Effects of dihydropyridines on calcium release from the isolated membrane complex consisting of the transverse tubule and sarcoplasmic reticulum

We have investigated a) the effects of the dihydropyridines (DHPs) nifedipine and nimodipine on depolarization-induced (T-tubule-mediated) Ca2+ release in the vesicles consisting of the complex of the T-tubule and SR, and b) the binding of [3H]nimodipine to these vesicles. These DHPs inhibited the slow but not the fast phase of depolarization-induced release, both of which are mediated via the T-tubule. The DHPs have no effect on caffeine-induced release in which T-tubules are not involved. There are two classes of DHP binding sites: one, with high affinity and small capacity, and another, exhibiting low affinity and a much larger capacity. The inhibition paralleled the low affinity binding of DHP with no correlation with the high affinity binding. These results suggest that the low affinity DHP binding sites located probably in the DHP receptor, rather than the high affinity DHP binding site, are responsible for the inhibition of e-c coupling.

Analysis of heterogeneous fluorescence photobleaching by video kinetics imaging: the method of cumulants

The method of cumulants has been applied to digital video fluorescence microscopy. The method is used to reconstruct the distribution of fluorescent molecules before the initiation of fluorescence photobleaching, and to characterize heterogeneous photobleaching by imaging one or more of the cumulants of the bleaching decay rate. Using the pipelined pixel processor of the image analysis system for the bulk of the calculations, rather than the general-purpose host-computer CPU, the video kinetics imaging can be performed in near real-time. The method is applied to chick embryo myotubes labelled with fluorescein-conjugated alpha-bungarotoxin. The pre-bleach fluorescence distribution is derived, and the image of fluorescein fluorescence is separated from glutaraldehyde-induced autofluorescence on the basis of the spatially resolved average photobleaching decay rate.

Regulation of the number of alpha-bungarotoxin binding sites in cultured chick myotubes by a 1,4 dihydropyridine calcium channel antagonist

Calcium has been suggested as the second messenger link between skeletal muscle activity and AChR gene expression and synthesis. We have compared the concentrations of the Ca2+ channel antagonists D600 and nisoldipine needed both to block Ca2+ uptake into cultured myotubes and to increase AChR expression. The good correspondence between these two measurements and the use of the highly specific Ca2+ channel antagonist nisoldipine strengthens the hypothesis that AChR expression is regulated by levels of intracellular Ca2+.

Identification of a approximately 170K subunit of the cardiac calcium channel using a monoclonal antibody to the skeletal muscle dihydropyridine receptor

Our newly isolated monoclonal antibody (#78) specifically interacts with the 170Kd 1,4 dihydropyridine binding component of the skeletal muscle calcium channel. Dihydropyridine receptor (DHPR) from rabbit skeletal muscle and canine cardiac membranes were purified by monoclonal antibody #78 affinity chromatography. We show that DHPR from canine cardiac membranes like DHPR from rabbit skeletal membranes contain a approximately 170Kd polypeptide to which antibody #78 immunoblots under both reducing and non-reducing conditions.

Photoaffinity labelling of a 33-35,000 dalton protein in cardiac, skeletal and smooth muscle membranes using a new 125I-labelled 1,4-dihydropyridine calcium channel antagonist

The binding sites for Ca2+ channel antagonists were probed using Bay P 8857 [2-iodoethyl isopropyl 1,4-dihydropyridine-2,6-dimethyl-4-(3-nitrophenyl)-pyridine-3,5-dicarbox ylate] that has been radiolabelled with 125I. This drug was shown to bind with high affinity to cardiac, smooth, and skeletal muscle membranes, with a KD approximately equal to 0.3 nM. A protein of molecular weight 33-35,000 daltons was specifically and irreversibly radiolabelled after irradiation of cardiac, skeletal and aortic smooth muscle membranes, incubated with the [125I]-Bay P 8857. The peptide labelled by 1,4-dihydropyridine binding therefore appears similar in size for cardiac, skeletal, and smooth muscle. This data suggests that of the three peptide subunits which reportedly comprise the skeletal and cardiac muscle 1,4-dihydropyridine receptor complex, the 33-35,000 dalton peptide contains the dihydropyridine binding site.

