Different mechanisms of action of agents acting on β-adrenoceptors in barium-stimulated and electrically-stimulated rat vas deferens

Abstract

1. The relaxation induced by beta-adrenoceptor agonists in rat vas deferens was examined under two different experimental conditions: on electrically-induced twitch responses (35 V, 3 ms, 0.07 Hz), and on contractions induced by single doses of barium chloride (300 microM). The experiments were performed in vasa of reserpine-treated rats, after blockade of alpha-adrenoceptors and extraneuronal uptake with dibenamine (10 microM, 30 min), and neuronal uptake with cocaine (10 microM). 2. When twitch responses were used, the values of pD2, interpolated from cumulative concentration-response curves for isoprenaline (Iso), adrenaline (Ad), and noradrenaline (NA) showed a rank order of potency consistent with the presence of beta 2-adrenoceptors (Iso greater than Ad much greater than NA). 3. When twitch responses were used, the non-selective beta-antagonist, propranolol, caused a concentration-dependent parallel shift to the right of Iso concentration-response curves. Similar shifts were obtained by use of the beta 2-antagonist, isopropylmethoxamine (IMA), and higher doses of the beta 1-antagonist, practolol, according to the expectations from receptor occupation theory. Practolol presented the lowest value of pKB, 5.03, corroborating the presence of beta 2-adrenoceptors. 4. When twitch responses were used, and Ad or NA employed instead of Iso, the antagonists produced shifts of concentration-response curves which were smaller than expected from theory, precluding the determination of pKB values. This indicates that other mechanisms are involved besides an interaction with a single population of postsynaptic beta 2-adrenoceptors. 5. When barium chloride was used instead of twitch responses, although the potencies of Iso and Ad were increased respectively by about 30 fold and 5 fold, the rank order of potency was still consistent with an interaction with beta 2-adrenoceptors. In addition, the antagonists produced parallel and concentrationdependent shifts of the curves of all the agonists, as expected from receptor theory. The values of pKB for a given antagonist were not modified by interchanging the agonists used, indicating a typical interaction with a single population of beta 2-adrenoceptors. When compared to the field-stimulated vas, the values of pKB for propranolol and IMA against isoprenaline were respectively 1.3 and 0.6 log units larger. These results suggest the beta l-adrenoceptor agents act by different mechanisms of action in barium-stimulated and electrically-stimulated vas. 6. It is suggested that when barium is used, the effects of agents acting on beta l-adrenoceptors are mediated only by postsynaptic beta 2-receptors, while other complicating factors, probably nerve-dependent presynaptic mechanisms, may be involved with electrical stimulation.