The action of ions and lipoids upon the frog's heart

Ionized concentrations in Ca2+ and Mg2+ buffers must be measured, not calculated

NEW FINDINGS

What is the topic of this review? The [Ca²⁺ ]/[Mg²⁺ ] in buffers are usually calculated using one of eight programs. These all give different values, thus [Ca²⁺ ]/[Mg²⁺ ] must be measured. What advances does it highlight? The ligand optimization method (LOM) using electrodes is an accurate method to do this. The limitations of the method are described. The LOM has been generalized to include calibration of fluorochromes and aequorin. It is the method of choice to measure intracellular equilibrium constants. Owing to the uncertainties for the values of resting [Ca²⁺ ], ∆[Ca²⁺ ] and the pK' values for intracellular Ca²⁺ /Mg²⁺ binding used in modelling, these values must now be re-examined critically.

ABSTRACT

Modelling intracellular regulation of Ca²⁺ /Mg²⁺ is now an established part of physiology. However, the conclusions drawn from such studies depend on accurate knowledge of intracellular [Ca²⁺ ], ∆[Ca²⁺ ] and the pK' values for the intracellular binding of Ca²⁺ /Mg²⁺ . Calculation of [Ca²⁺ ]/[Mg²⁺ ] in buffers is normal. The eight freely available programs all give different values for the [Ca²⁺ ]/[Mg²⁺ ] in the buffer solutions, varying by up to a factor of 4.3. As a result, concentrations must be measured. There are two methods to do this, both based on the ligand optimization method (LOM): (1) calibration solutions from 0.5 to 4 mmol l^-1 ; and (2) calibration solutions from 0.1 µmol l^-1 to 2 mmol l^-1 . Both methods can be used to calibrate Ca²⁺ /Mg²⁺ electrodes. Only Method 2 can be used directly to calibrate fluorochromes and aequorin. Software in the statistical program R to calculate the [Ca²⁺ ]/[Mg²⁺ ] in buffers is provided for both methods. The LOM has now been generalized for use with electrodes, fluorochromes and aequorin, making it the ideal method to determine the pK' values for intracellular binding of Ca²⁺ /Mg²⁺ . The [Ca²⁺ ]/[Mg²⁺ ] in buffers must be measured routinely, which is best done by calibrating electrodes with the LOM and software written in R. If [Ca²⁺ ]/[Mg²⁺ ] in buffers are calculated, the parameters used in modelling show the same degree of variability as the software programs. Uncritical acceptance of such parameters means that conclusions reached from such studies are relative, not absolute, and must now be re-examined.

Contractures in a superfused frog's ventricle

1. A new perfused preparation of frog's ventricle is described, whose main advantage is that there are short diffusion distances between the cells and the washing fluid.2. This preparation responds within a few seconds to alterations in sodium, potassium or calcium concentration, or alteration in the osmotic pressure of the bathing fluid. The speed of these alterations is consistent with the diffusional distances involved measured histologically.3. On depolarizing the preparation with Ringer + excess potassium, tension development starts at about -25 mV and is still increasing at -8 mV (the lowest voltage studied). As in earlier experimetns in frog ventricle (Niedergerke, 1956b) and in skeletal muscle (Hodgkin & Horowicz, 1960) the relation between potential and tension is very steep.4. Sodium free contractures (Lüttgau & Niedergerke, 1958) are very rapid in onset with this preparation. To maintain the tension developed, a rapid rate of washing is necessary for 1-2 min. After this, slow washing is sufficient. It is proposed that this is due to the leaching out of cellular sodium.5. In fresh preparations the twitch and maximum K contracture are of similar size when stimulated at 30/min in 1-2 mM-Ca. With the onset of hypodynamia the twitch tension falls, but the K contracture remains unaltered. In hypodynamic ventricles the sensitivity of the twitch to the ratio [Ca]/[Na](2) declines, whereas that of the K contractures remains unaltered. Hypodynamia therefore does not affect the contractile elements themselves.6. During the staircase phenomenon the K contracture and twitch size alter in a similar manner, as previously described by Niedergerke. The sodium free contracture, however, remains unaltered. This tends to favour the hypothesis that the staircase phenomenon is due to effects at the cell membrane rather than in the cell interior.7. Maximum K contractures occur at a [Ca]/[Na](2) ratio of about 0.7 x 10(-4) mM(-1), a figure similar to that obtained in earlier experiments (Lüttgau & Niedergerke, 1958).8. The length-tension curves of K contractures are similar to those previously described for heart muscle, using single twitches to generate tension.9. Application of K free solutions produced no rapid contractures in this preparation.

