Tachyphylaxis and sensitization to nicotine-induced tachycardiac and pressor effects after nicotine infusions.

This work examined the effects of nicotine on mean arterial pressure and heart rate in non-anesthetized spinal rats. Nicotine (200 mg/kg) was administered as a single bolus, as infusions lasting 7.5, 15 or 30 min, and as a post-infusion bolus. A nicotine bolus increased pressure and rate. These effects were less marked as the rate of infusion decreased. The infusions affected differentially the effects of a subsequent bolus. Thus, while tachycardia was decreased, the blood pressure rise was increased. An initial transient bradycardia was observed after bolus administration, but not during infusions; this effect was unchanged after post-infusion boluses. Pharmacological analysis indicated that tachycardia and bradycardia were predominantly due to ganglionic stimulation, while adrenal and sympathetic nerve catecholamine release played a major role in the pressor response. These results indicate that slow nicotine infusions do not induce tachyphylaxis for all of the cardiovascular effects of a subsequent bolus, and that development of acute tolerance appears to depend on the mechanism of action of the response.