The effects of caffeine on the contraction of the frog heart

Extensive eccentric contractions in intact cardiac trabeculae: revealing compelling differences in contractile behaviour compared to skeletal muscles

Force enhancement (FE) is a phenomenon that is present in skeletal muscle. It is characterized by progressive forces upon active stretching-distinguished by a linear rise in force-and enhanced isometric force following stretching (residual FE (RFE)). In skeletal muscle, non-cross-bridge (XB) structures may account for this behaviour. So far, it is unknown whether differences between non-XB structures within the heart and skeletal muscle result in deviating contractile behaviour during and after eccentric contractions. Thus, we investigated the force response of intact cardiac trabeculae during and after isokinetic eccentric muscle contractions (10% of maximum shortening velocity) with extensive magnitudes of stretch (25% of optimum muscle length). The different contributions of XB and non-XB structures to the total muscle force were revealed by using an actomyosin inhibitor. For cardiac trabeculae, we found that the force-length dynamics during long stretch were similar to the total isometric force-length relation. This indicates that no (R)FE is present in cardiac muscle while stretching the muscle from 0.75 to 1.0 optimum muscle length. This finding is in contrast with the results obtained for skeletal muscle, in which (R)FE is present. Our data support the hypothesis that titin stiffness does not increase with activation in cardiac muscle.

Interactions between Plant Metabolites Affect Herbivores: A Study with Pyrrolizidine Alkaloids and Chlorogenic Acid

The high structural diversity of plant metabolites suggests that interactions among them should be common. We investigated the effects of single metabolites and combinations of plant metabolites on insect herbivores. In particular we studied the interacting effects of pyrrolizidine alkaloid (PAs), and chlorogenic acid (CGA), on a generalist herbivore, Frankliniella occidentalis. We studied both the predominantly occurring PA N-oxides and the less frequent PA free bases. We found antagonistic effects between CGA and PA free bases on thrips mortality. In contrast PA N-oxides showed synergistic interactions with CGA. PA free bases caused a higher thrips mortality than PA N-oxides while the reverse was through for PAs in combination with CGA. Our results provide an explanation for the predominate storage of PA N-oxides in plants. We propose that antagonistic interactions represent a constraint on the accumulation of plant metabolites, as we found here for Jacobaea vulgaris. The results show that the bioactivity of a given metabolite is not merely dependent upon the amount and chemical structure of that metabolite, but also on the co-occurrence metabolites in, e.g., plant cells, tissues and organs. The significance of this study is beyond the concerns of the two specific groups tested here. The current study is one of the few studies so far that experimentally support the general conception that the interactions among plant metabolites are of great importance to plant-environment interactions.

The time-dependent and dose-dependent effects of caffeine on the contraction of the ferret heart

1. Trabeculae isolated from ferret heart and from other mammalian hearts have been mounted in a way that enables the tension generated to be measured while the composition of the bathing fluid is rapidly altered.2. Application of caffeine to these trabeculae initiates a rapid transient contracture and depresses the strength of regularly evoked heart beats.3. The strength of the contractures, the rate of tension development and the rate of spontaneous relaxation are all increased by raising the concentration of the applied caffeine.4. The strength of the caffeine contracture is relatively unaffected by changes in the bathing Na(+), K(+) or Ca(2+) concentrations, but is reduced by exposure to the free-base form of local anaesthetics.5. Lowering of the temperature has complex effects on the amplitude of the caffeine contracture due to the differing temperature sensitivities of the contraction and spontaneous relaxation.6. Following a caffeine contracture, a period of perfusion by caffeinefree solution is required before a full-sized contracture can be evoked by the re-application of caffeine. This re-priming of the caffeine contracture has a sigmoidal time course that can be fitted by a two compartment model. The rate constants of the filling of each of the compartments can be obtained analytically, and are found to be increased by raising the extracellular calcium concentration, [Ca](o), by stimulating the preparation or by raising the temperature. Reducing the [Na](o) or raising the [K](o) has little effect on these processes.7. The presence of traces of caffeine in the perfusing fluid between the conditioning and test challenges with the caffeine contracture solution reduces the extent of the re-priming without much affecting its rate.8. The behaviour of several model systems have been compared with that of the heart with the aid of an analogue computer. A four compartment closed system has been found to simulate the results presented in this paper.9. It appears that caffeine has its effects by acting to increase the rate of release of activator calcium from one part of a non-homogeneous intracellular relaxing system present within the mammalian heart, which is likely to be the sarcoplasmic reticulum.

Location of ryanodine and dihydropyridine receptors in frog myocardium

Frog myocardium depends almost entirely on calcium entry from extracellular spaces for its beat-to-beat activation. Atrial myocardium additionally shows internal calcium release under certain conditions, but internal release in the ventricle is absent or very low. We have examined the content and distribution of the sarcoplasmic reticulum (SR) calcium release channels (ryanodine receptors, RyRs) and the surface membrane calcium channels (dihydropyridine receptors, DHPRs) in myocardium from the two atria and the ventricle of the frog heart using binding of radioactive ryanodine, immunolabeling of RyR and DHPR, and thin section and freeze-fracture electron microscopy. In cells from both types of chambers, the SR forms peripheral couplings and in both chambers peripheral couplings colocalize with clusters of DHPRs. However, although a low level of high affinity binding of ryanodine is detectable and RyRs are present in peripheral couplings of the atrium, the ventricle shows essentially no ryanodine binding and RyRs are not detectable either by electron microscopy or immunolabeling. The results are consistent with the lack of internal calcium release in the ventricle, and raise questions regarding the significance of DHPR at peripheral couplings in the absence of RyR. Interestingly, the free SR membrane in both heart chambers shows a low but equal density of intramembrane particles representing the Ca(2+) ATPase.

SR calcium depletion following reversal of the Na+/Ca2+-exchanger in rat ventricular myocytes

We previously reported that cytosolic calcium transiently increases after reversal of the sarcolemmal Na+/Ca2+-exchanger. Calcium released from sarcoplasmic reticulum (SR) constituted the major part of this cytosolic transient. The aim of this study was to test whether reversal of the Na+/Ca2+-exchanger affects SR calcium content, and whether altered SR calcium content is associated with direct triggering of SR calcium release or calcium release secondary to SR calcium overload. To this purpose we studied the change of SR calcium content after reversal of the Na+/Ca2+-exchanger and the dependence on the magnitude of change of its free energy (delta Gexch) in isolated rat ventricular myocytes. The Na+/Ca2+-exchanger was reversed by abrupt reduction of extracellular sodium ([Na+]o). The magnitude of change of deltaGexch was varied with [Na+]o. Cytosolic free calcium ([Ca2+]i) was measured with indo-1 and SR calcium content was estimated from the increase of [Ca2+]i after rapid cooling (RC). SR function was manipulated either by blockade of the SR Ca2+-ATPase with thapsigargin or by blockade of SR calcium release channels with tetracaine. Reversal of the Na+/Ca2+-exchanger caused a transient increase of [Ca2+]i of about 180 s duration with a time to peak of about 30 s. During the first 30 s rapid small amplitude cytosolic calcium fluctuations were superimposed on this transient. The magnitude of the response of [Ca2+]i to RC, during the course of the cytosolic [Ca2+]i transient, also transiently increased from 174 in control myocytes to 480 nmol/l at the time of the peak value. After correction of [Ca2+]i data for the fraction of mitochondrially compartmentalized indo-1 and mitochondrial calcium, total calcium released from SR after RC was calculated with the use of literature data on cytosolic calcium buffer capacity. Contrary to the measured RC-dependent increase of measured [Ca2+]i, after reversal of the Na+/Ca2+-exchanger, calculated total calcium released from SR transiently decreased. The extent of SR calcium depletion after reversal of the Na+/Ca2+-exchanger increased with the magnitude of change of deltaGexch. Restitution of [Na+]o 30 s after reversal of the Na+/Ca2+-exchanger, greatly accelerated both recovery of [Ca2+]i and SR calcium content. Pretreatment of myocytes with thapsigargin caused almost entire depletion of SR and substantial reduction of the cytosolic transient of [Ca2+]i following reversal of the Na+/Ca2+-exchanger. Application of tetracaine hardly affected SR calcium content, but caused an increase of the SR calcium content following reversal of the Na+/Ca2+-exchanger, while the cytosolic transient increase of [Ca2+]i was substantially reduced. We conclude that reversal of the Na+/Ca2+-exchanger directly triggers SR calcium release and decreases SR calcium content in a deltaGexch dependent manner.