EFFECTS OF pH AND OF VARIOUS CONCENTRATIONS OF SODIUM, POTASSIUM, AND CALCIUM CHLORIDE ON MUSCULAR ACTIVITY OF THE ISOLATED CROP OF PERIPLANETA AMERICANA (ORTHOPTERA)

1. Twenty-five solutions which contained KCl (0.0, 0.2, 0.4, 0.6, and 0.8 gm. per liter), in combination with CaCl₂ (0.0, 0.2, 0.4, 0.6, and 0.8 gm. per liter), 10.0 gm. of NaCl, and 0.2 gm. of NaHCO₃ per liter of solution were tested in order to determine satisfactory KCl/CaCl₂ ratios in an insect physiological salt mixture for the maintenance of muscular activity by the isolated crop of the American roach. Satisfactory activity products (0.390 to 0.549) were obtained in seven mixtures with KCl/CaCl₂ ratios of 0.2/0.2, 0.4/0.4, 0.6/0.6, 0.8/0.8, 0.2/0.4, 0.4/0.6, and 0.6/0.8, expressed as gram per liter. These ratios lie between 0.50 and 1.00. In solutions which contained calcium, but no potassium, approximately 50 per cent of the crops exhibited an initial tone increase and were arrested in rigor. See Fig. 2. In solutions which contained potassium, but no calcium, all crops showed an initial loss of tone and arrest in relaxation. See Fig. 2. 2. Seven KCl/CaCl₂ ratios (see paragraph 1 above) were tested with eight NaCl concentrations (1.0, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, and 1.8 per cent) at a pH of 8.0. In these mixtures, the ones with KCl/CaCl₂ ratios of less than 1.0 produced higher activity products than those with ratios equal to 1.00. The highest average activity product (0.849) was obtained in the solutions with 0.2 gm. of KCl and 0.4 gm. of CaCl₂ per liter. 3. Four KCl/CaCl₂ ratios (0.2/0.2, 0.4/0.4, 0.2/0.4, and 0.4/0.6 gm. per liter) were tested with 1.4, 1.5, and 1.6 per cent NaCl at a pH of 7.5. When analyzed with data from comparable solutions at a pH of 8.0, it was found that 1.4 per cent NaCl afforded an optimum environment for isolated crop activity. 4. Effects of hydrogen and hydroxyl ion concentrations were studied at pH values of 6.8, 7.5, 8.0, and 8.9. The highest average activity product, 1.011, was produced at a pH of about 8.0. 5. A satisfactory physiological salt solution for the isolated foregut of the American roach, Periplaneta americana, would contain 14.0 gm. of NaCl, 0.4 gm. of CaCl₂, 0.2 gm. of KCl, and 0.2 gm. of NaHCO₃ per liter of solution. This mixture should have a pH value between 7.8 and 8.2. 6. Durations of crop activity extending over periods as long as 25 hours were quite common, and several crops maintained contractions for more than 30 hours. The greatest longevity was for crop 814, from a female, which continued activity for slightly more than 47 hours. 7. A significant difference between the activity products of the crops from males and the crops from females was recorded. Although there was not a significant difference in the amount of food ingested by males and females, 12 hours after feeding there was more food in the females' crops, and the food progressed more rapidly through the males' crops than through the females'. In addition, crops from the two sexes reacted differently to the effects of day old solutions. This sex difference is apparently related to an inherently increased activity of the crop from the male roach.

Physiological effects and biochemical properties of a serum protein that produces positive inotropic and chronotropic effects on isolated guinea pig atria

Isolated left and right guinea pig atria were used as a bioassay for the detection of an endogenous cardioactive substance in bovine serum. Serum, buffer exchanged to Krebs-Henseleit solution, produced positive inotropic and chronotropic effects on the isolated guinea pig atria. The cardiotonic effects were unaffected by the combined presence of propranolol and methysergide (both 10(-6)M) and were also dissimilar in time course from other known cardiotons such as catecholamines and cardiac glycosides. Following ultrafiltration (using XM100A Amicon membranes), activity was found solely in the retentate fractions and was therefore probably due to a large molecular weight (> 100 kDa) substance or a small molecule bound to a large protein. The cardioactive factor (CF) in the whole serum was heat labile, sensitive to acidification, exposure to potassium bromide and equilibration to physiological buffers of a low ionic strength. Isolation by conventional protein purification techniques was unsuccessful due to the labile nature of the active molecule(s) when exposed to non-physiological experimental conditions. Physical and biochemical properties of the CF which may help avoid inactivation are discussed for future experiments aimed at elucidating the nature and identity of the cardiotonic principle.

