Calcium-dependent potassium conductance in the photoresponse of a nudibranch mollusk

• 1.1. The response to light of Hermissenda photoreceptors when recorded intracellularly without interference from synaptic and action potentials consisted of three phases: an early depolarization (ED) followed by hyperpolarization (dip) and subsequent depolarization (tail).
• 2.2. The ED and the dip were associated with increased membrane conductance while decreased membrane conductance was involved with the tail.
• 3.3. The dip reversal potential was − 82.1 ± 5.3 mV and its amplitude varied inversely with the log of [K⁺].
• 4.4. Perfusing with agents which block K⁺ current like 4AP, Quinine, Quinidine or injection of TEA eliminated the dip and its associated increased membrane conductance, thus further supporting the role of K⁺ conductance in producing the dip.
• 5.5. The dip was enhanced by increased [Ca²⁺]_o, reduced by decreased [Ca²⁺]_o and abolished together with its associated increased membrane conductance when perfused with either D600, Cd²⁺, Mg²⁺, Mn²⁺, or Co²⁺, which block transmembrane Ca²⁺ current.
• 6.6. The dip and its associated increased membrane conductance were abolished by intracellular injection of EGTA and enhanced by perfusion with Ruthenium red.
• 7.7. Intracellular injection of Ca²⁺ mimicked the dip: membrane conductance was increased and the cell hyperpolarized.
• 8.8. These results indicate that the increase in intracellular [Ca²⁺] is primarily responsible for the light-induced increase of K⁺ conductance during the dip. The possible source of the Ca²⁺ is, at least in part, extracellular due to activation of an inward Ca²⁺ current.

     

Bioelectrical potentials across the mantle epithelium of Achatina fulica

• 1.1. The shell side of the mantle of Achatina fulica is several millivolts positive to the blood side in vitro.
• 2.2. The electrical potential does not depend on Na⁺, Ca²⁺, Mg²⁺, K⁺ or HCO₃⁻ but requires the presence of chloride on the shell side.
• 3.3. The potential difference and short-circuit current ranged from 3.0 to 30.0 mV and 15.0 to 75 μA/cm² with averages at 10m V and 50 μA/cm² respectively.
• 4.4. The electrical gradient is reduced by 2,4-dinitrophenol, thiocyanate and furosemide but not by ouabain, CO₂ or acetozolamide.
• 5.5. It is suggested that the nature and mechanism of electrogenesis in Achatina parallels that of the Helix mantle.

     

The exchange of radioactive cations by somatic and cardiac muscles of the crayfish

• 1.1. Intracellular concentrations of Na⁺, K⁺, Ca²⁺ and Mg²⁺ were measured in a somatic muscle and in the heart of the crayfish. The uptake and the efflux of Na²⁴, K⁴², Ca⁴⁵ and of Sr⁸⁹ were also measured.
• 2.2. The initial influx rates of the ions from van Harreveld's solution into resting somatic muscle (in μEq/g cell water/hr) are: K⁺ = 25; Na⁺ = 56; Ca²⁺ = 38. Similar figures were obtained for the heart muscle.
• 3.3. The calculated permeability constants (× 10⁸ cm/sec) are: P_K = 64; P_Na = 30 P_Ca = 10; P_Sr = 1·5.
• 4.4. The stimulation of the muscle fiber leads to an additional Ca²⁺ influx of about 2·8 pEq/cm² fiber surface. The additional Ca²⁺ uptake is sufficient to account for the change in potential on the membrane.
• 5.5. When muscles were immersed in Sr²⁺ solutions, no additional Sr⁸⁹ uptake was found with stimulation. However, there is a high resting Sr⁸⁹ uptake and the muscle in Sr²⁺ has a long refractory period, so a reasonable increase in Sr⁸⁹ uptake would not be detectable.
• 6.6. The results are discussed in relation to the divalent cation mechanism for generating action potentials and to the part played by Ca²⁺ in triggering contraction.

