INTRODUCTORY REMARKS

Calmodulin-binding proteins in granule and plasma membranes from bovine chromaffin cells

Calmodulin-binding proteins in chromaffin granule membrane and chromaffin cell plasma membranes have been investigated and compared. Chromaffin granules were purified by centrifugation over a 1.7 M sucrose layer. Plasma membranes were obtained in a highly purified form by differential and isopycnic centrifugation. Enzymatic determinations of 5'-nucleotidase, a generally accepted plasma membrane marker, showed a 40-50-fold enrichment as compared to the cell homogenate. Marker enzyme studies demonstrated only minimal contamination by other subcellular organelles. After solubilization with Triton X-100, calmodulin-binding proteins were isolated from chromaffin granule membranes and plasma membranes by affinity chromatography on a calmodulin/Sepharose 4B column. On two-dimensional polyacrylamide gelelectrophoresis a prominent protein (Mr = 65,000, pI ranging from 5.1 to 6) consisting of multiple spots, was present in the calmodulin-binding fraction from chromaffin granule membranes as well as from plasma membranes. Besides this 65 kDa protein both fractions had at least four groups of proteins in common. Also, proteins typical for either preparation were observed. In the calmodulin-binding protein preparations from chromaffin granule membranes a prominent spot with Mr = 80,000 and a pH ranging from 5.0 to 5.7 was present. This protein was enzymatically and immunologically identified as dopamine-beta-monooxygenase.

Differential effects of triorganotins on calmodulin activity

In vitro effects of three triorganotins--tributyltin (TBT), triethyltin (TET), and trimethyltin (TMT)--on calmodulin (CaM) activity were studied. Stimulation of Ca2(+)-ATPase of rat brain synaptic membranes and phosphodiesterase (PDE) of bovine brain were assayed as indicators of CaM activity. The rat synaptic membranes were prepared and CaM was depleted by washing with 1 mM EGTA. All the three organotins inhibited the basal as well as CaM-stimulated Ca2(+)-ATPase in a concentration-dependent manner, suggesting their interaction with calcium pump. However, CaM-stimulated Ca2(+)-ATPase was more sensitive than the basal enzyme. The order of potency of the three organotin compounds was TBT greater than TET greater than TMT. The IC50 values of Ca2(+)-ATPase (basal) were 0.63, 35, and approximately 800 microM, respectively, whereas the values for CaM-stimulated Ca2(+)-ATPase were 0.05, 0.8, and 18 microM for TBT, TET, and TMT, respectively. CaM-deficient PDE did not show any sensitivity to these three organotin compounds, while TBT and TET significantly decreased the CaM-stimulated PDE activity. TMT, which was the least effective inhibitor of Ca2+ pump, did not alter PDE activity. Further, the inhibition of CaM-stimulated Ca2(+)-ATPase activity by these organotins could be reversed by excess addition of CaM. These results suggest that the organotins interact with CaM activity, as evidenced by their potent effect on CaM-dependent Ca2(+)-ATPase and PDE activities.

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.

Chlordecone inhibition of calmodulin activated calcium ATPase in rat brain synaptosomes

The effect of chlordecone on Ca2+-ATPase of rat brain synaptosomes in the absence and presence of calmodulin was determined in vitro and in vivo. A 50% activation of Ca2+-ATPase was obtained at 5 micrograms calmodulin per 1.5 ml reaction mixture containing 50 micrograms synaptosomal protein. Chlordecone inhibited Ca2+-ATPase in the absence of calmodulin with an IC50 of 10 microM, whereas chlordecone at 1.0 microM, which had no effect on the basal enzyme activity, completely inhibited the calmodulin activated Ca2+-ATPase. Chlordecone-treated rats showed a significant reduction in calmodulin levels in brain P2 fraction. Brain synaptosomal Ca2+-ATPase in chlordecone-treated rats showed a 50% reduction, which was restored by exogenously added calmodulin. These results suggest that chlordecone may be altering calmodulin-regulated synaptic processes in the brain.

The effect of trifluoperazine on maturation of Xenopus laevis oocytes

Full grown Xenopus oocytes were incubated with trifluoperazine (TFP) or injected with TFP. Incubation of oocytes in TFP resulted in normal-appearing meiotic maturation, as judged by the presence of the white spot and the absence of the germinal vesicle. Cortical granule breakdown in TFP-incubated oocytes was not normal. Abnormal cortical granule breakdown was also observed when progesterone-maturated oocytes were activated in the presence of TFP. Oocytes microinjected with TFP and incubated with progesterone appeared to mature in a normal manner, as judged by the absence of the germinal vesicle; these underwent cortical granule breakdown following activation, but frequently lacked the white spot. Oocytes microinjected with TFP did not mature in the absence of progesterone. We conclude that incubation, although not microinjection, of oocytes with TFP induces essentially normal resumption of meiotic maturation.