Metabolic plasticity and the promotion of cardiac protection in ischemia and ischemic preconditioning

The concept of metabolic protection of the ischemic myocardium is in constant evolution and has recently been supported by clinical studies. Historically, enhanced glucose metabolism and glycolysis were proposed as anti-ischemic cardioprotection. This hypothesis is supported by the sub-cellular linkage between key glycolytic enzymes and the activity of two survival-promoting membrane-bound pumps, namely the sodium-potassium ATPase, and the calcium uptake pump of the sarcoplasmic reticulum. Moreover, improved resistance against ischemia follows the administration of glucose-insulin-potassium in a variety of animal models and in patients following acute myocardial infarction. The metabolic plasticity paradigm has now been expanded to include (1) the benefit of improved coupling of glycolysis to glucose oxidation, which explains the action of anti-ischemic fatty acid inhibitors such as trimetazidine and ranolazine; (2) the role of malonyl CoA in the glucose-fatty acid interaction; and (3) the anti-apoptotic role of insulin. Furthermore, we argue for a protective role of increased glucose uptake in the preconditioning paradigm. Additionally, we postulate an adaptive role of mitochondrial respiration in the promotion of cardioprotection in the context of ischemic preconditioning. The mechanisms driving these mitochondrial perturbations are still unknown, but are hypothesized to involve an initial modest uncoupling of respiration from the production of mitochondrial ATP. These perturbations are in turn thought to prime the mitochondria to augment mitochondrial respiration during a subsequent ischemic insult to the heart. In this review we discuss studies that demonstrate how metabolic plasticity can promote cardioprotection against ischemia and reperfusion injury and highlight areas that require further characterization.

Anaerobic and aerobic activity in isolated muscle

It is well known that a muscle excited anaerobically fails to contract any more when the concentration of lactic acid inside it rises to about 0·3 per cent. In the presence of oxygen the lactic acid disappears and the muscle recovers, but this does not prove that recovery is due solely to the removal of the acid; it might be argued that some other mechanism also needs restoration before the muscle can function normally again. Now it is possible, by suspending a sufficiently thin muscle in oxygen-free Ringer's solution and exciting it sufficiently slowly, to allow diffusion to remove the lactic acid as it is formed. Under such conditions the muscle is found to liberate 2 to 3 times as much energy as it would it diffusion were not permitted. It appears to continue working until all its carbohydrate is used up. Since, therefore, nothing is known to pass into the Ringer's solution except lactic acid, we may, until some other substance is found, attribute the absence of fatigue to the absence of lactic acid in the muscle, and conclude that when diffusion is not allowed fatigue is due primarily to lactic acid. In a recent paper (1, p. 51) the case was considered of a muscle excited by single shocks at regular intervals in oxygen-free Ringer's solution, attaining a "steady state" in which outward diffusion of lactic acid balances new formation. If a plane sheet of muscle of thickness 2b, exposed on both sides to Ringer's solution, produce α gm. of lactic acid per c. c. per minute, and ifkbe the diffusion constant, thegreatestconcentration attained anywhere inside it, viz., in its middle plane, is (in gms. per c. c.) αb2/2k.

The isolated and perfused working heart of the frog, Rana esculenta: an improved preparation

1. An in vitro preparation of the intact heart of the frog Rana esculenta was set up. 2. The isolated heart, perfused at constant pressure, was spontaneously beating and able to generate physiological values of output pressure, cardiac output, ventricle work and power. It showed the typical phenomenon of the "hypodynamic state" after a relatively constant time from the onset of the perfusion. 3. Perfusion with air-saturated saline and 99.5% oxygen-saturated saline did not show significant differences in the recorded parameters. 4. This experimental model represents a useful tool for physiological and pharmacological studies, especially when the direct analysis of the effects of hormones, mediators or drugs requires an intact heart preparation.

