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Results for “Calmodulin”

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PubMed1989

Effect of nickel(II)acetate on distribution of calmodulin in the rat kidney.

The effect of subcutaneous injection of a single dose of nickel(II)acetate (95 mumol/kg body wt.) on the distribution of calmodulin in the membrane and cytosolic fractions of kidneys of F344/NCr rats was studied with the use of radioimmunoassay. Over the first 8 h post-injection, the membrane-bound calmodulin concentration increased by 20% versus the control value of 3.84 +/- 0.54 SE micrograms/g wet tissue. That increase was then followed by a gradual decrease to the control value in 7 d. At the same time intervals, the cytosolic calmodulin concentration first decreased by 13% from the control value of 8.81 +/- 2.1 SE micrograms/g wet tissue and then slowly increased, reaching a level of 12% above the control at day 7 post-injection. Statistical analysis of the results after 8 h revealed a downward trend in the membrane-bound (P less than 0.0014) and upward trend in the cytosolic (P less than 0.0207) calmodulin concentrations until day 7. However, the total renal calmodulin calculated from the membrane/cytosolic distribution data did not show any statistically significant treatment-related differences among the mean values (13.39 micrograms/g for the nickel(II)acetate-treated rats vs. 12.56 micrograms/g wet tissue for the controls between days 1 and 7). Therefore, the observed effect of nickel(II)acetate must be attributed only to temporary transfer of calmodulin from the soluble cytosolic form into an insoluble, membrane-bound form, without any detectable influence on its degradation or synthesis rate.

Acetates↗

PubMed1994

Calcium and calmodulin inhibit phosphorylation of a novel auditory nerve protein.

The growing use of cochlear prosthetic devices and demonstrations of direct ototoxic insult to spiral ganglion neurons make it imperative to gain an understanding of intracellular biochemical regulation in primary sensory neurons. Calcium and calmodulin regulate many aspects of neuronal cellular physiology through stimulation of protein kinase activity. We have previously demonstrated the presence of calmodulin-dependent protein kinase substrates in the guinea pig modiolus and, additionally, the presence of two proteins (12 kDa and 81 kDa, designated as p12 and p81) whose phosphorylation is blocked by calcium and calmodulin (Coling and Schacht, 1991). Here, we investigate three models for this unusual regulatory mechanism. The effects of calcium, calmodulin and trifluoperazine on dephosphorylation of both proteins suggests that calmodulin inhibits protein kinase activity. P81 was identified by immunoprecipitation as the myristoylated alanine-rich C kinase substrate (MARCKS), a ubiquitous actin-binding protein. Two observations indicate that MARCKS may be regulated differently in acoustic nerve than in cerebral cortex. 32P incorporation was significantly higher in acoustic nerve than in brain. The calmodulin-dependent block of MARCKS phosphorylation was observed only in acoustic nerve. p12 shares several characteristics with myelin basic protein (MBP). We used a double label assay with 32P autoradiography and immunoblotting to show that p12 is in fact distinct from MBP. We suggest that either p12 or p12 kinase may be either specific to the peripheral auditory system or novel marker proteins for that tissue.

Animals↗

PubMed1990

Calcium and calmodulin levels in late embryonic and early hatched Japanese quail brain.

Characterizing the last phases of embryonic avian brain development are increased brain activity and increased absorption of shell calcium. The calcium-binding protein, calmodulin, regulates many activities of calcium. In neural tissue, calmodulin modulates neural transmission, and is required for the phosphorylation of synaptosomal proteins. Therefore, the objective was to compare levels of brain calcium and calmodulin in the Japanese quail. Brain extracts from embryonic days 11, 15, hatch, and 5 days post-hatch (n = 7/group) were analysed for calcium, protein and calmodulin. Despite increases in protein between embryonic (X = 0.126 and 0.145 mg/mg wet wt) and hatched groups (X = 0.183 and 0.221 mg), no significant increases in calmodulin were observed (237-279 ng/mg protein). Calcium levels in the brain were U-shaped with low levels at embryonic day 1 (341 micrograms/mg wet wt) and post-hatch day 5 (315 micrograms/mg wet wt) with higher levels on embryonic day 15 (425 micrograms/mg wet wt) and at hatch (433 micrograms/mg wet wt). Calmodulin levels do not show a developmental pattern similar to calcium and protein levels or with reports of brain activity.

