Stimulatory effects of DB-c-AMP and adrenaline on myocardial contraction and 45Ca exchange. Experiments at reduced calcium concentration and low frequencies of stimulation

Effect of norepinephrine and heart rate on intracellular sodium activity and membrane potential in beating guinea pig ventricular muscle

The effect of 3 microM norepinephrine (NE) on intracellular sodium activity (aiNa) and resting membrane potential was studied by continuous intracellular recordings with a conventional and an ion-selective microelectrode. The electrodes were impaled simultaneously in small (diameter, 0.3 mm) superfused trabeculae of the beating guinea pig ventricle at 37 degrees C. In the absence of NE, changes of the beating rate produced an increase of aiNa by 1.5 +/- 0.17 mM (from 0 to 1 Hz) and 1.9 +/- 0.47 mM (from 0 to 2 Hz). In the presence of NE, there was a very small significant increase of aiNa during constant stimulation (1 Hz) and at at [K+]o of 4.7 and 11.5 mM. After 7 minutes of exposure, aiNa increased by 0.5 +/- 0.19 mM (mean +/- SEM, n = 4) at [K+]o of 4.7 mM and by 0.5 +/- 0.22 (n = 6) at [K+]o of 11.5 mM. Resting membrane potential became more positive by 1 mV at both levels of [K+]o. The effect of NE became also clearly manifest from the configurational changes of action potentials (profound increase in plateau height and duration). Stimulation of the Na(+)-K+ pump by NE became manifest from the changes of resting membrane potential and aiNa after abrupt cessation of stimulation. The magnitude and the rate of the decrease in aiNa and the initial rate of hyperpolarization were significantly greater in the presence of NE than in its absence.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiating effects of forskolin on the cardiovascular teratogenicity of ephedrine in chick embryos

The effects of forskolin on the teratogenicity of ephedrine in the developing chick heart were studied. Forskolin was administered to 4-day chick embryos (Hamburger-Hamilton stage 24) together with ephedrine at doses at which each agent alone caused minimal embryotoxicity. The embryos were examined for malformations on day 14 of incubation. The frequency of malformed embryos exposed to ephedrine (0.5 or 5 mumol) alone was 8 and 26%, respectively, and significantly increased to 47-72% in the presence of 1 nmol forskolin. Forskolin (1 nmol) alone did not induce a significant number of cardiac malformations. These results suggest that the increase in cAMP through stimulation of adenylate cyclase by forskolin is associated with the potentiation of ephedrine-induced cardiovascular malformations in the chick.

Channel-mediated calcium current in the heart

Calcium ions play an important role in the regulation of heart functions. Calcium ions may enter or leave the myocardial cell through various mechanisms, including several exchange mechanisms and pumps. This review concentrates on the influx of calcium ions through channels in the sarcolemma, resulting in an electric current flow. The calcium current plays an important role in the maintenance of the action potential duration, in the generation of pacemaker activity, and in the initiation of contraction. The calcium current displays both activation and a subsequent inactivation when the membrane potential is changed in a stepwise fashion. Previously, the activation was thought to occur rather slowly, hence the name "slow inward current." Recent evidence suggests that the calcium current occurs much faster and that two types of calcium currents might exist, differing in their selectivity to other ions and in their sensitivity to membrane potential and to drugs. The calcium current is modulated by several factors. Beta-adrenergic stimulation increases the calcium current by increasing the opening probability of the calcium channel. The effects of acetylcholine are less well described. There also exists a class of drugs, called calcium channel blockers (or calcium antagonists) that decrease the flow of calcium ions through calcium channels. It is not quite clear how the calcium current is changed during myocardial ischemia. Factors that may reduce the calcium current during ischemia are the increased extracellular potassium concentration, metabolic inhibition and a decreased ATP level, and acidosis. Raised levels of intracellular cAMP, however, should lead to an increased calcium current.

The effect of stimulation frequency and calcium concentration on maintenance of developed tension in isolated heart muscle preparations

The ability of isolated cardiac muscle preparations to maintain force of contraction was assessed at different extracellular Ca2+ concentrations and at various stimulation frequencies. Preparations were incubated in Krebs-Henseleit buffer at 32 degrees C and electrically stimulated at frequencies between 0.25 and 3.0 Hz. Extra-cellular Ca2+ was maintained at given concentrations between 0.8 and 2.4 mM, a range that included the plasma-ionized Ca2+ levels determined for rat and guinea pig. Developed tension of atrial or papillary muscle preparations of guinea pig heart was more stable at higher stimulation frequencies. Rat atria and ventricular strips were more stable at 0.25 than at 0.5 or 1.0 Hz, and the stability increased at 2.0 and 3.0 Hz. Guinea pig atrial muscle was maintained for a longer time at higher media Ca2+ concentrations; however, no such effect was observed with rat atrial tissue. Thus, the decrementing at developed tension in isolated cardiac muscle preparations is dependent on stimulation frequency and Ca2+ concentrations and can be retarded by selection of appropriate incubation conditions.

Calcium-dependent chronotropic action of bovine parathyroid hormone (1-34) in isolated atria of Japanese quail, Coturnix coturnix japonica

The involvement of extracellular calcium in the chronotropic action of parathyroid hormone (PTH) in isolated quail atria was studied. Bovine (b) PTH-(1-34) caused dose-dependent increases in the atrial beating rate. The PTH-induced positive chronotropism was significantly attenuated by lowering the extracellular calcium concentration or by using the calcium channel antagonists verapamil and D600. This suggests the importance of calcium influx in the cardiac action of PTH. beta-Adrenergic blockade with propranolol did not affect the responses to bPTH-(1-34). This further substantiates the previous findings that the PTH action is independent of cardiac beta-adrenoceptor stimulation.

