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

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 (greater than 10 mumol/l, peptidase inhibitors present) inhibited the stimulation-evoked efflux of 3H-noradrenaline in a concentration-dependent manner, but not morphine up to 10 mumol/l. The inhibition by ethylketocyclazocine, MR 2033, and etorphine was antagonized by naloxone 1 mumol/l. Similarly, the MR 2033 effect was antagonized by SKF 10047 1 mumol/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 mumol/l) and (+)-MR 2267 (0.3-10 mumol/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 kappa-subtype) of opioid receptors is present at guinea-pig atrial noradrenergic nerves.