Effects of caffeine on crayfish muscle fibers. I. Activation of contraction and induction of Ca spike electrogenesis

Regulation of excitation-contraction coupling at the Drosophila neuromuscular junction

The Drosophila neuromuscular system is widely used to characterize synaptic development and function. However, little is known about how specific synaptic alterations effect neuromuscular transduction and muscle contractility, which ultimately dictate behavioural output. Here we develop and use a force transducer system to characterize excitation-contraction coupling at Drosophila larval neuromuscular junctions (NMJs), examining how specific neuronal and muscle manipulations disrupt muscle contractility. Muscle contraction force increased with motoneuron stimulation frequency and duration, showing considerable plasticity between 5 and 40 Hz and saturating above 50 Hz. Endogenous recordings of fictive contractions revealed average motoneuron burst frequencies of 20-30 Hz, consistent with the system operating within this plastic range of contractility. Temperature was also a key factor in muscle contractility, as force was enhanced at lower temperatures and dramatically reduced with increasing temperatures. Pharmacological and genetic manipulations of critical components of Ca²⁺ regulation in both pre- and postsynaptic compartments affected the strength and time course of muscle contractions. A screen for modulators of muscle contractility led to identification and characterization of the molecular and cellular pathway by which the FMRFa peptide, TPAEDFMRFa, increases muscle performance. These findings indicate Drosophila NMJs provide a robust system to correlate synaptic dysfunction, regulation and modulation to alterations in excitation-contraction coupling. KEY POINTS: Larval muscle contraction force increases with stimulation frequency and duration, revealing substantial plasticity between 5 and 40 Hz. Fictive contraction recordings demonstrate endogenous motoneuron burst frequencies consistent with the neuromuscular system operating within the range of greatest plasticity. Genetic and pharmacological manipulations of critical components of pre- and postsynaptic Ca²⁺ regulation significantly affect the strength and time course of muscle contractions. A screen for modulators of the excitation-contraction machinery identified a FMRFa peptide, TPAEDFMRFa and its associated signalling pathway, that dramatically increases muscle performance. Drosophila serves as an excellent model for dissecting components of the excitation-contraction coupling machinery.

Caffeine-induced contracture in oesophageal striated muscle of normotensive and hypertensive rats

To elucidate whether properties of the sarcoplasmic reticulum are altered, not only in vascular smooth muscle, but also in visceral striated muscle of spontaneously hypertensive rats (SHR), caffeine-induced contractures in oesophageal striated muscle of Wistar Kyoto rats (WKY) and stroke-prone SHR (SHRSP) were compared. In both preparations, 30 mM caffeine induced a contracture with two components. The second component, which was diminished by extracellular Ca(2+) removal or Ni(2+) but not by verapamil, was much smaller in SHRSP. Both components and differences between WKY and SHRSP coincided with changes in intracellular Ca(2+). Although membrane potential was identical between these preparations, caffeine induced slight depolarization only in WKY preparations. Similar depolarization was observed with 10 mM K(+), which induced no contraction. It is suggested that the first and the second components of caffeine-induced contracture were induced by Ca(2+) released from sarcoplasmic reticulum and by Ca(2+) that entered through channels activated by sarcoplasmic reticulum Ca(2+) depletion, respectively. In SHRSP preparations, Ca(2+) from the latter pathway was clearly decreased, although this change is thought not to be related to the initiation of hypertension. These results suggest that Ca(2+) handling properties of cell membrane and sarcoplasmic reticulum are generally altered in muscles of SHRSP.

Caffeine-evoked contractures in single slow (tonic) muscle fibres of the frog (Rana temporaria and R. esculenta)

Single slow (tonic) muscle fibres were dissected from cruralis muscles of Rana temporaria and R. esculenta. Increasing concentrations of caffeine were applied in Ringer solution, and contractures were measured isometrically. Sigmoid caffeine concentration-response curves were obtained, the threshold value being near 1.2 mmol/l, and maximum contractures being obtained with 10 to 20 mmol/l concentrations of caffeine. Contracture solutions were modified by varying the Ca2+ concentration or by replacing Ca2+ with 1.8 mmol/l Mg2+, Ni2+, Co2+ or with 0.1-5.0 mmol/l La3+. The effects of low pH (5.3), K+ (6,10 and 95 mmol/l), adenosine (10 mmol/l) and gallopamil (D600; 30 micromol/l) were examined too. The caffeine threshold was lowered by Mg2+, K+, 0 .1 mmol/l La3+ and D600, while all other substances including 0.5-5.0 mmol/l La3+ increased it. The amplitude of contractures evoked by high caffeine concentrations was unaffected. Caffeine (1-40 mmol/l) was also pressure injected into slow fibres. The composition of the solution was modified in a number of ways, but a contractile response was not observed or measured. Extracellular application of caffeine from the same pipettes evoked local contractures. Similar injection experiments in twitch fibres revealed the same results. These observations suggest that an extracellular binding site seems to be involved in the initiation of caffeine-evoked contractures in intact frog muscle fibres. Possible reasons for the ineffectiveness of intracellular caffeine are discussed.