Effects of serum, its protein and lipid extracts, and commercial serum proteins and lipid on the isolated frog heart

This study investigates the inotropic effects of serum, its protein and lipid extracts, and commercial serum proteins and lipid on the isolated, spontaneously-beating heart and superfused, hypodynamic ventricle of the frog. Serum taken from either man, horse, calf, frog, or rabbit evoked marked positive inotropic responses which were unaffected by cholinergic, serotonergic, and adrenergic receptor antagonists. Dialysed serum (dialisand) and void volume fractions from Sephadex G200-120 columns corresponding to large molecular weight constituents evoked marked positive inotropic responses. When serum was separated into fractions containing either proteins or lipids/lipoproteins by high-density ultracentrifugation or activated charcoal, both extracts evoked marked positive inotropic responses. Commercial serum globulins and serum containing a high proportion of immunoglobulins elicited large increases in contractile force, whereas serum albumin evoked a negative inotropic effect. Serum which was either boiled and/or treated with chymotrypsin to denature proteins also caused a marked increase in isometric twitch tension in the frog heart. Similar inotropic response was obtained with fractions of boiled serum eluted on columns of Sephadex G200-120. These fractions corresponded to molecular weight in the region of 60-70 kDa. However, the inotropic effect of boiled serum was abolished following pretreatment with lipase. Superfusion of frog hearts with commercial cardiolipin resulted in marked dose-dependent increases in contractile force. The results demonstrate the presence of at least two large molecular weight cardioactive principles in serum. These substances are comparable in size to constituents of serum proteins (e.g., globulins and immunoglobulins) and serum lipids/lipoproteins (e.g., cardiolipin) and may serve as physiological regulators of cardiac function.

Endogenous digitalis-like substance in pig left ventricle

A crude fraction was isolated from pig heart left ventricle (150 g) homogenates after extraction of lipids, chromatographic separation and desalting. The extract contained an ionic content of 0.21, 0.27, 0.33 and 1.7 mM respectively for Mg2+, Ca2+, K+, and Na+. The albumin extract, used as a reference control, contained an ionic content of 0.88 and 2.1 mM respectively for K+ and Na+ and negligible amounts of Mg2+ and Ca2+. The isolated fraction exhibited digitalis-like properties in the inhibition of sarcolemmal Na+, K+-ATPase in a dose dependent manner, the displacement of [3H]-ouabain binding from membrane receptor sites and produced +ve inotropic response in isolated perfused heart in a dose dependent manner. The albumin extract tested in the same manner showed no digitalis-like properties. The ventricular fraction was unable to displace (-) 3H-DHA binding from membrane sites and its inotropic action was not blocked by propranolol. The data suggests that the fraction isolated from pig heart left ventricle contains a substance which has some properties like digitalis.

Inotropic responses of the isolated frog ventricle to serum

A study has been made of the effects of newborn calf-serum on the isolated frog ventricle. Preparations were superfused with solutions containing different concentrations of serum and the changes in contractility measured. Ordinary calf-serum evoked a dose-dependent increase in contractile force which was unaffected by adrenergic receptor antagonists (propranolol and phentolamine). Serum which was either used in growing HeLa cells or incubated with chymotrypsin failed to elicit a positive inotropic response. However, if prostaglandin-like materials were extracted from the serum then its ability to potentiate the twitch remained unaltered. These results suggest that the cardioactive-factor present in calf-serum is most probably related to a polypeptide, and possibly a growth factor.

Effects of exogenous neurotransmitters on contractility and cyclic nucleotide metabolism in the isolated frog ventricle

A study has been made of the effects of acetylcholine and adrenaline on the metabolism of 3',5'-cyclic nucleotides by the isolated frog ventricle. Measurements of adenosine 3',5'-cyclic monophosphate (cyclic AMP) and guanosine 3',5'-cyclic monophosphate (cyclic GMP) were made: (a) at various times during responses produced by a given concn of acetylcholine (10(-7) M) or adrenaline (10(-6) M); and (b) at fixed times following exposure to different concns of the two agonists. The negative inotropic effect of acetylcholine is accompanied by reciprocal changes in the two cyclic nucleotides: cyclic GMP levels rise and cyclic AMP falls. Both effects are dose-related. A more complex pattern of changes occurs during responses produced by adrenaline. The initial increase in twitch tension is accompanied by a transient reduction in cyclic GMP and by a large increase in cyclic AMP. The rate at which cyclic AMP accumulates is greatest at the time when cyclic GMP levels are maximally depressed. An increase in cyclic GMP, seen later, coincides with a gradual fall in both twitch tension and cyclic AMP. The increments in both cyclic AMP and cyclic GMP, measured when the twitch is maximally potentiated, are dose-related. The magnitude of the decrease in peak twitch tension produced by acetylcholine and of the increase produced by adrenaline correlate closely with corresponding changes in the relative proportion of cyclic AMP: cyclic GMP present in the fibres. The possible significance of the changes in cyclic AMP and cyclic GMP in modulating the response of the ventricle is discussed.