Animals↗

PubMedDate not supplied

EGTA-extractable calmodulin in platelet membrane is lower in alcoholics than in controls.

The purpose of this study was to determine calmodulin distribution in platelets of alcoholics. Eight alcoholics were diagnosed by DSM-III (Diagnostic and Statistical Manual of Mental Disorders, 3rd edition) criteria prior to admission to the Alcohol Unit, VA Hospital, San Antonio, TX. Whole blood was drawn after a five day washout period, and again after 30 days of disulfiram treatment. Platelets were prepared from the whole blood, suspended in Ringer's-citrate-dextrose buffer (RCD), divided into 2 aliquots, then sonicated and centrifuged (100K X g). Supernatants were assayed for calmodulin content by RIA. The pellets were resuspended by sonication in 1 mM EGTA in RCD or 1% (w/v) Lubrol-PX in RCD, then centrifuged at 100K X g. Calmodulin was measured in the EGTA and Lubrol supernatants. EGTA-extractable calmodulin in pre-disulfiram, post-disulfiram, and control subjects was 5.07 +/- 2.2 (SD), 5.19 +/- 1.7 (SD), and 10.5 +/- 6.7 (SD) ng calmodulin/mg whole platelet protein. The EGTA-extractable calmodulin was significantly lower in alcoholics pre- or post-disulfiram than in controls (p less than 0.025). These preliminary results suggest an alteration in platelet membranes of alcoholics that affects the binding of calmodulin.

Adult↗

PubMed1989

Influence of dopamine receptor agonists and antagonists on calmodulin translocation in different brain regions.

Acute and chronic experiments were performed to study the effects of intraperitoneally administered dopamine receptor agonists and antagonists on the translocation of cytosolic and membrane-bound calmodulin in the striatum and hippocampus of the rat brain. Single doses of apomorphine and a low-dose amphetamine (1.25 mg/kg) resulted in a translocation of calmodulin, as measured by a decrease in membrane-bound and increase in cytosolic calmodulin in the striatum, whereas bromocryptine was ineffective. Amphetamine exerted a similar effect in the hippocampus and striatum. However, a high-dose amphetamine (5 mg/kg) had an opposite effect on translocation, in that there was an increase in membrane-bound calmodulin. Chronically applied amphetamine (5 mg/kg) and haloperidol (1 mg/kg), i.e. under conditions of dopamine receptor supersensitivity, tended to decrease cytosolic calmodulin in the striatum and hippocampus, and to increase membrane-bound calmodulin. The findings and, especially, the chronic effects of haloperidol and the high dose of amphetamine are interpreted in the light of the current concept which suggests that transmitters and intraneuronal signalling systems converge, thereby influencing the more complex processes of neuronal plasticity, rather than receptor sensitivity only.

Amphetamine↗

PubMed1991

Interactions of calmodulin antagonists with calcium antagonists binding sites.

Calmodulin antagonists have calcium entry blocking properties. In order to quantitatively investigate the interactions of these drugs with calcium channels, their effect on [3H]nitrendipine and [3H]d-cis-diltiazem binding to rat cerebral cortex membrane preparations was compared to their inhibitory effect on the activation of cyclic nucleotide phosphodiesterase by calmodulin. The potency of most antagonists to inhibit [3H]nitrendipine binding was correlated with their calmodulin inhibitory potency. However, bepridil (K0.5 = 280 nM), chlorpromazine (K0.5 = 3 microM), triflupromazine (K0.5 = 1.5 microM), imipramine (K0.5 = 3 microM) and propranolol (K0.5 = 14 microM) were much more active on [3H]d-cis-diltiazem binding than on either [3H]nitrendipine binding or calmodulin, suggesting that these compounds bind to higher affinity sites on the calcium antagonist target protein. Moreover, the potencies of these compounds to compete with [3H]d-cis-diltiazem and to inhibit calcium-induced contractions in depolarized smooth muscle were correlated (R = 0.76, p less than 0.02). These data suggest that low concentrations of these hydrophobic drugs which have calcium and calmodulin antagonistic properties inhibit smooth muscle contraction through calcium entry blockade, not calmodulin antagonism.

2',3'-Cyclic-Nucleotide Phosphodiesterases↗

PubMed1992

Inactivation of glibenclamide-sensitive K+ channels in Xenopus oocytes by various calmodulin antagonists.