     

The possibility that Ca2+-ATPase from the plasma membrane-rich fraction of bovine parotid gland is ecto-Ca2+-dependent nucleoside triphosphatase

• 1.1. Parotid plasma membrane nonpump low-affinity Ca²⁺-ATPase, which possesses high-affinity (Ca²⁺ + Mg²⁺ )-ATPase activity, was characterized.
• 2.2. Purified Ca²⁺-ATPase hydrolyzed the nucleoside triphosphates, GTP, ITP, CTP, UTP, TTP (67–93% of ATP) and nucleoside diphosphates, ADP. GDP, IDP, CDP, TDP (12–40% of ATP) but not AMP and p-NPP.
• 3.3. The maximum activities of Ca²⁺- and (Ca²⁺ +Mg²⁺ )-ATPases were obtained in the presence of 1 mM and 0.13 μ M Ca²⁺, respectively.
• 4.4. The K_m values for Ca²⁺ in Ca²⁺- and (Ca²⁺+ Mg²⁺ )-ATPases were 0.2 mM and 22 nM. respectively.
• 5.5. The activities of both Ca²⁺- and (Ca²⁺ + Mg²⁺ )-ATPases were found in the right-side-out-vesicles obtained from the plasma membrane-rich fraction.
• 6.6. These features suggest that Ca²⁺-ATPase is an ecto-Ca²⁺-dependent nucleoside triphosphatase.

     

Myofibril and sarcoplasmic reticulum changes during muscle development: Activity vs inactivity

• 1.1. The purpose of this study was to determine whether biochemical changes of skeletal muscle that occur as a result of exercise in young rats persist into adulthood.
• 2.2. Littermates (10 days old) were assigned to a 3, 6 and 12 week control or training group. In addition, a rest-exercise group (R-E) and exercise-rest (E-R) group were included.
• 3.3. The rest-exercise and exercise-rest rats were maintained for the 12 weeks with the first 6 weeks being either rest or exercise and the condition reversed during the last 6 weeks of the experiment.
• 4.4. Myofibril ATPase activity of rat plantaris increased from the 10d to 12 week animals (P < 0.05). As anticipated, training resulted in a lowered activity at 6 and 12 weeks compared to controls.
• 5.5. The Ca²⁺ uptake and Ca²⁺-ATPase activity of the sarcoplasmic reticulum followed a similar pattern.
• 6.6. With regard to the exercise-rest rats, the myofibril and SR ATPase activities at 12 weeks were comparable to the 12 weeks control rats.
• 7.7. The rest-exercise group approximated the 12 week training group with regard to myofibril and SR ATPase activities (P > 0.05).
• 8.8. The results suggest that the training adaptations that occur during development of skeletal muscle return to normal, when training ceases in the adult rat.
• 9.9. Furthermore, animals that started to train prior to puberty do not have a greater capacity to adapt than animals which initiated training during adulthood.

     

Phospholipase A activity of culture supernatants from Streptococcus mutans strain 6715

• 1.1. Phospholipase A activity was found in the culture broth of growing cultures of Streptococcus mutans strain 6715.
• 2.2. The amount of enzyme activity was proportional to the cell density of the cultures.
• 3.3. The enzyme had a pH optimum of 7.0 and was inactivated at temperatures greater than 45°C.
• 4.4. The enzyme was Ca²⁺-dependent, since both EDTA and EGTA were inhibitory and Ca²⁺ was stimulatory.
• 5.5. Analysis of the fatty acid products resulting from the enzyme's action on 1-palmitoyl-2-oleoyl phosphatidylcholine indicated the enzyme to be a phospholipase A₁, (EC 3.1.1.32).

     

Mechanisms of membrane potential oscillation in bursting neurons on the snail,Helix pomatia