Calcium-induced calcium release in crayfish skeletal muscle

1. Cut crayfish skeletal muscle fibres were mounted in a triple Vaseline-gap voltage clamp with the Ca(2+)-sensing dye Rhod-2 allowed to diffuse in via the cut ends. Ca2+ currents across the surface/T-tubule membranes (ICa) were recorded simultaneously with changes in myoplasmic Ca2+ concentration (Ca2+ transients). 2. Excitation-contraction coupling in crayfish skeletal muscle fibres is abolished when calcium in the extracellular solution is replaced by Mg2+. 3. The amplitude of the Ca2+ transients elicited by voltage clamp pulses closely followed the amplitude of the peak calcium currents recorded simultaneously across the surface/T-tubule membranes. This included decreases in both parameters as the pulse potential approached ECa (reversal potential for Ca2+), as well as secondary Ca2+ transients accompanying large tail calcium currents occurring upon repolarization from very large depolarizations. 4. A large contribution of sarcoplasmic reticulum (SR) Ca2+ release to the Ca2+ transients was revealed by a large decrease in the transient caused by the calcium-induced calcium release (CICR) blockers procaine and tetracaine. 5. Short pulses which interrupted the calcium current while SR Ca2+ release was in progress at high rates caused the Ca2+ transient to stop rising nearly immediately after the end of the pulse in most fibres. In about 15% of the fibres the Ca2+ transients continued to rise, albeit at a slower rate, for 10-20 ms after the end of the pulse, as if released Ca2+ was able to elicit some further Ca2+ release from the SR for a while. 6. Even with fibres displaying little sign of continued release after termination of short pulses under control conditions, procaine accelerated the decay of Ca2+ transients elicited by short pulses, indicating that continued release was taking place even as the transient was declining. 7. These results suggest that CICR in crayfish fibres is more closely controlled by a small entry of Ca2+ via surface/T-tubule membrane Ca2+ current than by a larger amount of Ca2+ released from the SR. The limited positive feedback of released Ca2+ on further Ca2+ release allows CICR to remain graded (according to ICa) rather than all-or-none.

Retinomotor movements in the frog retinal pigment epithelium: dependence of pigment migration on Na+ and Ca2+

The ionic dependence of the screening-pigment migrations in the frog retinal epithelium (RPE) was quantitatively studied with eyecups incubated in media of different compositions. Typical migrations in response to light and darkness, equivalent to those observed in the intact animal, were fully accomplished and maintained for up to 6 hr by the isolated organ bathed in Ringer solution rich with O2. Pigment migration in either direction was completed under the appropriate illumination conditions at any time during the day, indicating that circadian influences, if present in the intact animal, can be overridden in the isolated organ by light or darkness alone. Pigment aggregation toward the dark-adapted position was inhibited by: (a) low external Ca2+, (b) high external Na+, and (c) drugs expected to increase the cytoplasmic levels of either Na+, or Ca2+, like ouabain, caffeine and the ionophore A23187. However, the inhibition caused by low Ca2+ did not occur if Na+ was also reduced in the incubation medium. On the other hand, an increase in the concentration of external Ca2+ or the addition of Co2+ to the normal Ringer facilitated pigment aggregation in the dark. Pigment dispersion to the light-adapted position was unaffected by any of the above conditions. This is the first report of full and stable pigment responses in the RPE of vertebrate eyes incubated under simple physiological conditions. The results seem to conciliate a discrepancy of previous reports on the Ca2+ dependence of RPE movements, and are compatible with current views on ionic mechanisms in analogous systems of intracellular transport.

Octopamine action on the contractile system of crustacean skeletal muscle

1. In the opener muscle of walking legs of crayfish (Astacus leptodactylus) octopamine (OA) greatly enhances the contractions resulting from brief applications of L-glutamate or of elevated K-concentrations. Synephrine is as effective as OA. 2. In the case of potentiation of responses to high-K applications a presynaptic component of the OA action was excluded by first desensitising the muscle fibres to the action of the natural transmitter, using a high concentration (1 mM) of glutamate. 3. The Ca-antagonists Co, Ni and Mn (1 mM) reduced the effects of glutamate and of elevated K to about one-half. In preparations treated with OA, the same Ca-antagonists also depressed the potentiated contractural responses to glutamate and to elevated K, again to about one-half. 4. OA also enhanced contractions resulting from the application of caffeine. 5. With 5-hydroxytryptamine (5-HT) application, the same postsynaptic effects were obtained as described for OA, except that the 5-HT actions were much weaker. 6. With OA, maximal effects were obtained with concentrations of 5 x 10(-6)-10(-5) M; maximally effective concentrations of 5-HT were around 10(-5) M. 7. The lowest effective concentrations of OA were around 10(-8) M; those of 5-HT were around 10(-7) M. 8. In the same preparation, 5-HT is far more effective in enhancing transmitter release (presynaptic action) than OA, the lowest effective concentration being around 10(-11) M while no presynaptic effects of OA were seen at concentrations below 10(-8) M, in some cases even below 10(-5) M.

Caffeine contracture in the cultured chick myotube

A possible function of Ca store site in cultured chick myotubes was examined by recording contraction of the myotube with special reference to the effect of caffeine. Caffeine at low concentrations (below 1 mM), applied focally on the myotube through a micropipette with a pressure pulse, elicited focal contraction without membrane potential changes. Procaine inhibited the caffeine contracture. Deuterium oxide also inhibited the caffeine contracture at low concentrations, but enhanced the maximal contracture. These observations are similar to those in the mature frog muscle fiber in which the sarcoplasmic reticulum (SR) is a main site of caffeine action. On the basis of these similarities, it was considered that caffeine acts on SR to elicit contracture in the myotube. The ability of SR to accumulate and release Ca ion seemed to be low, because caffeine contracture decreased or disappeared in a Ca-free solution in many myotubes.

