Discrimination between phenylephrine-triggered contractile events by atrial natriuretic factor in rat aorta

The effects of atrial natriuretic factor (ANF) on contractile events triggered by phenylephrine (PE) were investigated in rat aorta. Isometric contraction of endothelium-free rat aorta rings was recorded in Ca2+-free medium containing 1 mM EGTA. PE (1 microM) induced a phasic contraction and a sustained contraction following addition of Ca2+ (2.5 mM) to the medium. The phasic contraction was due to intracellular Ca2+ release whereas the sustained one was dependent on extracellular Ca2+ influx. ANF (1-3 nM) and prazosin (3-10 nM) both reduced the two types of contraction. However, during the sustained contraction, a rhythmic activity was observed when ANF, but not prazosin, was used as an inhibitory agent. The calcium antagonist diltiazem (0.1-1 microM) abolished this rhythmic activity, which was attributed to a Ca2+ influx through potential-dependent channels. The results indicate that ANF, unlike prazosin, may discriminate between PE-triggered contractile events in the rat aorta.