• 1.1. The mechanism of generation of membrane potential (MP) oscillations was studied in identified bursting neurons from the snail Helix pomatia.
• 2.2. Long-lasting stimulation of an identified peptidergic interneuron produced a persistent bursting activity in a non-active burster.
• 3.3. External application of calcium channel blockers (1 mM Cd²⁺ or 5 mM La²⁺) resulted in a transient increase in the slow-wave amplitude and subsequent prevention of pacemaker activity generation in bursting neurons. Application of these blockers together with endogenous neuropeptide initiating bursting activity generation, increased MP wave amplitude without prevention of bursting activity generation.
• 4.4. Replacement of all NaCl in normal Ringer's solution with isoosmotic CaCl₂, glucose or Tris-HCl produced a reversible block of bursting activity generation. Stationary current-voltage relation (CVR) of bursting neuron membrane has a region of negative resistance (NRR) and does not intersect the potential axis in threshold region for action potential (AP) generation in normal Ringer's solution. In Na-free solution stationary CVR is linear and intersects the potential axis near — 52 mV.
• 5.5. Novel potential- and time-dependent outward (E_rev = − 58 mV) current, I_B, activated by hyperpolarization was found in the bursting neuron membrane. Having achieved a maximal value, this current decayed with a time constant of about 1 sec. Hyperpolarization inactivated maximal conductance, g_B, responsible for I_B, and depolarization abolished inactivation of g_B.
• 6.6. Short-lasting (0.01 sec) hyperpolarization of the bursting neuron membrane by inward current pulse induced the development of prolonged hyperpolarization wave lasting up to 10 sec.
• 7.7. These results suggest that: (a) persistent bursting activity of RPal neuron in the snail Helix pomatia is not endogenous but is due to a constant activation of peptidergic synaptic inputs of these neurons; (b) Ca²⁺ ions do not play a pivotal role in the ionic mechanism of MP oscillations but play a determining role in the process of secretion of a peptide initiating bursting activity by the interneuron presynaptic terminal; (c) depolarizing phase of the MP wave is due to specific properties of stationary CVR and hyperpolarization phase is due to regenerative properties of hyperpolarization-activated outward current I_B. The minimal mathematical version of MP oscillations based on the experimental data is presented.

     

Ca2+ - and Mg2+-dependent ATPase activities in the deoxycholate-treated rat heart sarcolemma

• 1.1. Isolated rat heart sarcolemma was treated with different concentrations of an ionic detergent, deoxycholate (DOC) and ATP hydrolysis in the presence of Ca²⁺ or Mg²⁺ was determined.
• 2.2. Both Ca²⁺-dependent ATPase and Mg²⁺-dependent ATPase activities were decreased in the DOC-treated membranes; however, the depression of Mg²⁺-dependent ATPase activity was greater than that of Ca²⁺-dependent ATPase.
• 3.3. The differential changes in Ca²⁺-dependent ATPase and Mg²⁺-dependent ATPase activities were apparent when incubations with DOC were carried out for different time intervals and at different temperatures.
• 4.4. In DOC-treated preparations, the K_m value for Ca²⁺-dependent ATPase was decreased whereas that for Mg²⁺-dependent ATPase was increased. The half maximal velocities of the Ca²⁺-dependent ATPase and Mg²⁺-dependent ATPase enzyme reactions in the treated preparations were obtained at a DOC: membrane protein ratio of 3.0 and 0.6, respectively.
• 5.5. In the DOC-treated membranes exhibiting the half maximal velocities of enzyme reactions, the K_i value for Ca²⁺-dependent ATPase was drastically reduced but remained unchanged for Mg²⁺-dependent ATPase.
• 6.6. The DOC treatment was associated with a loss of protein as well as phospholipids and resulted in changes in the ultrastructural integrity of the membrane.
• 7.7. Varying degrees of decreases in the activities of sarcolemmal adenylate cyclase. (Na⁻-K⁺)-ATPase. 5'-nucleotidase and calcium binding were seen upon DOC treatment.
• 8.8. The extent of reduction in Ca²⁺-dependent ATPase and Mg²⁺-dependent ATPase activities were also different when the membrane was treated with a non-ionic detergent, Lubrol PX.
• 9.9. These data suggest that Ca²⁺-dependent ATPase in heart sarcolemma is more resistant than Mg²⁺-dependent ATPase to detergent treatments and further indicate some differences in the properties of these enzymes.

     

Erythrocyte membrane anion-sensitive Mg2+-ATPase—Identity with monovalent cation sensitive Ca2+-Mg2+-ATPase

• 1.1. Activation of Mg²⁺-ATPase of rabbit and guinea-pig erythrocyte membrane by bicarbonate or chloride could be completely abolished by ethylene-glycol-bis-(β-aminoethylether)-N,N'-tetraacetic acid. The anion stimulation was actually an activation of contaminating Ca²⁺ -stimulated Mg²⁺-ATPase by monovalent cations associated with the anions.
• 2.2. Guinea-pig red cell Ca²⁺-Mg²⁺-ATPase could be activated by both sodium and potassium while the rabbit enzyme was sensitive only to sodium. The concentrations of monovalent cations for half-maximal stimulation of Ca²⁺-Mg²⁺-ATPase are: k_na+ = 40.8 mM, k_k+ = 12.2 mM (guinea-pig); K_Na+ = 13.3mM (rabbit).
• 3.3. Potassium enhanced activation of rabbit erythrocyte membrane Ca²⁺-Mg²⁺-ATPase by red cell Ca²⁺-Mg²⁺-ATPase activator protein. With the guinea pig enzyme, neither sodium nor potassium enhanced activator stimulation of Ca²⁺-Mg²⁺-ATPase.
• 4.4. Ca²⁺-Mg²⁺-ATPase of aged rabbit erythrocyte membrane responded to sodium but not to activator protein.
• 5.5. Triton X-100 solubilized rabbit erythrocyte membrane Ca²⁺-Mg²⁺-ATPase has an apparent molecular weight of 371,000. It did not respond to the activator.
• 6.6. One major and three minor proteins, visualized by SDS-polyacrylamide gel electrophoresis, were extracted from rabbit erythrocyte membrane by 50 μM chlorpromazine.