Caffeine inhibition of calcium accumulation by the sarcoplasmic reticulum in mammalian skinned fibers

Oxalate-supported Ca accumulation by the sarcoplasmic reticulum (SR) of chemically skinned mammalian skeletal muscle fibers is activated by MgATP and Ca2+ and partially inhibited by caffeine. Inhibition by caffeine is greatest when Ca2+ exceeds 0.3 to 0.4 microM, when free ATP exceeds 0.8 to 1 mM, and when the inhibitor is present from the beginning of the loading period rather than when it is added after Ca oxalate has already begun to precipitate within the SR. Under the most favorable combination of these conditions, this effect of caffeine is maximal at 2.5 to 5 mM and is half-maximal at approximately 0.5 mM. For a given concentration of caffeine, inhibition decreases to one-half of its maximum value when free ATP is reduced to 0.2 to 0.3 mM. Varying free Mg2+ (0.1 to 2 mM) or MgATP (0.03 to 10 mM) has no effect on inhibition. Average residual uptake rates in the presence of 5 mM caffeine at pCa 6.4 range from 32 to 70% of the control rates in fibers from different animals. The extent of inhibition in whole-muscle homogenates is similar to that observed in skinned fibers, but further purification of SR membranes by differential centrifugation reduces their ability to respond to caffeine. In skinned fibers, caffeine does not alter the Ca2+ concentration dependence of Ca uptake (K0.5, 0.5 to 0.8 microM; Hill n, 1.5 to 2.1). Reductions in rate due to caffeine are accompanied by proportional reductions in maximum capacity of the fibers, and this configuration can be mimicked by treating fibers with the ionophore A23187. Caffeine induces a sustained release of Ca from fibers loaded with Ca oxalate. However, caffeine-induced Ca release is transient when fibers are loaded without oxalate. The effects of caffeine on rate and capacity of Ca uptake as well as the sustained and transient effects on uptake and release observed under different conditions can be accounted for by a single mode of action of caffeine: it increases Ca permeability in a limited population of SR membranes, and these membranes coexist with a population of caffeine-insensitive membranes within the same fiber.

Calcium and magnesium contents and volume of the terminal cisternae in caffeine-treated skeletal muscle

(a) The effects of caffeine on the composition and volume of the terminal cisternae (TC) of the sarcoplasmic reticulum (SR) in frog skeletal muscle were determined with rapid freezing, electron microscopy, and electron probe analysis. (b) Caffeine (5 mM) released approximately 65% of the Ca content of the TC in 1 min and 84% after 3 min. The release of Ca from the TC was associated with a highly significant increase in its Mg content. This increase in Mg was not reduced by valinomycin. There was also a small increase in the K content of the TC at 1 min, although not after 3 min of caffeine contracture. (c) On the basis of the increase in Mg content during caffeine contracture and during tetanus (Somlyo, A. V., H. Gonzalez-Serratos, H. Shuman, G. McClellan, and A. P. Somlyo, 1981, J. Cell Biol., 90:577-594), we suggest that both mechanisms of Ca release are associated with an increase in the Ca and Mg permeability of the SR membranes, the two ions possibly moving through a common channel. (d) There was a significant increase in the P content of the TC during caffeine contracture, while in tetanized muscle (see reference above) there was no increase in the P content of the TC. (e) Mitochondrial Ca content was significantly increased (at 1 and at 3 min) during caffeine contracture. Valinomycin (5 microM) blocked this mitochondrial Ca uptake. (f) The sustained Ca release caused by caffeine in situ contrasts with the transient Ca release observed in studies of fragmented SR preparations, and could be explained by mediation of the caffeine-induced Ca release by a second messenger produced more readily in intact muscle than in isolated SR. (g) The TC were not swollen in rapidly frozen, caffeine-treated muscles, in contrast to the swelling of the TC observed in conventionally fixed, caffeine-treated preparation, the latter finding being in agreement with previous studies. (h) The fractional volume of the TC in rapidly frozen control (resting) frog semitendinosus muscles (approximately 2.1%) was less than the volume (approximately 2.5%) after glutaraldehyde-osmium fixation.

Permeability changes induced by L-glutamate at the crayfish neuromuscular junction

Two different methods are described which allow the reversal potential (Er) for the channels opened by L-glutamate at the voltage-clamped, crayfish neuromuscular junction to be measured accurately. In both cases the value of Er was found to be about +6 mV. Reversal potentials were also measured in solutions where Na+ was replaced by K+, Ca2+, or Mg2+; or in which Cl- was replaced by isethionate. In solutions where Na+ was partially replaced by K+, the measured reversal potentials were compared to theoretical values predicted by both the constant-field and equivalent-circuit equations. The experimental values were more accurately described by the constant-field equation. Permeability ratios (PX/PNa) for K+, Ca2+, Mg2+, and Cl- were calculated using the constant-field equation. K+ and Na+ were equally permeant while Ca2+ and Mg2+ were about half as permeant as the monovalent cations. Cl- was impermeant. The results of these experiments indicate that the L-glutamate activated channel is non-selective for cations. Furthermore, the value of the permeability ratios for the physiological cations tested are very similar to those obtained for the acetylcholine activated channel in vertebrate skeletal muscle.

