Differential response to chloroethylclonidine in blood vessels of normotensive and spontaneously hypertensive rats: role of alpha 1D- and alpha 1A-adrenoceptors in contraction

The effects of chloroethylclonidine on alpha(1)-adrenoceptor-mediated contraction in endothelium-denuded caudal arteries and aorta from normotensive Wistar and Wistar Kyoto (WKY), and from spontaneously hypertensive (SHR) rats were evaluated. Chloroethylclonidine elicited concentration-dependent contractions. Maximal contraction was similar in caudal arteries among strains ( approximately 40% of noradrenaline effect). However, chloroethylclonidine elicited a higher contraction in aorta from SHR than from normotensive rats. In Wistar aorta chloroethylclonidine produced the smallest contractile response. In SHR aorta, BMY 7378 and 5-methylurapidil blocked chloroethylclonidine-elicited contraction, while (+)-cyclazocine did not inhibit it; while in caudal arteries, 5-methylurapidil blocked chloroethylclonidine action; the other antagonists had no effect. In chloroethylclonidine-treated aorta noradrenaline elicited biphasic contraction-response curves, indicating a high affinity (pD(2), 8.5 - 7.5) chloroethylclonidine-sensitive component and a low affinity (pD(2), 6.3 - 5.2) chloroethylclonidine-insensitive component. The high affinity component was blocked by chloroethylclonidine; while in caudal arteries noradrenaline elicited monophasic contraction-response curves with pD(2) values (6.5 - 5.7) similar to the low affinity component in aorta. Chloroethylclonidine inhibition of noradrenaline response was greater in aorta than in caudal arteries. Chloroethylclonidine increased the EC(50) values of noradrenaline approximately 1000 fold in aorta and approximately 10 fold in caudal arteries. In SHR aorta BMY 7378 protected alpha(1D)-adrenoceptors and in caudal arteries 5-methylurapidil protected alpha(1A)-adrenoceptors from chloroethylclonidine alkylation, allowing noradrenaline to elicit contraction. These results show marked strain-dependent differences in the ability of chloroethylclonidine to contract aorta; moreover, chloroethylclonidine stimulates alpha(1D)-adrenoceptors in aorta and alpha(1A)-adrenoceptors in caudal arteries. The higher contraction observed in aorta from SHR and WKY suggests an augmented number of alpha(1D)-adrenoceptors in these strains.

Direct block of native and cloned (Kir2.1) inward rectifier K+ channels by chloroethylclonidine

1. We have investigated the inhibition of inwardly rectifying potassium channels by the alpha-adrenergic agonist/antagonist chloroethylclonidine (CEC). We used two preparations; two-electrode voltage-clamp of rat isolated flexor digitorum brevis muscle and whole-cell patch-clamp of cell lines transfected with Kir2.1 (IRK1). 2. In skeletal muscle and at a membrane potential of -50 mV, chloroethylclonidine (CEC), an agonist at alpha2-adrenergic receptors and an antagonist at alpha1x-receptors, was found to inhibit the inward rectifier current with a Ki of 30 microM. 3. The inhibition of skeletal muscle inward rectifier current by CEC was not mimicked by clonidine, adrenaline or noradrenaline and was not sensitive to high concentrations of alpha1-(prazosin) or alpha2-(rauwolscine) antagonists. 4. The degree of current inhibition by CEC was found to vary with the membrane potential (approximately 70% block at -50 mV c.f. approximately 10% block at -190 mV). The kinetics of this voltage dependence were further investigated using recombinant inward rectifier K+ channels (Kir2.1) expressed in the MEL cell line. Using a two pulse protocol, we calculated the time constant for block to be approximately 8 s at 0 mV, and the rate of unblock was described by the relationship tau=exp((Vm+149)/22) s. 5. This block was effective when CEC was applied to either the inside or the outside of patch clamped cells, but ineffective when a polyamine binding site (aspartate 172) was mutated to asparagine. 6. The data suggest that the clonidine-like imidazoline compound, CEC, inhibits inward rectifier K+ channels independently of alpha-receptors by directly blocking the channel pore, possibly at an intracellular polyamine binding site.