Dual effects of tetracaine on spontaneous calcium release in rat ventricular myocytes

1. Confocal microfluorometry was used to study the effects of tetracaine on spontaneous Ca2+ release from the sarcoplasmic reticulum (SR) in isolated rat ventricular myocytes. 2. At low concentrations (0.25-1.25 mM), tetracaine caused an initial inhibition of spontaneous release events (Ca2+ sparks) and Ca2+ waves, which was followed by a gradual increase in Ca2+ release activity. The frequency and magnitude of sparks were first decreased and then increased with respect to control levels. At high concentrations (> 1.25 mM), tetracaine abolished all forms of spontaneous release. 3. Exposure of the myocytes to tetracaine resulted in a gradual increase in the SR Ca2+ load as indexed by changes in the magnitude of caffeine-induced Ca2+ transients. 4. In cardiac SR Ca(2+)-release channels incorporated into lipid bilayers, tetracaine (> 0.25 mM) induced a steady inhibition of channel activity. Addition of millimolar Ca2+ to the luminal side of the channel caused an increase in channel open probability under control conditions as well as in the presence of various concentrations of tetracaine. 5. We conclude that the primary effect of tetracaine on SR Ca(2+)-release channels is inhibition of channel activity both in vitro and in situ. The ability of tetracaine to reduce spark magnitude suggests that these events are not due to activation of single channels or non-reducible clusters of channels and, therefore, supports the multichannel origin of sparks. We propose that the paradoxical late potentiation of release by submaximal concentrations of tetracaine is caused by a gradual increase in SR Ca2+ load and subsequent activation of the Ca(2+)-release channels by Ca2+ inside the SR.

Characteristics of cocaine block of purified cardiac sarcoplasmic reticulum calcium release channels

We have examined the effects of cocaine on the SR Ca2+ release channel purified from canine cardiac muscle. Cocaine induced a flicker block of the channel from the cytoplasmic side, which resulted in an apparent reduction in the single-channel current amplitude without a marked reduction in the single-channel open probability. This block was evident only at positive holding potentials. Analysis of the block revealed that cocaine binds to a single site with an effective valence of 0.93 and an apparent dissociation constant at 0 mV (Kd(0)) of 38 mM. The kinetics of cocaine block were analyzed by amplitude distribution analysis and showed that the voltage and concentration dependence lay exclusively in the blocking reaction, whereas the unblocking reaction was independent of both voltage and concentration. Modification of the channel by ryanodine dramatically attenuated the voltage and concentration dependence of the on rates of cocaine block while diminishing the off rates to a lesser extent. In addition, ryanodine modification changed the effective valence of cocaine block to 0.52 and the Kd(0) to 110 mM, suggesting that modification of the channel results in an alteration in the binding site and its affinity for cocaine. These results suggest that cocaine block of the SR Ca2+ release channel is due to the binding at a single site within the channel pore and that modification of the channel by ryanodine leads to profound changes in the kinetics of cocaine block.

Major difference between rat and guinea-pig ureter in the ability of agonists and caffeine to release Ca2+ and influence force

1. We have investigated the internal Ca2+ store and its ability to affect contraction by simultaneously measuring force and Ca2+ in the ureter from guinea-pig and rat. Both species responded in a similar manner to electrical stimulation and depolarization with high-K+, generating plateau-type action potentials and increasing intracellular calcium ([Ca2+]i) and force. 2. In the guinea-pig, carbachol had no effect on [Ca2+]i and force in the resting ureter. In contrast, resting rat ureter always responded with a large [Ca2+]i rise and maintained force to carbachol in Ca(2+)-containing solution, and in Ca(2+)-free solution it showed a transient increase in [Ca2+]i and force. This Ca2+ release and force development was also present in both polarized and high-K(+)-depolarized preparations and was insensitive to nifedipine, suggesting the presence of a receptor-coupled pathway of Ca2+ release in rat ureter. 3. Caffeine was able to produce a release of Ca2+ from the internal store of guinea-pig ureter and elicit contraction. However, rat ureter failed to respond to caffeine. In the presence of La3+, the caffeine response in the guinea-pig ureter and carbachol response in the rat ureter, elicited in Ca(2+)-free solutions, were always increased and prolonged and could be repeatedly evoked, suggesting similarity in Ca2+ uptake behaviour of the store in both species. 4. Ryanodine blocked the caffeine responses of the guinea-pig ureter elicited both in Ca(2+)-containing and Ca(2+)-free solutions, both in the absence and presence of La3+. However, ryanodine failed to prevent the rat ureter responding to carbachol, suggesting that carbachol was releasing Ca2+ from a ryanodine-insensitive channel in the sarcoplasmic reticulum (SR). 5. Cyclopiazonic acid, which inhibits the SR Ca(2+)-ATPase, abolished the effects of both caffeine and carbachol in Ca(2+)-free solutions in guinea-pig and rat, respectively. 6. We conclude that there is a major difference in the mechanisms of Ca2+ release in the internal Ca2+ store of smooth muscle from guinea-pig and rat ureter. The data suggest that the guinea-pig store is purely a calcium-induced calcium release (CICR)-type store and that the rat store is a pure receptor-operated Ca2+ store.

Microwave influence on the isolated heart function: II. Combined effect of radiation and some drugs

The combined effects of microwave radiation and some drugs were studied in an isolated frog auricle preparation. The experiments established that exposure to pulse-modulated 915 MHz microwaves for up to 40 min had no effect on either the rate or the amplitude of spontaneous auricle twitches, unless the average absorbed power was high enough to produce preparation heating. Treatment of the preparation with saline containing (0.6-3.0) 10(-5) M of propranolol or (0.5-1.5) 10(-7) M of atropine altered neither its pacemaker nor its contractile functions; these drugs also had no effect when they were combined with nonthermal microwave irradiation. Caffeine (1 mM) strongly increased the average heart power, which was calculated as the product of twitch rate and amplitude. The caffeine effect appeared to be significantly augmented (by about 15%, P < 0.02) under exposure to burst-type pulsed microwaves (pulse width, 1.5 msec; pause, 2.5 msec; 8 pulses/burst, 16 bursts/s; average SAR, 8-10 W/kg). By itself, this modulation was not effective; the heating of the preparation and saline during exposure was approximately 0.1 degrees C, which could not account for the detected changes. The experimental results demonstrate that caffeine treatment increases the microwave sensitivity of the frog auricle preparation and reveals primarily subthreshold, nonthermal microwave effect.

