EFFECTS OF CHRONIC NICOTINE TREATMENT ON NICOTINIC RECEPTORS IN THE RABBIT URINARY-BLADDER

Nicotine induced a phasic contraction in the rabbit urinary bladder. The response was abolished by hexamethonium and partially reduced by atropine and capsaicin. Simultaneous atropine and capsaicin treatment did not abolish the contraction. These findings suggest that the response to nicotine is due to acetylcholine, tachykinins, and unknown mediator release. In contrast, nicotine-induced contraction diminished following the chronic nicotine treatment without a change of its pharmacological properties. These results suggest the possibility that chronic nicotine treatment causes a decrease in nicotinic receptor numbers. Therefore, the binding properties of (-)-[3H]nicotine on rabbit urinary detrusor muscle membrane fractions were studied to evaluate the effects of chronic nicotine treatment on nicotinic receptor. Specific (-)-[3H]nicotine binding reached saturation and Scatchard plots were curvilinear, suggesting the existence of two different affinity sites for (-)-[3H]nicotine. Dissociation constants (K(D)) and maximum binding sites (B(max)) were K(D1) = 4.91 ± 1.88 nM, B(max1) = 2.42 ± 0.22 fmol/mg protein and K(D2) = 263 ± 56 nM, B(max2) = 25.0 ± 4.3 fmol/mg protein. In urinary bladder membrane fractions from chronic nicotine-treated rabbits, K(D) and B(max) values were K(D1) = 3.96 ± 0.38 nM, B(max1) = 1.07 ± 0.25 fmol/mg protein and K(D2) = 249 ± 12 nM, B(max2) = 10.8 ± 1.5 fmol/mg protein. Dissociation constants for both sites following chronic nicotine treatment did not change but maximum binding site numbers for both sites significantly decreased (p < 0.05). These results suggest that the decrease in contractile response evoked by nicotine after chronic nicotine treatment in rabbit urinary bladder is due to a decrease in numbers of nicotinic receptors.