UCSF

Persons exposed to nicotine develop tolerance to many of its effects. When heart rate and forearm venous blood concentration are plotted against time after intravenous administration of nicotine, a greater increase in heart rate is seen for a given nicotine concentration during the rising phase of nicotine concentrations than during the decreasing phase. This could be due to acute tolerance or to more rapid distribution of drug to effect site (brain) than to venous blood. To distinguish between these possibilities, six rabbits were given nicotine intravenously. Blood samples were taken from the internal jugular vein (reflecting brain concentration), and the femoral vein and artery. Brain concentrations peaked before femoral venous concentrations. Seven men received intravenous infusions of nicotine. Peripheral venous blood concentrations and cardiovascular responses were measured. Heart rate peaked before venous concentrations. A physiological kinetic model, fit to the rabbit data, was scaled to humans and used to predict "brain" concentrations in them. Heart rate and predicted brain concentrations peaked simultaneously. We conclude that the rapid development of tolerance to the cardioaccelerating effect of nicotine can be attributed, at least in part, to its distribution kinetics.