Functional evidence equating the pharmacologically‐defined α_1A‐ and cloned α_1C‐adrenoceptor: studies in the isolated perfused kidney of rat

Abstract

1 The present study characterizes and classifies ax-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 al-adrenoceptor subtypes, native aIA-adrenoceptors in submaxillary gland of rat, and MIA-adrenoceptors in several other tissues of rat).2 Concentration-effect curves to noradrenaline in the presence of 5-methyl-urapidil were biphasic, indicating al-adrenoceptor heterogeneity. The al-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 aI-adrenoceptor subtype or subtypes was abolished by nitrendipine (1 JLM), thereby allowing characterization of the second, high affinity site for 5-methyl-urapidil.3 The following antagonists interacted competitively with noradrenaline at the al-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 with a,-adrenoceptors. Dehydroniguldipine acted as a competitive antagonist (pKB =9.0). Thus, the results with antagonists define the aI-adrenoceptor as an cXIA-adrenoceptor. 4 An agonist 'fingerprint' was constructed in the presence of nitrendipine to define further the aIA-adrenoceptor. The following order and relativity of agonist potency was obtained: cirazoline (1)t adrenaline (2)>noradrenaline (5) 6 The present study demonstrates that the MlA-adrenoceptor is the predominant a-adrenoceptor subtype mediating vasoconstrictor responses to exogenously administered noradrenaline in the isolated perfused kidney of rat. More importantly, OIA-adrenoceptors mediating vasoconstrictor responses to noradrenaline exhibited a pharmacological equivalency to the cloned bovine xic-adrenoceptor. Thus, definitive functional pharmacological data are provided for equating the two receptors and support results derived recently from molecular and radioligand binding studies.

Modulation of noradrenaline-induced microvascular constriction by protein kinase inhibitors

“…While all three can mediate vasoconstriction, their degree of involvement depends on the species and vascular bed under investigation (Vargas and Gorman 1995). Noradrenaline-induced constriction of rat mesenteric and renal resistance arteries does not involve α 2 -adrenoceptors and occurs predominantly if not exclusively via α 1A -adrenoceptors (Blue et al 1992(Blue et al , 1995Chen et al 1996Chen et al , 1997Ipsen et al 1997;Williams and Clarke 1995). The present study has used both types of microvessels to investigate modulation of noradrenaline-induced vasoconstriction by inhibitors of PKC (H7, staurosporine, bisindolylmaleimide I, Gö 6976), of tyrosine kinases (genistein, tyrphostin 23), and of the ERK cascade (PD 98059).…”

Effect of methoxamine on maximum urethral pressure in women with genuine stress incontinence: A placebo-controlled, double-blind crossover study

“…Among available agonists, methoxamine fits the selectivity requirements most closely. Methoxamine is selective for ␣ 1A -adrenoceptors over the ␣ 1B -adrenoceptor [Minneman et al, 1994;Blue et al, 1995], where it exhibits little or no activity. Buckner et al [1996] found methoxamine to be equipotent at ␣ 1D -adrenoceptors of the rat aorta and ␣ 1A -adrenoceptors in the rat vas deferens, whereas Minneman et al [1994], using cloned bovine ␣ 1A -and rat ␣ 1D -adrenoceptors, reported 25-fold ␣ 1A -selectivity.…”

Is α1D-adrenoreptor protein detactable in rat tissues?

“…For example α 1A -adrenoceptor protein is found in rat cerebral cortex, heart, vas deferens and kidney while α 1B -adrenoceptor protein is found in rat cerebral cortex, heart and kidney as well as in liver and spleen where it apparently is the only α 1 -adrenoceptor subtype at the protein level. At the functional level, contraction of rat vas deferens (Han et al 1987; and mesenteric (Chen et al 1996) and renal resistance vessels (Blue et al 1995) is mainly mediated by α 1A -adrenoceptors, while cardiac contraction (Michel et al 1994b) and phosphorylase activation in rat liver (Garcia-Sainz et al 1992) are mainly mediated by α 1B -adrenoceptors. To date only a single function has been attributed to α 1D -adrenoceptors in the rat, which is vascular smooth muscle contraction in the arterioles of cremaster muscle (Leech and Faber 1996) or in the aorta Deng et al 1996;Saussy et al 1996;van der Graaf et al 1996); however, even this has been controversial (Vargas and Gorman 1995).…”