Analysis of alpha 1L-adrenoceptor pharmacology in rat small mesenteric artery

1. To illuminate the controversy on alpha 1A- or alpha 1L-adrenoceptor involvement in noradrenaline-mediated contractions of rat small mesenteric artery (SMA), we have studied the effects of subtype-selective alpha 1-adrenoceptor agonists and antagonists under different experimental conditions. 2. The agonist potency order in rat SMA was: A61603 >> SKF89748-A > cirazoline > noradrenaline > ST-587 > methoxamine. Prazosin antagonized all agonists with a low potency (pA2: 8.29-8.80) indicating the involvement of alpha 1L-rather than alpha 1A-adrenoceptors. 3. The putative alpha 1L-adrenoceptor antagonist JTH-601, but not the alpha 1B-adrenoceptor antagonist chloroethylclonidine (10 microM) antagonized noradrenaline-induced contractions of SMA. The potency of the selective alpha 1D-adrenoceptor antagonist BMY 7378 against noradrenaline (pA2 = 6.16 +/- 0.13) and of the selective alpha 1A-adrenoceptor antagonist RS-17053 against noradrenaline (pKB = 8.35 +/- 0.10) and against the selective alpha 1A-adrenoceptor agonist A-61603 (pKB = 8.40 +/- 0.09) were too low to account for alpha 1D- and alpha 1A-adrenoceptor involvement. 4. The potency of RS-17053 (pKB/pA2's = 7.72-8.46) was not affected by lowering temperature, changing experimental protocol or inducing myogenic tone via KCl or U46619. 5. Selective protection of a putative alpha 1A-adrenoceptor population against the irreversible action of phenoxybenzamine also failed to increase the potency of RS-17053 (pA2 = 8.25 +/- 0.06 against A61603). 6. Combined concentration-ratio analysis demonstrated that tamsulosin, which does not discriminate between alpha 1A- and alpha 1L-adrenoceptors, and RS-17053 competed for binding at the same site in the SMA. 7. In summary, data obtained in our experiments in rat SMA indicate that the alpha 1-adrenoceptor mediating noradrenaline-induced contraction displays a distinct alpha 1L-adrenoceptor pharmacology. This study does not provide evidence for the hypothesis that alpha 1L-adrenoceptors represent an affinity state of the alpha 1A-adrenoceptor in functional assays. Furthermore, there is no co-existing alpha 1A-adrenoceptor in the SMA.

Characterization of alpha1-adrenoceptor subtypes mediating vasoconstriction in human umbilical vein

1. The present study attempted to characterize pharmacologically the subtypes of alpha-adrenoceptors mediating contractions in human umbilical vein (HUV). 2. HUV rings were mounted in isolated organ baths and cumulative concentration-response curves were constructed for the alpha-adrenoceptor agonists phenylephrine and adrenaline. Adrenaline was more potent than phenylephrine (pD2=7.29 and 6.04 respectively). 3. Isoproterenol exhibited no agonism on KCl pre-contracted HUV rings. Propranolol (1 microM) and rauwolscine (0.1 microM) did not affect the concentration-response curves to adrenaline. These results demonstrate the lack of involvement of functional beta-or alpha2-adrenoceptors in adrenaline-induced vasoconstriction. 4. The non subtype selective alpha1-adrenoceptor antagonist prazosin was evaluated on phenylephrine and adrenaline concentration-response curves. The effects of the competitive alpha1A and alpha1D-adrenoceptor antagonists, 5-methyl urapidil and BMY 7378 and the irreversible alpha1B selective compound chloroethylclonidine (CEC) were also evaluated on adrenaline concentration-response curves. 5. The potencies of prazosin against responses mediated by adrenaline (pA2= 10.87) and phenylephrine (pA2= 10.70) indicate the involvement of prazosin-sensitive functional alpha1-adrenoceptor subtype in vasoconstriction of the HUV. 6. The potencies of 5-methyl urapidil (pA2 = 6.70) and BMY 7378 (pA2= 7.34) were not consistent with the activation of an alpha1A- or alpha1D-adrenoceptor population. 7. Exposure to a relatively low CEC concentration (3 microM) abolished the maximum response to adrenaline suggesting that this response was mediated by an alpha1B-adrenoceptor subtype. 8. We conclude that HUV express a prazosin-sensitive functional alpha1-adrenoceptor resembling the alpha1B-subtype according with the low pA2 values for both 5-methyl urapidil and BMY 7378 and the high sensitivity to CEC.

