Role of Na+/Ca2+ exchange in [Ca2+]i clearance in rat culture Purkinje neurons requires reevaluation

Previous studies have shown that in contrast to other neuronal cells, Na+/Ca2+ exchange contributes little to Ca-i(2+) homeostasis in rat cerebellar Purkinje neurons under intracellular perfused conditions and at room temperature [Fierro et al.: J Physiol (Lond) 510: 499-512, 1998]. The purpose of this study was to clarify the role of this transporter in cerebellar Purkinje neurons by using intact cells at nearly physiological body temperature. Using Fluo-3 microfluorometry, we have examined the role of the Na+/Ca2+ exchange in the buffering of calcium loads in cultured rat Purkinje neurons at two temperatures: 20 and 34 degreesC. At 20 degreesC, the recovery of the K+-induced [Ca2+](i) signal was little affected by the presence of external Na+ (tau(e) = 35.5+/-1.2 s [n=49]), or by its absence (tau(e) = 36.6+/-2.2 s [n=29]), i.e. in a Li+-containing medium. In contrast, at 34 degreesC, the recovery of the [Ca2+](i) signal was highly dependent on external Na, i.e. tau(e) = 19.9 +/- 1.2s (n = 119) and tau(e) = 41.7 +/- 2.6 s (n = 39), in Li+-containing media, respectively. A comparison of the rate of clearance of [Ca2+](i) in Na+ or Li+ media, shows that at a room temperature of 20 degreesC, the Na+/Ca2+ exchange contributes at most to 15-20% of the total [Ca2+](i) clearance, compared to 55-65% at 34-36 degreesC. We also demonstrate that under normal physiological conditions forward and reverse Na+/Ca2+ exchanges operate in the same neuron. We conclude that the Na+/Ca2+ exchange is strongly suppressed at room temperature and therefore its role should be reevaluated among different neuronal preparations.