Studies on heart sarcolemma: vesicles of opposite orientation and the effect of ATP on the Na+/Ca2+ exchanger

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

Structural and functional aspects of calcium homeostasis in eukaryotic cells

The maintenance of a low cytosolic free-Ca2+ concentration, ([Ca2+]i) is a common feature of all eukaryotic cells. For this purpose a variety of mechanisms have developed during evolution to ensure the buffering of Ca2+ in the cytoplasm, its extrusion from the cell and/or its accumulation within organelles. Opening of plasma membrane channels or release of Ca2+ from intracellular pools leads to elevation of [Ca2+]i; as a result, Ca2+ binds to cytosolic proteins which translate the changes in [Ca2+]i into activation of a number of key cellular functions. The purpose of this review is to provide a comprehensive description of the structural and functional characteristics of the various components of [Ca2+]i homeostasis in eukaryotes.

Subfractionation of cardiac sarcolemma with wheat-germ agglutinin

The properties of highly purified bovine cardiac sarcolemma subfractionated with the lectin, wheat-germ agglutinin (WGA) were studied. Two different membrane subfractions were isolated, one which was agglutinated in the presence of 1.0 mg of WGA/mg of protein (WGA+ vesicles) and a second fraction which failed to agglutinate (WGA- vesicles). These two membrane fractions had quantitatively different rates of Na+/K+-dependent, ouabain-sensitive ATPase and Na+/Ca2+ exchange activities, yet a similar protein composition, which suggests that they were both derived from the plasma membrane. WGA- vesicles had a decreased number of [3H]quinuclidinyl benzilate-binding sites and no detectable [3H]nitrendipine-binding sites. Electron-microscopic and freeze-fracture analysis showed that the WGA+ fraction was composed of typical spherical sarcolemmal vesicles, whereas the WGA- fraction primarily contained elongated tubular structures suggestive of the T-tubule vesicles which were previously isolated from skeletal muscle. Assays of marker enzymes revealed that these fractions were neither sarcoplasmic reticulum nor plasma membrane from endothelial cells. Moreover, WGA agglutination did not result in the separation of right-side-out and inside-out vesicles. On the basis of these findings we propose that the WGA+ fraction corresponds to highly purified sarcolemma, whereas the WGA- fraction may be derived from T-tubule membranes.

Low-sodium contractures indicating sarcolemmal Na/Ca-exchange in the frog heart

1. In the frog heart, Ca2+ enters the cell by the slow inward current (Isi) and by an electrogenic, carrier-mediated, and passive Na-out/Ca-in-exchange. 2. The latter reverses to Na-in/Ca-out-exchange during depolarization and thereby controls relaxation. 3. The exchange ratio is 3 Na+ for 1 Ca2+. 4. The Na/Ca-exchange is not inhibited by organic Ca-antagonists in frog myocardium, indicating that the initiation of the heart beat may mainly depend on Isi. 5. This is not necessarily in contradiction with the Na-Ca-antagonism, since there also exists an antagonism between Na+ and Ca2+ in the slow channel. 6. However, the contractures caused by a decrease of NaO+ are mediated by the Na/Ca-exchange.

Separation and characteristics of inside-out and right side-out vesicles from a rat cardiac sarcolemma preparation

Purified cardiac sarcolemma (SL) vesicles are highly suitable to study various Ca2+-transport systems present in the SL. We describe in this paper the separation of the Inside-Out (IO) and Right side-Out (RO) oriented vesicle subpopulations from a purified rat heart SL preparation. The isolated subfractions were characterized with respect to the number of beta-adrenergic binding sites and the Ca2+-uptake and (Ca2+-Mg2+)-ATPase activities. It was found that the Ca2+-uptake and the (Ca2+-Mg2+)-ATPase activities reside in the IO fraction and are virtually absent in the RO fraction, confirming that the active Ca2+-uptake represents the outward directed sarcolemmal Ca2+-flux.

