Mobilization of Stored Calcium for Phasic Contraction Induced by Norepinephrine in Rabbit Aorta

In Ca-free solution norepinephrine (NE) produced only a phasic contraction in the media-intima layer of rabbit aorta. The second application of NE was almost ineffective. Incubation of the muscle with Ca for a short period (Ca loading) restored the ability to produce a phasic contraction in Ca-free solution. Various ions and compounds were applied to the muscle prior to the Ca loading or prior to NE application and it was found that these treatments variously affected the NE-induced phasic contraction. The data suggest that the NE-induced phasic contraction in Ca-free solution results from Ca release from cellular sites and that a hyperbolic relationship exists between the amount of Ca at these sites and the magnitude of the contraction. These sites take up extracellular Ca by a process sensitive to La3+ but not to D600; the Ca influx stimulated by high K solution, which is sensitive to D600 and also contributes to refilling of these sites; NE releases this stored Ca by a process sensitive to procaine, caffeine and theophylline and, in this manner, elicits a phasic contraction.