1 The action of 'selective' agonists and antagonists at muscarinic receptors mediating ileal contractions, and the rate and force of atrial contractions has been assessed. 2 The effect of nicotinic receptor stimulation, catecholamine release and acetylcholinesterase (AChE) action on muscarinic activity has also been assessed. 3 The nicotinic actions of carbachol did not affect its agonist potency nor the antagonist affinity data obtained when this agonist was used in atrial and ileal preparations. 4 Antagonist data indicated that muscarinic receptors mediating the rate and force of atrial contractions did not differ. Differences in agonist potencies at these two muscarinic receptors were attributable to either differences in intrinsic efficacy or susceptibility to the action of acetylcholinesterase. The small differences in agonist potency observed between atrial and ileal muscarinic receptors were considered not sufficient to indicate receptor heterogeneity. 5 The pirenzepine affinity data indicated that all three receptors are of the M2 type. Affinity data using secoverine and 4-diphenyl-acetoxy-N-methyl piperidine methiodide indicated that ileal and atrial muscarinic receptors differ. Data obtained using gallamine, pancuronium and stercuronium cannot be regarded as indicative of receptor affinity since the antagonism is not competitive; it did nonetheless corroborate the conclusion that ileal and atrial muscarinic receptors are different.

1 RS-15385-197 ((8aR, 12aS, 13aS)-5,8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(methylsulphonyl RS-15385-197. 5 RS-15385-197 was non-selective for the a2A-and M2B-adrenoceptor subtypes in that the pKi for the aX2A-adrenoceptor in human platelets was 9.90 and the pKi for the x2B-adrenoceptor in rat neonate lung was 9.70. However, RS-15385-197 showed lower affinity for the m2-adrenoceptor subtype in hamster adipocytes (pKi 8.38). 6 In anaesthetized rats, RS-15385-197 was a potent antagonist of the mydriasis response induced by UK-14,304 or clonidine (AD50 5 and 7 pg kg-', i.v., respectively; 96 1g kg-', p.o.) and of induced pressor responses in pithed rats (ADI0 7 pg kg-', i.v.); the compound therefore is both centrally and orally active. Even at a high dose (10mg kg-', i.v.), RS-15385-197 did not antagonize pressor responses to cirazoline in pithed rats, indicating that the selectivity for a2 vs. xl-adrenoceptors was maintained in vivo.8 RS-15385-197 is therefore a very potent, selective, competitive m2-adrenoceptor antagonist, both in vitro and in vivo, is orally active and readily penetrates the brain. It will thus be a powerful pharmacological tool for exploring the various physiological roles of aX2-adrenoceptors.