In follicle-enclosed Xenopus oocytes, extracellular application of cromakalim (a K+ channel opener) or intracellular injection of cAMP induces the smooth outward K+ current which is inactivated by glibenclamide. We found that cromakalim- or cAMP-induced K+ currents in the oocytes were rapidly, reversibly and dose-dependently blocked by various drugs having a calmodulin antagonizing activity in common, namely, by a selective calmodulin antagonist (W-7), antipsychotics (trifluoperazine, chlorpromazine, haloperidol), an antidepressant (amitriptyline), a beta-adrenoceptor blocker (propranolol), a local anesthetic (lidocaine) and a calcium antagonist (prenylamine). W-7, trifluoperazine, chlorpromazine and prenylamine were relatively potent blockers. For example, IC50 values to block cromakalim (100 microM)-induced K+ currents were 12 microM for trifluoperazine and 16 microM for W-7, which were close to their IC50 values to inhibit Ca2+/calmodulin-dependent phosphodiesterase (an index of the potency of calmodulin antagonists). IC50 values to inhibit cAMP (20 pmol/oocyte)-induced K+ currents were 126 microM for prenylamine and 129 microM for chlorpromazine. The IC50 values of all drugs tested to block cromakalim or cAMP responses were significantly correlated with their calmodulin-antagonizing potencies. Isoproterenol-induced K+ currents in the oocytes were also dose-dependently inhibited by glibenclamide, W-7 and trifluoperazine. These results suggest the possibility that the activity of glibenclamide-sensitive K+ channels in follicle-enclosed oocytes are regulated by calmodulin or a calmodulin-dependent process.

Animals↗

PubMed1994

Calcium channel blockers inhibit the (Ca2+ + Mg2+)-ATPase activity and the 125I-calmodulin binding in brain membranes.

Ca2+ channel blockers belonging to three distinct chemical groups (dihydropyridines, phenylalkylamines and diphenylalkylamines) differentially inhibit the (Ca2+ + Mg2+)-ATPase activity of synaptic plasma membranes (Santos et al., J. Neurochem. 52, S49D, 1989). We now report that (-)-desmethoxyverapamil and flunarizine are the most potent inhibitors of the Ca(2+)-activated ATPase activity of synaptic plasma membranes, decreasing the Vmax by 41% and 37%, respectively, with no significant effects on the Km for Ca2+ (162.7 +/- 14.9 nM free [Ca2+]), while nitrendipine did not affect these parameters. Trifluoperazine was the most potent inhibitor of the Ca(2+)-activated ATPase of synaptic plasma membranes with an IC50 of 8-10 microM. To clarify whether the inhibitory effects of Ca2+ channel blockers and of trifluoperazine on the (Ca2+ + Mg2+)-ATPase occur through the inhibition of the interaction of calmodulin with the enzyme, we studied their effects on the binding of 125I-calmodulin to the membrane proteins separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), followed by electrotransfer to nitrocellulose and autoradiography. The autoradiograms revealed Ca(2+)-dependent CaM binding proteins of about 140, 70 and 55 kDa. Trifluoperazine (30-40 microM) inhibited by 50-60% the binding of 125I-calmodulin to the 140 kDa band, which probably includes the (Ca2+ + Mg2+)-ATPase protein. Flunarizine and (-)-desmethoxyverapamil (100 microM) inhibited the 125I-calmodulin binding to the 140 kDa peptides by 100 and 90%, respectively, and they inhibited by 55 and 40%, respectively, the binding of 125I-calmodulin to the peptides in the 70-55 kDa range, whereas nitrendipine did not show any effect. The results suggest that the inhibitory effects of (-)-desmethoxyverapamil and flunarizine, as well as trifluoperazine, on the (Ca2+ + Mg2+)-ATPase activity of synaptic plasma membranes are mediated by inhibition of the calmodulin interaction with the enzyme.

Animals↗

PubMed2006

Inhibition of calmodulin-dependent cyclic AMP phosphodiesterase by phenoxazines.

