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The relationship between noradrenaline‐induced contraction and 45Ca efflux stimulation in rabbit mesenteric artery

1 Cellular Ca2+ recycling in a branch of the rabbit mesenteric artery was investigated by measuring the time‐ and concentration‐dependent effects of noradrenaline (NA) on contraction and 45Ca efflux in Ca2+‐free solution. 2 When NA was present continuously (15 min), both force development and 45Ca efflux stimulation consisted of a fast and a slow (often oscillatory) component. These components were sensitive to caffeine and are probably both related to Ca2+ release from the intracellular Ca2+ store, presumably sarcoplasmic reticulum (s.r.). 3 When NA was applied for shorter time periods, both tension and stimulated 45Ca efflux decreased similarly. 4 Repetitive short (30 s) NA applications resulted in repeated contractions and stimulations of 45Ca efflux. 5 The NA‐stimulated 45Ca efflux was not inhibited when external Ca2+ was present or in Na+‐free medium. 6 Loading the cell with Ca2+ (with physiological salt solution for 3 h or with a high K+ depolarizing solution) increases the number of subsequent NA‐induced repeated contractions in Ca2+‐free solution. 7 The Ca2 + content of the sarcoplasmic reticulum (s.r.) in the smooth muscle cells of this small artery was estimated to be at least 50 μmol kg−1 wet weight, corresponding to an s.r. Ca2+ concentration of about 3.1 mM. 8 These results indicate that the NA‐induced increase in cytosolic free Ca2+ (as measured by force development) is accompanied by an increase in Ca2+ extrusion (as measured by stimulation of 45Ca efflux). This suggests that at least part of the activator Ca2+ cycles through the extracellular space during hormone‐induced activation of vascular smooth muscle.