Contractions induced by sodium withdrawal in crab (Callinectes danae) muscle fibres

A study was made of the effects of Na removal on the resting tension of single muscle fibres of the crab Callinectes danae. Reduction of [Na]o (replacement with Li, Tris or choline) below a threshold value, typical for each fibre, induced spontaneous, local contractions randomly dispersed along the fibres; this was followed by propagated contractile waves and tension oscillations. Sustained contractures were occasionally seen at threshold [Na]o and were consistently observed when [Na]o was further reduced. The Na withdrawal contractions depended on [Ca]o and were abolished in Ca-free media; they were restored within seconds after the addition of Ca (3-12 mM) or Sr (15-25 mM), but not Ba (10-100 mM), to the media. Caffeine (0.2-1.0 mM) facilitated, whereas La (2-5 mM), procaine (1 mM) or lidocaine (10 mM) inhibited the Na-withdrawal contractions. It is concluded that increased Ca influx across the sarcolemma and release of stored Ca from the sarcoplasmic reticulum are involved in the contractions induced by Na-deficient solutions in crab fibres.

Structure of membranes in crayfish muscle: comparison of phasic and tonic fibres

Membranes of two crayfish muscles with different contraction speeds were studied with freeze-fracture replicas and thin sections. A fast-contracting, short sarcomere phasic muscle, the tail flexor, and a slowly contracting long sarcomere tonic muscle, the carpopodite flexor, were chosen for this study. Membranes examined included the plasmalemma, clefts, T-system, Z-tubules and sarcoplasmic reticulum (SR). We found distinct differences in the distribution of these membranes: T-system and clefts are more elaborate in the tail flexor, while SR is more extensive in the leg flexor. The tail flexor apparently lacks Z-tubules. These differences were more obvious in freeze-fracture replicas than in thin sections. In freeze-fracture replicas, both junctional and non-junctional T-tubule membranes can be distinguished from Z-tubules by content of intramembranous particles. The junctional regions of T-system and surface membranes contain large (10-11 nm) intramembranous particles that are absent from non-junctional parts of these membranes. There is also a class of particles on the junctional SR fracture faces that differs from intramembranous particles on non-junctional SR. These junctional specialization are similar in long and short sarcomere fibres.

Action of caffeine in excitation-contraction coupling of frog skeletal muscle fibres

1. Frog sartorius muscle bathed in 1 mM-caffeine generates brief asynchronous contraction of individual sarcomeres, 'sarcomeric oscillations', and propagated contractile waves. 2. Analysis of cinematographic records shows that during sarcomeric oscillations the sarcomere length decreases and the distribution of sarcomere lengths is wider than in controls. 3. Caffeine can produce sarcomeric oscillations in K depolarized muscle fibres and, to a limited extent, in glycerol-treated muscle fibres. 4. Treatment of muscle with dantrolene sodium blocks production of sarcomeric oscillations by caffeine. 5. In caffeine-treated muscle fibres, electrically produced depolarization could initiate or increase the frequency of sarcomeric oscillations, and electrical hyperpolarization diminishes the frequency or stops sarcomeric oscillations. 6. Caffeine solution bathing a muscle undergoing sarcomeric oscillations (the perfusate), when applied to a fresh muscle, initiates sarcomeric oscillations with a relatively short latency. 7. An U.V.-absorbance peak at 245 nm develops in the caffeine solution bathing a muscle undergoing sarcomeric oscillations. 8. It was found that a contraction-regulating substance (oscillogen) is released from a muscle undergoing sarcomeric oscillations. From results of selective dialysis and gel permeation chromatography, the molecular weight of the oscillogen is estimated to be between 700-1000 daltons. 9. It is proposed that the oscillogen is a normally occurring substance which functions in excitation-contraction coupling at the T-tubule terminal cistern junction in skeletal muscle.

Caffeine blocks the delayed K+ outward current of molluscan neurons

Extracellular application of caffeine inhibits the delayed K+ outward current of Aplysia neurons in a dose dependent manner without changing the kinetics. Half-maximum blockade is produced with a concentration of 16.0 +/- 0.7 mM (S.E.M.) caffeine after 1-2 min. Intracellular injection of caffeine has an almost immediate blocking effect. The evidence suggests that the blocking site is at or close to the inner surface of the cellular membrane.

Synaptic potentials in sympathetic ganglia: are they mediated by cyclic nucleotides?

The hypothesis that cyclic nucleotides are intracellular second messengers mediating the generation of synaptic potentials was studied in the sympathetic ganglia of the bullfrog. Synaptic potentials and the effect of administering cyclic nucleotides and agents which affect cyclic nucleotide metabolism were recorded by the sucrose gap technique. The administration of adenosine 3',5'-monophosphate (cyclic AMP), guanosine 3',5'-monophosphate (cyclic GMP), or several of their derivatives produced little or no change in membrane potential. Prostaglandin E1 did not block the generation of postsynaptic potentials. Theophylline produced membrane effects that were different from those associated with postsynaptic potential generation; it also reduced the slow excitatory postsynaptic potential (EPSP) and potentiated the slow inhibitory postsynaptic potential (IPSP). The administration of papaverine, however, reduced both the slow EPSP and the slow IPSP. Although synaptic stimulation increases both cyclic GMP and cyclic AMP in these neurons, these results raise the possibility that these cyclic nucleotides may have functionla roles other than mediation of synaptic potentials.

