Abstract
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. 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.). When NA was applied for shorter time periods, both tension and stimulated 45Ca efflux decreased similarly. Repetitive short (30 s) NA applications resulted in repeated contractions and stimulations of 45Ca efflux. The NA-stimulated 45Ca efflux was not inhibited when external Ca2+ was present or in Na+-free medium. 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. 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 mumol kg-1 wet weight, corresponding to an s.r. Ca2+ concentration of about 3.1 mM. 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.