Potentiation of the twitch responses by inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase in frog atrial fibres

In frog atrial fibres, cyclopiazonic acid as well as thapsigargin, which are inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase, induced a significant increase in the twitch amplitude without detectable changes in its kinetics. The measurements performed on chemically skinned fibres show that cyclopiazonic acid has no effect on the properties of contractile proteins. In the presence of a T-type Ca2+ channel blocker or L-type Ca2+ channel blocker, cyclopiazonic acid still induced a potentiation of the twitch while no effect was found in the presence of a Na(+)-Ca2+ exchange blocker. The effect of cyclopiazonic acid was not related to any modification in myofibrillar Ca2+ sensitivity or in Ca2+ influx through Ca2+ channels. It is proposed that the inhibition of the sarcoplasmic reticulum Ca(2+)-ATPase resulted in a potentiation of the effect of the Ca2+ influx and that the major role of the sarcoplasmic reticulum was to limit the intracellular Ca2+ concentration.

Procaine effects on single sarcoplasmic reticulum Ca2+ release channels

The effects of the Ca(2+)-induced Ca2+ release blocker procaine on individual sarcoplasmic reticulum Ca2+ release channels have been examined in planar lipid bilayers. Procaine did not reduce the single channel conductance nor appreciably shorten the mean open times of the channel; rather, it increased the longest closed time. These results indicated that procaine interacted selectively with a closed state of the channel rather than with an open state. Gating of the sarcoplasmic reticulum Ca2+ release channel was described by a modified scheme of Ashley and Williams (1990. J. Gen. Physiol. 95:981-1005), including an additional long-lived closed state. Computer simulations determined that procaine was more likely to interact with this long-lived Ca(2+)-bound closed state of the channel rather than with other states of the channel. Simulations with the same model were also able to reproduce a prominent Ca(2+)-sensitive transition between "random" and "bursting" forms of gating of the channel, variations of which may account for "gearshift" behavior reported in studies with this and other single channels.

Effects of caffeine and ryanodine on low pHi-induced changes in gap junction conductance and calcium concentration in crayfish septate axons

Electrical uncoupling of crayfish septate axons with acidification has been shown to cause a substantial increase in [Ca2+]i which closely matches in percent the increase in junctional resistance. To determine the origin of [Ca2+]i increase, septate axons have been exposed either to drugs that influence Ca2+ release from internal stores, caffeine and ryanodine, or to treatments that affect Ca2+ entry. A large increase in junctional resistance and [Ca2+]i maxima above controls resulted from addition of caffeine (10-30 mM) to acetate solutions, while a substantial decrease in both parameters was observed when exposure to acetate-caffeine was preceded by caffeine pretreatment. In contrast, ryanodine (1-10 microM) always caused a significant decrease in junctional resistance and [Ca2+]i maxima when applied either together with acetate or both before and with acetate. Calcium channel blockers such as La3+, Cd2+ and nisoldipine had no effect, while an increase in the [Ca2+] of acetate solutions either decreased junctional resistance and [Ca2+]i maxima or had no effect. The data suggest that cytoplasmic acidification causes an increase in [Ca2+]i by releasing Ca2+ from caffeine and ryanodine-sensitive Ca2+ stores. The increase in [Ca2+]i results in a decrease in gap junction conductance.

The role of [Ca2+]i and [Ca2+] sensitization in the caffeine contracture of rat myocytes: measurement of [Ca2+]i and [caffeine]i

1. Fluorescence measurements have been made in single, isolated rat ventricular myocytes using the Ca2(+)-sensitive indicators Fura-2 and Indo-1. In Fura-2-loaded cells, the application of caffeine (2-20 mM) produced a change of fluorescence indicating an increase of [Ca2+]i which then spontaneously decayed to control levels. These changes of [Ca2+]i were accompanied by a contracture. 2. In contrast, in Indo-1-loaded cells, in addition to the changes of fluorescence expected for the transient increase of [Ca2+]i produced by caffeine, there was a maintained decrease of fluorescence. 3. Measurements in vitro showed that caffeine quenches the fluorescence of Indo-1 (but not of Fura-2) in a [Ca2+]-and wavelength-independent manner. Caffeine therefore had no effect on the ratio of Indo-1 fluorescence measured at two wavelengths. This inhibition by caffeine could be described by an apparent Ki of 4 mM. In the cell the Ki was considerably larger (18 mM). 4. We have separated the Indo-1 fluorescence changes into caffeine- and [Ca2+]i-dependent components. The time course of change of intracellular caffeine was calculated. When [caffeine]o was rapidly increased, [caffeine]i changed with a rate constant of 8 s-1 giving an apparent permeability to caffeine of 2 x 10(-3) cm s-1. 5. This method was used to measure [caffeine]i and [Ca2+]i simultaneously during caffeine-induced contractures. The shape of the caffeine contracture was found to depend on both the speed of application of caffeine and the concentration applied. If caffeine was applied quickly then the contracture developed within 1 s to a maximum level and then relaxed to a lower maintained level. With slower application, there was a more complete relaxation of the initial contraction followed by a slower redevelopment of contraction. 6. Despite the difference in contraction time course, irrespective of the flow rate, [Ca2+]i decayed monotonically. The slow secondary development of contraction has the same time course as the increase of [caffeine]i. The caffeine contracture can be reproduced by a model in which both [Ca2+]i and [caffeine]i affect contraction. 7. The increase of [Ca2+]i is not greatly affected by altering the caffeine concentration from 2.5 to 50 mM. In contrast the maintained level of contraction increases over this range showing that the Ca2(+)-independent effects of caffeine on the myofilaments have a low affinity for caffeine.

Caffeine suppresses length dependency of Ca2+ sensitivity of skinned striated muscle

Freeze-dried skinned cardiac and skeletal muscle preparations of the rabbit were immersed in Ca2+-containing solutions with different concentrations of caffeine. The relation between the negative logarithm of the Ca2+ concentration (pCa) and normalized developed force was studied. The exact position of these Ca2+-sensitivity functions proved to be dependent on both the sarcomere length (monitored by means of laser diffraction) and caffeine concentration. High concentrations of caffeine induce a reversible fall in tension, particularly at low binding site saturation (low pCa) and long sarcomere lengths. At a concentration of 10 mM caffeine, the sarcomere length dependency of the Ca2+-sensitivity curves is markedly reduced for the rising part of the curve. Only the depressive effect of caffeine at high pCa remains. A possible mechanism of caffeine action is discussed.

