| alpha2-Adrenoceptors are G protein-coupled receptors (GPCRs) that are classified to three subtypes, alpha2A/D-, alpha 2B- and alpha2C-subtypes. The alpha2D-subtype in rodent is a species homolog of the human alpha2A-subtype. Constriction of cutaneous veins mediated by alpha2-adrenoceptors plays an important role in thermoregulation. The aim of this work was to understand how cooling affects alpha2-adrenoceptor-mediated venous contraction using the isolated rat tail vein. A series of experimental conditions to study alpha 2-adrenoceptor-mediated contraction of the rat tail vein were established in preliminary studies. Cooling from 37°C to 28°C increased the potencies and maximal contractions to the alpha2-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 alpha2 -adrenoceptors. The affinities (KB values) of a panel of competitive alpha2-adrenoceptor antagonists (RX821002, non-selective; ARC239, alpha2B-subtype selective; MK912, alpha2C-subtype selective; rauwolscine, alpha2C-subtype selective and BRL44408, alpha 2A/D-subtype selective) were not consistently altered by cooling. The affinities of these antagonists were consistent with the alpha2C-subtype causing contraction at both 37°C and 28°C. 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 alpha 2C-adrenoceptor selective agents and for studying the thermoregulatory function of cutaneous veins. |