Effects of extracts of rat brain on the digitalis receptor

There is a possibility that an endogenous substance exists which interacts with a ouabain binding site on Na+, K+-ATPase. Recently, several reports have appeared suggesting the presence of an endogenous digitalis-like substance in acid-acetone extracts of brain. We have demonstrated that in preparing an acid-acetone extract, peroxidized lipids and lysophospholipids are produced, both of which inhibit Na+, K+-ATPase, thereby complicating interpretation. Preliminary evidence suggests, however, that when rat brains are extracted with an aqueous-acetone mixture under nitrogen, a principle is obtained which specifically inhibits Na+, K+-ATPase.

Release of prostaglandins from the isolated frog ventricle and associated changes in endogenous cyclic nucleotide levels

1. A study has been made of the decline in contractility and some associated metabolic changes which occur in the isolated frog ventricle during the development of hypodynamic depression. 2. The release of two identified prostaglandins (PG), E1 and E2, together with several as yet unknown prostaglandin-related substances (PRS), accompanies the development of hypodynamic depression. There is a close correlation between the extent to which the isometric twitch is depressed and the quantity of prostaglandin released into the superfusate. 3. Fractionation of extracts of 'used' superfusates, using preparative-scale thin-layer chromatography, revealed the presence of six major components, four of which (PGE1 and PGE2 and two unidentified components) were found to be cardioactive and potentiated contraction when tested subsequently on hypodynamic preparations. 4. Two agents which influence prostaglandin biosynthesis, arachidonic acid and indomethacin, are found to affect both the rate at which the hypodynamic state develops and the extent to which the 'steady-state' twitch tension is depressed, in a dose-dependent manner. Indomethacin, a PG-synthetase inhibitor, accelerates the decay and depresses the final 'steady-state' tension attained, whereas arachidonic acid, the principal precursor for prostaglandin biosynthesis, has the converse effects. 5. Measurements of endogenous 3'5'-cyclic nucleotide levels reveal a time-dependent decrease in intracellular adenosine 3'5'-cyclic monophosphate (3'5'-cyclic AMP) and a concomitant increase in guanosine 3'5' cyclic monophosphate (3'5'-cyclic GMP). The decline in isometric twitch tension is paralleled almost exactly by an equivalent reduction in the ratio 3'5'-cyclic AMP: 3'5'-cyclic GMP. 6. Superfusion of isolated ventricles with Ringer solution containing exogenous, lipid-soluble derivatives of 3'5'-cyclic AMP and 3'5'-cyclic GMP affects both the rate of decline of the isometric twitch and the steady-state tension ultimately reached: thus, 8-bromo-3'5'-cyclic GMP accelerates the decline in contractility and depresses the steady-state level, whereas dibutyryl 3'5'-cyclic AMP delays the development of hypodynamic depression, and elevates the final twitch tension. The effects of both 3'5' cyclic nucleotide derivatives are dose-dependent. 7. The possible involvement of prostaglandins and 3'5'-cyclic nucleotides as causal agents in the mechanism of hypodynamic depression is discussed. The biochemical basis for the implied antangonistic effects of 3'5'-cyclic AMP and 3'5'-cyclic GMP in regulating ventricular contractility is considered in the following paper (Flitney & Singh, 1980).

The energetics of Na-dependent solute transport in isolated cells [proceedings]

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The influence of calcium and beta-sympathomimetics on force-frequency relationship and resting potentiation in guinea pig papillary muscle

The influence of various calcium concentrations and of the beta-sympathomimeticum orciprenaline (Alupent) on the force-frequency relationship and on the resting potentiation of isolated isometrically contracting papillary muscles of the guinea pig heart was studied. Elevation of Ca concentration and/or addition of orciprenaline augment the amplitudes of contractions elicited by rhythmical stimulation, but they diminish the amplitudes of test contractions elicited after a test interval of 1-2 s. It is suggested that calcium and orciprenaline strengthen the calcium loss from the sarcoplasmic reticulum out of the cell during a prolonged interval.