1 The effects of x2-adrenoceptor agonists and antagonists on rat tail skin temperature (t,,), an indicator of local cutaneous blood flow, were studied in conscious and anaesthetized rats and in the isolated, Krebs perfused, vascular bed of the rat tail.2 In conscious rats, at an ambient temperature of 18.5-20'C, t,, was 21.0 ± 0.2'C and core (rectal) temperature (tJ) was 38.2 ± 0.04'C (n = 126). The M2-adrenoceptor antagonist, delequamine (RS-15385-197 per dose).3 The maximum increases in t,, in response to a dose of 1 mg kg', s.c. of z2-adrenoceptor antagonists were as follows (n = 6 per drug): delequamine (+ 9.6 ± 0.8'C)> yohimbine (+ 9.0 ± 1.0C) > WY-26703 (+ 7.9 ± 1.3C)> piperoxan (+ 5.6 ± 1.7C)> idazoxan (+ 4.6 ± 1.3C)> imiloxan (+ 4.1 ± 1.3C)> SKF 104078 (+ 2.0 ± 1.9°C) > BDF-6143 (+ 1.3 ± 0.8°C).4 Prazosin (0.3 mg kg-', s.c.), hydralazine (10 mg kg-', s.c.) and nifedipine (3 mg kg', s.c.) did not increase tt,, whereas propranolol (10 mg kg-', s.c.) evoked a small increase in tt, (+ 2.9 ± 1.0C). Pentolinium (2-10 mg kg-', s.c.) elicited a dose-related increase in t,,, which was elevated by 4.4 ± 1.3°C after a dose of 10 mg kg-'; tc was reduced in a dose-related manner. Drug vehicles (1 ml kg'-, s.c.) had no effect on t,, or tc.5 In anaesthetized rats, idazoxan (300 pg, i.v.) produced a rapid increase in t,, which was detectable 2 min after beginning the injection, reaching a peak after 7 min. When the same dose was administered i.c.v., tt, also rose, but more slowly. The peak response (+ 3.6 ± 0.70C, n = 5) was significantly smaller than when idazoxan was administered intravenously (+ 6.3 ± 1.2°C, n = 5), which suggests that the increase in tt, following systemic administration of M2-adrenoceptor antagonists is not due to a central effect. The change in tt, was not secondary to changes in blood pressure. 6 In the isolated, Krebs perfused, tail vascular bed of the rat, at an ambient temperature of 20-21C, under constant flow conditions (3.5-4.0 ml min-'; n = 4), baseline perfusion pressure was 57 ± 4 mmHg. 5-Hydroxytryptamine (5-HT; 70-150 nM) increased perfusion pressure by 56± 9 mmHg. The M2-adrenoceptor agonist, UK-14,304 (10 nmol), elicited a further increase in perfusion pressure by 27.5 ± 15 mmHg but had no effect in the absence of 5-HT; this response to UK-14,304 was abolished by rauwolscine (1 JAM). 7 Under constant pressure conditions (-I100 mmHg; n = 9), baseline mean perfusion flow was 2.1 ± 0.2 ml min-', and mean tail skin temperature was 31.6 ± 0.6C. 5-HT (119 ± 28 nM) decreased t. by 3.3 ± 2.0°C and reduced flow by 1.2 ± 0.3 ml min-'. UK-14,304 (10 nmol) further reduced t,, by 3.0 ± 0.3°C without significant effect on flow; this effect was also abolished by 1 JAM rauwolscine.8 We conclude that post-junctional M2-adrenoceptors in the vasculature of the rat tail have a major vasoconstrictor role, controlling both the flow and distribution of blood within the tail and thereby thermoregulatory heat loss from its surface.

1 RS-45041-190 (4-chloro-2-(imidazolin-2-yl)isoindoline) showed high affinity for 12 imidazoline receptors labelled by [3H]-idazoxan in rat (pKi = 8.66 ± 0.09), rabbit (pKi = 9.37 ± 0.07), dog (pKi = 9.32 ± 0.18) and baboon kidney (pKi = 8.85 ± 0.12), but had very low affinity for a2-adrenoceptors in rat cerebral cortex (pKi = 5.7 ± 0.09).2 RS-45041-190 showed low affinity for other adrenoceptors, dopamine, 5-hydroxytryptamine, and muscarinic receptors and dihydropyridine binding sites (selectivity ratio>1000). 3 RS-45041-190 showed moderate potency for the inhibition of monoamine oxidase A in vitro (pIC50= 6.12), but had much lower potency for monoamine oxidase B (pICs = 4.47), neither of which equated with its affinity for I2 receptors. 4 RS-45041-190 (0.001 to 3 mg kg-', i.v. and 1 ng-50 yg i.c.v.) had only small, transient effects on blood pressure and heart rate in anaesthetized rats. In conscious rats, RS-45041-190 had no effect on body core temperature or tail skin temperature (1 mg kg-', s.c.) or on activity or rotarod performance (10 mg kg-1, i.p.). There were also no effects on barbiturate sleeping time in mice after doses of 1-10 mg kg-, i.p.5 RS-45041-190 (10 and 25 mg kg-', i.p.) significantly increased food consumption in rats for up to 4 h after dosing, but unlike idazoxan (10 mg kg-, i.p.) did not increase water consumption. 6 RS-45041-190 is therefore a selective, high-affinity ligand at I2 imidazoline receptors and its hyperphagic effect may suggest a role for I2 imidazoline receptors in the modulation of appetite. However, in the absence of a selective agonist it is unclear whether this ligand is an agonist or an antagonist at I2 receptors.

The 5max is the number of fmoles of [3 4H]dihydroalprenolol which binds per mg of protein. A Bmu of 50 was determined for desipramine after 14 days of once daily dosing at 7.5 mg/kg ip.