     

The role of phosphoinositide metabolism in Ca2+ signalling of skeletal muscle cells

• 1.1. The mobilization of Ca²⁺ from intracellular stores by d-myo-inositol 1,4,5-triphosphate[Ins(1,4,5)P₃] is now widely accepted as the primary link between plasma membrane receptors that stimulate phospholipase C and the subsequent increase in intracellular free Ca²⁺ that occurs when such receptors are activated (Berridge, 1993). Since the observations of VoIpe et al. (1985) which showed that Ins(1,4,5)P₃ could induce Ca²⁺ release from isolated terminal cisternae membranes and elicit contracture of chemically skinned muscle fibres, research has focused on the role of Ins(1,4,5)P₃ in the generation of SR Ca²⁺ transients and in the mechanism of excitation-contraction coupling (EC-coupling).
• 2.2. The mechanism of signal transduction at the triadic junction during EC-coupling is unknown. Asymmetric charge movement and mechanical coupling between highly specialized triadic proteins has been proposed as the primary mechanism for voltage-activated generation of SR Ca²⁺ signals and subsequent contraction. Ins(1,4,5)P₃ has also been proposed as the major signal transduction molecule for the generation of the primary Ca²⁺ transient produced during EC-coupling.
• 3.3. Investigations on the generation of Ca²⁺ transients by Ins(1,4,5)P₃ have been conducted on ion channels incorporated into lipid bilayers, skinned and intact fibres and isolated membrane vesicles. Ins(1,4,5)P₃ induces SR Ca²⁺ release and the enzymes responsible for its synthesis and degradation are present in muscle tissue. However, the sensitivity of the Ca²⁺ release mechanism to Ins(l,4,5)P₃ is highly dependent on experimental conditions and on membrane potential.
• 4.4. While Ins(1,4,5)P₃ may not be the major signal transduction molecule for the generation of the primary Ca²⁺ signal produced during voltage-activated contraction, this inositol polyphosphate may play a functional role as a modulator of EC-coupling and/or of the processes of myoplasmic Ca²⁺ regulation occurring on a time scale of seconds, during the events of contraction.

     

The effect on creatine kinase release of altered conditions in the Ca2+ paradox

• 1.1. Release of creatine kinase (CK) in the Ca²⁺ paradox of the Langendorff-perfused rat heart is dependent on the conditions of Ca²⁺ depletion and Ca²⁺ repletion.
• 2.2. CK release is reduced by raising [Ca²⁺]_o during Ca²⁺ depletion and progressively increased by extending the Ca²⁺ free period from 2 to 5 min.
• 3.3. CK release is reduced by decreasing the electrochemical gradient for Ca²⁺ during Ca²⁺ repletion.
• 4.4. The findings are discussed in the light of current hypotheses for the biochemical mechanisms that underlie the Ca²⁺ paradox.

     

Synaptic transmission at high pressure: Effects of [Ca2+]o

• 1.1. The effects of pressure on synaptic currents were examined in crayfish abdominal muscles.
• 2.2. Helium pressure (10.1 MPa) considerably decreased extracellulariy-recorded excitatory junctional potentials associated with increased short-term facilitation.
• 3.3. These effects could be mimicked by a reduction of [Ca²⁺]_o, and partially compensated by an increase in [Ca²⁺]_o.
• 4.4. Pressure also reduced the amplitude of the extracellular nerve terminal potentials (ENTP) by up to 25%, and significantly increased synaptic delay in a [Ca²⁺]_o-dependent manner.
• 5.5. The interaction between compression and various [Ca²⁺]_o were analysed in terms of an existing model of transmitter release. The results were consistent with the hypothesis that high pressure decreases the maximal Ca²⁺ influx into nerve terminals.
• 6.6. The decreased ENTP and increased synaptic delay suggest that additional processes may be involved in pressure effects on synaptic transmission.

