Sodiumcalcium exchange in sarcolemmal vesicles from tracheal smooth muscle

Abstract

Sarcolemmal vesicles prepared by a new procedure from bovine tracheal smooth muscle were found to have a NaCa exchange activity that is significantly higher than that reported for different preparations from other types of smooth muscle. The exchange process system co-purified with 5′-nucleotidase, a plasma membrane marker enzyme, and was significantly enriched (over 100-fold) compared to mitochondria ( cytochrome-c oxidase) but only slightly enriched (4-fold) compared to sarcoplasmic reticulum ( NADPH-cytochrome-c reductase). The Na + dependence of Ca 2+ transport was demonstrated through both uptake and efflux procedures. The uptake profile with respect to Ca 2+ was monotonic with a linear ν o vs. ν o · S −1 plot. The resultant K m of Ca 2+ from the airway sarcolemmal vesicles (20 μM) was similar in magnitude to the K m of cardiac sarcolemmal vesicles (30 μM). Tracheal vesicles demonstrated a V max of 0.3–0.5 nmol · mg −1 · s − which is significantly higher than that reported in preparations from other smooth muscle types. Furthermore, two processes found to stimulate cardiac Na-Ca exchange, pretreatment with either a mixture of dithiothreitol and Fe 2+ or with chymotrypsin, were ineffective on the tracheal smooth muscle. Thus, the Na-Ca exchanger identified in tracheal smooth muscle appears to be different from that observed in cardiac muscle, implying that regulation of this activity may also be different.