Low-sodium contractures indicating sarcolemmal Na/Ca-exchange in the frog heart

1. In the frog heart, Ca2+ enters the cell by the slow inward current (Isi) and by an electrogenic, carrier-mediated, and passive Na-out/Ca-in-exchange. 2. The latter reverses to Na-in/Ca-out-exchange during depolarization and thereby controls relaxation. 3. The exchange ratio is 3 Na+ for 1 Ca2+. 4. The Na/Ca-exchange is not inhibited by organic Ca-antagonists in frog myocardium, indicating that the initiation of the heart beat may mainly depend on Isi. 5. This is not necessarily in contradiction with the Na-Ca-antagonism, since there also exists an antagonism between Na+ and Ca2+ in the slow channel. 6. However, the contractures caused by a decrease of NaO+ are mediated by the Na/Ca-exchange.

Prevention of ventricular fibrillation after myocardial revascularization

Ventricular fibrillation during reperfusion after aortic cross-clamping for coronary artery bypass grafting may cause subendocardial injury. We investigated the use of lidocaine to prevent ventricular fibrillation during this period. In a blind, prospective, randomized trial, 91 consecutive patients undergoing elective coronary artery bypass graft procedures were given lidocaine (2 mg/kg) or normal saline immediately before removal of the aortic cross-clamp. The groups were similar with respect to demographic, clinical, and intraoperative variables. Myocardial preservation techniques were similar in both groups. Of 47 patients receiving lidocaine, 38 recovered a supraventricular rhythm without ventricular fibrillation, compared with only 5 of 44 patients in the control group (p less than .001). When ventricular fibrillation occurred, patients in the control group required a greater number of direct-current countershocks (2.31 versus 1.86) to convert to sinus rhythm. Transient heart block, requiring temporary pacing, developed in 3 patients in the lidocaine group, compared with 1 patient in the control group. There was no significant difference between the groups in the requirement for perioperative inotropic support (6 of 47 versus 6 of 44) or the number of myocardial infarctions (2 of 47 versus 1 of 44), and there were no deaths in either group. Lidocaine infusion immediately before removal of the aortic cross-clamp significantly reduces the incidence of ventricular fibrillation during the reperfusion period after cardiopulmonary bypass.

The role of calcium channel in the effect of nicotine on contractility in isolated toad ventricle

The mechanism of the positive inotropic effect produced by nicotine (6.2 X 10(-5) mol/l to 4.9 X 10(-4) mol/l) on electrically driven toad ventricles was investigated. The response to nicotine was not affected by 6-hydroxydopamine pretreatment, bretylium (2.4 X 10(-4) mol/l) exposure or tyramine tachyphylaxis. Following desensitisation by isoprenaline (4.2 X 10(-6) mol/l) of the beta-adrenoceptor in the ventricles, the response to nicotine was no affected. However, the response was antagonised by ethylene diamine tetraethyl acetate (2.3 X 10(-4) mol/l), verapamil (0.4 X 10(-5) mol/l) or calcium-free Ringer. Nicotine prolonged the action potential duration and enhanced the force of contraction. Nicotine induced slow action potentials in partially depolarized (in high potassium solution) ventricles and this was antagonised by verapamil (0.4 X 10(-5) mol/l). These results suggest that the effects of nicotine are mediated by a direct interaction with the Ca2+ channels at the cell surface.

Modification of caffeine-induced injury in Ca2+-free perfused rat hearts. Relationship to the calcium paradox

The pathogenesis of the calcium paradox has not been established. In calcium-free perfused hearts, caffeine, which releases calcium from the sarcoplasmic reticulum, causes severe myocardial injury, with creatine kinase (CK) release and contraction band necrosis similar in many respects to the calcium paradox. It has been postulated that contracture, initiated by a small rise in intracellular calcium, may cause sarcolemmal injury in both the calcium paradox and caffeine-induced myocardial injury. The present study was initiated to determine whether interventions which modulate caffeine-induced contracture will also correspondingly alter cellular injury. The effects of caffeine dose, procaine, extended calcium-free perfusion, elevated potassium, temperature, and increasing intracellular sodium on caffeine-induced contracture were examined in Langendorff-perfused adult rat hearts. Caffeine-induced contracture at 22 C increased over a dose range of 5-40 mM caffeine. Procaine, which inhibits caffeine-induced calcium release at doses between 5 and 20 mM, progressively reduced contracture caused by addition of 20 mM caffeine at 22 C. Hearts perfused with calcium-free solution containing 16 mM K+ showed a reduction in caffeine-induced contracture. Extended calcium-free perfusion (20 minutes) at temperatures from 18 to 37 C resulted in a progressive reduction of caffeine-induced contracture. Each of these interventions was also found to inhibit caffeine-induced injury at 37 C. Low temperature was found to have complex effects. Hypothermia enhanced caffeine contractures but also protected hearts from cell separations and CK release. Increasing intracellular sodium was found to enhance caffeine-induced contracture at 37 C. There was a direct correlation between measured intracellular sodium levels and the magnitude and duration of caffeine-induced contracture. These results demonstrate a direct correlation between the magnitude of contracture and myocardial injury in calcium-free hearts. It is proposed that contracture is the primary mediator of sarcolemmal membrane injury in hearts with intercalated disks weakened by prior calcium-free perfusion.

Effects of procaine on calcium accumulation by the sarcoplasmic reticulum of mechanically disrupted rat cardiac muscle

The ability of the sarcoplasmic reticulum of skinned cardiac muscle of the rat to accumulate and release Ca2+ was studied in the presence and absence of procaine. Ca2+ accumulation was estimated from the magnitude of the caffeine- (30 mM) induced force transient in a weakly Ca2+ buffered solution. The relative area under the caffeine-induced force transient was up to 4-fold greater when 5 mM-procaine had been present during the preceding period of Ca2+ loading, than that after an equivalent period of Ca2+ loading in the absence of procaine. Procaine antagonized the caffeine-induced release of Ca2+ when present in the Ca2+ releasing solution, however, the ability of procaine to attenuate the caffeine-induced Ca2+ release diminished as the extent to which the sarcoplasmic reticulum was loaded with Ca2+ increased. In the presence of 1 mM-Mg2+ procaine also markedly attenuated the small spontaneous force oscillations (5-10% P0) associated with the cyclic release and reuptake of Ca2+ by the sarcoplasmic reticulum. When the Mg2+ concentration was reduced to 0.1 mM, procaine initially suppressed the small spontaneous oscillations in force, however, large force oscillations (40-80% P0) of lower frequency were invariably initiated after 20-60 s exposure to 5 mM-procaine. Procaine (5 mM) produced a slight shift (approximately 0.04 pCa unit) of the force-pCa relation toward lower Ca2+ concentrations. This effect is too small to influence in any substantial way the apparent effects of procaine on the sarcoplasmic reticulum. The results indicate that whilst procaine is indeed able to suppress Ca2+ release under certain circumstances, in its presence the net accumulation of Ca2+ by the sarcoplasmic reticulum can be markedly enhanced.