Unusual thick and thin filament packing in a crustacean muscle

The proximal accessory flexor (PAF) of the myochordotonal organ (MCO) in the meropodite of crayfish walking legs contains two populations of muscle fibers which are distinguishable by their diameters. The large accessory (LA) fibers are 40-80 micrometer in diam and are similar in ultrastructure to other slow crustacean fibers. The small accessory (SA) fibers are 1-12 micrometer in diam and have a unique myofilament distribution at normal body lengths. There is extensive double overlap of thin filaments at these lengths, and some of them form bundles that may extend the length of the sarcomere. In the middle of the sarcomeres, thick and thin filaments are totally segregated from each other. When the fibers are stretched to lengths beyond double overlap length, the myofilament patterns are conventional. The segregated pattern is reestablished when stretched fibers are allowed to shorten passively. The length-tension relationship of the SA fibers is described by a linear ascending branch, a plateau, and a linear descending branch. The ascending branch encompasses normal body lengths from slack length (Ls) with maximum double overlap to the length at which double overlap ceases (1.8 X Ls). The descending phase is comparable to that of other skeletal muscles. That is, tension decreases in proportion with the reduction in thick-thin filament interdigitation (2 X Ls to 3 X Ls).

Recovery of ultrastructural changes accompanying caffeine contractures in isolated muscle fibres of the crayfish

Reversible caffeine contractures in isolated muscle fibres of the crayfish (Astacus fluviatilis; m. extensor carpopoditi) are accompanied by swelling of the diadic cisternae of the sarcoplasmic reticulum and by constriction of the T-system tubules. The membranes of the tubules may coalesce and form pentalaminar structures. These changes in the diadic region can be found already in fibres fixed 10 s after the sudden introduction of 4 mM or 6 mM caffeine solution. The changes are completely reversible. The recovery period after a 6 mM caffeine contracture lasts 20 min. The restitution of ultrastructural changes induced by caffeine correlates well with the recovery of responsiveness to a repeated application of caffeine. The pretreatment of fibres with procaine prevents both the generation of caffeine contractures and the occurrence of changes in the ultrastructure. No changes or a very slight enlargement of diadic cisternae were found during potassium contractures of comparable size and duration to caffeine contractures. The effect of procaine as well as the restitution of ultrastructural changes after the recovery of responsiveness to caffeine support the view that the cisternal swelling is a direct consequence of the calcium releasing action of caffeine. Isolated swelling of diadic cisternae and the absence of any changes in the remaining parts of the sarcoplasmic reticulum during reversible caffeine contractures could point to the existence of functional differentiation of the respective parts of the sarcoplasmic reticulum. The absence of ultrastructural changes during potassium contractures might indicate a different source of the activating calcium for contraction and/or a different mechanism of its release.

Cyclic nucleotides, thioldisulfide status of proteins, and cellular control processes

It is shown that cyclic nucleotides can have a variety of effects on cell division, cell shape, cell adhesion, and cell movement, depending on the cells selected and the conditions under which they are used. For example, while CHO cells elongate under the influence of exogenous dibutyryl CAMP, Y-1 adrenal tumor cells round up and polyoma-transformed 3T3 cells show no change in shape. The totality of experience with cyclic nucleotides suggests that where they have been used by cells as control elements involving the four processes listed above, they are superimposed on basic cellular processes that progress in their absence--that is, they must be acting indirectly. In attempting to understand the inhibitory action of methyl xanthines on egg development, we were forced to abandon the idea that they acted through cyclic nucleotides. We found that methyl xanthines inhibited the activation of glutathione reductase and that glutathione oxidizing agents act as mitotic inhibitors. Further, we found that tubulin polymerizability, NAD-kinase activity, and a mitotic apparatus associated Ca+2-ATP-ase were all inhibited by oxidation of some of their sulfhydryls and were activated by reduction of the resulting disulfides. These results are discussed in terms of reported cycles and activations of glutathione reductase (GR) in cells and reports that mixed disulfides of glutathione and proteins can act as substrates for GR. Using the fact that a CAMP-dependent protein kinase has been reported to be activated by glutathione, we have suggested potential sites where sulfhydryl control processes and cyclic nucleotide control processes and cyclic nucleotide control processes may interact in certain restricted cases.

Sites of action of D2O in intact and skinned crayfish muscle fibers

The effect on tension development of replacing 90% of the H2O of the bathing saline with D2O was studied on intact single fibers, and on skinned fibers before and after the latter were treated so as to eliminate Ca-accumulation by the sarcoplasmic reticulum (SR). Excitation-contraction coupling (ECC) of intact fibers is not abolished, but is depressed by D2O so that higher depolarizations are required to elicit a given tension. The reduction in tension at a given level of depolarization is not due to inhibition of the contractile system. The latter showed an enhanced Ca sensitivity; that is, skinned fibers respond to Ca concentrations that are 1-2 orders of magnitude smaller in D2O than in H2O saline. When bathed in D2O saline, intact fibers or skinned fibers with functional SR can still accumulate and release Ca in sufficient quantities to allow repeated induction of maximum tensions. Relaxation is slowed in all three types of preparation, perhaps because of an increased affinity of troponin to Ca in D2O salines.