Phenoxazine derivatives were examined for their ability to inhibit the calmodulin-mediated activation of phosphodiesterase, which is based on the hydrolysis of cAMP to AMP by phosphodiesterase in the presence or absence of inhibitor, followed by quantitative analysis by HPLC method. Anticalmodulin activity of phenoxazines with respect to substitution at C-2 position follows the order: 2-trifluoromethyl>2-chloro>unsubstituted phenoxazines. The interaction of phenoxazines with calmodulin using fluorescence spectroscopy has been performed. Binding study showed that calmodulin has two types of binding sites for phenoxazines. One is high affinity binding site (Kd value 0.07-0.46 microM) and the other, a low affinity binding site (Kd value 0.7-34.5 microM). The change in secondary structure of calmodulin upon binding to phenoxazines was studied by circular dichroism (CD) method, which showed that the percentage of helicity decreased with an extensive change in tertiary structure of calmodulin. Kinetic analysis of the phenoxazine-calmodulin interaction showed that phenoxazines competitively inhibited the activation of phosphodiesterase without affecting Vmax. Thus, these studies showed a good correlation between the ability of phenoxazines to block the activation of phosphodiesterase and their ability to bind to the activator.

3',5'-Cyclic-AMP Phosphodiesterases↗

PubMed2005

Calmodulin interacts with the cytoplasmic tails of the parathyroid hormone 1 receptor and a sub-set of class b G-protein coupled receptors.

Parathyroid hormone (PTH) binds to its receptor (PTH 1 receptor, PTH1R) and activates multiple pathways. The PTH1R, a class b GPCR, contains consensus calmodulin-binding motifs. The PTH1R cytoplasmic tail interacts with calmodulin in a calcium-dependent manner via the basic 1-5-8-14 motif. Calcium-dependent calmodulin interactions with the cytoplasmic tails of receptors for PTH 2, vasoactive intestinal peptide, pituitary adenylate cyclase activating peptide, corticotropin releasing hormone, calcitonin, and the glucagon-like peptides 1 and 2 are demonstrated. The cytoplasmic tails of the secretin receptor and the growth hormone releasing hormone receptor either interact poorly or not at all with calmodulin, respectively. Fluphenazine, a calmodulin antagonist, enhances PTH-mediated accumulation of total inositol phosphates, suggesting that calmodulin regulates signaling via phospholipase C.

Amino Acid Sequence↗

PubMed2002

Calmodulin-dependent regulation of hypotonicity-induced translocation of ENaC in renal epithelial A6 cells.

Hypotonicity stimulates translocation of epithelial Na(+) channel (ENaC) to the apical membrane from the intracellular store site of ENaC by activating protein tyrosine kinase (PTK) in renal epithelial A6 cells. Based upon the fact that calmodulin shows its action on other enzymes through PTK caused phosphorylation of tyrosine residues of calmodulin itself, we studied whether a calmodulin-dependent pathway is involved in the action of hypotonicity on ENaC. W7, an antagonist of calmodulin, diminished the stimulatory action of hypotonicity on ENaC, irrespective of W7 treatment before or after application of hypotonicity. Calmodulin is known to regulate three pathways: (1) protein phosphatase 2B (PP2B), (2) Ca(2+)/calmodulin-dependent protein kinase II (CaMK II), and (3) myosin light chain kinase (MLCK). Pretreatment with cyclosporin A, an inhibitor of PP2B, did not influence the hypotonicity action on ENaC. The hypotonicity action on ENaC was partially inhibited by pretreatment with KN93, an inhibitor of CaMK II, but not by addition of KN93 after hypotonic stimulation had been applied. ML-7, an inhibitor of MLCK, showed the action similar to KN93. These observations indicate that: (1) the hypotonicity-induced translocation of ENaC depends on CaMK II and MLCK and (2) ENaC translocated to the apical membrane by hypotonicity is maintained in its activity and/or stability at the apical membrane through a calmodulin-dependent pathway.

Calmodulin↗

PubMed1983

Calmodulin-NADH semidehydroascorbate oxidoreductase interactions of clathrin coated vesicles.

