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

1 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). 2 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. 3 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. 4 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). 5 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). 6 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.