Molecular cloning, expression and characterization of cDNA encoding a mouse alpha1a-adrenoceptor

1. In this study, we have cloned, expressed, and characterized an alpha1a-adrenoceptor gene from the mouse. We designed oligonucleotide PCR primers complementary to regions of the rat alpha1a-adrenoceptor sequence and amplified cDNA fragments from total RNA of mouse cerebral cortex, liver and kidney by reverse transcription-polymerase chain reaction (RT PCR). 2. Both the nucleotide and deduced peptide sequences of the cDNA showed high sequence identity with those of cloned alpha1a-adrenoceptors from other species. The cDNA clone had an open reading frame of 1398 nucleotides encoding a 466 amino acid peptide which had 97%, 92% and 90% identity with the deduced amino acid sequences of the rat, human and bovine alpha1a-adrenoceptor, respectively. 3. The amplified mouse cDNA was inserted into a mammalian expression vector pcDNA3.1(+) and expressed in COS-1 cells. The pharmacological properties of the mouse cDNA clone were examined in radioligand binding studies and functional assays. The expressed mouse protein had a high affinity for [3H]-prazosin (Kd = 0.48 nM) and pattern of affinity for antagonists in competition studies that is similar to that of the rat alpha1a-adrenoceptor. Chloroethylclonidine (CEC) could slowly alkylate the expressed protein, with a rate similar to that of the rat alpha1a-adrenoceptor. 4. The expressed receptors were able to mediate noradrenaline (NA) stimulation of the production of inositol phosphates in COS-1 cells, consistent with coupling to phospholipase C. This response to NA could be reversed by pretreatment of the transfected cells with prazosin. 5. Based on the above evidence, we concluded that the cloned cDNA is that of the mouse alpha1a-adrenoceptor.

Pharmacological classification of alpha 1-adrenoceptors mediating contractions of rabbit isolated ear artery: comparison with rat isolated thoracic aorta

1. The present study attempted to classify pharmacologically the alpha 1-adrenoceptor subtype(s) present in two isolated, vascular ring preparations, the rabbit ear artery and rat thoracic aorta. 2. In the ear artery, the agonist effects of phenylephrine were antagonized by 5-methyl urapidil (pA2 = 7.90; Schild slope = 0.85) and BMY 7378 (pA2 = 6.11; Schild slope = 0.80) but not in a simple competitive manner. The shallow Schild slopes are consistent with the activation of a heterogeneous receptor population. Indeed the 5-methyl urapidil data set could be fitted to a two-receptor model yielding a high antagonist affinity (pKBH) estimate of 7.85 and a low affinity (pKBL) estimate of 6.03. 3. The effects of clonidine in the ear artery were competitively antagonised by 5-methyl urapidil (pKB = 7.91) and BMY 7378 (pKB = 5.53). These data are consistent with contractions to clonidine being mediated by a single receptor subtype. 4. In the aorta, the effects of phenylephrine were antagonized by 5-methyl urapidil (pA2 = 7.95; Schild slope = 1.11) and BMY 7378 (pA2 = 9.08; Schild slope = 0.73). Neither data set was consistent with a simple competitive interaction. The BMY 7378 data suggested again, that phenylephrine was acting at a heterogeneous receptor population. Subsequent analysis by the two-receptor model yielded a high affinity (pKBH) estimate of 8.95 and a low affinity (pKBL) estimate of 7.00. 5. The alkylating agent, chloroethylclonidine (CEC) elicited concentration-dependent contractions in the ear artery with a potency (p[A]50) of 5.57. Pretreatment of this tissue with CEC (5 microM, 30 min incubation) had no effect on subsequent responses to phenylephrine. In contrast, in the aorta, CEC demonstrated no agonism but pretreatment with this agent (5 microM, 15 min incubation) caused a rightward shift and depression of subsequent phenylephrine concentration-effect curves. 6. The affinity of clonidine in the rabbit ear artery (pKA = 6.17) was found to be significantly different to its affinity in the rat thoracic aorta (pKA = 7.12) suggesting that this agonist activates different alpha 1-adrenoceptor subtypes in the two tissues. 7. These results suggest that heterogeneous populations of alpha 1-adrenoceptors are present in both tissues. In the ear artery, the profile of antagonist and agonist activity is most consistent with alpha 1A-adrenoceptors being the predominant receptor subtype. The second receptor population does not appear to correspond to any of the recognized alpha 1-adrenoceptor subtypes. In the aorta alpha 1D-adrenoceptors appear to predominate, with alpha 1A-adrenoceptors being the most likely candidate for the second receptor population.