     

Luminescence of isolated photophores and supracaudal gland from myctophum punctatum: Electrical stimulation

• 1.1. Isolated photophores of the living bathypelagic fish Myctophum punctatum respond to train of weak (10–25 V) and short (2–5 msec) electrical stimuli applied at different frequencies (8–100/sec) by a luminus response.
• 2.2. This response is characterized by a short emission latency time, the peak of light develops within about 2 sec after the beginning of the electrical stimulation: afterwards the light decreases to a constant level within about 8 sec.
• 3.3. Stimuli of higher strength (60–75 V) and longer duration (8–16 msec) applied at different rates (1–100/sec) evoke brief flashes.
• 4.4. The isolated supracaudal gland emits flashes in response to electrical stimulation whatever the strength and the duration of stimuli.
• 5.5. The flash of the supracaudal gland differs from the flash of the isolated photophores in three respects: lower threshold, higher magnitude and longer duration.

     

Inhibitors of active Ca2+ uptake by fragments of the sarcoplasmic reticulum of lobster muscle

• 1.1. Vesicles from the sarcoplasmic reticulum of lobster muscle accumulate Ca²⁺ if supplied with ATP as an energy source. A search was undertaken for inhibitors of Ca²⁺ transport.
• 2.2. p-Hydroxymercuribenzoate can completely inhibit Ca²⁺ transport and ATP hydrolysis. 2–4 Dinitrophenol inhibits uptake but not hydrolysis.
• 3.3. Sr²⁺, Ba²⁺ and Zn²⁺ inhibit uptake, perhaps by competing with Ca²⁺ for a carrier.
• 4.4. The vesicles contain acetylcholinesterase. Anticholinesterases can reduce —but not abolish—Ca²⁺ uptake. Acetylcholine has no effect on the activity of the vesicles.
• 5.5. Ca²⁺ uptake is not affected by Mn²⁺, glutamate, pilocarpine, carnosine, caffeine, strophanthidin or tetraethylammonium.
• 6.6. K⁺ is needed for maximal activity of the uptake system but not for ATP hydrolysis. Apparently K⁺ enhances the coupling between the energy supply and the carrier mechanism.

     

High-affinity Ca2+-Mg2+-ATPase in kidney of euryhaline Gillichthys mirabilis: kinetics,subcellular distribution and effects of salinity

• 1.1. Two components of Ca²⁺-Mg²⁺-ATPase are observed in kidneys of G. mirabilis. The high-affinity component has a K_0.5Ca of 0.23μM; the low-affinity activity K_0.5Ca is 90–110μM. The high-affinity activity requires Mg²⁺, displays Michaelis-Menten kinetics, has peak activity at 1.2 μM Ca²⁺, and is insensitive to ouabain and Na⁺ azide.
• 2.2. In subcellular fractions, the high-affinity component segregates with Na⁺-K⁺-ATPase and is localized predominantly in BLM. The low-affinity component is broadly distributed among membranous organelles, including brush border, and may be equivalent to alkaline phosphatase.
• 3.3. Specific activity of the high-affinity Ca²⁺-Mg²⁺-ATPase is modestly increased following adaptation of fish to FW, but total renal high-affinity activity is greatest in the hypertrophied kidneys of FW-adapted fish and is least in kidneys of fish adapted to 200% SW.
• 4.4. High-affinity Ca²⁺-Mg²⁺-ATPase may be associated with active Ca²⁺ transport or with regulation of intracellular Ca²⁺ concentration of tubular cells.

     

An alkaline phosphatase from Thermus sp strain Rt41A

• 1.1. A thermostable orthophosphoric monoester phosphohydrolase (EC 3.1.3.1) from Thermus sp strain Rt41A has been purified 400-fold to give a specific activity of 25 U/mg at 60°C in IM diethanolamine (pH 11.1).
• 2.2. The enzyme has a M_r of 160,000 and is trimeric.
• 3.3. The half-life of the enzyme is 5 min at 85°C.
• 4.4. The enzyme has a wide specificity for a number of phosphate monoesters.
• 5.5. The H_m of the enzyme is pH dependent, so the pH optimum of the enzyme is affected by the substrate concentration.
• 6.6. The enzyme is inhibited 50% by 20 mM Ca²⁺ or Mg²⁺.
• 7.7. The K_i for phosphate, EDTA-di sodium salt and arsenate (in 1 M diethanolamine, pH 11.1) is approx 1.2, 1.6 and 4mM respectively.
• 8.8. Urea (200 mM) is not inhibitory.

