Factors affecting the tissue binding of nicotine in various species.

The binding of [¹⁴C]nicotine to various tissue fractions has been studied in the cat, pigeon and rat using the technique of equilibrium dialysis. Liver homogenates from all three species bound the drug to varying extents though the distribution of radioactivity showed species differences. Cat and rat liver binding appeared to reside predominantly in the microsomal fraction whereas in the pigeon the activity was essentially present in the cytosol. Lung tissue from all three species showed only a small binding capability which was not associated with the microsomes or soluble cell components. Brain homogenates in cat and rat showed a similar small degree of binding but there was a significant degree of binding in pigeon brain homogenate. Cat and pigeon kidney homogenates showed a small degree of tissue binding which appeared to be uniformly distributed in the cell fractions. Rat kidney, however, exhibited an extremely large in vitro binding capability which was apparently associated with the cytosol. The percentage binding was increased with increasing buffer pH over range pH 6–8 corresponding to an increase in the proportion of free base. A Scatchard plot over the concentration range 62–1540 nM gave a linear response, calculations from which indicated a low affinity, high capacity, binding capability for nicotine. In vivo distribution studies in the rat after subcutaneous administration of 0.4 mg/kg [¹⁴C]nicotine revealed a high degree of localisation of radioactivity in the kidney relative to other tissues and subsequent tissue fractionation confirmed the in vitro observations. Plasma from all three species showed no significant binding properties. The nature of the binding entity is not yet known though it can be concentrated by chromatography on Sephadex G-100 and is associated with a fraction of relatively low molecular weight containing little, if any, lipid.