ROLE OF EXTRACELLULAR CALCIUM IN CONTRACTIONS PRODUCED BY ACTIVATION OF POSTSYNAPTIC ALPHA-2 ADRENOCEPTORS IN THE CANINE SAPHENOUS-VEIN

  • 1 January 1985
    • journal article
    • research article
    • Vol. 234  (1) , 161-165
Abstract
Canine saphenous vein (CSV) contains both postsynaptic .alpha.-1 and .alpha.-2 adrenoceptors. Activation of postsynaptic .alpha.-1 adrenoceptors in this tissue utilizes both extracellular and intracellular Ca2+ to produce contractions. The source of Ca mobilized by activation of postsynpatic .alpha.-2 adrenoceptors in CSV was elucidated. Contractions of tissue rings to the supramaximal concentrations of 3 selective .alpha.-2 agonists, B-HT 920, M-7 [5,6-dihydroxy-2-dimethylaminotetralin] and clonidine, were determined in the absence and presence of 5 mM La3+. In the presence of La3+, clonidine and M-7 produced small but statistically significant contractions (8-14% of control) which were abolished when the .alpha.-1 adrenoceptors were inactivated by phenoxybenzamine (10-7 M, 30 min). Contractions of B-HT 920 were abolished completely in the presence of La3+. All the 3 .alpha.2 agonists stimulated 45Ca2+ uptake into CSV (0.3-0.4 mmol/kg wet weight, 10 min). 45Ca2+ efflux studies demonstrated that the selective .alpha.-2 agonist, B-HT 920 (10-5 M plus 10-7 M phenoxybenzamine), did not induce an increase in the rate of 45Ca2+ efflux. An augmented 45Ca2+ efflux rate was observed with the .alpha.-1 agonist, phenylephrine (10-4 M plus 10-7 M rauwolscine). Activation of postsynaptic .alpha.-2 adrenoceptors in CSV apparently utilizes primarily extracellular Ca2+ to produce contractions.