Mechanism of activation of cyclic nucleotide phosphodiesterase: requirement of the binding of four Ca2+ to calmodulin for activation

Is calcium more important than sodium in the pathogenesis of essential hypertension?

The hypothesis that abnormalities of calcium homeostasis at both an organ and cellular level are a primary factor in the pathogenesis of human and experimental hypertension forms the basis of this review. The rapidly expanding data base relating disordered calcium metabolism to altered vascular smooth muscle function and increased peripheral vascular resistance is summarized and integrated with the observations that reduced dietary calcium intake is the most consistent nutritional correlate of hypertension in the United States. The role of sodium and sodium chloride in pathogenesis of hypertension is reassessed in the light of new data from epidemiological clinical research, experimental models, and cell physiology investigations. The data supporting the thesis that the effects of sodium or chloride or both on blood pressure may represent, in selected situations, secondary influences mediated through induced changes in calcium homeostasis are presented. The interface between these nutritional factors and the normal regulation of vascular smooth muscle is discussed, providing a theoretical framework in which to assess the current information and to formulate the necessary future research.

Differential regulation of bovine brain calmodulin-dependent cyclic nucleotide phosphodiesterase isoenzymes by cyclic AMP-dependent protein kinase and calmodulin-dependent phosphatase

Purified bovine brain calmodulin-dependent cyclic nucleotide phosphodiesterase (3',5'-cyclic-nucleotide 5'-nucleotidohydrolase, EC 3.1.4.17) contains isozymes that are composed of two distinct subunits with molecular masses of 60,000 and 63,000 daltons. Analysis by NaDodSO4 gel electrophoresis and autoradiography of a phosphodiesterase sample phosphorylated in the presence of [32P]ATP and bovine heart cAMP-dependent protein kinase catalytic subunit revealed that only the 60-kDa subunit was phosphorylated. By using an isozyme preparation greatly enriched with the 60-kDa subunit, the following observations regarding the subunit phosphorylation were made. First, the phosphorylation resulted in the maximal incorporation of about 2 mol of phosphate per mol of subunit. Second, complete inhibition of 60-kDa subunit phosphorylation was approached at a saturating concentration of Ca2+ when a molar ratio of calmodulin to phosphodiesterase of 2:1 was used. No inhibition was observed in the presence of either Ca2+ or calmodulin alone. Third, the phosphorylation was accompanied by a decrease in the enzyme affinity for calmodulin; calmodulin concentrations required for 50% activation of nonphosphorylated and maximally phosphorylated phosphodiesterase isozyme samples were 0.51 and 9.3 nM, respectively. Fourth, the phosphodiesterase isozyme could be dephosphorylated by the calmodulin-dependent phosphatase (calcineurin) in the presence of Ni2+ or Mn2+, the dephosphorylation being associated with an increase in the enzyme affinity for calmodulin. Fifth, peak II rabbit liver phosphoprotein phosphatase catalytic unit did not catalyze the dephosphorylation of the phosphodiesterase isozyme.

Calcium-dependent calmodulin binding to chromaffin granule membranes: presence of a 65-kilodalton calmodulin-binding protein

The presence of calmodulin-binding sites on chromaffin granule membranes has been investigated. Saturable, high-affinity 125I-calmodulin-binding sites (KD = 9.8 nM; Bmax = 25 pmol/mg protein) were observed in the presence of 10(-4) M free calcium. A second, nonsaturable, calmodulin-binding activity could also be detected at 10(-7) M free calcium. No binding occurred at lower calcium levels. When chromaffin granule membranes were delipidated by solvent extraction, calmodulin binding was observed at 10(-4) M free calcium. However no binding was detected at lower calcium concentrations. Thus it appears that a calcium concentration of 10(-7) M promotes the binding of calmodulin to some solvent-soluble components of the chromaffin granule membrane. Calmodulin-binding proteins associated with the granule membrane identified by photoaffinity cross-linking. A calmodulin-binding protein complex, of molecular weight 82K, was formed in the presence of 10(-4) M free calcium. This cross-linked product was specific because it was not detected either in the absence of calcium, in the presence of nonlabeled calmodulin, or in the absence of cross-linker activation. When solvent-treated membranes were used, a second, specific, calmodulin-binding protein complex (70K) was formed. Since the apparent molecular weight of calmodulin in our electrophoresis system was 17K, these experiments suggested the presence of two calmodulin-binding proteins, of molecular weights 65K and 53K, in the chromaffin granule membrane. This result was confirmed by the use of calmodulin-affinity chromatography. When detergent-solubilized membranes were applied on the column in the presence of calcium, two polypeptides of apparent molecular weights of 65K and 53K were specifically eluted by EGTA buffers. Since detergent treatments or solvent extractions are necessary to detect the 53K calmodulin-binding protein, it is concluded that only the 65K calmodulin-binding polypeptide may play a role in the interaction between calmodulin and secretory granules in chromaffin cells.

