Release and distribution of [3H]norepinephrine in nonpigmented and pigmented rabbit iris.

Unequal accumulation of drugs in pigmented and nonpigmented iris has been known for some time. However, at low concentrations of the sympathetic neurotransmitter norepinephrine, the accumulation is equal in both types of irides. The spontaneous and potassium-induced release of 3H was studied in isolated rabbit irides labeled with 0.1 micron [3H]norepinephrine ([3H]NE). The spontaneous outflow and the tissue 3H content were similar in both types of irides, but the potassium-induced release was significantly higher in nonpigmented iris. When irides were labeled with [3H]NE in the presence of 1 micron desmethylimipramine, the potassium-induced release was still significantly higher in nonpigmented than in pigmented iris, although the tissue 3H content was higher in the latter. Subcellular distribution of [3H]NE in both types of irides is presented. The data indicate that [3H]NE is distributed unequally in both types of irides in spite of equal tissue accumulation. The binding of NE by the pigment cell suggests an additional site of loss, or site of uptake, of the transmitter in melanin-containing tissues. The physiological significance of such an unequal release in relation to the evolutionary adaptation of the eye to light is discussed.