Calcium channels in planar lipid bilayers: insights into mechanisms of ion permeation and gating

Electrophysiological recordings were used to analyze single calcium channels in planar lipid bilayers after membranes from bovine cardiac sarcolemmal vesicles had been incorporated into the bilayer. In these cell-free conditions, channels in the bilayer showed unitary barium or calcium conductances, gating kinetics, and pharmacological responses that were similar to dihydropyridine-sensitive calcium channels in intact cells. The open channel current varied in a nonlinear manner with voltage under asymmetric (that is, physiological) ionic conditions. However, with identical solutions on both sides of the bilayer, the current-voltage relation was linear. In matched experiments, calcium channels from skeletal muscle T-tubules differed significantly from cardiac calcium channels in their conductance properties and gating kinetics.

cAMP, not Ca2+/calmodulin, regulates the phosphorylation of acetylcholine receptor in Torpedo californica electroplax

The regulation of the phosphorylation of the acetylcholine receptor in electroplax membranes from Torpedo californica and of purified acetylcholine receptor was investigated. The phosphorylation of the membrane-bound acetylcholine receptor was not stimulated by Ca2+/calmodulin, nor was it inhibited by EGTA, but it was stimulated by the catalytic subunit of cAMP-dependent protein kinase, and was blocked by the protein inhibitor of cAMP-dependent protein kinase. Purified acetylcholine receptor was not phosphorylated by Ca2+/calmodulin-dependent protein kinase activity in electroplax membranes, nor by partially purified Ca2+/calmodulin-dependent protein kinases from soluble or particulate fractions from the electroplax. Of the four acetylcholine receptor subunits, termed alpha, beta, gamma and delta, only the gamma- and delta-subunits were phosphorylated by the cAMP-dependent protein kinase (+ cAMP), or by its purified catalytic subunits.

Ketamine and ditran block end-plate ion conductance and [3H]phencyclidine binding to electric organ membrane

Alterations by ketamine (10-100 microM) and ditran (50-100 microM) of end-plate currents were studied using transected cutaneous pectoris muscles. Both drugs reduced peak current and shortened the time constant for end-plate current decay (tau). Ketamine was more effective at pH 5.3 than at 7.4 or 9.1. Recovery from blockade was asymmetrical in that tau recovered more quickly than did peak current when the drugs were removed from the bath. By contrast, 4-aminopyridine antagonized the depression of peak current by ketamine, but not the reduction of tau. Both ketamine and ditran disrupted the voltage dependence of tau. The binding to microsacs prepared from electric organs of [3H]phencyclidine ([3H]PCP) was blocked by ketamine and ditran. In microsacs treated with carbachol, the IC50 for ketamine block of [3H]PCP binding was 6.6 X 10(-6) M. For ditran, the IC50 for block of [3H]PCP binding in the presence of carbachol was 1.7 X 10(-6) M. The binding of [alpha-125I]bungarotoxin to the microsacs or to the cultured chick myotubes was reduced only slightly by ketamine. Because ketamine has no effect on transmitter release and little effect on [alpha-125I]bungarotoxin binding, it is concluded that, like PCP, ketamine and ditran block open channels in the end-plate. In addition, the asymmetrical recovery of end-plate current parameters suggests that ketamine may block closed channels. The recovery from block of closed channels (caused by either a direct action on closed channels or a very slow channel unblocking rate) proceeds more slowly than does the block of open channels.

Regulation of acetylcholine receptor phosphorylation by calcium and calmodulin

Acetylcholine receptor-enriched membranes prepared from frozen electric organ of Torpedo californica by differential centrifugation and density step gradient centrifugation were assayed for endogenous phosphorylation in the absence and presence of calmodulin and calcium. Each of the membrane fractions exhibited a 3- to 6-fold stimulation of endogenous phosphorylation by calcium and calmodulin. Both calcium and calmodulin were needed for maximal stimulation although calcium alone afforded a small, reproducible stimulation of endogenous phosphorylation. In the presence of fluoride, a phosphatase inhibitor, the calmodulin plus calcium stimulation was increased an additional 3-fold. The phosphorylation reaction was rapid, and maximal phosphorylation was achieved in 2 min. Stimulation of phosphorylation by calcium and calmodulin was completely inhibited by 25 microM trifluoperazine; at 50 microM it inhibited basal phosphorylation by 60%, suggesting that most of the basal phosphorylation may be due to the endogenous calmodulin present in our membrane preparation. NaDodSO4/polyacrylamide gel electrophoresis revealed that at least three of the phosphorylated species (both in the presence and in the absence of calcium and calmodulin) correspond to subunits of the purified acetylcholine receptor from T. californica (i.e., 65,000, 58,000, and 50,000 daltons) which are the beta, gamma, and delta subunits of the receptor.

Postsynaptic inhibition of neuromuscular transmission by trifluoperazine

The effect of trifluoperazine (TFP) on neuromuscular transmission was investigated on chick biventer cervicis and frog cutaneous pectoris and sartorius nerve-muscles. In the chick, TFP inhibited indirectly elicited twitches in a frequency-dependent manner. Inhibition was much more rapid at higher frequencies of stimulation. Directly elicited twitches, KCl contracture and action potentials of desheathed frog sciatic nerve and sartorius muscles were unaffected by TFP, suggesting an action of TFP on neuromuscular transmission. TFP depressed end plate potential amplitude and miniature end plate potential (MEPP) amplitude without affecting MEPP frequency. When MEPP frequency was increased by high Na+ Ringer, depression of MEPP amplitude was much more rapid. Similarly, at high frequencies of stimulation (100 Hz), TFP rapidly depressed end plate currents. TFP inhibited contractures induced by bath-applied acetylcholine (ACh); depressed ACh potentials produced by iontophoretically applied ACh; decreased ionic current and time constant of decay of end-plate currents of transected muscle; and inhibited [alpha-125I]bungarotoxin binding to ACh receptor. These data suggest that TFP acts postsynaptically in a frequency-dependent manner to inhibit neuromuscular transmission. Based on recent evidence that TFP is a potent antagonist of calmodulin and that calmodulin is localized mainly to postsynaptic regions, we postulate that the postsynaptic inhibitory actions of TFP may be mediated through antagonism of calmodulin, which in turn may regulate ACh receptor function.