Transmission at voltage-clamped giant synapse of the squid: evidence for cooperativity of presynaptic calcium action

Synaptic transmission was studied at the squid giant synapse with voltage clamp control of both presynaptic and postsynaptic cells. Because presynaptic voltage gradients can complicate interpretation of electrophysiological data obtained from this preparation, we used local Ca application to restrict Ca influx and transmitter release to a short and relatively isopotential portion of the elongated presynaptic terminal. Under these conditions, we found that postsynaptic current varies approximately as the third power of presynaptic Ca current. This finding is consistent with the hypothesis that several Ca ions cooperate in triggering secretion of a single transmitter quantum.

Characterization of phosphodiesterase catalytic sites by means of cyclic nucleotide derivatives

Cyclic nucleotide derivatives have been used as a tool to characterize distinct catalytic sites on phosphodiesterase enzyme forms: the cGMP-stimulated enzyme from rat liver and the calmodulin-sensitive enzyme from rat or bovine brain. Under appropriate assay conditions, the analogues showed linear competitive inhibition with respect to cAMP (adenosine 3',5'-monophosphate) as substrate. The inhibition sequence of the fully activated cGMP-stimulated phosphodiesterase was identical to the inhibition sequence of the desensitized enzyme, i.e. the enzyme which has lost its ability to be stimulated by cGMP. The inhibition pattern could, therefore, not be attributed to competition with cGMP at an allosteric-activating site. Also, the inhibition sequence of the calmodulin-sensitive phosphodiesterase was maintained whether activity was basal or fully stimulated by calmodulin. When cAMP and cGMP, with identical chemical ligands substituted at the same position, were compared as inhibitors of the calmodulin-sensitive phosphodiesterase, the cGMP analogues were always the more potent suggesting that, for that enzyme, the catalytic site was sensitive to a guanine-type cyclic nucleotide structure. Comparing the two phosphodiesterases, it was possible to establish both similar and specific inhibitor potencies of cyclic nucleotide derivatives. In particular, the two enzymes exhibited large differences in analogue specificity modified at C-6, 6-chloropurine 3',5'-monophosphate or purine 3',5'-monophosphate.

Correlation between calmodulin-dependent increase in the rate of calcium transport and calmodulin-dependent phosphorylation of cardiac sarcoplasmic reticulum. Characterization of calmodulin-dependent phosphorylation

The aim of the present study was to prove a correlation between the calmodulin-dependent increase in the rate of calcium transport by dog cardiac sarcoplasmic reticulum and calmodulin-dependent phosphorylation. The dependence of phosphorylation on the total calmodulin concentration at 75 microM and 1 microM free calcium gave apparent calmodulin half-saturation constants Km (CaM) of 9.4 nM and 181 nM, respectively, whilst the apparent Km (CaM) for the rate of calmodulin-stimulated calcium transport carried out at 1 microM calcium, but phosphorylated prior to the calcium uptake at 75 microM or 1 microM calcium, were 12.5 nM and 127 nM, respectively. A positive correlation was obtained between calmodulin-dependent increase in the rate of calcium transport and hydroxylamine-insensitive phosphoester formed by the calcium/calmodulin-regulated, membrane-bound protein kinase. More than 90% of incorporated [32P]phosphate is confined to a 26-28-kDa or 9-11-kDa protein as determined by polyacrylamide gel electrophoresis following solubilization in sodium dodecyl sulfate at 37 degrees C and at 100 degrees C, respectively, similar to the results obtained by phosphorylation with cAMP-dependent protein kinase. The data indicate that calmodulin-dependent phosphorylation of the above protein(s) is causally related to the stimulation of the rate of calcium transport by cardiac sarcoplasmic reticulum, which is at least partially due to a shift in the calcium dependence of the rate of calcium transport to lower free calcium concentrations, K(Ca), of 1.25 microM and 0.61 microM in controls and calmodulin-dependent phosphorylation, respectively. Activation of calmodulin-dependent phosphorylation by free calcium at total calmodulin concentrations of 300 nM, 100 nM and 30 nM gave apparent K(Ca) values of 0.83 microM, 1.44 microM and 2.3 microM and Hill coefficients of 4.13, 3.76 and 3.79, respectively, indicating that all four calcium binding sites of calmodulin have to be saturated to obtain activation of the calcium/calmodulin-regulated protein kinase. The calmodulin-dependent modulation of calcium transport in vivo is, therefore, determined to great extent by the total calmodulin concentration present in the sarcoplasm.

