Pharmacological characterization of two distinct α1‐adrenoceptor subtypes in rabbit thoracic aorta
Authors: Oshita Masafumi , Kigoshi Shigeru , Muramatsu Ikunobu 
Published: July 2012

α1‐Adrenoceptor subtypes in rabbit thoracic aorta have been examined in binding and functional experiments.

[3H]‐prazosin bound to two distinct populations of α1‐adrenoceptors (pKD,high = 9.94, Rhigh = 79.2 fmol mg−1 protein; pKD,low = 8.59, Rlow = 215 fmol mg−1 protein). Pretreatment with chloroethylclonidine (CEC, 10 μm) almost inactivated the prazosin‐high affinity sites and reduced the number of the low affinity sites without changing the pKD value.

In the displacement experiments with CEC‐untreated membranes, unlabelled prazosin, WB4101 and HV723 displaced the binding of 200 pm [3H]‐prazosin monophasically; the affinities for WB4101 (pK1 = 8.88) and HV723 (8.49) were about 10 times lower than that for prazosin (9.99). In the CEC‐pretreated membranes also, the antagonists inhibited the binding of 1000 pm [3H]‐prazosin monophasically; the pK1 values for prazosin, WB4101 and HV723 were 9.09, 8.97 and 8.17, respectively. These results suggest that the prazosin‐high and low affinity sites can be independently appraised in the former and latter experimental conditions. Noradrenaline, but not methoxamine, showed slightly higher affinity for the prazosin‐high affinity site than for the low affinity site.

In the functional experiments, noradrenaline (0.001–100 μm) and methoxamine (0.1–100 μm) produced concentration‐dependent contractions. Pretreatment with CEC inhibited the contractions induced by low concentrations of noradrenaline but without effect on the responses to methoxamine. Prazosin inhibited the concentration‐response curves for noradrenaline in the CEC‐untreated aorta in a manner which was not consistent with competitive antagonism at a single site, and two distinct affinity constants (pKB = 9.71 and 8.74) were obtained. However, after CEC‐pretreatment, Schild plots for prazosin were not significantly different from unity (pKB = 8.50). WB4101 and HV723 competitively inhibited the noradrenaline‐induced contraction with low pKB values (approximately 8.30), irrespective of CEC‐pretreatment. Methoxamine‐induced contractions were competitively inhibited by prazosin, WB4101 and HV723 with low pKB values similar to those obtained when noradrenaline was used as the agonist. These were not affected by CEC‐pretreatment.

The affinity constant for noradrenaline (pKA = 6.40) in CEC‐untreated aorta was slightly greater than that obtained in CEC‐pretreated aorta (5.78). On the other hand, methoxamine showed a similar affinity in CEC‐untreated and pretreated aortae (pKA = approximately 4.5).

Nifedipine (1 μm) slightly attenuated the contractile responses to noradrenaline and methoxamine in CEC‐untreated and pretreated aortae, suggesting that nifedipine cannot discriminate between α1‐adrenoceptors involved in CEC‐sensitive and ‐resistant contractions.

From these results it is suggested that in the rabbit thoracic aorta there are two distinct α1‐adrenoceptor subtypes (presumably α1B and α1L subtypes according to recently proposed subclassification), both of which are involved in noradrenaline‐induced contraction. The α1L subtype predominantly mediates the contraction induced by methoxamine.