Submaximal sodium-lack contractures in rapidly perfused frog ventricular strips

Rapid perfusion of strips of frog ventricular muscle (width less than or equal to 8 mm) by sodium-free Ringer yielded contractures with very fast rates of rise (half time approximately equal to 2 s) and flat plateaus. Tension in lithium Ringer was 0.64 (0.38-0.77) of the maximal tension the muscle could produce [i.e., for an extracellular potassium concentration ([K]o) of 160 mM, and an [Na]o of 0 mM at less than 15 degrees C], showing that intracellular calcium concentration ([Ca]i) in zero [Na]o and normal [K]o was insufficient to saturate the myofilaments. Tension was related to [Ca]o in an S-shaped manner (apparent Km = 1.3 microM), was rapidly increased by electrical depolarization, high potassium, or low temperature, but was unaffected by up to 10(-5) M dihydroouabain. From these data was derived the working hypothesis that the value of [Ca]i during the contracture plateau is a steady-state value due to influx through a sodium-dependent mechanism and calcium uptake or efflux via a sodium-independent mechanism. Also, 1 microM dihydroouabain increased peak twitch tension by 60% but did not potentiate tension during the plateaus of contractures obtained by partial replacement of Nao. This result supports the working hypothesis and suggests that the potentiating effect of raising Nai may depend on the membrane potential (D. A. Eisner, W. J. Ledener, and R. D. Vaughan-Jones. J. Physiol. Lond. 335: 723-743, 1983).

Field stimulation responses of rat urinary bladder detrusor smooth-muscle. Dependence upon slow calcium channel activity determined by K+ depolarization and calcium antagonists

Field stimulation responses of rat urinary bladder detrusor smooth muscle were unaffected by atropine, phentolamine, propranolol, caffeine and theophylline. After caffeine or theophylline treatment, previously quiescent detrusor preparations developed pronounced spontaneous contractile activity. Both natural spontaneous contractile activity and field stimulation responses were eliminated in Ca-free EGTA media. After washout, both kinds of activity returned but over quite different time scales. At moderate levels of K+-induced depolarization which inactivate fast Ca-channels, field stimulation responses persisted and additively superimposed upon the K+-induced tension, suggesting that field stimulation-induced depolarization had activated slow voltage-dependent Ca-channels. Almost 61% of detrusor muscle preparations examined showed some spontaneous contractile activity. Such preparations were more sensitive to K+-induced depolarization than quiescent preparations and K+-depolarization greatly accelerated the spontaneous activity of these muscle strips. The slow Ca-channel blockers nifedipine and verapamil eliminated field stimulation responses in the 10(-6)-10(-5) M range, but octylonium bromide was far less effective even at 10(-4) M. The inorganic Ca-channel blocker Mn2+ inhibited field stimulation responses at 0.5-1.0 mM but La3+ was surprisingly far less effective than Mn2+ in this concentration range, requiring a 1 mM level to achieve a 50% inhibition of response. It is concluded that field stimulation induced a depolarization of rat urinary bladder detrusor muscle which activates slow voltage-dependent Ca-channels. Field stimulation responses and natural spontaneous activity appear to strongly depend upon a sustained Ca influx through these channels for the sustained release of intracellular calcium and the maintenance of contractile force.

The mechanism of the increase of tonic tension produced by caffeine in sheep cardiac Purkinje fibres

The effects of caffeine were examined on contraction and membrane current in voltage-clamped sheep cardiac Purkinje fibres. The photoprotein aequorin was injected into several cells in order to measure the intracellular ionized Ca concentration [( Ca2+]i). When the Na-K pump was inhibited, depolarization produced a twitch followed by a tonic component of tension. Repolarization produced an after-contraction. These components of tension were accompanied by corresponding increases of aequorin light. Caffeine (10 mM) decreased both the twitch and the after-contraction while increasing the tonic component. The application of caffeine also produced a transient increase of aequorin light, both during depolarization and at rest, which was followed by a maintained decrease in all three components of the light signal. In particular, although caffeine decreased the rise of aequorin light during prolonged depolarization it increased the tonic tension. The possibility that the effects of caffeine on tonic tension could be due to suppression of spontaneous Ca oscillations was rejected for the following reasons. (i) Ryanodine (which also abolishes Ca oscillations) decreased the magnitude of the tonic tension. (ii) Caffeine still increased tonic tension when it was added to a fibre exposed to ryanodine (1-10 microM). In the presence of ryanodine it was possible to measure [Ca2+]i and tonic tension without interference from Ca oscillations. The increase of tonic tension produced by caffeine could not be accounted for by a rise of [Ca2+]i. The results showed that, at a given level of Ca, caffeine increased tension. The results show that a large part of the increase of tonic tension produced by caffeine is due to an increase of the Ca sensitivity of the contractile apparatus rather than to changes of [Ca2+]i. The consequence of this observation for the experimental use of caffeine is discussed.

The effects and interactions of caffeine, lignocaine and carbachol at the chick neuromuscular junction

The effects and interactions of lignocaine, carbachol and caffeine on the contractile response and endplate potential were investigated in the isolated chick biventer cervicis nerve-muscle preparation using a moving fluid electrode technique. Lignocaine (37 microM-1.8 mM) produced a dose-dependent contracture response that was not related to an endplate depolarization and appeared to inhibit non-competitively the usual depolarization and contractile response to carbachol (5.5 microM-0.8 mM). In contrast, lignocaine enhanced the contractile response to caffeine (0.54-10.8 mM) possibly by lowering the threshold and mainly by increasing the maximal response. The peak effects occurred at the same caffeine concentration regardless of the presence of lignocaine. The possibility that both lignocaine and caffeine may produce contractures in the chick muscle by acting on intracellular calcium concentration is discussed.

Comparative studies on the role of calcium in triggering subcellular damage in cardiac muscle

Isolated cardiac muscle strips from amphibians and mammals, together with isolated frog hearts, have been used as model systems for studying the action of elevated [Ca2+]i in promoting severe damage. A23187 and caffeine are believed to cause a rise in [Ca2+]i. Elevated [Ca2+]i causes characteristic damage which has been categorized and includes hypercontraction, Z-line damage and myofilament dissolution. The damage closely resembles that described in the isolated mammalian heart and in skeletal muscle preparations when [Ca2+]i is raised dramatically. Damage can therefore be triggered by releasing Ca2+ from intracellular sites, as distinct from increasing Ca2+ entry (as in the Ca2+-paradox). DNP and ruthenium red also cause identical damage and the results suggest that whilst the fall in pHi associated with ischaemia is probably the consequence of Ca2+/2H+ exchange at the mitochondria, coupled with ATP hydrolysis, lowered pHi by mitochondrial action is probably not the only cause of myofilament dissolution. Damage is not prevented by pretreatment with leupeptin, an inhibitor of Ca2+-activated neutral proteases, and it is concluded that the latter are probably not implicated in rapid and dramatic damage. The possible involvement of lysosomal enzymes in damage triggered by high [Ca2+]i is discussed.

The neuromuscular effects of lignocaine

The local anesthetic drug lignocaine, in concentrations which completely block both nerve and muscle action potentials, produces concentration-dependent contractures in the vertebrate skeletal and smooth muscle fibres. The contractures produced by lignocaine were not related to membrane depolarization. Although lignocaine greatly reduced the depolarizations and contractures produced by depolarizing agents, such ACh or carbachol, it had a differential effect on the responses produced by tetraethylammonium (TEA) at the chick neuromuscular junction. Lignocaine reduced the TEA-induced depolarizations while it markedly enhanced the contracture responses. In the absence of membrane depolarization, the contractures produced by lignocaine may be attributed to an effect on release of intracellular calcium (Ca2+) from the sarcoplasmic reticulum (SR). The results suggested a multiple site of action of lignocaine at the vertebrate neuromuscular junction.

