AN EXAMINATION OF THE SOURCES OF CALCIUM FOR CONTRACTIONS MEDIATED BY POSTJUNCTIONAL ALPHA-1-ADRENOCEPTORS AND ALPHA-2-ADRENOCEPTORS IN SEVERAL BLOOD-VESSELS ISOLATED FROM THE RABBIT

1. The roles of intracellular and extracellular-derived Ca2+ in α-adrenoceptor-mediated contractions to noradrenaline (NA) have been investigated in several isolated blood vessels from the rabbit by examining responses in the presence of a modified Krebs-Henseleit saline with 2.5 mM Ca2+ and a Ca2+-buffered saline with 0.1 μM free Ca2+. 2. NA was tested in preparations of the abdominal aorta, distal saphenous artery, renal vein, lateral saphenous vein, plantaris vein and ear vein exposed to a Ca2+-buffered saline with 0.1 μM [Ca2+]. A concentration of NA which was maximally effective in modified Krebs-Henseleit saline, produced an initial transient contraction (ITC) followed by a relaxation towards baseline. This is evidence that α-adrenoceptor-mediated responses in all these blood vessels depend upon calcium from both sources. 3. The ITC was particulary pronounced in the arteries and was associated more closely with the α1-receptor subtype. In the abdominal aorta, distal saphenous artery and renal vein the ITC can almost exclusively be attributed to an α1-adrenoceptor (prazosin-sensitive, rauwolscine-resistant). In the ear vein, and to a lesser extent the plantaris vein, the ITC was mediated in part by an α2-adrenoceptor (prazosin-resistant, rauwolscine-sensitive). 4. α2-Adrenoceptors in the lateral saphenous vein largely account for the response to NA in modified Krebs-Henseleit saline, but α1-adrenoceptors mediate the ITC in Ca2+-buffered saline. After selective inactivation of α1-adrenoceptor with a combination of phenoxybenzamine and rauwolscine, responses to NA in modified Krebs-Henseleit saline are slow in onset and there is no ITC in Ca2+-buffered saline. 5. The possible significance of the coupling of postjunctional α2-adrenoceptors to dual sources of Ca2+ is discussed in relation to the interaction between α-adrenoceptor subtypes and the ease of demonstrating functional α2-adrenoceptors in isolated blood vessels.