The effect of chloroethylclonidine on nodrenaline-induced contractions of the rat aorta was studied. Chloroethylclonidine (1.5 × 10-5 M) shifted noradrenaline dose-response curve to the right approximately 5000-fold without depressing the maximum. The response to noradrenaline after chloroethylclonidine was not antagonized by phenoxybenzamine (10-7 M), prazosin (10-7 M), WB 4101 (10-7 M) nor yohimbine (10-5 M) and is therefore not mediated via α1-adrenoceptors. These results would suggest that there is a homogenous population of chloroethylclonidine-sensitive α1-adrenoceptors in the rat aorta and that chloroethylclonidine treatment reveals a non-α-adrenoceptor noradrenaline binding site in this tissue.

Interaction with chlorethylclonidine has been used to subdivide populations of α1-adrenoceptors in some tissues. WB 4101 can distinguish high and low affinity states of the receptor. The present study was carried out to determine if different populations or affinity states of α1-adrenoceptors distinguished by either of these compounds, could explain the variation in α1-adrenoceptor agonist affinity found amongst rabbit arteries. Five arteries were studied whose affinity for noradrenaline vary between 4.8 and 6.4. These were the thoracic aorta, renal, superior mesenteric, ear and ovarian arteries. WB 4101 was found to be equally effective in antagonizing noradrenaline on all arteries. Chlorethylclonidine caused a 20-fold rightward shift of the noradrenaline dose-contraction curve in the thoracic aorta; but had little or no effect on the other vessels. Thus, the combination of different proportionsof subsets of α1-adrenoceptors distinguished by WB 4101 or chlorethylclonidine does not explain the variation in α1-adrenoceptor affinity found in these rabbit arteries.

The ability of derivatives of 2-aminotetralins (2AT), cis- or trans-isomers of octahydrobenzo[f]quinolines (BfQ) and clonidine to modulate acetylcholine release was studied using field-stimulated guinea pig ilea (GPI). Antihistaminic and antiacetylcholine activities were also determined using isolated superfused segments of GPI. Hydroxylated 2AT, BfQ and clonidine inhibited field stimulation-induced contractions through α-adrenoceptor mechanisms which were antagonized by phentolamine. In contrast, the inhibition produced by nonhydroxylated 2AT was not attenuated by α-adrenoceptor antagonism. 2AT, trans-7,8-dihydro-BfQ and cis-8,9-dihydroxy-BfQ inhibited contractions induced by nicotine bitartrate using superfused GPI. Clonidine was inactive as an antinicotinic agent and there was no correlation between a compound's ability to inhibit contractions induced by field stimulation and its antinicotinic activity. Various 2AT derivatives demonstrated weak antimuscarinic and/or antihistaminic activities on superfused ileal segments. These data demonstrate that these agents posses a spectrum of pharmacological activity.

The role of protein kinase C (PKC) in mediating contractile responses in the rat vas deferens was studied. Phorbol-12,13-di-acetate (PDA) in the presence of 20 mM K+ elicited a concentration-dependent response with an EC50 of 190 nM. The non-PKC activator 4α-phorbol (2 μM) was unable to elicit contraction in 20 mM K+ buffer. Incubation of rat vas deferens with the PKC inhibitor iso-H7 (30 μM) attenuated the response to norepinephrine (NE) ane neurokinin A, with maximal effects depressed to 42 and 39% of control, respectively. Responses to 60 mM K+ and 2 μM PDA (20 mM K+) was also significantly inhibited by iso-H7. In the presence of 2 μM PDA and 20 mM K+, the NE concentration-effect curve was shifted 3,6-fold to the right of the control curve in a parallel manner. 4α-Phorbol (20 mM K+) at the same concentration did not produce this effect. These results suggest a significant role for PKC in the contractile response of the rat vas deferens.