FUNCTIONAL EVIDENCE EQUATING THE PHARMACOLOGICALLY-DEFINED ALPHA(1A)-ADRENOCEPTOR AND CLONED ALPHA(1C)-ADRENOCEPTOR - STUDIES IN THE ISOLATED-PERFUSED KIDNEY OF RAT

1 The present study characterizes and classifies alpha(1)-adrenoceptor-mediated vasoconstriction in the isolated perfused kidney of rat using quantitative receptor pharmacology and compares the results to radioligand binding studies (made in cloned alpha(1)-adrenoceptor subtypes, native alpha(1A)-adrenoceptors in submaxillary gland of rat, and alpha(1A)-adrenoceptors in several other tissues of rat). 2 Concentration-effect curves to noradrenaline in the presence of 5-methyl-urapidil were biphasic, indicating alpha(1)-adrenoceptor heterogeneity. The alpha(1)-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 this alpha(1)-adrenoceptor subtype or subtypes was abolished by nitrendipine (1 mu M), thereby allowing characterization of the second, high affinity site for 5-methyl-urapidil. 3 The following antagonists interacted competitively with noradrenaline at the alpha(1)-adrenoceptor for which 5-methyl-urapidil exhibits high affinity (pK(B) value): WB 4101 (10.3) > prazosin (9.5) approximate to HV 723 (9.3) approximate to 5-methyl-urapidil (9.2) > phentolamine (8.6) > spiperone (pA(2) = 8.1)approximate to oxymetazoline (7.9). In contrast, insurmountable antagonism was seen with S(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 with alpha(1)-adrenoceptors. Dehydroniguldipine acted as a competitive antagonist (pK(B) = 9.0). Thus, the results with antagonists define the alpha(1)-adrenoceptor as an alpha(1A)-adrenoceptor. 4 An agonist 'fingerprint' was constructed in the presence of nitrendipine to define further the alpha(1A)-adrenoceptor. The following order and relativity of agonist potency was obtained: cirazoline (1) approximate to adrenaline (2) > noradrenaline (5) > phenylephrine (23) approximate to amidephrine (31) > methoxamine (71)much greater than isoprenaline (1456) approximate to dopamine (2210). 5 A high correlative association was shown between the affinity of antagonists obtained functionally in the isolated perfused kidney of rat and pK(i) values obtained from binding experiments with the cloned bovine alpha(1C)-adrenoceptor (R(2) = 0.85), native alpha(1A)-adrenoceptors in submaxillary gland of rat (R(2) = 0.79), and alpha(1A)-adrenoceptors from several other tissues of rat (values taken from the literature, R(2) = 0.89). 6 The present study demonstrates that the alpha(1A)-adrenoceptor is the predominant alpha(1A)-adrenoceptor subtype mediating vasoconstrictor responses to exogenously administered noradrenaline in the isolated perfused kidney of rat. More importantly, alpha(1A)-adrenoceptors mediating vasoconstrictor responses to noradrenaline exhibited a pharmacological equivalency to the cloned bovine alpha(1A)-adrenoceptor. Thus, definitive functional pharmacological data are provided for equating the two receptors and support results derived recently from molecular and radioligand binding studies.