Antagonism of enkephalin action on acetylcholine release by methylxanthines: lack of a purine link

Theophylline (Theo) and caffeine antagonized the inhibitory effect of methionine (Met)-enkephalin, leucine (Leu)-enkephalin and morphine on the twitch height of the field stimulated myenteric plexus longitudinal muscle (MPLM) preparation of the guinea-pig ileum. Antagonism by Theo was observed only in tissues stimulated submaximally, but that by caffeine was observed in tissues stimulated submaximally and supramaximally. Injection of Theo (20, 40 mg kg-1) or caffeine (40 mg kg-1) reversed or blocked the inhibitory effects of Leu-enkephalin (50 micrograms i.c.v.) and a systemically active enkephalin FK 33,824 (0.5 mg kg-1) on the release of acetylcholine (ACh) from the rat cortex in vivo. Injections of morphine (2.5, 5.0 mg kg-1 i.v.) did not modify the in vivo release of radiolabelled purines from the cerebral cortex prelabelled with [3H]-adenosine (2.8 X 10(-7) M). Application of K+ (60 mM) to the cortex readily stimulated this release. Injection of morphine (5.0 mg kg-1 i.v.) increased the spontaneous release of radiolabelled purines from the cortex prelabelled with a higher concentration of [3H]-adenosine (10(-4) M) in six out of eleven experiments. Under similar conditions neither Leu-enkephalin (50 micrograms i.c.v.) nor FK 33,824 (0.5 mg kg-1 i.v.) stimulated purine release. It is concluded that methylxanthines can antagonize the inhibitory action of opioids on the peripheral and central release of ACh. However, this antagonism does not reflect an intermediary purine step in the action of opioids on the release of ACh.

Effects of caffeine on Ca-activated force production in skinned cardiac and skeletal muscle fibres of the rat

Skinned fibres prepared by mechanical and/or chemical means from cardiac and skeletal muscles of the rat were activated in solutions strongly buffered for Ca2+ (with 50 mM EGTA) in the absence or presence of caffeine 5-40 mM. In all preparations caffeine was found to reversibly shift the relation between steady-state force and free [Ca2+] toward lower free [Ca2+] in a dose dependent manner. This increase in apparent Ca2+ sensitivity was not antagonized by procaine and was the same, within each muscle type, irrespective of the manner in which the skinned fibre was prepared, and consequently the degree to which it retained cellular membranes. The effect was more pronounced in cardiac and slow-twitch than in fast-twitch, myofibrillar preparations. At equivalent concentrations theophylline mimicked this effect of caffeine in all preparations, however, addition of exogenous cyclic AMP neither mimicked nor modified, in any way, the effect. Maximum Ca2+ -activated force production was not affected by caffeine below 20 mM but was depressed by concentrations of 20 mM and above. The increase in apparent Ca2+ sensitivity produced by caffeine can not be the result of a mobilization of some cellular store of Ca2+ but must arise from a direct effect of caffeine on the myofilaments which leads to a change in the apparent affinity constant of the force controlling sites for Ca2+.

Effect of vanadate and of removal of extracellular Ca2+ and Na+ on tension development and 45Ca efflux in rat and frog myocardium

Vanadate in the range 0-5 mM has positive inotropic effects on myocardial strips of frog and to a lesser extent on those of rat. Inhibiting the sarcolemmal Na+, Ca2+ exchange by a solution free of Ca2+ and Na+ caused a drop in 45Ca efflux and a transient increase in resting tension. These effects were more expressed for the frog than for the rat myocardium, which suggests that the Na+ for Ca2+ exchange across the cell membrane is more important in the frog than in the rat myocardium. A subsequent addition of vanadate at 2 or 5 mM had no effect on 45Ca efflux, while it increased the resting tension. This increase was higher for the frog than for the rat myocardium. These results suggest that the inotropic effects of vanadate may be due to an effect on membrane-bound Ca2+-ATPase.

Cellular calcium turnover in the perfused rat heart: modulation by caffeine and procaine

Pool A is a rapidly exchangeable cellular pool of Ca++ whose release is triggered by a mechanism involving extracellular Ca++ (Hunter et al., 1981). We have now found that pool A is rapidly released from the perfused rat heart when 10 mm caffeine is added to the perfusate. Pool A release by caffeine was demonstrated during a Ca++ free perfusion. When the perfusate contained 2.5 mm Ca++, caffeine induced an immediate contractile failure. The steady state level of pool A (normally 69 +/- 10 nmol Ca++/g wet wt heart) was also decreased by 60%. Pool A was similarly depleted in hearts perfused with medium containing 0.2 mm Ca++. Procaine (5 mm) inhibited by 50% the release of pool A triggered by caffeine and inhibited by 86% the release triggered by extracellular Ca++. The inhibition of Ca++-induced release of pool A by procaine was partially relieved by externally stimulating the hearts. External stimulation also decreased the inhibition by procaine of Ca++ uptake by pool A from 83% to 28%. These results are further evidence that pool A is located in the sarcoplasmic reticulum and that release of pool A to the myofibriles is triggered by excitation-dependent Ca++ influx.

Intracellular calcium and sodium activity in sheep heart Purkinje fibres. Effect of changes of external sodium and intracellular pH

Intracellular Ca, Na and H selective microelectrodes were used to study the effects of reduction of the extracellular Na concentration, [Na]0, on the free intracellular Ca concentration, [Ca]i, Na activity, (aiNa), and intracellular pH (pHi) in sheep heart Purkinje fibres. 1. Reduction of [Na]0 from 140 mM to 14 mM produced a decrease of aiNa, and increase of free [Ca]i, and normally an increase of resting tension. 2. Inhibition of the Na-K pump by 10(-5) M acetyl-strophanthidin produced a slow rise of [Ca]i and resting tension. 3. The magnitude of the increase of free [Ca]i (and tension) produced by [Na]0 reduction was greatly enhanced when the Na-K pump is inhibited by either acetylstrophanthidin or K-free solutions. 4. We suggest that this enhanced rise of free [Ca]i in the presence of Na-K pump inhibition is due to Ca loading of intracellular Ca buffering systems during the pump inhibition. 5. Addition of NH4Cl produced a transient decrease of free [Ca]i that accompanied an alkaline change in pHi. Removal of NH4Cl (which produced a transient intracellular acidification) produced a transient increase of free [Ca]i. We conclude that a close relationship exists between the control of free [Ca]i and pHi which may be due to competition at, or common use of, intracellular buffering systems.

Analysis of caffeine action in single trabeculae of the frog heart

Effects of caffeine on contractile tension and on intracellular action and resting potentials were examined in single frog heart trabeculae suspended in a rapid perfusion chamber. Trabeculae from atria responded more readily than those from ventricles and were therefore studied in greater detail. Both the contracture and twitch responses, the one obtained at high (greater than or equal to 10mM), the other at low (less than or equal to 10mM) caffeine concentrations, consisted of a transient tension rise followed by a maintained phase of lower, but still enhanced, tension. The hypothesis was tested that the transient response is due to the release of calcium from the sarcoplasmic reticulum (s.r.) whereas the maintained tension results from enhanced calcium influx through the cell surface. Support for these ideas was obtained by examining the response to step changes of external calcium and caffeine concentrations, applied in various combinations, simultaneously and in sequence. It also emerged tht the effects on twitch tension of calcium derived from (a) s.r. discharge and (b) influx are additive, to a first approximation. A test procedure for monitoring the s.r. store content was evolved to follow the accumulation of s.r. calcium after a preceding depletion. The results obtained, and others, suggest that the s.r. calcium pump can be operative in atrial heart cells and capable, after store depletion, of reabsorbing up to some 40% of calcium activating a twitch, the remainder being, presumably, extruded from the cells.

