Functional evidence equating the pharmacologically‐defined α1A‐ and cloned α1C‐adrenoceptor: studies in the isolated perfused kidney of rat

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Abstract
The present study characterizes and classifies α1‐adrenoceptor‐mediated vasoconstriction in the isolated perfused kidney of rat using quantitative receptor pharmacology and compares the results to radioligand binding studies (made in clonedαl‐adrenoceptor subtypes, nativeα1A‐adrenoceptors in submaxillary gland of rat, andα1A‐adrenoceptors in several other tissues of rat). Concentration‐effect curves to noradrenaline in the presence of 5‐methyl‐urapidil were biphasic, indicating αl‐adrenoceptor heterogeneity. Theαl‐adrenoceptor subtype mediating the first phase (low affinity for 5‐methyl‐urapidil) could not be ‘isolated’ for detailed pharmacological characterization but was defined by a sensitivity to inhibition by chloroethylclonidine and an inability of methoxamine to activate the site. Additionally, vasoconstriction mediated by thisα1‐adrenoceptor subtype or subtypes was abolished by nitrendipine (1 μm), thereby allowing characterization of the second, high affinity site for 5‐methyl‐urapidil. The following antagonists interacted competitively with noradrenaline at theα?l‐adrenoceptor for which 5‐methyl‐urapidil exhibits high affinity (pKB value): WB 4101 (10.3)>prazosin (9.5) ∼ HV 723 (9.3) ∼ 5‐methyl‐urapidil (9.2)> phentolamine (8.6)> spiperone (pA2 = 8.1) ∼ toxymetazoline (7.9). In contrast, insurmountable antagonism was seen with S(+)‐ and R(−)−niguldipine, the S(+)‐isomer being approximately 30 fold more potent than the R(−)−isomer. Receptor protection experiments indicated that S(+)‐niguldipine interacted directly withα1‐adrenoceptors. Dehydroniguldipine acted as a competitive antagonist (pKB = 9.0). Thus, the results with antagonists define theα1‐adrenoceptor as anα1A‐adrenoceptor. An agonist ‘fingerprint’ was constructed in the presence of nitrendipine to define further theα1A‐adrenoceptor. The following order and relativity of agonist potency was obtained: cirazoline (1) ∼ adrenaline (2)> noradrenaline (5)> phenylephrine (23) ∼ amidephrine (31)> methoxamine (71)>> isoprenaline (1456) ∼ dopamine (2210). A high correlative association was shown between the affinity of antagonists obtained functionally in the isolated perfused kidney of rat and pKi values obtained from binding experiments with the cloned bovineαlC‐adrenoceptor (R2 = 0.85), nativeαlA‐adrenoceptors in submaxillary gland of rat (R2 = 0.79), andα1A‐adrenoceptors from several other tissues of rat (values taken from the literature, R2 = 0.89). The present study demonstrates that theα1A‐adrenoceptor is the predominant α‐adrenoceptor subtype mediating vasoconstrictor responses to exogenously administered noradrenaline in the isolated perfused kidney of rat. More importantly, α1A‐adrenoceptors mediating vasoconstrictor responses to noradrenaline exhibited a pharmacological equivalency to the cloned bovineα1C‐adrenoceptor. Thus, definitive functional pharmacological data are provided for equating the two receptors and support results derived recently from molecular and radioligand binding studies.