The effect of calmodulin on histamine release in the rat peritoneal mast cell

To investigate the role of the Ca2+-binding protein calmodulin on histamine release in the rat peritoneal mast cell, we exposed cells to exogenous calmodulin in the presence of a variety of histamine secretagogues. Histamine release stimulated by compound 48/80, polymyxin B and ionophore A23187 was inhibited while concanavalin A-stimulated release was not affected. Calmodulin in the presence of the secretagogues did not affect cell viability and calmodulin alone had no effect on histamine release. No direct interaction between calmodulin and the secretagogues was observed. Exogenous calmodulin does not appear to be incorporated into the cell. The inhibition of histamine release by calmodulin can be explained as a labile interaction between the protein and the cell that requires externally-bound Ca2+. These experiments demonstrate the use of exogenous calmodulin as a probe in the study of the mechanism of histamine release.

Dictyostelium calmodulin: affinity isolation and characterization

The Ca2+-binding regulatory protein calmodulin (CaM) has been purified from the cellular slime mold, Dictyostelium discoideum. Isolation of homogeneous Dictyostelium CaM was accomplished in high yield by ion-exchange chromatography and Ca2+-dependent affinity chromatography on phenothiazine-Sepharose 4B. This isolate has been demonstrated to possess the following physicochemical and functional properties characteristic of other CaM isolates: (i) a molecular weight ca. 16,000; (ii) an amino acid composition similar to other CaMs--with the notable exception that Dictyostelium CaM, as first determined by Bazari and Clarke [(1981) J. Biol. Chem. 256, 3598-3603] lacks the single trimethylated lysine (Tml) residue identified in nearly all CaMs purified to date; (iii) a CNBr peptide map similar to that of other CaMs; (iv) a Ca2+-dependent shift in migration during native- and sodium dodecyl sulfate-polyacrylamide gel electrophoretic analyses; (v) ability to form Ca2+-dependent complexes with rabbit skeletal muscle troponin I; and (vi) ability to activate in a Ca2+-dependent manner bovine brain cyclic nucleotide phosphodiesterase.

Detection of intracellular Ca2+ transients in sympathetic neurones using arsenazo III

Changes in cytosolic calcium ion concentration ([Ca2+]i) have been implicated in a wide variety of cellular stimulus--transduction roles. In nerve cells, it is believed that electrical activity raises [Ca2+]i by allowing influx of Ca2+ through voltage-dependent channels in the surface membrane. Elevation of neuronal [Ca2+]i may also occur due to release of Ca2+ from intracellular storage sites. Transient increases in [Ca2+]i are thought to trigger neurotransmitter release, and to modulate axonal transport, energy metabolism and growth cone movement. Intracellular Ca2+ also appears to regulate membrane potassium channels and thereby to regulate electrical excitability. Although [Ca2+]i transients have been measured in a few giant invertebrate neurones, detection of such transients in a vertebrate neurone has not been previously reported. We have measured [Ca2+]i in bullfrog sympathetic neurones by photometry of a microinjected calcium indicator dye, arsenazo III (refs 14-16), and report here that action potentials and voltage-clamped depolarizations cause long-lasting increases in [Ca2+]i. Aslo, exposure to the drug theophylline can cause spontaneous periodic increases in [Ca2+]i. Comparisons of [Ca2+]i signals with simultaneous intracellular recordings of membrane potential suggest that the kinetics of the post-tetanic hyperpolarization (PTH) following a series of action potentials or the spontaneous hyperpolarizations induced by theophylline directly reflect the kinetics of the [Ca2+]i transient.

Some further observations on the stimulation by high external potassium of the sodium efflux in barnacle muscle fibers

A study has been made in single barnacle muscle fibers of the mechanism underlying the stimulatory response of the ouabain-insensitive Na efflux to high K0. Fibers poisoned with ouabain and preinjected with GTPNa2 show a biphasic stimulatory response to 100 mM K0 which is practically abolished by exposure of these fibers to verapamil. Prior or post-injection of cAMP-protein kinase type II regulatory subunits reduces the size of the response to 100 mM K0. Prior injection of cAMP-protein kinase catalytic subunits (CSU) attenuates the response to 100 mM K0 in fibers showing moderate sensitivity to CSU. In fibers showing extreme sensitivity to injected CSU, elevation of K0 partially reverses the sustained stimulatory response to CSU. Prior injection of Mg2+ leads to almost complete abolition of the response to 100 mM K0. External and internal application of F- reduces the response to 100 mM-K0. Injection of KF following decay of the response to high K0 results in a sustained stimulatory response. The drugs, chlorpromazine, trifluoperazine, imipramine and diphenylhydantoin are able to appreciably reduce the response to 100 mM K0. Taken together, these observations support the idea that the stimulatory response to high K0 is primarily due to activation by newly formed cAMP of cAMP-protein kinase, and that the transitory nature of the response and variable sensitivity of these fibers to high K0 is closely linked to the activity of a putative Mg2+-dependent protein phosphatase.