NADH semidehydroascorbate oxidoreductase activity is present in clathrin coated vesicles isolated from rat liver. The activity of the enzyme on the coated vesicles and Golgi apparatus but not that of endoplasmic reticulum is stimulated by calmodulin and is inhibited by three different drugs which are known inhibitors of calmodulin function including trifluoperizine, pimozide and R24571. Extraction of clathrin from the vesicles causes a decrease in activity which can be partially restored when the extracted clathrin is added back. Added calmodulin also restores much of the activity which is lost when the clathrin is removed and the specific activity of added pure calmodulin is similar to that of the crude clathrin on a protein basis. There is a decrease in enzyme activity if coated vesicles or Golgi apparatus are treated with a calcium antagonist (8-[N,N-diethylamino]-octyl-3,4,5-trimethoxybenzoate) (TMB-8). However, the enzyme activity can be recovered to that of the untreated control if calcium (6.0 mM) is added. An additive stimulatory effect on enzyme activity is also observed when both calcium (1.0 mM) and calmodulin (40 micrograms/ml) are present in the vesicles simultaneously. The results show that the NADH-semidehydroascorbate oxidoreductase of coated vesicles and Golgi apparatus have regulatory properties different from those of the microsomal electron transport system. Calmodulin-calcium control mediated through the semidehydroascorbate reductase, may be among the components that regulate Golgi apparatus secretion.

Animals↗

PubMed1998

MARCKS, a major protein kinase C substrate, assumes non-helical conformations both in solution and in complex with Ca2+-calmodulin.

MARCKS, a major cellular substrate for protein kinase C, plays important roles in various cellular functions and its functions are regulated by calmodulin. We have studied the conformational properties of recombinant human MARCKS in solution and in complex with calmodulin. Circular dichroism (CD) spectra showed a high content of random coil in physiological solution. When MARCKS or MARCKS-derived calmodulin-binding peptide was complexed with Ca2+-calmodulin, little change was observed in the CD spectra, suggesting that MARCKS binds with calmodulin in a non-helical conformation, which is unique among the calmodulin-binding proteins.

Amino Acid Sequence↗

PubMed1999

Calmodulin inhibits inositol 1,4,5-trisphosphate-induced calcium release through the purified and reconstituted inositol 1,4,5-trisphosphate receptor type 1.

Our previous studies have demonstrated that calmodulin binds to IP3R type I (IP3R1) in a Ca2+ dependent manner, which suggests that calmodulin regulates the IP3R1 channel. In the present study, we investigated real-time kinetics of interactions between calmodulin and IP3R1 as well as effects of calmodulin on IP3-induced Ca2+ release by purified and reconstituted IP3R1. Kinetic analysis revealed that calmodulin binds to IP3R1 in a Ca2+ dependent manner and that both association and dissociation phase consist of two components with time constants of k(a) = 4.46 x 10(2) and > 10(4) M(-1) s(-1) k(d) = 1.44 x 10(-2) and 1.17 x 10(-1) s(-1). The apparent dissociation constant was calculated to be 27.3 microM. The IP3-induced Ca2+ release through the purified and reconstituted IP3R1 was inhibited by Ca2+/calmodulin, in a dose dependent manner. We interpret our findings to mean that calmodulin binds to IP3R1 in a Ca2+ dependent manner to exert inhibitory effect on IP3R channel activity. This event may be one of the mechanisms governing the negative feedback regulation of IP3-induced Ca2+ release by Ca2+.

Animals↗

PubMed1987

Independent conformational changes caused by calmodulin and calcium in the cardiac microsomal (Ca2+, Mg2+)-ATPase during its ATP hydrolysis-ATP synthesis cycle.

The cardiac sarcoplasmic reticulum preparation employed in this study presented two fractions of associated calmodulin, one easily removable and the other tightly bound. The latter one was resistant to treatment with EGTA and high ionic strength. The forward or hydrolytic component of the (Ca2+, Mg2+)-ATPase cycle of calmodulin-free vesicles was stimulated when preincubated with trypsin, as opposed to the inhibition of this activity when the intact or calmodulin containing vesicles were employed in parallel assays. Interestingly, the reverse reaction of the ATPase cycle or ATP in equilibrium Pi exchange reaction found in both types of vesicles, was more sensitive to exposure to trypsin. Although a drastic inhibition of this exchange reaction was observed independent of the presence of the modulator, this reaction was transiently stimulated when the calmodulin-free vesicles were preincubated with a low trypsin to protein ratio. The differential effect of trypsin upon each reaction indicates that the equilibrium between the E1 and the E2 states of the enzyme is displaced. [125I]calmodulin was found to bind equally to the Ca X E1 approximately P and the E2 - P intermediates of the isolated enzyme formed with the ATP and Pi respectively. It is suggested the formation of an independent overall conformational state for the (Ca2+, Mg2+)-ATPase in the presence of calmodulin.

Adenosine Triphosphate↗

PubMed2001

Calmodulin effect on purified rat cortical plasma membrane Ca(2+)-ATPase in different phosphorylation states.

