Comparison of bovine platelet and teat alpha 2-adrenoceptor characteristics by [3H]rauwolscine binding

Alpha 2-adrenoceptors on platelet membranes, isolated from "slaughterhouse cows," were characterized by [3H]-rauwolscine binding. There were 30 +/- 3 [3H]rauwolscine sites per platelet, corresponding to 194 +/- 18 fmol/mg protein. These had an equilibrium dissociation constant (KD) of 5.25 +/- 0.56 nM, a Hill coefficient of 1.00 +/- 0.01 (n = 9), and were competed for by adrenergic compounds stereoselectively. The rank order of catecholamines coincided with that expected for alpha 2-adrenoceptors. Ki-values for inhibition of specific [3H]rauwolscine binding by 7 agonists and 6 antagonists were in the same range as previously observed in teat tissue. It is therefore concluded that quite similar [3H]rauwolscine binding sites were identified and characterized in both bovine teats and platelets. Direct comparison of binding characteristics between alpha 2-adrenoceptors from teat tissue and platelets of the same "primiparous cows," revealed no difference in binding affinity, although a significant higher platelet receptor number was noted in the platelets (platelets: 353 fmol/mg protein; teat tissue: 91 fmol/mg protein; n = 10).