Calcium-sodium antagonism on the frog's heart: a voltage-clamp study

1. In double sucrose-gap voltage-clamped frog atrial fibres the influence of [Ca]o and [Na]o on membrane current and contraction was investigated. 2. The slow (secondary) inward current varied with [Ca]o but was almost insensitive to changes in [Na]o. In contrast, the phasic (transient) contraction initiated by the slow inward current was affected by both [Ca]o and [Na]o. 3. With moderate changes of [Ca]o and [Na]o from normal, the strength of phasic contraction at a given depolarization followed the [Ca]o/[Na]2o ratio approximately. This was best seen at membrane potentials near zero level. 4. Under the same conditions, tonic (sustained) contractions associated with prolonged depolarizations were strictly correlated to the [Ca]o/[Na]2o ratio at any potential. No interrelation between tonic tension and steady-state current was found. 5. With extensive changes in [Ca]o and [Na]o, the sensitivity of both phasic and tonic tension to the [Ca]o/[Na]2o ratio declined, the negative effect of [Na]o becoming smaller than was expected from this ratio. 6. In Na-free choline-Ringer, a strong contracture developed followed by a spontaneous relaxation. Starting from the relaxed state, application of depolarizing clamps gave rise to phasic contractions with a very slow relaxation while tonic contractions were apparently lacking. 7. The results are interpreted in terms of an energy-dependent carrier mechanism exchanging one Ca for two Na ions across the cell membrane. The model implies a strong asymmetry in the rate constants governing the chemical reactions on both sides of the membrane. The system is thought to operate close to equilibrium at any potential, thereby determining the steady level of myoplasmic Ca. The equilibrium itself is considered to shift upon depolarization. Assuming that [Na]i is constant, the steady level of [Ca]i is expected to be proportional to the [Ca]o/[Na]2o ratio, the scale factor being a function of membrane potential. 8. The carrier model suggests the occurrence of a depolarization-induced inward transfer of Ca which might be involved in the generation of tonic contractions. 9. The apparent lack of tonic contractions in the absence of external Na ions may be explained by a suppression of carrier-mediated Ca influx normally occurring upon depolarization. 10. The antagonistic effects of [Ca]o and [Na]o on phasic contraction are understood as being due to alterations of the Ca pumping system rather than changes in slow inward current.

Coronary vasodilation by fatty acids

Fatty acids increase the coronary flow rate of rat hearts, perfused according to the Langendorff technique. Long-chain and medium-chain fatty acids are more effective vasodilators than short-chain fatty acids. The vasodilatation by fatty acids does not proceed through the intermediate formation of the vasodilator adenosine, nor by stimulation of adenylcyclase activity. Since at low Ca2+ concentrations fatty acids not only stimulate the coronary flow rate but also cardiac contractility, it is suggested that especially the lipophilic fatty acids have calcium ionophoric properties leading to increased Ca2+ removal from smooth muscle cytosol and hence to vasodilatation. Preliminary experiments, moreover, indicate that both medium- and long-chain fatty acids, like prostaglandin E1 and Ca2+, inhibit membrane ATPase(s) of aorta smooth muscle cells, suggesting increased Ca2+ binding to vascular smooth muscle cell membranes.

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.

Membrane current and contraction in frog atrial fibres

1. Membrane current and mechanical activity were recorded from short segments of frog atrial muscle strips using a double sucrose gap voltage clamp arrangement. Experiments were performed at 4-7 degrees C. Two types of contraction were observed dependent upon the duration of the clamp.2. Short-lasting depolarizations caused a flow of Ca inward current, I(Ca), and development of a phasic contraction. Time to peak tension approximated 400 msec. Both I(Ca) and contraction, as functions of membrane potential, had a threshold of about - 40 mV and were maximal at inside positive potentials in normal Ringer fluid. Peak tension decreased at strong depolarizations.3. The minimum time of depolarization required for initiation of a phasic contraction was 40-70 msec. The time necessary for full activation of contraction was 200-300 msec and comparable to the period of time covered by the flow of I(Ca).4. There was no marked change in peak tension upon repetitive depolarization to the same membrane potential.5. Restoration of (phasic) contractility after a preceding contraction was strongly dependent on the level of membrane potential between conditioning and test pulse. Restoration was half complete at potentials around - 45 mV.6. Long-lasting depolarizations generated tonic (sustained) contractions superimposed on the phasic (transient) ones. Threshold potential for initiation of tonic contractions was usually positive to the threshold of phasic contractions. The time taken to attain the final level of tension ranged between 0.7 and 3 sec. Plateau tension, as a function of membrane potential, increased with increasing depolarization and reached a flat maximum at about + 50 mV in normal Ringer fluid.7. At membrane potentials near zero level, plateau tension developed by the tonic mechanism was about twice peak tension due to phasic contraction.8. Removal of Ca ions from the external medium resulted in an almost complete abolition of phasic contraction within 1-2 min and a gradual decrease of tonic contraction during the first 10 min. Application of a ;Ca inhibitor' to normal Ringer fluid caused a strong reduction of both I(Ca) and phasic contraction without affecting tonic contractions.9. It is concluded that phasic contractions are directly activated by the flow of I(Ca). Generation of tonic contractions may be attributed to a Ca transfer mechanism different from I(Ca) or a release of Ca from intracellular stores.

