenaline (NA) was significantly shifted to the left 7 days after denervation (ca. x 10) or by treatment with guanethidine (ca. x 10), but those for methoxamine (Meth) and acetylcholine (ACh) were not. Cocaine did not further sensitize the denervated muscle to NA, but it sensitized the normal muscle to an extent similar to that caused by denervation. Cocaine did not affect the sensitivity to Meth in both control and denervated muscles. The maximum response to NA decreased by about 30% or 20% after surgical or chemical denervation, respectively, while those to Meth, ACh and Ca2+ did not alter significantly. These results indicate that in the dilator muscle, the sympathetic denervation caused a purely specific supersensitivity to NA which was simply explained by a presynaptic mechanism: an impairment of the neuronal uptake process.