Specific Role of α2A- and α2B-, but not α2C-, Adrenoceptor Subtypes in the Inhibition of the Vasopressor Sympathetic Out-flow in Diabetic Pithed Rats

Abstract

Several lines of evidence have shown an association of diabetes with a catecholamines' aberrant homeostasis involving a drastic change in the expression of adrenoceptors. This homeostatic alteration includes, among other things, atypical actions of α2-adrenoceptor agonists within central and peripheral α2-adrenoceptors (e.g. profound antinociceptive effects in diabetic subjects). Hence, this study investigated the pharmacological profile of the α2-adrenoceptor subtypes that inhibit the vasopressor sympathetic out-flow in streptozotocin-pre-treated (diabetic) pithed rats. For this purpose, B-HT 933 (up to 30 μg/kg min) was used as a selective α2-adrenoceptor agonist and rauwolscine as a non-selective α2A/2B/2C-adrenoceptor antagonist; in addition, BRL 44408, imiloxan and JP-1302 were used as subtype-selective α2A-, α2B- and α2C-adrenoceptor antagonists, respectively (all given i.v.). I.v. continuous infusions of B-HT 933 inhibited the vasopressor responses induced by electrical sympathetic stimulation without affecting those by i.v. bolus injections of noradrenaline in both normoglycaemic and diabetic rats. Interestingly, the ED50 for B-HT 933 in diabetic rats (25 μg/kg min) was almost 1-log unit greater than that in normoglycaemic rats (3 μg/kg.min). Moreover, the sympatho-inhibition induced by 10 μg/kg min B-HT 933 in diabetic rats was (i) abolished by 300 μg/kg rauwolscine or 100 and 300 μg/kg BRL 44408; (ii) partially blocked by 1000 μg/kg imiloxan; and (iii) unchanged by 1000 μg/kg JP-1302. Our findings, taken together, suggest that B-HT 933 has a less potent inhibitory effect on the sympathetic vasopressor responses in diabetic (compared to normoglycaemic) rats and that can probably be ascribed to a down-regulation of α2C-adrenoceptors.