The dependence of the contractile force generated by frog auricular trabeculae upon the external calcium concentration

1. A method is described by which the solutions bathing single auricular trabeculae, isolated from the heart of the frog, can be rapidly altered while the tension generated and the membrane potential can be measured simultaneously.2. Changes of the [Ca](o) result in changes of the twitch strength similar to that reported for frog ventricle.3. At [Ca](o) of less than 1 mM, the isometric contracture tension generated during application of K-rich solutions, and the maximum rate of tension development, are proportional to [Ca](o) (3).4. This relationship is not the consequence of (a) the hypertonicity of the K-rich solutions, (b) the dependence of the membrane potential on [Ca](o), or (c) the facilitation due to a twitch response at the initiation of the contracture.5. Reduction of the [Na](o) increases the strength of the high-K contractures according to the ratio of [Ca](o)/[Na](o) (2); Na ions in the bathing medium are shown to competitively inhibit the potentiating action of Ca ions on the force generated during contractures.6. An equation is derived which assumes that three Ca compounds act co-operatively at some stage in the process of excitation-contraction coupling.7. Two hypotheses are discussed. The first proposes that the sarcoplasmic [Ca] established during depolarization of the muscle membrane depends upon [Ca](o) (3), and tension generated by the contractile elements on a first order reaction with ionic Ca. The second suggests that if the sarcoplasm [Ca] established during excitation is proportional to [Ca](o), then three Ca ions are required to activate the contractile response at the unit level.

Interaction between heart rate and calcium concentration in the control of contractile strength of the frog heart

1. The changes in twitch tension during the ascending and descending staircase of the frog heart have been examined under various experimental conditions including the hypodynamic and prehypodynamic state.2. The descending staircase resembles the tension change after reduction of external calcium concentration in showing two consecutive phases, an initial rapid and later slow phase of tension decline.3. The time course of the ascending staircase depends on the condition of the heart; it is slow after, and rapid before, development of the hypodynamic state. It is also rapid when elicited after conditioning periods of increased heart rate.4. The tension transients in response to brief concentration steps of external calcium concentration were examined at various levels of heart rate. The results indicate that at high heart rates the sensitivity of heart cells to external calcium is increased.5. The results are interpreted along the lines of a previous hypothesis relating tension development to the cooperative action of two intracellular calcium compounds. The additional assumption is made that an intermediary exists which facilitates calcium movements in heart cells to an extent depending on the level of heart rate.

Effects of calcium on the contraction of the hypodynamic frog heart

1. The time course of build-up or decline of twitch tension in response to enhancement or reduction of the external calcium concentration was determined in frog heart ventricles, in the hypodynamic state as well as under non-hypodynamic conditions.2. The tension build-up induced by high calcium was greatly slowed when the heart passed into the hypodynamic state, whereas the decline in low calcium was little altered by this condition.3. The tension decline consisted of two approximately exponential phases, an initial rapid phase (t((1/2)) between 3 and 10 sec) and a later slow phase (t((1/2)) between 50 and 180 sec).4. A similar composite time course of tension build-up only occurred under certain conditions: (a) in ventricles in which the hypodynamic state had not developed; (b) after a conditioning period of exposure to enhanced calcium or reduced sodium concentrations.5. The results are explained on the assumption that contraction is brought about by the co-operative action inside heart cells of two calcium compounds whose concentrations change at different rates after variation of the external calcium concentration.6. Formal relationships describing the dependence of twitch tension on the concentration of these two hypothetical compounds are obtained.A tentative explanation of the development of the hypodynamic state is also proposed.

The effect of adrenaline on the tension developed in contractures and twitches of the ventricle of the frog