Differential effects of ouabain and 2,4-dinitrophenol on contractile tension of and on sodium and calcium efflux from frog heart ventricular strips

1 The efflux, from heart ventricular strips of Rana pipiens, of sodium ((22)Na) and calcium ((45)Ca) was measured simultaneously.2(22)Na efflux could be resolved into two first order kinetic components: k(I) = 0.105 min(-1), thought to represent efflux from the extracellular space, and k(II) = 0.0182 min(-1) representing efflux from the cells.3(45)Ca efflux was also resolved into an extracellular component, k(I) = 0.1216 min(-1); and an intracellular one, k(II) = 0.0102 min(-1). (45)Ca k(II) was greatly increased by 2,4-dinitrophenol (DNP), but unchanged by caffeine. This suggests that it represents a mitochondrial calcium compartment.4(22)Na k(II) was not changed by DNP. This indicates that, at the time of DNP addition, (22)Na was passively bound to undefined intracellular components.5 Ouabain (10(-6) M) decreased (45)Ca efflux (k(II)) initially but at later periods slightly increased it. The former effect is thought to be due to an action at the plasma membrane level, while the latter probably represents an increased exchangeability of mitochondrial calcium. The same effects were always found when ouabain was applied at different times of strip superfusion.6 Ouabain (0.25 to 4 muM) did not decrease the k(II) of (22)Na efflux. Kinetic reasons are presented which indicate that, in this preparation, the activity of the sodium pump may be too fast to be measured by means of (22)Na efflux, therefore these findings do not necessarily mean that ouabain does not inhibit active sodium transport.7 The time course of the inotropic effect of ouabain was also studied in ventricular strips of Rana pipiens heart that were stimulated at 0.2 Hz with biphasic, 2 ms pulses of supramaximal intensity, and incubated in Ringer solution containing 1.1 mM calcium, or in ;calcium-free' Ringer (residual calcium: 5.2 muM), or in ;calcium-free' Ringer with 0.1 mM of the calcium chelator ethyleneglycol bis (beta-aminoethylether) N,N'-tetraacetic acid (EGTA).8 In Ringer, the inotropic effect of ouabain was already observed at 5-10 s after steroid addition, even with the lowest concentration tested (0.25 muM), while signs of toxicity appeared only after 15 min in 4 muM ouabain, the highest concentration used.9 When the strips were incubated in ;calcium-free' Ringer solution, force of contraction decayed to 1-2% of that in 1.1 mM calcium. Addition of 4 muM ouabain to these hypodynamic strips led to a progressive increase in contractile force of up to 300%, that started after a 50 s latency period. No signs of toxicity were observed.10 Incubation of the strips in EGTA-Ringer also reduced contractile force to about 2% of that in Ringer, and 4 muM ouabain also increased force of contraction by approximately the same amount as seen in ;calcium-free' Ringer, but the effect began after a 10 min latency period. The concentration of calcium ion (Ca(2+)) in the extracellular space of strips incubated in EGTA-Ringer, was approximately 800 fold lower than in Ringer, and 60 fold lower than in ;calcium-free' Ringer solution.11 Caffeine (20 mM) induced, in strips previously incubated for 1 h in 4.4 mM calcium Ringer solution plus 10(-6) M ouabain, a marked initial contracture, that relaxed spontaneously, and was followed by slow waves of contracture. This was not observed if the strips were incubated, prior to caffeine, in 4.4 mM calcium Ringer without ouabain, or in 1.1 mM calcium Ringer solution that contained 10(-6) M ouabain.12 Based on these findings, a hypothesis that can explain the inotropic effect of cardioactive steroids is presented.

The tension-depolarization relationship of frog atrial trabeculae as determined by potassium contractures

1. In the presence of extracellular Na ions K contractures evoked from isolated frog atrial trabeculae show an initial phasic and a subsequent tonic contractile response. 2. The phasic response shows a steep dependence on membrane potential, persists in Na-free fluid, but is blocked by Mn ions, D600 and tetracaine. It has an indirect dependence on the [Ca]0 and would seem to be associated with both the secondary inward current and the release of Ca2+ from intracellular stores. 3. The tonic component of the K contracture is unaffected by D600 or tetracaine, shows a shallow dependence on membrane potential but is absent in Na-free fluid. Its tension-depolarization curve is immediately affected by alteration of either the [Ca]0 or the [Na]0. The form of the tension-depolarization relationship and the effects of [Ca]0 and [Na]0 are consistent with the strength of the tonic tension being determined by a 3Na+ for 1Ca2+ exchange across the cell membrane. 4. The results agree well with those obtained previously with voltage-clamp experiments on the same tissue, and may also help with the interpretation of Ca-flux experiments.

Adenine nucleotides and synaptic transmission in the in vitro rat hippocampus

1 The effects of adenosine and various derivatives were examined in the in vitro hippocampal slice preparation from rat.2 The amplitudes of extracellularly recorded field potentials from the CA1 region were depressed by adenosine, and this effect could be antagonized by methylxanthines. Because presynaptic field potentials were unaffected by adenosine, while the field e.p.s.p. was depressed, adenosine would appear to act at a synaptic site to depress transmission.3 Adenosine deaminase, which breaks down adenosine to inosine, increased the amplitude of synaptic responses, while hexobendine, which blocks reuptake of adenosine, had a depressant effect. This strongly suggests that the endogenous release of adenosine from the hippocampal slice preparation is sufficient to exert a tonic inhibitory influence on the amplitude of synaptic responses.4 Cyclic adenosine 3',5'-monophosphate (cyclic AMP) and its dibutyryl derivative had depressant effects on the amplitude of field responses which were blocked by theophylline, suggesting that they are able to act at the extracellular adenosine receptor. (-)-Isoprenaline (which raises tissue cyclic AMP levels), and the 8-p-chlorophenylthio derivative of cyclic AMP both increased the amplitude of population spike responses, and these effects were not blocked by theophylline, suggesting that the physiological effects of adenosine are not mediated via a cyclic AMP-dependent mechanism.5 Since adenosine is not the transmitter at this CA1 pyramidal cell synapse, but is apparently present in the extracellular compartment in sufficient concentrations to affect the synaptic physiology of this region, this provides strong evidence in favour of the concept of a neuromodulatory role for adenosine in the central nervous system.