Characterization of the ATP-promoted aspect of Na+-Ca2+ exchange present in squid retinal nerve axolemma

Using an in vitro system which consists of an axolemma-rich vesicle fraction prepared from squid retinal nerve fibers, an Na+-Ca2+ exchange process has been characterized and appears identical with that reported in squid giant axon. This exchange is absolutely dependent on the establishment of an Na+ gradient, shows monovalent and divalent cation specificity and is highly sensitive to monensin, A23187 and valinomycin but not to ouabain, digitoxigenin, vanadate, pentylenetetrazole, tetrodotoxin or tetraethylammonium. Furthermore, it was found that the exchange process is enhanced by the addition of ATP. This ATP-promoted aspects of Na+-Ca2+ exchange shares many similar characteristics with Na+-Ca2+ ATP hydrolysis and may indicate a common mechanism for both activities via a protein phosphorylation-dephosphorylation event.

Na-Ca exchange in cardiac tissues

Our awareness of the importance of Na-Ca exchange in cardiac muscle has progressed from early observations of Na-Ca antagonism in the activation of contractile force. This was followed by demonstrations of actual Na-Ca ion countertransport across cell membranes and later functional studies in which manipulation of intracellular and extracellular Na and Ca concentrations has permitted a better characterization of the exchange process and its contribution to contractile force. The recent development of vesicle preparations from cardiac sarcolemmal membranes has, despite some drawbacks, produced useful information on the electrogenicity of the exchange mechanisms and on the relative affinity of the exchange carrier compared to the ATPase-driven Ca pump. These studies confirmed earlier estimates of the approximate exchange ratio of the Na-Ca countertransport system and have demonstrated its large maximum transport rate capabilities. The application of ion-sensitive microelectrodes in recent years has enabled measurements of the actual ion-activity gradients across the sarcolemmal membrane. These activity gradients together with the membrane potential control the rate and direction of the Na-Ca exchange. Despite the wide range of techniques employed to tackle the problem, the exchange ratio of Na to Ca movement is still in some doubt, with most estimates ranging between 5:2 and 4:1.

Kinetic properties of the ATP-dependent Ca2+ pump and the Na+/Ca2+ exchange system in basolateral membranes from rat kidney cortex

Basolateral plasma membranes from rat kidney cortex have been purified 40-fold by a combination of differential centrifugation, centrifugation in a discontinuous sucrose gradient followed by centrifugation in 8% percoll. The ratio of leaky membrane vesicles (L) versus right-side-out (RO) and inside-out (IO) resealed vesicles appeared to be L:RO:IO = 4:3:1. High-affinity Ca2+-ATPase, ATP-dependent Ca2+ transport and Na+/Ca2+ exchange have been studied with special emphasis on the relative transport capacities of the two Ca2+ transport systems. The kinetic parameters of Ca2+-ATPase activity in digitonin-treated membranes are: Km = 0.11 microM Ca2+ and Vmax = 81 +/- 4 nmol Pi/min X mg protein at 37 degrees C. ATP-dependent Ca2+ transport amounts to 4.3 +/- 0.2 and 7.4 +/- 0.3 nmol Ca2+/min X mg protein at 25 and 37 degrees C, respectively, with an affinity for Ca2+ of 0.13 and 0.07 microM at 25 and 37 degrees C. After correction for the percentage of IO-resealed vesicles involved in ATP-dependent Ca2+ transport, a stoichiometry of 0.7 mol Ca2+ transported per mol ATP is found for the Ca2+-ATPase. In the presence of 75 mM Na+ in the incubation medium ATP-dependent Ca2+ uptake is inhibited 22%. When Na+ is present at 5 mM an extra Ca2+ accumulation is observed which amounts to 15% of the ATP-dependent Ca2+ transport rate. This extra Ca2+ accumulation induced by low Na+ is fully inhibited by preincubation of the vesicles with 1 mM ouabain, which indicates that (Na+-K+)-ATPase generates a Na+ gradient favorable for Ca2+ accumulation via the Na+/Ca2+ exchanger. In the absence of ATP, a Na+ gradient-dependent Ca2+ uptake is measured which rate amounts to 5% of the ATP-dependent Ca2+ transport capacity. The Na+ gradient-dependent Ca2+ uptake is abolished by the ionophore monensin but not influenced by the presence of valinomycin. The affinity of the Na+/Ca2+ exchange system for Ca2+ is between 0.1 and 0.2 microM Ca2+, in the presence as well as in the absence of ATP. This affinity is surprisingly close to the affinity measured for the ATP-dependent Ca2+ pump. Based on these observations it is concluded that in isolated basolateral membranes from rat kidney cortex the Ca2+-ATPase system exceeds the capacity of the Na+/Ca2+ exchanger four- to fivefold and it is therefore unlikely that the latter system plays a primary role in the Ca2+ homeostasis of rat kidney cortex cells.