1. The effect of adrenaline on contracture and twitch tension in frog's ventricle has been examined, using the superfused preparation.2. In 1 mM-Ca Ringer, contractures induced with excess KCl concentrations from 50 to 200 mM, are reduced by 1 x 10(-6) g/ml. adrenaline to an average of 0.62 of control values, in marked contrast to the well known positive inotropic effect of adrenaline on the heart twitch. This effect of adrenaline is directly dose dependent. Increasing [Ca](o) diminishes the effect of adrenaline on contracture tension, and on the twitch tension.3. Adrenaline has a significantly greater effect on the KCl contracture tension than noradrenaline or isoprenaline.4. In 1 mM-Ca Ringer, Na-free contractures are reduced to 0.72 of controls by 1 x 10(-6) g/ml. adrenaline. Adrenaline also significantly reduces tension in contractures induced by 50 c/s alternating current.5. The action of adrenaline on contracture tension is largely complete in 1-2 min at various rates of stimulation and calcium concentrations. A similar time course has been found for the effect of adrenaline on membrane potential.6. Pronethalol blocks the action of adrenaline on both twitch and contracture. The action on the contracture can also be blocked by ouabain (1 x 10(-5)M), and exposure of the tissue to K-free or Na-free Ringer solution.7. Adrenaline hyperpolarizes the membrane potential with a range of [K](o) from 0 to 200 mM. This effect is blocked by pronethalol and ouabain. After exposure to ouabain, adrenaline causes a significant decrease in the membrane potential. This may be due to an increase in the sodium permeability.8. At low values of the [Ca]/[Na](2) ratio, adrenaline takes a relatively constant number of beats for full action, but at high values of the ratio the development of full effect is largely time dependent.9. The time course of the effect on the twitch of changing from 0.5 to 2 mM-Ca Ringer has been studied at various rates of stimulation. The equilibration time has been found to depend on the heart rate.10. The effect on the contracture suggests that adrenaline decreases the calcium permeability. It is further suggested that the development of twitch tension is not due to direct Ca entry but is due to the release of Ca from a local store within or between the cells. The inotropic action of adrenaline is explained in terms of this store.

The efflux of potassium, sodium, chloride, calcium and sulphate ions and of sorbitol and glycerol during the cardiac cycle in frog's ventricle

1. The exchanges of potassium and various other substances have been measured in beating frog's ventricles, using both superfused and distended preparations. In both preparations the high fluid flow rates used (1 ml./sec) cleared the ventricular cavity with a half-time (T(1/2)) of about 130 msec.2. Histological sections show that the modal strand radius in the relaxed or contracted distended ventricle is 17.5 mu, and in the relaxed and contracted superfused ventricle is 17.5 and 27.5 mu respectively.3. In quiescent ventricles the resting potassium influx and efflux are approximately equal at about 16 p-mole/cm(2).sec. This figure is computed from Niedergerke's (1963b) estimate of a cell size of 3.5 mu taken from electron-micrographs. If the older figure of 9.2 mu from single isolated cells is used (Skramlik, 1921) then the fluxes are about 44 p-mole/cm(2).sec. To allow for some cell damage in these preparations a further increase in flux of about 30% may be necessary.4. Contraction leads to a diminution of both potassium influx and efflux. Measurements made at 100 msec intervals throughout the cardiac cycle have demonstrated (a) that this decreased K efflux occurs at the same time as the mechanical twitch, and (b) that the size of the decrease is dependent on the external calcium concentration. Other experiments show that a similar decrease can be obtained by inducing a contracture at a constant membrane potential. It is concluded that the decreased K efflux during contraction is due to mechanical distortion of the tissue. This leads to a further slowing of the K diffusion and allows considerable reabsorption of K to occur into the cells.5. Efflux analysis suggests that normal K diffusion in the extracellular space may be about 1/10 of that in free solution. If this is correct the true membrane fluxes may be x 5 those measured.6. Phasic efflux measurements of Na, Ca, K, Cl, SO(4), sorbitol and erythritol show that a peak of efflux occurs just after the point of maximum rate of contraction of the ventricle. The peak efflux of K is least but all the other substances show similar patterns. In calcium-free solutions these phasic changes are absent. It is concluded that these effects are mechanical.7. Net K and Na changes were measured in ventricles poisoned by ouabain. The computed net changes for quiescent ventricles were a gain of 2.8 p-mole/cm(2).sec of Na and a loss of 5.3 p-mole/cm(2).sec of K. On stimulation a further increase in Na uptake of 8 p-mole/cm(2) occurred with no further loss of potassium. These results are computed for a cell diameter of 3.5 mu, for the larger diameter of 9.2 mu appropriate values of Na and K are 7.4 and 13.4 p-mole/cm(2).sec respectively for quiescent ventricles and an extra Na uptake of 21 p-mole/cm(2) per action potential. These results: (a) show that no large degree of single-file interaction occurs on the K movements, (b) are in agreement with the hypothesis that the membrane K fluxes are underestimated and (c) show that sufficient Na enters the cells per action potential to discharge a capacity of about 4 muF/cm(2).8. A general conclusion reached in these experiments is that ion movements during the long cardiac action potential cannot easily be measured because of mechanical artifacts.