α-Adrenoceptor subtypes of dog saphenous vein: Another unusual property

The functional relationship between vascular smooth muscle alpha1- and alpha2-adrenoceptor (AR) subtypes was investigated by simultaneous measurement of contractile and fluorescence ratio in fura-2 loaded rings of dog saphenous vein (DSV). Prazosin, as well as rauwolscine, at 0.1 microM, substantially antagonized contractions and associated cytosolic [Ca2+] rises induced by UK 14304, while rauwolscine, as well as prazosin, antagonized similar effects of phenylephrine (PE). These antagonisms were characterized by a parallel rightward shift of the concentration-response curves. In the absence of extracellular Ca2+, PE as well as UK 14304 caused simultaneous transient elevation of contractile force and cytosolic [Ca2+], although the UK 14304 responses were smaller than PE responses. We propose that DSV smooth muscle cells possess interacting alpha1- and alpha2-ARs which have overlapping functional domain sensitive to the agonists and antagonists of either alpha-AR subtype. Both alpha-AR subtypes appear to utilize similar signaling mechanisms via Ca2+ release from the same intracellular stores and Ca2+ entry across the plasma membrane.