Sodium/calcium exchange in smooth-muscle microsomal fractions

Abstract

1. The existence of Na+-dependent Ca2 + transport was investigated in microsomal fractions from the longitudinal smooth muscle of the guinea-pig ileum and from the rat aorta, and its activity was compared with that of the plasmalemmal ATPdependent Ca2+ pump previously identified in these preparations. 2. The rate of Ca2 + release from plasmalemmal vesicles previously loaded with Ca2 + through the ATP-dependent Ca2 + pump was transiently faster in the presence of 150 mM-NaCl in the medium than in the presence of 150mM-KCl or -LiCl or 300mM-sucrose. 3. Na+-loaded vesicles took up Ca2 + when an outwardly directed Na+ gradient was formed across the membrane. The Ca ionophore A23 187 induced a rapid release of 85% of the sequestered Ca2 +, whereas only 15% was displaced by La3 +. Ca2 + accumulated by the Na + -induced Ca2 + transport was released by the addition of NaCl, but not KCI, to the medium. 4. Ca2 + uptake in Na+-loaded vesicles was inhibited in the presence of increasing NaCl concentration in the medium. Half-maximum inhibition was observed with 28 mM-NaCl. Data fitted the Hill equation, with a Hill coefficient (h) of 1.9. 5. Na +-induced Ca2 + uptake was a saturable function of Ca2 + concentration in the medium. Half-maximum activity was obtained with 18 M-Ca2+ in intestinalsmooth-muscle microsomal fraction and with 50,uM-Ca2 + in aortic microsomal fraction. 6. The results suggest that in these membrane preparations a transmembrane movement of Ca2+ can be driven by a Nal gradient. However, the Na+-induced Ca2 + transport had a lower capacity, a lower affinity and a slower rate than the ATP-dependent Ca2 + pump.

Ion transport defects and hypertension. Where is the link?

“…Na + -Ca 2+ exchange has a well-established role in Ca 2+ metabolism in nerve 33 and cardiac muscle 34 where it was originally described. Although Na + -Ca 2+ exchange activity has been found by a number of investigators both in intact vascular smooth muscle cells 35 and in plasma membranes from these cells, 36 58 and other vessels. 56 Such pH,-induced changes in vascular tone may be mediated by effects of H on myosin ATPase activity 59 or on the Ca 2+ affinity for Ca 2+ binding proteins.…”

Regulation of cell calcium and contractility in mammalian arterial smooth muscle: the role of sodium‐calcium exchange.

“…Moreover, recent sarcolemmal vesicle studies demonstrate directly the presence of an Na-Ca exchange mechanism in various types of smooth muscle, including vascular smooth muscle (e.g. Daniel, Grover & Kwan, 1982;Morel & Godfraind, 1984;Casteels, Raeymaekers, Droogmans & Wuytack, 1985).…”

“…Two mechanisms mediate Ca2+ extrusion in many types of cells, including smooth muscle cells: an ATP-driven, calmodulin-regulated Ca2+ pump, and Na-Ca exchange (Carafoli, 1984; Blaustein, 1984;Morel & Godfraind, 1984;Sheu & Blausfein, 1986). The Na-Ca exchange system can move (net) Ca2+ in either direction across the plasmalemma, depending upon the prevailing Na+ electrochemical gradient, AftNa (cf.…”

Sodium‐dependence of the non‐specific desensitization of the guinea‐pig ileum induced by acetylcholine and histamine

“…A similar effect occurs in the action of histamine, but with less intensity than in the case of acetylcholine (Bolton, 1973 (Brading, 1981), it appears to occur in guinea-pig ileum (Morel & Godfraind, 1984 (Figure 2). The similar to responses to prostaglandin E2 were particularly sencoincides sitive to changes in the Na+ gradient (Figures 5,6a and the secon7c) and were severely impaired in preparations previously submitted to desensitizing treatment with either histamine or acetylcholine (Figure 3c).…”