Pathophysiological and pharmacological mechanisms of acute cocaine toxicity in conscious rats

Abstract

In conscious rats, continuous i.v. infusion of cocaine (2 mg/kg/min) produced a marked increase in blood pressure, an initial moderate increase followed by a decrease in heart rate, tonic-clonic convulsions and, finally, a lethal episode of status epilepticus. No change in rectal temperature was observed. Infusion of cocaine methiodide (2 mg/kg/min), a quaternary derivative of cocaine, also produced a lethal episode of status epilepticus, but it was 6 times less potent than cocaine on a molar basis. In pentobarbital-anesthetized, spontaneously breathing rats, cocaine produced death by respiratory failure. Artificial ventilation of pentobarbital-anesthetized rats elevated the lethal dose of cocaine by 15-fold and these animals died of marked hypotension. In conscious rats, pretreatment with dl-, d- or l-propranolol or the alpha 2-selective adrenoceptor antagonist yohimbine enhanced the convulsive and lethal effects of cocaine. In contrast, the alpha 2-selective adrenoceptor agonist clonidine or the alpha 1-selective adrenoceptor antagonist prazosin attenuated these effects. Yohimbine antagonized the protective effect of clonidine. The nonselective alpha adrenoceptor antagonist phentolamine, the autonomic ganglionic blocker chlorisondamine and various calcium channel blockers had no effect on the convulsive or lethal doses of cocaine. The pressor response to cocaine was attenuated by calcium channel blockers, clonidine, phentolamine and dl- or l-propranolol, but not by d-propranolol. The pressor response to cocaine was abolished by chlorisondamine, reversed to a depressor response by prazosin and enhanced by yohimbine. The initial tachycardiac response to cocaine was reversed to bradycardia by dl- and l-propranolol, prazosin, yohimbine or high doses of the calcium channel blockers, but was unaffected by phentolamine, d-propranolol, clonidine or chlorisondamine. These results indicate that in spontaneously breathing animals, acute i.v. infusions of lethal doses of cocaine produce death primarily by central effects, namely by status epilepticus in conscious rats and by respiratory arrest in pentobarbital-anesthetized rats. In artificially ventilated, pentobarbital-anesthetized rats, however, cocaine produces death by effects on the cardiovascular system. In conscious rats, endogenous alpha 1 adrenoceptors exert a deleterious influence on cocaine-induced convulsive and lethal effects, whereas alpha 2 adrenoceptors provide protective influence. Propranolol appears to enhance cocaine-induced acute lethality through a mechanism independent of beta adrenoceptors. Calcium channel blockers appear ineffective in antagonizing cocaine's lethality.