Abstract
α2-Adrenoceptors are G protein-coupled receptors (GPCRs) that are classified to three subtypes, α2A/D-, α2B- and α2C-subtypes. The α2D-subtype in rodent is a species homolog of the human α2A-subtype. Constriction of cutaneous veins mediated by α2-adrenoceptors plays an important role in thermoregulation. The aim of this work was to understand how cooling affects α2-adrenoceptormediated venous contraction using the isolated rat tail vein. A series of experimental conditions to study α2-adrenoceptor-mediated contraction of the rat tail vein were established in preliminary studies. Cooling from 37 ºC to 28 °C increased the potencies and maximal contractions to the α2-adrenoceptor agonists UK14304, guanabenz, BHT933, and dexmedetomidine; but not to other GPCR agonists including 5-HT, angiotensin II, arginine vasopressin and uridine-5’-triphospate. Thus cooling selectively increased contraction activated by α2-adrenoceptors. The affinities (KB values) of a panel of competitive α2-adrenoceptor antagonists (RX821002, non-selective; ARC239, α2B-subtype selective; MK912, α2C-subtype selective; rauwolscine, α2C-subtype selective and BRL44408, α2A/D-subtype selective) were not consistently altered by cooling. The affinities of these antagonists were consistent with the α2C-subtype causing contraction at both 37 ºC and 28 ºC. The affinity (KA value) of UK14304 was not changed by cooling. However the affinities (KP values) of guanabenz and BHT933 were increased by cooling, in parallel with a change in their potencies. Analysis of receptor occupancy-response relationships for guanabenz and BHT933 indicated that their increased potencies could be explained by increased affinities after cooling. Analysis using the operational model of drug-receptor theory found that cooling increased the relative efficacy of UK14304 but not that of guanabenz and BHT933, suggesting that the increased relative efficacy of UK14304 contributed to its increased potency. Other mechanisms possibly associated with increased contraction caused by cooling were also studied but the results were inconclusive. These studies include new receptors possibly translocated from intracellular receptor pool, receptor coupling to G proteins other than Gi proteins and involvement of Rho kinase activation. The rat tail vein is a useful model for screening α2C-adrenoceptor selective agents and for studying the thermoregulatory function of cutaneous veins.