effect of cocaine on kinetics of α₁-adrenergic contractile response in different portions of the rat vas deferens

The effect of cocaine (10 µM) on the kinetics of contractile response to noradrenaline (NA) was studied in the rat epididymal and prostatic vas deferens. Cocaine caused an acute increase in vas deferens adrenergic sensitivity primarily due to blockage of NA neuronal capture. The presynaptic action prevailed in the epididymal half: the EC 50 value decreased 32-fold with a slight increase of the maximum adrenergic response more evident in the prostatic half. In the presence of cocaine, the prostatic contraction to NA was mediated not by a single pool of α₁-adrenoceptors like in epididymal vas deferens but by two. Its high affinity pool had the same affinity as α₁-adrenoceptors of the epididymal half, the affinity value of the low one was 36-fold less, and the total maximal response of both pools increased 4.5-fold. The differences in cocaine effect on the rat epididymal and prostatic vas deferens contractions to NA appear to be caused by the different sources of Ca²⁺ involved in these responses.