Molecular and mechanistic heterogeneity of the Na(+)-Ca2+ exchanger

Extracellular sodium concentration has diverse effects on the hypoxia-induced increase in intracellular Ca2+ in rat hippocampal slices

The intracellular free Ca2+ concentration ([Ca2+]i) in the CA1 pyramidal cell layer was measured using fura-2-loaded hippocampal slices prepared from adult rats. Hypoxia (oxygen-glucose deprivation) elicited a gradual increase in [Ca2+]i in normal Krebs solution. With a high extracellular sodium concentration ([Na+]o), the hypoxia-induced response was attenuated, its onset-latency was longer and the time constant of its decay phase was shorter than in controls. In contrast, hypoxia in low [Na+]o elicited a significantly enhanced response with a short onset-latency and delayed decay phase. This exaggerated response to hypoxia in low [Na+]o was reversed by pre-incubation of the slice in low [Na+]o prior to the hypoxic insult. Some possible mechanisms and the functional significance of the observed effects of [Na+]o on the hypoxia-induced increase in [Ca2+]i are discussed, with particular emphasis on the putative participation of the glutamate transporter and the sodium-calcium exchanger in hypoxia-induced neuronal injury.

Characteristics of Na(+)-Ca2+ exchange in frog skeletal muscle

1. Fluxes studies were carried out to investigate the Na(+)-dependent outward movement of Ca2+ in intact frog sartorius muscle from Leptodactylus ocellatus, a preparation for which published data on the subject are sparse. 2. Under normal resting conditions the Na(+)-Ca2+ exchange was not readily detectable. 3. When muscles were exposed to 4 mM caffeine, the rate of fractional loss of Ca2+ (kCa,o) increased by about 50%. Most of this increase exhibits characteristics typical of the Na(+)-Ca2+ antiport working in the forward mode found in other cells. 4. The increase in kCa,o promoted by caffeine was decreased by: (a) 72% in the absence of external Na+ (Nao+); (b) 73% in Na(+)-loaded muscles ([Na+]i = 98 mM); (c) 70% when fibres were depolarized to -27 mV ([K+]o = 50 mM); and (d) 80% in the presence of 5 mM amiloride. 5. Ni2+ (5 mM), an inhibitor of the Na(+)-Ca2+ exchanger current, unexpectedly increased the caffeine-promoted rise in kCa,o. This effect of Ni2+ was associated with a concomitant caffeine-stimulated Ni2+ influx. In the absence of caffeine Ni2+ did not affect kCa,o. 6. It was concluded that: (a) under resting conditions the sarcolemmal Ca2+ pump suffices to handle the cytosolic calcium concentration ([Ca2+]i); (b) Na(+)-Ca2+ activity becomes apparent when [Ca2+]i is substantially increased by caffeine-induced Ca2+ release from the sarcoplasmic reticulum; and (c) the blocking effect of Ni2+ on the current generated by a Na(+)-Ca2+ exchange with a coupling ratio > 2 might actually represent a shift of the antiport mode toward an electroneutral 1 Ni(2+)-1Ca2+ exchange.

Cardiac sarcolemmal Na/Ca-inhibiting peptides XIP and FMRF-amide also inhibit Na/Ca exchange in squid axons

The effect of two cardiac sarcolemmal inhibitory peptides, the 20-amino acid exchange inhibitory peptide (XIP) and the molluscan cardioexcitatory tetrapeptide amide Phe-Met-Arg-Phe-NH2 (FMRFa), were tested in dialyzed squid giant axons. XIP injected into axons causes a maximal inhibition of 52 +/- 8% (n = 6) in the external Na (Nao)-dependent Ca efflux. The inhibitory effect was the same in axons dialyzed with saturating intracellular Ca (Cai) concentration (100 microM) and no MgATP or in axons containing submicromolar Cai concentrations (0.7 microM) and 2 mM MgATP. FMRFa, a peptide that shows no obvious homology with XIP, also causes a marked inhibition in Nao-dependent Ca efflux. As in cardiac sarcolemmal vesicles, the peptide inhibits with low apparent affinity (Ki = 1.9 microM; n = 5). Like XIP, FMRFa has the same effect in axons dialyzed with or without MgATP. The data indicate that XIP, which resembles an endogenous calmodulin binding site that may have an autoregulatory function, and the tetrapeptide FM-RFa, which binds to a putative opiate site, both inhibit Na/Ca exchange in squid axons. The sites at which these peptides bind are not related to the nucleotide (MgATP) regulation of Na/Ca exchange. We therefore suggest that these two sites in the vertebrate cardiac Na/Ca exchange are conserved in the invertebrate axon exchanger.

Regional distribution in the rat central nervous system of a mRNA encoding a portion of the cardiac sodium/calcium exchanger isolated from cerebellar granule neurons

The cardiac Na+/Ca2+ exchanger is a bidirectional electrogenic ion transporter that exchanges three Na+ ions for each Ca2+ ion and plays a critical role in returning sarcolemma Ca2+ concentrations to their resting levels. Because of the importance that the Na+/Ca2+ exchanger may play in maintaining neuronal Ca2+ homeostasis in the central nervous system, we subcloned a 456 bp portion of the Na+/Ca2+ exchanger cDNA from RNA isolated from primary cultures of rat cerebellar granule neurons using the polymerase chain reaction (PCR). This cDNA fragment was sequenced and shown to share 91.4% sequence identity with the human and 88% sequence identity with the canine cardiac Na+/Ca2+ exchangers. The PCR amplification product was used to analyze the distribution of this portion of the Na+/Ca2+ exchanger mRNA in various regions of the CNS by both Northern blotting and in situ hybridization histochemistry. The Northern analysis showed that the rank order of abundance of this mRNA was: hippocampus > cortex > cerebellum > hypothalamus > midbrain > striatum. The in situ hybridization data indicated that the corresponding mRNA containing this portion of the exchanger was present in numerous brain regions including multiple cortical layers, the hippocampus, septal nuclei, various thalamic nuclei, cerebellum, hypothalamus, olfactory bulb, brainstem, in various regions of the thoracic spinal cord and to a lesser extent in the striatum. The differential distribution of the mRNA as revealed by the in situ hybridization pattern suggests that either additional molecular variants exist or that different Na+/Ca2+ exchange mechanisms may be operative in those cell types that contain low amounts of this fragment of the exchanger mRNA.