On the opioid receptor subtype inhibiting the evoked release of 3H-noradrenaline from guinea-pig atria in vitro

Abstract

Guinea-pig isolated atria were incubated and loaded with ³H-(−)-noradrenaline. The intrinsic nerves were stimulated with trains of 5 or 35 field pulses (4 Hz), and the evoked efflux of ³H-noradrenaline and of total tritium was determined in the presence of atropine, corticosterone, desipramine, and phentolamine by liquid scintillation spectrometry. Ethylketocyclazocine (1.4 nmol/l, IC₅₀), MR 2033 (9.1 nmol/l), dynorphin A (1–13) (25 nmol/l, peptidase inhibitors present), etorphine (71 nmol/l), and [d-Ala², d-Leu⁵]-enkephalin (>10 μmol/l, peptidase inhibitors present) inhibited the stimulation-evoked efflux of ³H-noradrenaline in a concentration-dependent manner, but not morphine up to 10 μmol/l. The inhibition by ethylketocyclazocine, MR 2033, and etorphine was antagonized by naloxone 1 μmol/l. Similarly, the MR 2033 effect was antagonized by SKF 10047 1 μmol/l. All antagonists investigated failed to affect the evoked ³H-noradrenaline efflux when present in the absence of exogenous agonists. Arunlakshana-Schild plots were calculated for the antagonism between ethylketocyclazocine and a pair of stereoisomers, (−)-MR 2266 (20 nmol/l–5 μmol/l) and (+)-MR 2267 (0.3–10 μmol/l) at the presynaptic opioid receptor, and pA₂ values were estimated. The isomeric affinity ratio was 60, with pA₂ values of (−)-MR 2266, 9.06, and (+)-MR 2267, 7.28, respectively. The results show that the ³H-noradrenaline release can be inhibited via activation of presynaptic opioid receptors. Under the conditions presently investigated endogenous opioids do not modulate the evoked transmitter release. The results favour the idea that a single population (presumably of the K-subtype) of opioid receptors is present at guinea-pig atrial noradrenergic nerves.