NOREPINEPHRINE INCREASES NA-CA EXCHANGE IN RABBIT ABDOMINAL-AORTA

Na-Ca exchange was measured as intracellular Na+ (Na(i)+)-dependent Ca-45 uptake in rabbit abdominal aortic rings. The amount of Na(i)+-dependent Ca-45 uptake was proportional to both the concentration of Na+ in the Na+-loading solution and the concentration of Ca2+ in the assay medium. Na(i)+-dependent Ca-45 uptake was inhibited by incorporation of Na+ in the assay medium and by amiloride analogues. Norepinephrine significantly enhanced the rate of Ca-45 uptake in Na+-loaded tissue but had no effect on Na(i)+-independent Ca-45 uptake. The effect of norepinephrine was prevented by phentolamine but not by propranolol. The stimulatory effect of norepinephrine was absent when the concentration of extracellular Ca2+(Ca(o)2+) was 0.3 mM or lower but became significant at 0.6 mM and higher. Na-Ca exchange was also increased by phorbol 12,13-dibutyrate but not by its inactive analogue (4-alpha-phorbol 12,13-didecanoate). 1-(5-Isoquinolinylsulfonyl)-2-methylpiperazine, an inhibitor of protein kinase C, blocked the stimulatory effect of norepinephrine on Na-Ca exchange. It is suggested that alpha-adrenoceptor stimulation increases Na-Ca exchange in rabbit abdominal aorta in a Na(i)+- and Ca(o)2+-dependent fashion. This effect is possibly mediated through the activation of protein kinase C.