Evidence for functional alpha 2D-adrenoceptors in the rat intestine

1. Peristaltic contractions were induced in segments of rat ileum by raising the intramural pressure. A mean pressure of 4.3 +/- 0.2 cmH2O (n = 112) was required to initiate rhythmic contractions of the longitudinal muscle (preparatory phase) and associated volume expulsions caused by circular muscle contraction. The frequency of peristalsis remained constant over two 15 min periods of stimulation. 2. The alpha 2-adreoceptor agonists, clonidine, UK 14,304, B-HT 920, B-HT 933 and the selective alpha 2A-agonists, oxymetazoline and guanfacine, caused concentration-dependent inhibition of peristaltic contractions. The potency order and IC50 values (nM) were: clonidine (2.81) > or = oxymetazoline (4.23) > or = UK 14,304 (4.48) > or = guanfacine (5.51) > B-HT 920 (78.72) > B-HT 933 (442.48). 3. High concentrations of clonidine, amounting to more than 10 and 100 times the IC50 value in the peristaltic reflex (30 and 300 nM respectively), failed to inhibit the cholinergic contractile response to transmural electrical stimulation over the range of 2.5 to 40 Hz. 4. alpha 2-Adrenoceptor antagonists were used to determine the subtype of presynaptic alpha 2-adrenoceptor in rat ileum. All the antagonists tested caused parallel shifts to the right of the concentration-response regression line to clonidine and none, except ARC 239, influenced the rate of peristalsis. However, ARC 239 significantly decreased the frequency of control period peristaltic contractions. The order of affinity of the antagonists against clonidine (pKB values in parentheses) was RX 821002 (8.99) > phentolamine (8.07) > BRL 44408 (7.43) > or = rauwolscine (7.41) > or = yohimbine (7.28) > prazosin (5.86) > or = ARC 239 (5.74). 5. These results, when compared with binding and functional data from various other tissues and cell lines, are consistent with the presence of presynaptic alpha 2D-adrenoceptors in rat ileum. Further evidence is provided that this subtype of alpha 2-adrenoceptor is probably located proximal to the final cholinergic neurones in the reflex arc.