The regulation of the Na+ -Ca2+ exchanger of heart sarcolemma

The Na+/Ca2+-exchange of calf-heart sarcolemma is activated by a treatment with ATP, Mg2+, and Ca2+, and deactivated by a treatment with phosphorylase phosphatase. The effect of the latter can be substituted by a treatment with Mg2+, Ca2+, and calmodulin. The activating treatment does not require added calmodulin, but is inhibited by calmodulin antagonists. Evidently, endogenous calmodulin is required and sufficient. Activation is half-maximal at about 2 microM Ca2+. Added calmodulin, however, decreases the Km (Ca2+) of the activating process to about 0.8 microM. Deactivation is half-maximal, at optimal calmodulin concentrations, at about 1.5 microM Ca2+. Experiments with adenosine 5'-[gamma-thio]triphosphate have shown that the activating treatment is mediated by a kinase and the deactivating treatment by a phosphatase. The concerted operation of the two enzymes is made possible by their different Ca2+ affinity. At saturating Ca2+ concentrations, the level of ATP may also influence the balance of the two enzymes.

Modulation by calcium of the potassium permeability of dog heart sarcolemmal vesicles

The movement of K+ in heart sarcolemmal vesicles has been followed through the opposing movement of the tetraphenylphosphonium ion. Ca2+ (5-50 microM) stimulates the efflux of K+ from K+-loaded vesicles [Km(Ca2+) approximately equal to 10 microM]. and the activation requires that Ca2+ be present inside the vesicles together with K+. The efflux of 86Rb+ from K+-, Rb+-loaded vesicles is similarly stimulated by 5-50 microM Ca2+ [Km(Ca2+) approximately equal to 10 microM]. The Ca2+-induced increase of K+ permeability does not become spontaneously inactivated. The effects of some inhibitors have been tested under conditions in which Ca2+ promotes the entry of K+ into vesicles. In this system, direct interaction of the drug with the Ca2+ and K+ membrane binding site(s) was therefore studied. Tetraethylammonium ion, which inhibits the potential-dependent K+ channel, does not interfere with the effect of Ca2+ whereas quinidine (IC50 = 12 microM) and trifluoperazine (IC50 = 8 microM at 50 micrograms of sarcolemmal protein per ml) inhibit.

Solubilization and reconstitution of membranes containing the Na+ -Ca2+ exchange carrier from rat brain

The Na+ -Ca2+ exchange carrier from brain plasmalemma was solubilized in cholate and reconstituted into asolectin vesicles by the cholate dilution method. Optimal solubilization and reconstitution required the presence of high NaCl (greater than or equal to 1.3 M). The reconstituted vesicles rapidly accumulated 45Ca2+ in the presence of an outward directed Na+ gradient. Other monovalent ion gradients (K+, Li+ or cholate+) did not drive transport. Further, Mg2+ X ATP did not drive Ca2+ uptake in the reconstituted vesicles. Uptake was temperature dependent with highest uptake occurring at 37 degrees C. Intravesicular Ca2+ accumulated by the Na+ -dependent process could be released by the Ca2+ ionophore A23187 or by extravesicular Na+ but not by external EGTA. Ca2+ uptake was inhibited by extravesicular Li+ or Na+. The Ki for Na+ inhibition was 35 mM for both the original membrane vesicles from brain plasmalemma and for the reconstituted vesicles. Ca2+ uptake was saturable with respect to extravesicular Ca2+ (Km(Ca2+) = 27 microM).