Kinetic studies of the intracellular transport of procollagen and fibronectin in human fibroblasts. Effects of the monovalent ionophore, monensin

The monovalent ionophore, monensin, has been found to inhibit the secretion of both procollagen and fibronectin from human fibroblasts in cell culture. The kinetics of inhibition, as well as those for the release of inhibition, suggested that both proteins may be cotransported in the cell. In the present study, we have examined the intracellular translocation and release into the culture medium of procollagen and fibronectin, in the presence or absence of monensin. Pulse-chase studies were combined with subcellular fractionation of the fibroblasts to determine the rates of intracellular movement. We found that monensin did not significantly affect either the subcellular fractionation, or the distribution of organelle marker enzymes along the gradient, and that procollagen moved from a region of high buoyant density (primarily endoplasmic reticulum), through a mid-density region (primarily Golgi elements), to a region of low buoyant density before exiting from the cell. Monensin markedly decreased the rate of transit from one region to the other; kinetic analysis of the data showed a greater than 3-fold decrease in the rate constants for intracellular movement into the low buoyant density components and thence to the culture medium. The density gradient distribution of fibronectin was affected by monensin in similar fashion, indicating that it and procollagen probably follow the same intracellular route. Since monensin causes both proteins to accumulate in highest abundance in regions of the density gradient corresponding to Golgi and endoplasmic reticulum, the site of ionophore blockade appears to be within the Golgi apparatus. Thus, procollagen and fibronectin share a common intracellular route prior to entering the Golgi region of the cell.

Monovalent ionophores inhibit secretion of procollagen and fibronectin from cultured human fibroblasts

Procollagen and fibronectin are major products of confluent fibroblasts in culture and both are released from the cells. Procollagen is secreted by known pathways, while the mechanism of fibronectin release is controversial. We find that the secretion of both these proteins can be reduced to 20% by low concentrations (0.1-1 muM) of ionophores that have affinity for monovalent cations. In contrast, little effect upon secretion was found for similar concentrations of an ionophore that binds divalent cations. Electron microscopy showed that the inhibition of secretion is accompanied by accumulation of membranous vacuoles. We believe that the ionophores impede secretion by acting on the secretory structures rather than on the proteins themselves. Biochemical studies supported this interpretation because no changes were detected in hydroxylation or glycosylation of procollagen or glycosylation of fibronectin, nor were significant changes in cellular amino acid incorporation observed. Pulse-chase studies indicated that the rates of secretion were impaired by the ionophore without enhancing intracellular degradation. The decreased secretory rates accounted for the lower levels of procollagen and fibronectin in the culture medium; no evidence for increased catabolism of the secreted proteins was found. Secretion could be readily restored by removing the ionophore from the culture medium. The results indicate that procollagen and fibronectin may be simultaneously secreted, possibly utilizing a common pathway for secretion; the ionophores effectively interfere with cellular secretory pathways without impairing protein synthesis or protein glycosylation or altering protein catabolism.

Bacteriophage f1 infection: fate of the parental major coat protein

The major coat protein of infecting f1 phage is incorporated into the inner membrane of the host cell, even in the absence of phage f1DNA penetration and replication. The major coat protein monomers are reutilized in the assembly of new phage. They are not conserved as a single unit but behave as independent units which are slowly incorporated into newly manufactured phage.

Bacteriophage f1 infection and colicin tolerance

Bacteriophage f1-infected cells are relatively insensitive to a variety of colicins including E1, E2, and K; an f1 gene product is responsible for this insensitivity. A colicin-tolerant mutant cannot grow f1 phage; f1 DNA does not penetrate this mutant properly. One interpretation of these results is that an f1 gene product blocks the penetration of colicins which is necessary for their action, whereas the colicin-tolerant mutation blocks the penetration of colicins as well as phage f1 DNA.

Synthesis of the major bacteriophage fl coat protein

A method which allows one to follow the synthesis of the major f1 coat protein in normal, unirradiated f1-infected cells is reported. The N-terminal tryptic peptide of this protein, labeled with (14)C-lysine, has a negative charge at pH 4.5 and is readily separated from the contaminating peptides of host cell proteins. This technique was used to study several aspects of the synthesis of the major f1 coat protein in infected cells.