     

The digestive enzymes of the pacific brown shrimp Penaeus californiensis.: I—Properties of amylase activity in the digestive tract

• 1.1. Crude extract of the whole digestive tract from the brown shrimp (P. californiensis) was investigated for digestive amylase activity.
• 2.2. Considerable amylase activity was found at pH 6.5–8.0, with optimum pH at around 7.5.
• 3.3. Optimum temperature was found between 30–40°C, similar to amylases from other crustaceans.
• 4.4. Amylase activity was highly halotolerant, having 50% maximum activity at 3 M NaCl.
• 5.5. Maximum amylase activity was found at 0.01 M NaCl.
• 6.6. Amylase activity was partially inhibited by the divalent ions Hg²⁺, Zn²⁺, Cu²⁺ and Cr²⁺.
• 7.7. Mg²⁺ and Ca²⁺ ions seemed to enhance amylase activity.

     

Crayfish retinular cells: Influence of extracellular sodium and calcium upon receptor potential

• 1.1. In crayfish, light stimulation of the retinular cells induces a depolarizing receptor potential.
• 2.2. Experiments were designed to determine the role of Na⁺ and Ca²⁺ on receptor potential during dark And light states.
• 3.3. Depolarization depends on Na⁺ and Ca²⁺ availability to the retinular cell.
• 4.4. Repolarization velocity and response duration depend on extracellular Ca²⁺ availability.
• 5.5. Light adaptation increases receptor potential dependence on calcium and sodium ions.
• 6.6. We analyse these results with respect to other invertebrate photoreceptors.

     

PLA2 activity in rat liver nuclei

• 1.1. Subcellular fractions of rat liver were assayed for PLA₂ activity.
• 2.2. The PLA₂ assay measures the release of [³ H]oleic acid from phospholipids, using labeled E. coli as substrate.
• 3.3. Nuclear fractions contained PLA₂ activity, which was Ca²⁺ dependent and could not be explained from mitochondrial, microsomal or plasma membrane contamination.
• 4.4. The V_max value of nuclear PLA₂ is 0.30 ± 0.04 pmol oleic acid/min/mg protein; its K_m value is 0.86±0.12μM, similar to that of mitochondrial PLA₂.
• 5.5. We conclude that rat liver nuclei contain PLA₂ activity.

     

Biochemical characterization of the plasma membrane Ca2+ pumping ATPase activity present in the gill cells of Mytilus galloprovincialis LAM

• 1.1. In the plasma membrane of mussel gill cells an ouabain insensitive, Ca²⁺-activated ATPase activity is present. The ATPase has high Ca²⁺ affinity (K_ma = 0.3 μM).
• 2.2. The optimum assay conditions to evaluate the enzymatic activity of the Ca²⁺-stimulated ATPase at 19°C are: 120–300 mM KCl ionic strength, pH 7.0 and 2 mM ATP. As for mammalian enzymes, the Ca²⁺ ATPase activity is stimulated by DTT (0.5–1 mM) and it is inhibited by low concentrations of vanadate (10–50 μM) and -SH inhibitors such as PCMB and PCMBS (10 μM); the enzyme appears to be calmodulin insensitive.
• 3.3. Electrophoretic analyses of plasma membrane proteins demonstrate that: (a) Ca²⁺ at n-μM concentrations is necessary to activate ATP hydrolysis with consequent formation of the enzyme-phosphate complex; (b) the steady state concentration of the phosphorylated intermediate is increased in the presence of La³⁺; (c) the mol. wt of Ca²⁺ ATPase is about 140 kDa.
• 4.4. Low Ca²⁺ concentrations (n-μM) are sufficient to stimulate the ATP-dependent Ca²⁺ uptake by plasma membrane inside-out vesicles.
• 5.5. The results indicate that the Ca²⁺ pump present in the gill plasma membranes could be responsible for Ca²⁺ extrusion and therefore involved in maintaining the cytosolic Ca²⁺ concentration within physiological levels.