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1 Department of Pharmacology and Physiology, UMDNJ - NJ Medical School, Newark, NJ, USA
* To whom correspondence should be addressed. E-mail: reeves{at}umdnj.edu.
La3+ uptake was measured in fura-2 loaded Chinese hamster ovary cells expressing the bovine cardiac Na+/Ca2+ exchanger (NCX1.1). La3+ was taken up by the cells after an initial lag phase of 50-60 s and achieved a steady-state within 5-6 min. Neonatal cardiac myocytes accumulated La3+ in a similar manner. La3+ uptake was due to the activity of the exchanger, since no uptake was seen in non-transfected cells, or in transfected cells that had been treated with gramicidin to remove cytosolic Na+. The low rate of La3+ uptake during the lag period resulted from insufficient cytosolic Ca2+ to activate the exchanger at its regulatory sites, as shown by the following observations. La3+ uptake occurred without a lag period in cells expressing a mutant of NCX1.1 that does not exhibit regulatory activation by cytosolic Ca2+. The rate of La3+ uptake by wild-type cells was increased, and the lag phase was reduced or eliminated, when the cytosolic Ca2+ concentration was increased prior to initiating La3+ uptake. La3+ could substitute for Ca2+ at very low concentrations to activate exchange activity. Thus, pre-loading cells expressing NCX1.1 with a small quantity of La3+ increased the rate of exchange-mediated Ca2+ influx by 20-fold; in contrast, cytosolic La3+ partially inhibited Ca2+ uptake by the regulation-deficient mutant. Based on an estimated KD of 30 pM for the binding of La3+ to fura-2, we conclude that cytosolic La3+ activates exchange activity at picomolar concentrations. We speculatively suggest that endogenous trace metals might activate exchange activity under physiological conditions.
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