Characterization of postsynaptic alpha-1 and alpha-2 adrenoceptors in the feline saphenous and femoral veins [corrected].

Experiments were designed to characterize the effects mediated by alpha-1 and alpha-2 adrenoceptors in saphenous and femoral veins of the cat. Ring segments of saphenous and femoral veins were mounted for isometric tension recording in modified Krebs-bicarbonate solution, gassed with 95% O2-5% CO2 and maintained at 37 degrees C. Norepinephrine (a mixed alpha 1 and alpha 2 agonist), phenylephrine (a preferential alpha 1 agonist) and clonidine (a preferential alpha 2 agonist) caused dose (concentration)-dependent contractions in saphenous and femoral veins. The maximal contractions produced by clonidine were significantly less than those produced by norepinephrine or phenylephrine in both veins. However, threshold dose and EC50 values indicated that clonidine was more potent than norepinephrine and phenylephrine. Contractile responses to these agonists were attenuated when the veins were pretreated with alpha 1-or alpha 2-adrenoceptor antagonists, prazosin and yohimbine, respectively. The contractile responses to norepinephrine and tyramine were inhibited to a greater extent by yohimbine than by prazosin in both saphenous and femoral veins, suggesting that norepinephrine released from perivascular nerve terminals activates preferentially postsynaptic alpha 2-adrenoceptors. Further examination of alpha-adrenoceptor subtypes was achieved by comparing pA2 values of prazosin and yohimbine from Arunlakshana and Schild plots. Chronic sympathetic denervation by removing lumbar sympathetic chain significantly reduced the contractile responses evoked by tyramine. Denervation did not significantly affect the concentration-response curve to phenylephrine but significantly augmented the contractile responses evoked by clonidine in both veins.(ABSTRACT TRUNCATED AT 250 WORDS)