The plasma membrane Ca(2+)-ATPase in neuronal tissue plays an important role in fine tuning of the intracellular Ca(2+) concentration. The enzyme exhibits a high degree of tissue specificity and is regulated by several mechanisms. Here we analysed the relationship between separate modes of Ca(2+)-ATPase regulation, i.e., reversible phosphorylation processes mediated by protein kinases A and C, protein phosphatases PP1 and PP2A, and stimulation by calmodulin. The activity of PKA- or PKC-phosphorylated Ca(2+)-ATPase was influenced by the further addition of calmodulin, and this effect was more pronounced for PKC-phosphorylated calcium pump. In both cases the fluorescence study revealed the increased calmodulin binding, and for PKA-mediated phosphorylation it was correlated with a higher affinity of calcium pump for calmodulin. The incubation of Ca(2+)-ATPase with CaM prior to protein kinases action revealed that CaM presence counteracts the stimulatory effect of PKA and PKC. Under the in vitro assay cortical Ca(2+)-ATPase was a substrate for PP1 and PP2A. Protein phosphatases decreased both the basal activity of Ca(2+)-ATPase and its affinity for calmodulin. Fluorescence analysis confirmed the lowered ability of dephosphorylated Ca(2+)-ATPase for calmodulin binding. These results may suggest that interaction of CaM with calcium pump and its stimulatory action could be a partly separate phenomenon that is dependent on the phosphorylation state of Ca(2+)-ATPase.

Animals↗

PubMed1998

Identification of a neuronal calmodulin-binding peptide, CAP-19, containing an IQ motif.

Neurons produce polypeptides which can bind the calcium-poor or pre-activated form of calmodulin. It is expected that this class of peptide will serve an important role in maintaining cellular homeostasis since it would modulate calcium-dependent target regulation and redirect intracellular signaling. The lack of conserved sequence has made the identification of these peptides difficult, consequently leading us to exploit their property of binding calcium-poor calmodulin as a means of finding new species. A new peptide termed Calmodulin-Associated Peptide-19 (CAP-19) was purified and characterized. The protein-sequence information was employed in order to recover a cDNA clone from rat which included the entire reading frame for the peptide. Like its counterparts, neuromodulin (GAP-43), neurogranin (RC3) and PEP-19, it contains an IQ motif although the remainder of the peptide is quite different. Northern blot analysis of ribonucleic acid (RNA) from animals of differing ages indicated that the message appears at birth and then persists into adulthood. Antibodies to synthetic peptide were employed for localizing CAP-19. The results indicated that the peptide was localized to neurons in several brain regions. CAP-19 is similar to other calmodulin-binding proteins in that the domain spanning the IQ motif was demonstrated to participate in binding to calmodulin. Database searching showed CAP-19 to be homologous to the silkworm protein, multiprotein bridging factor 1 (MBF1). This homology suggests a potential new role for calmodulin-associated proteins in cellular homeostasis.

Amino Acid Sequence↗

PubMed2002

Calmodulin dissociation mediates desensitization of the cADPR-induced Ca2+ release mechanism.

Ryanodine receptor (RyR) activation by cyclic ADP-ribose (cADPR) is followed by homologous desensitization. Though poorly understood, this "switching off" process has provided a key experimental tool for determining the pathway through which cADPR mediates Ca(2+) release. Moreover, desensitization is likely to play an important role in shaping the complexities of Ca(2+) signaling involving cADPR, for example, localized release events and propagated waves. Using the sea urchin egg, we unmask a role of calmodulin, a component of the RyR complex and a key cofactor for cADPR activity, during RyR/cADPR desensitization. Recovery from desensitization in calmodulin-depleted purified endoplasmic reticulum (microsomes) is severely impaired compared to that in crude egg homogenates. An active, soluble factor, identified as calmodulin, is required to restore the capacity of microsomes to recover from desensitization. Calmodulin mediates recovery in a manner that tightly parallels its time course of association with the RyR. Conversely, direct measurement of calmodulin binding to microsomes reveals a loss of specific binding during cADPR, but not IP(3), desensitization. Our results support a mechanism in which cycles of calmodulin dissociation and reassociation to an endoplasmic reticulum protein, most likely the RyR itself, mediate RyR/cADPR desensitization and resensitization, respectively.

Animals↗