An examination of the sources of calcium for contractions mediated by postjunctional α1‐ and α2‐adrenoceptors in several blood vessels isolated from the rabbit

Abstract

1. The roles of intracellular and extracellular-derived Ca2+ in alpha-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 microM 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 microM [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 alpha-adrenoceptor-mediated responses in all these blood vessels depend upon calcium from both sources. 3. The ITC was particularly pronounced in the arteries and was associated more closely with the alpha 1-receptor subtype. In the abdominal aorta, distal saphenous artery and renal vein the ITC can almost exclusively be attributed to an alpha 1-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 alpha 2-adrenoceptor (prazosin-resistant, rauwolscine-sensitive). 4. alpha 2-Adrenoceptors in the lateral saphenous vein largely account for the response to NA in modified Krebs-Henseleit saline, but alpha 1-adrenoceptors mediate the ITC in Ca2(+)-buffered saline. After selective inactivation of alpha 1-adrenoceptors 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 alpha 2-adrenoceptors to dual sources of Ca2 + is discussed in relation to the interaction between alpha-adrenoceptor subtypes and the ease of demonstrating functional alpha 2-adrenoceptors in isolated blood vessels.