The effect of sodium, calcium and metabolic inhibitors on calcium efflux from goldfish heart ventricles

1. 45Ca efflux and tissue Ca content were examined in goldfish ventricles under conditions known to affect cellular Ca movements. 2. EGTA or Ca-EGTA was added to the washout solutions in sufficient concentration (10 mM) to avoid retardation of the apparent tissue 45Ca efflux by extracellular 45Ca binding or backflux. 3. After a variable initial increase, the cellular Ca content usually stabilizes within 60 min when ventricles are immersed in Li- or K-substituted saline containing 1.8 mM Ca0 (under these conditions the internal Ca2+ concentration is below 10(-5) M). 4. 45Ca efflux is maximally activated by external concentrations of Ca2+ as low as 10(-6) M, in both Na-containing and Na-free saline. 5. 45Ca efflux decreases in Na-free solutions. It is reactivated by Na-saline. The effect of different external Na concentration on 45Ca efflux is comparable at external Ca2+ concentrations between 10(-6) M and 2 x 10(-3) M. 6. Reactivation of Ca efflux after Na0 readmission is inhibited by metabolic poisoning, or in the presence of 10 mM-caffeine. Loading with 45Ca at very low external Ca2+ concentration prevents the inhibition of Ca efflux in Na-free medium. 7. Caffeine (10 mM) produces contractions of about equla size when K-depolarized preparations are immersed in either Na- or Li-saline. At the same time there is a similar increase in 45Ca efflux in absence of Na0 and in its presence. 8. In the virtual absence of Ca2+0 (10(-5) M-Ca, 10(-2) M-EGTA) and Na+0, the residual 45Ca efflux is reversibly inhibited by cyanide (2 mM). 9. The results are roughly compatible with the general concept of ATP-dependent Na-Ca exchange in internal Ca2+ homeostasis. However, this hypothesis should probably be modified to account for the fact that under physiological concentrations Na+0 and Ca2+0 do not compete for activating 45Ca efflux. Metabolic products may be involved in Na0- and Ca0-dependent Ca efflux. It is therefore not excluded that a Na-independent active mechanism co-operates with Na-Ca exchange in Ca extrusion.

On the effects of divalent cations and ethylene glycol-bis-(beta-aminoethyl ether) N,N,N',N'-tetraacetate on action potential duration in frog heart

Resting and action potentials were recorded from superfused strips of frog ventricle. Reducing the bathing calcium concentration ([Ca2+]0) with or without ethylene glycol-bis(beta-aminoethyl ether)N,N,N',N'-tetraacetate (EGTA) prolongs the action potential (AP). The change in the duration of the AP extends over many minutes, but is rapidly reversed by restoring calcium ions. Other changes (e.g., in resting potential and overshoot) are, however, only more slowly reversed. Reducing [Ca2+]0 with 0.2, 2, or 5 mM EGTA produces progressively greater prolongation of AP; maximum values were well in excess of 1 min. This prolongation can be reversed by other divalent cations in EGTA (Mg2+, Sr2+) or Ca-free (Mn2+) solutions, or by acetylcholine. Barium ions increase AP duration in keeping with their known effect on potassium conductance. D600, which blocks the slow inward current in cardiac muscle, is without effect on the action potentials recorded in EGTA solutions, or on the time course and extent of the recovery to normal duration upon restoring calcium ions. It is concluded that divalent cations exert an influence on membrane potassium conductance extracellularly in frog heart. The cell membrane does not become excessively "leaky" in EGTA solutions.

The effects of manganese ions on the contraction of the frog's heart

1. The effects of Mn(2+) in particular but also of Ni(2+), Co(2+), Cd(2+), verapamil and D600 on the contraction of isolated frog atrial trabeculae have been investigated. Contraction was initiated either by electrical stimulation, or by raising the [K](o) or by lowering the [Na](o).2. Mn ions like Ca ions cause a hyperpolarization of the cell membrane and a rise in the threshold for the action potential and twitch. Mn ions, particularly at low concentrations, reduce the overshoot of the ventricular action potential.3. Mn ions reduce the strength of the regularly evoked heart beats. Prolonged exposure, in beating hearts, results in a rise in resting tension and often a small recovery of the heart beat.4. In normal Ringer solution and in Ringer free of Na ions, the addition of Mn causes the tension-depolarization curve to be displaced by an amount equivalent to an 18 mV hyperpolarization of the membrane potential for a tenfold increase in the divalent cation concentration.5. Mn, Co, Ni and Cd ions all cause a marked reduction in the tension generated by exclusion of Na ions from the bathing fluid. In the presence of these divalent cations the contracture divides into an initial phasic and a later tonic contraction. This inhibition is reversed by raising the [Ca](o), while the tension developed during the initial phasic contraction varies with the [Ca](o)/[Mn](o) quotient.6. A similar tonic contracture is initiated after exposure to Na-free fluid containing a high [Mn](o) by the addition of a small concentration of Na, Li, hydrazinium or hydroxylammonium ions.7. The organic ;Ca antagonists' verapamil and D600 have little effect on the contracture induced by lowering [Na](o) even after prolonged exposure at relatively high concentrations but they do inhibit the twitch contraction and the K contractures.8. The effects of Mn on the Na-withdrawal contracture of frog heart can be interpreted in terms of an exclusively extracellular effect where Mn ions resemble Na ions in their action, and both antagonize the movement of Ca across the cell membrane.9. The experimental evidence suggests that the K contracture in frog heart is initiated by a mechanism which is, in some ways different to that underlying the Na-withdrawal contracture, and may involve two different sources of activator Ca.10. The several different effects of Mn on the frog heart probably reflects the ability of this cation to interfere with many processes involving Ca, and that there are a number of such processes involved in the results described in this work. The effects of Mn are more complex than might be generally supposed.

The inotropic memory of amphibian myocardium

(1) Experimentally observed changes of contractile force induced by changes in the pattern of stimulation of frog ventricular myocardium were compared with predictions computed on the basis of a model for the contractile conditioning, proposed in a previous paper. (2) For this purpose, two functions (Potentiation and Inhibition) which describe in the model the effect of previous contractions, were determined experimentally. (3) It is shown that the model adequately predicts : force-frequency curve Inotropic Effect Curves at different basal frequencies, effect of the suppression of a contraction in a sequence of definite frequency, frequency-staircases, and strength-interval curves. (4) The formal characteristics of the model are discussed, and it is suggested that the potentiation mechanism results from the recirculation of activator calcium in two types of compartments, from which calcium passes to the myofilaments during the contractions or is lost to the exterior with first order kinetics during the rest period. Possible locations for these compartments are proposed. Various hypothesis on the nature of the inhibitory mechanism are considered.

The effects of very low external calcium and sodium concentrations on cardiac contractile strength and calcium-sodium antagonism

1. The tension levels of the contractures induced in frog atrial trabeculae by reduction of the bathing Na concentration have been investigated over a wide range of [Na+]o and [Ca2+]o, making use of EGTA to buffer [Ca2+]o under 10(-5)M. 2. The relationship between tension and the quotient [Ca2+]o/[Na+]2o is only an apparent one, and holds approximately for Na concentrations as low as 1-5 mM. 3. However, for low Na concentrations tension depends on the absolute values of both [Ca2+]o and [Na+]o. 4. Extracellular Ca is the essential factor in tension production. The results indicate an apparent threshold for tension near 5 X 10(-8) M-[Ca2+]o. 5. It can be shown that, under the present experimental conditions, the Na concentration around the cells must be lower than about 0-1 mM in 'zero' Na+ solutions. 6. It is shown that the over-all process leading to tension production can be explained in terms of a reaction involving two Ca ions. 7. To account for the results, a modification of the reaction scheme put forward by Lüttgau & Niedergerke (1958) is proposed, involving a competitive action between Ca and Na ions at a free receptor site. 8. To account for the scheme, an example of a mechanism involving both internal and external Na and Ca concentrations is considered.

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.