The effects of caffeine on the contraction of the frog heart

1. The force of contraction and the membrane potentials have been measured from preparations of frog heart, with methods that also allow the rapid exchange of the extracellular fluid.2. In regularly beating preparations, caffeine induces only weak contractures at temperatures above 15 degrees C, but it does cause a marked potentiation of the twitch responses; longer exposure results in a depression of contraction. The build up and decline of the twitch strength, on addition and on removal of caffeine approximates to a single exponential, time constant 26-45 sec, and this time constant is not altered by variation of the [Ca](o), the stimulus rate or the caffeine concentration. This time course of the change of twitch strength is less complex than the changes seen when either [Ca](o) or the stimulus rate is altered, suggesting a more direct action of caffeine on excitation-contraction coupling.3. Caffeine increases the strength of the contractures initiated by potassium-rich or sodium-depleted solutions in isolated atrial trabeculae.4. After the spontaneous relaxation of the contracture, evoked by either sodium-free or potassium-rich fluids, the application of caffeine initiates a redevelopment of tension. This caffeine contracture is transient and its strength is dependent on the caffeine concentration. The response in sodium-free solution can be elicited in the virtual absence of extracellular calcium.5. Local anaesthetics antagonize the caffeine contracture.6. The results suggest that the sarcoplasmic reticulum of frog heart muscle plays an important role in the initiation and control of contraction and relaxation.

Structure-activity relations for caffeine: a comparative study of the inotropic effects of the methylxanthines, imidazoles and related compounds on the frog's heart

1. The ability of several groups of compounds, related to caffeine, to induce contractures in isolated frog auricular trabeculae has been tested.2. Of the methylxanthines, theophylline, theobromine and paraxanthine are of similar potency to caffeine. This applies to contractures produced in either high-potassium or in sodium-free solution, and to the twitch responses in normal Ringer.3. Xanthines in which the 9-position nitrogen is combined and is, therefore, without an ethylene bond do not affect contraction.4. The hypothesis is put forward that a double-bonded nitrogen, in an imidazole ring, is required for activity of the methylxanthine. This hypothesis is supported by the ability of imidazole and several close derivatives (e.g. histamine), as well as imidazolines, to evoke contractures. As predicted by the hypothesis, imidazolidines and imidazolidones, in which all the nitrogen atoms have single bonds, fail to initiate tension development.5. The activity of histamine and histidine is only demonstrable at high pH ( bumpy equals, equals 9.0).6. Raising the pH in sodium-free solution induces a transient contracture.7. Several arguments suggest that cyclic AMP is probably not an intermediate in the response to the methylxanthines. The activity of cyclic AMP (and adenosine) in eliciting contractures is predicted by the hypothesis because they contain an imidazole moiety as part of their molecular structure.

The effects of caffeine on sodium transport, membrane potential, mechanical tension and ultrastructure in barnacle muscle fibres

1. The effects of graded concentrations of caffeine on the Na efflux were investigated. External application of 10 mM caffeine usually caused a biphasic response, viz. a fall, followed by a rise in the Na efflux. 1 and 5 mM caffeine usually caused stimulation. Only the stimulatory phase of this response depended on the presence of external Ca(2+).2. Internal application of 100 mM caffeine caused a small rise in the Na efflux, the magnitude of which was independent of external Ca(2+) and comparable to that obtained with external application of 1 mM caffeine. This action, however, could be greatly augmented by pre-treating the fibre with 5 x 10(-5)M ouabain.3. The rise in Na efflux caused by external application of 10 mM caffeine could be greatly augmented by pre-treating the fibre with 5 x 10(-5)M ouabain. The observed stimulatory response was biphasic, more so in the absence of external Ca(2+). Restoration of external Ca(2+) following the onset of the second stimulatory phase resulted in further rise of the Na efflux. Measurements of the Na efflux during treatment with graded concentrations of ouabain and 10 mM caffeine showed that the rate coefficient for Na efflux varied with the ouabain concentration in the range 10(-8)-10(-4)M. Measurements of the ouabain-insensitive Na efflux before and during treatment with 10 mM caffeine in bathing media containing varying concentrations of Ca, disclosed the existence of two Ca(2+)-thresholds, one in the 0-2.5 mM range and the other in the 12.5-15 mM range.4. Comparisons were made between the effects on the Na efflux of 10 mM caffeine followed by external acidification, and external acidification, followed by 10 mM caffeine. The magnitude of the response of the ouabain-insensitive Na efflux to external acidification before treatment with 10 mM caffeine was greater than that found when external acidification followed external application of the alkaloid. It also was considerably greater than that of the response to external application of 10 mM caffeine before external acidification.5. External application of 10 mM procaine prevented 10 mM caffeine from stimulating the Na efflux, and from inducing contractures. Internal application of 100 mM-EGTA reduced the response of the Na efflux to 10 mM caffeine, and also prevented the fibre from contracting. External application of 10(-4)M diphenylhydantoin reduced the response of the Na efflux to 10 mM caffeine but failed to prevent the development of contractures.6. Internal application of 0.05 M-cGMP, cAMP or its dibutyryl derivative caused a large rise in the Na efflux. The magnitude of the effects observed in ouabain-poisoned fibres was often greater than that in unpoisoned fibres. Internal application of 2.5 units/ml. phosphodiesterase beforehand failed to reduce the magnitude of the stimulatory response to injected cyclic nucleotides. Injected phosphodiesterase also failed to reduce the response of the Na efflux to 10 mM caffeine.7. External application of 10 mM caffeine to unpoisoned and ouabain-poisoned fibres caused a fall of approximately 10 mV in the membrane potential. In unpoisoned fibres this effect was transitory. The response of the membrane potential to internal application of graded concentrations of CaCl(2) was biphasic. When low concentrations of CaCl(2) were used the membrane potential underwent a small rise but when high concentrations were used the opposite was found. These results could not be repeated with graded concentrations of MgCl(2).8. The effects of graded concentrations of caffeine on tension development were also studied. Strong contractures were observed with caffeine concentrations as low as 4 mM, while peak tetanus tension was usually exceeded with 7-8 mM concentrations. The tension-external Ca(2+) curve was sigmoidal in shape.9. Electron microscopic studies showed that 10 mM caffeine in ASW caused little or no distension and disorganization of cisternal fine structure. Such structural changes, however, were far more pronounced in fibres suspended in Ca(2+)-free ASW and then treated with 10 mM caffeine in Ca(2+)-free ASW. Fibres soaked in Ca(2+)-free ASW had ruptured mitochondria and mitoplasts, whereas those additionally treated with 10 mM caffeine had relatively intact mitochondria.10. The main conclusions drawn from this work are: (i) that caffeine stimulates the ouabain-insensitive Na efflux (and inhibits the transport enzyme) by raising the internal free Ca(2+) concentration; (ii) that in the presence of inhibition of the transport enzyme, the magnitude of the stimulatory response to 10 mM caffeine depends not only on the external Ca(2+) concentration but mainly on the residual level of activity of the transport enzyme; (iii) that the Ca(2+)-sensitive and CO(2)-sensitive components of the ouabain-insensitive Na efflux, though not the same, may overlap at the level of the plasma membrane or share a common metabolic step away from the membrane; (iv) that cyclic nucleotides participate in the control of the magnitude of the ouabain-insensitive Na efflux, and that the phosphodiesterase system under the present experimental conditions does not seem to be involved in the mechanism underlying the stimulatory action of caffeine; (v) that the membrane potential changes caused by caffeine fail to explain the stimulatory response of the Na efflux, as well as contracture of these fibres; (vi) that the contractile machinery in these fibres is considerably more sensitive to caffeine than that in vertebrate muscle and the muscles of other arthropods, and (vii) that the Ca(2+) released by the sarcoplasmic reticulum which leads to a biphasic response of the Na efflux is the result of cisternal disorganization caused by caffeine.