Ion binding to calmodulin. A comparison with other intracellular calcium-binding proteins

Over the past few years calcium has emerged as an important bioregulator. Upon external stimulation, the cell generates a transient Ca2+ increase, which is transformed into a cellular event through a molecular cascade. The first step in this cascade is the binding of calcium to proteins present in the cytosol. These proteins capable of binding Ca2+ under physiological conditions all belong to the same evolutionary family that evolved from a common ancestor. However, they strongly differ in the properties of their calcium binding sites. Calmodulin, the ubiquitous calcium binding protein present in all eukaryotic cells, is very close to the ancestor protein, presents four calcium binding sites which bind calcium, magnesium and monovalent ions competitively and is involved in the triggering of cellular processes. Parvalbumin, another member of the family, is more specialized and found mostly in fast-twitch skeletal muscle. It binds calcium and magnesium with high affinity and seems to be involved in muscle relaxation. On the other hand, troponin C which confers Ca2+ sensitivity to acto-myosin interaction exhibits both triggering and relaxing sites. The study of intracellular Ca2+ binding proteins has shown that calcium binding proteins have evolved from a simple common structure to fulfill different functions.

Drugs that block calmoduLin activity inhibit cell-to-cell coupling in the epidermis of Tenebrio molitor

In many cell systems, the permeability of membrane junctions is modulated by the cytoplasmic level of free Ca++. To examine whether the calcium-dependent regulatory protein calmodulin is involved in this process, the ability of anticalmodulin drugs to influence the cell-to-cell passage of injected current and an organic tracer was tested using standard intracellular glass microelectrode techniques. Several antipsychotics and local anesthetics were found to block junctional communication in the epidermis of the beetle Tenebrio molitor. Treatment of the epidermis with chlorpromazine (0.25 mM) raised intercellular resistance two- to threefold within 20 to 25 min; cell-to-cell passage of electrical current was abolished within 41 +/- 5 min. Loss of electrotonic coupling was accompanied by a block in the cell-to-cell movement of the organic tracer carboxyfluorescein. The reaction is fully reversible, with normal electrotonic coupling being restored within 2 to 4 hr. Other antipsychotics and local anesthetics had similar effects on cell coupling. The order of potency found was: trifluoperazine greater than thioridazine greater than D-butaclamol greater than chlorprothixine = chlorpromazine greater than L-butaclamol greater than dibucaine greater than tetracaine. The relative uncoupling potencies of these drugs correlate well with their known ability to inhibit calmodulin-dependent phosphodiesterase activity. Other anesthetic compounds, procaine and pentobarbital, did not block cell-to-cell communication. Altering the extracellular Ca++ concentration did not affect the rate of uncoupling by antipsychotics, while chelation of extracellular Ca++ with EGTA raised electrotonic coupling. The effect of three metabolic inhibitors on coupling was also examined. Iodoacetate uncoupled the epidermal cells while DNP and cyanide did not. These results are discussed in terms of possible mechanisms by which calmodulin may control junctional communication in this tissue.

Activation of human erythrocyte Ca2+-dependent Mg2+-activated ATPase by calmodulin and calcium: quantitative analysis

The effect of Ca2+ and calmodulin on (CaM) on the activation of Ca2+-dependent Mg2+-activated ATPase (Ca2+,Mg2+-ATPase; ATP phosphohydrolase, EC 3.6.1.3) has been carried out because of the finding that the CaM dependence of the activation varies with the concentration of free Ca2+, similarly to brain phosphodiesterase and adenylate cyclase. The study was carried out in the absence of chelating agents because they strongly interfere in the enzyme kinetics. Three main conclusions can be drawn (i) CaM-Ca3 and CaM-Ca4 together are the biochemically active species in vitro. (ii) These species bind in a non-cooperative way to the CaM-binding site of the enzyme with a dissociation constant of 6 x 10(-10) M or 1.1 x 10(-8) M, depending on whether Ca2+ saturates the substrate binding site of the enzyme or not. (iii) The binding of CaM-Ca3 to the enzyme lowers the dissociation constant of the enzyme for Ca2+ at the substrate binding site from 51.5 to 2.8 microM. Contrary to general belief, CaM does not induce pronounced positive cooperativity in the binding of Ca2+ to the enzyme. Such a cooperativity is seen only when the enzyme is incompletely saturated with the activator, but it disappears in the presence of saturating concentrations of CaM-Ca3. The rate equation proposed here accurately predicts the extent of enzyme activation over a wide range of Ca2+ and CaM concentration. In healthy erythrocytes the concentrations of Ca2+ and CaM are such that the Ca pump works with a minimal dissipation of energy, but a small increase in the intracellular Ca2+ concentration leads to a strong amplification of the pumping activity.