Calcium efflux from barnacle muscle fibers. Dependence on external cations

Calcium-45 was injected into single giant barnacle muscle fibers, and the rate of efflux was measured under a variety of conditions. The rate constant (k) for (45)Ca efflux into standard seawater averaged 17 x 10(-4) min(-1) which corresponds to an efflux of about 1-2 pmol/cm(2).s. Removal of external Ca (Ca(o)) reduced the efflux by 50%. In most fibers about 40% of the (45)Ca efflux into Ca-free seawater was dependent on external Na (Na(o)); treatment with 3.5 mM caffeine increased the magnitude of the Na(o)-dependent efflux. In a few fibers removal of Na(o), in the absence of Ca(o), either had no effect or increased k; caffeine (2-3.5 mM) unmasked an Na(o)-dependent efflux in these fibers. The Na(o)-dependent Ca efflux had a Q(10) of about 3.7. The data are consistent with the idea that a large fraction of the Ca efflux may be carrier-mediated, and may involve both Ca-Ca and Na-Ca counterflow. The relation between the Na(o)-dependent Ca efflux and the external Na concentration is sigmoid, and suggests that two, or more likely three, external Na(+) ions may activate the efflux of one Ca(+2). With a three-for-one Na-Ca exchange, the Na electrochemical gradient may be able to supply sufficient energy to maintain the Ca gradient in these fibers. Other, more complex models are not excluded, however, and may be required to explain some puzzling features of the Ca efflux such as the variable Na(o)-dependence.

Muscle contraction during hyperpolarizing currents in the crab

Isolated muscle fibers from the motor legs of the crab Trichodactilus dilocarcinus were submitted to strong hyperpolarizing currents of varied intensities which produced tension during the current pulse. Threshold for tension was obtained with intensities of about 0.2 x 10(-5) A, changing E(m) to ca. -150 mV (starting from a resting potential ofca. -80 mV). At the closure of the anodic square pulse, a second phase of tension usually appeared superimposed upon the one obtained during hyperpolarization. The first phase of tension increased with the increase of Ca(++) concentration in the bath. Sr(++) produced the same type of mechanical output as Ca(++). When added to the normal Ca(++) concentration, Ba(++) and Mn(++) in low concentrations (up to 21.5 mM) also increased the tension of this phase, but at higher concentrations they blocked both phases while Mg(++) did not alter the tension. Of all the divalent cations employed, only Sr(++) is capable of developing tension as a substitute for Ca(++) in the external media. Procaine administered in a dosage (5 x 10(-3) W/V)which would suppress the contracture due to caffeine (10 mM), did not modify the tension developed during the hyperpolarization. The preceding data indicate that the Ca(++) required for tension during hyperpolarization comes from sites which would differ from those usually postulated for tension due to depolarization in the muscle fibers of other crustaceans (American crayfish). Furthermore, the external source of Ca(++) appears to be one mainly implicated in the induction of tension due to inward current pulses.

The myofilament lattice: studies on isolated fibers. 3. The effect of myofilament spacing upon tension

The effect of ionic strength on the generation of tension and upon the interfilament spacing in living intact and skinned single striated muscle fibers from the walking leg of crayfish (Orconectes) were determined by isometric contraction studies correlated with low-angle X-ray diffraction. Sarcomere lengths were determined by light diffraction. Tensions were induced in intact fibers by caffeine in the bathing medium and by ionophoretic microinjection of calcium. Tensions were induced in skinned fibers by a buffered calcium-EGTA solution. The interfilament spacing of intact and skinned fibers over the range of ionic strengths investigated were determined by X-ray diffraction and correlated with the physiological data. It is demonstrated that the ionic strength affects the tension-generating capacity of the muscle as it affects the chemo-mechanical transform of excitation-contraction coupling. It is further demonstrated that interfilament spacing changes encountered during shortening and with variation in the osmotic strength have no effect upon the tension-generating capacity of muscle.

Effects of rapid cooling on the mechanical and electrical activities of smooth muscles of guinea pig stomach and taenia coli

The effects of rapid cooling on the mechanical and electrical activities of the guinea pig taenia coli and circular muscle of the stomach were investigated. Lowering the temperature from 32 degrees to 10 degrees C (cold shock) depolarized the membrane and increased the membrane resistance in both tissues. However, in the taenia coli, an initial reduction of membrane resistance was observed. In both tissues, contracture evoked by cold shock and rapid relaxation after rewarming, preceded the changes of membrane properties. Displacements of the membrane potential did not modify the amplitude of contracture under cold shock. Caffeine and thymol modified the membrane properties, but the effects of cold shock were still observed. The effects of cold shock were also observed on K-induced contracture. It was postulated that at least two different sites of sequestered bound Ca are located in these smooth muscles and are responsible for evoking the mechanical response. One component possesses a close relation to membrane and the other component is presumably sequestered within the muscle.

The interaction between caffeine and calcium in the desensitization of muscle postjunctional membrane receptors

The interaction between caffeine and calcium on the rate of desensitization of muscle postjunctional membrane (PJM) receptors during the sustained application of 0.27 mM carbamylcholine (CARB) has been studied in vitro on the sartorius muscle of the frog. The rate of PJM repolarization with CARB added to the solution bathing the muscle or the recovery of the effective transmembrane resistance (EMR) during the microperfusion of CARB directly onto the end-plate region of individual fibers was used as an index of the rate of desensitization. Caffeine (1.5 mM) increased the rate of PJM repolarization with bulk application of CARB in a 1.8 or 10 mM calcium Ringer solution but had no effect on PJM repolarization in a calcium-deficient, 4 mM magnesium Ringer solution. For EMR measurements the preparation was rendered mechanically quiescent by repeated challenges with isotonic KCl during an exposure of several hours to a calcium-free, 4 mM magnesium-1 mM EGTA Ringer solution. In these fibers, the microperfusion of 0.27 mM CARB together with 1.8 mM calcium plus 1.5 mM caffeine significantly increased the rate of EMR recovery above that observed in the absence of caffeine. Raising the calcium concentration to 10 mM had a similar effect; however, no additional increase was noted by the inclusion of 1.5 mM caffeine. It is suggested that the major role of caffeine in PJM desensitization is to increase the calcium permeability of the surface membrane. The transmembrane movement of calcium and the consequent intracellular accumulation of calcium is seen as a critical factor in controlling the rate of PJM desensitization.

Excitation-contraction coupling in amphioxus muscle cells

1. Excitation-contraction coupling was studied in myotomal muscles of amphioxus, Branchiostoma californiense.2. The action potential of a muscle cell produces a twitch with a rise time of 30-40 msec at 11 degrees C and its Q(10) is about 2.2.3. The twitch increases in amplitude with increasing external Ca concentration and is abolished in Ca-free saline (1 mM-EGTA and 55.7 mM-MgCl(2)); the twitch amplitude is suppressed by Co or La ions.4. Caffeine at concentrations above 1 mM in the external saline causes a prolongation of the action potential and a contracture which lasts several minutes.5. After exposure to caffeine the responsiveness of the muscle to subsequent applications of caffeine recovers in normal saline in 20-30 minutes but not in Ca-free saline.6. The amplitude of the caffeine contracture is independent of the external Ca concentration and is unaltered after the twitch is eliminated in Ca-free saline.7. After exposure to caffeine a full-sized twitch can be obtained before the responsiveness to caffeine shows any significant recovery.8. It is concluded that the twitch is produced by the Ca influx resulting from the increased permeability of the muscle cell membrane to Ca during the action potential and that the Ca mobilized by caffeine is not necessary to the initiation of the twitch.9. Electronmicroscopy shows the existence of sarcoplasmic reticulum.

The effect of quinine on tension development, membrane potentials and excitation-contraction coupling of crab skeletal muscle fibres

1. The effect of quinine on tension development and membrane potentials of crab skeletal muscle was examined using strain gauges and intracellular electrodes.2. In low concentrations (0.1-0.5 mM), quinine caused transient potentiation of twitch tension which then rapidly declined along with progressive depression of the tetanus. These actions are correlated with the decline of both action and resting potentials during quinine treatment.3. In moderate concentrations (1-5 mM), quinine induced phasic contractures, but the attendant depolarization made the muscles refractory to stimulation and potassium activation, but not to caffeine activation.4. Quinine did not induce contractures in depolarized muscle, which suggests that the action of quinine in inducing calcium release from the sarcoplasmic reticulum may be blocked by potassium depolarization, unlike the calcium-releasing action of caffeine. Quinine appeared to have no effect on the mechanical threshold of crab skeletal muscle fibres.5. To explain its depression of contractility in crab muscle, it is suggested that quinine may deplete the calcium store of the sarcoplasmic reticulum, leading to extinction of the terminal stages of the excitation-contraction coupling process and loss of contractility.