Myocardial calcium uptake in streptozotocin diabetic and control rats after dibutyryl-cAMP; alpha-adrenergic stimulation and calcium deprivation

Previous studies have shown a decreased myocardial calcium uptake after beta-adrenergic stimulation with isoproterenol in isolated perfused hearts from streptozotocin diabetic rats. Abnormalities in the beta-receptor-adenylate cyclase system could explain this but in order to circumvene the receptor we studied the effect of the permeable cAMP analogue, dibutyryl-cAMP on this calcium uptake. A marked increase was seen in control hearts while no increase could be registered in diabetic hearts. Defects in the protein kinase phosphorylation system or in the protein kinase substrate in the sarcolemma are suggested. alpha-Adrenergic stimulation with phenylephrine, being a cAMP independent positive inotropic agent, was also tested but no increase in calcium uptake was seen in either control or diabetic hearts. This could be due to a different effect on calcium currents during action potential after alpha-stimulation compared to the beta-adrenergic effect. Reexposure to calcium after calcium deprivation leads to excessive myocardial calcium uptake (calcium paradox), but the increase was significantly smaller in diabetic hearts, suggesting a differential responsiveness to the damage induced by this procedure. Early biochemical abnormalities in the basement membrane or in the composition and calcium binding ability of the sarcolemma could possibly constitute a common final site for the defect myocardial calcium uptake after isoproterenol, db-cAMP and calcium deprivation in streptozotocin diabetic rats.

Beta-adrenergic modulation of calcium channels in frog ventricular heart cells

Adrenergic modulation of calcium channels profoundly influences cardiac function, and has served as a prime example of neurohormonal regulation of voltage-gated ion channels. Channel modulation and increased Ca influx are mediated by elevation of intracellular cyclic AMP and protein phosphorylation. The molecular mechanism of the augmented membrane Ca conductance has attracted considerable interest. An increase in the density of functional channels has often been proposed, but there has previously been no direct evidence. Single-channel recordings show that isoprenaline or 8-bromocyclic AMP increase the proportion of time individual channels spend open by prolonging openings and shortening the closed periods between openings. To look for an additional contribution of changes in the number of functional channels, we applied ensemble fluctuation analysis to whole-cell recordings of cardiac Ca channel activity. Here we present evidence that in frog ventricular heart cells beta-adrenergic stimulation increases NF, the average number of functional Ca channels per cell. We also find that isoprenaline slows the time course of both activation and inactivation, and that the enhancement of peak current decreases gradually with greater membrane depolarization.

Mechanism of adenosine 3',5'-monophosphate (cAMP)-induced increase in Ca2+ uptake by the sarcoplasmic reticulum in functionally skinned myocardial fibers

The increased rate of Ca2+ uptake and ATPase activity in isolated cardiac sarcoplasmic reticulum (SR) by adenosine 3',5'-monophosphate (cAMP) has been shown to be activated by a cAMP-dependent protein kinase (cAMP kinase). Functionally skinned myocardial fiber preparations were used to study the mechanisms of cAMP action on the SR at the same time that tension was monitored. cAMP effects were studied on Ca2+ -activated tension of the contractile proteins, and on Ca2+ uptake and release from the SR using caffeine-induced tension transients. Neither cyclic AMP (0.1-5 microM) nor the catalytic subunit of cAMP kinase (0.1-1 microM) (PK-C) significantly changed either the maximal or the submaximal Ca2+ -activated tension. The areas of the tension transients were unchanged when cAMP was present in the releasing solution (release phase), and were significantly increased up to a mean of about 80% when cAMP or PK-C was present in the Ca2+ loading solutions (uptake phase). The increased tension transient was blocked by heat-stable inhibitor of cAMP kinase. We conclude that cAMP-induced increases in Ca2+ uptake by the SR could play an important role in the positive inotropic effect. cAMP kinase could thus play a crucial role in the regulation of myocardial contractility.

Phosphorylation of low-molecular-weight proteins in preparations of rat heart sarcolemma and sarcoplasmic reticulum

Two substrate proteins for cAMP-dependent protein kinase detected in a rat heart sarcolemma preparation displayed molecular weights of 24,000 and 9000 in sodium dodecyl sulfate gels and were shown to be interconvertible. The 9000-dalton protein could readily be separated from other low molecular weight phosphoproteins (mol. wt. 14,000 and 7000) by the use of 15% polyacrylamide gels. In addition to an endogenous cAMP-dependent protein kinase the membrane preparation also contained a protein-phosphorylation system that required Ca2+ and calmodulin. It appeared that both 24,000- and 55,000-dalton proteins were substrates for the endogenous Ca2+- and calmodulin-dependent protein kinase. Contaminating sarcoplasmic reticulum vesicles, first loaded with calcium oxalate, could be separated from the enriched sarcolemma preparation by sucrose gradient centrifugation. The separation was confirmed by comparative analysis of 5'-nucleotidase, Na+ -Ca2+ antiporter, and (Ca2+ + Mg2+)-dependent ATPase activities and by determination of gel electrophoretic (phospho)protein composition, sialic acid, cholesterol, and phospholipid contents. The 24,000-dalton phosphoprotein complex was equally distributed between sarcolemmal and sarcoplasmic reticulum fractions, whereas the 55,000- and 7000-dalton proteins were predominantly found in the sarcolemmal fraction. The 24,000-dalton protein was most likely phospholamban, because no other phosphoprotein was found in the 20,000 molecular weight range.

Mechanical response of rat myocardium to dibutyryl cyclic AMP in relation to effects of alpha-and beta-adrenoceptor stimulators

Dibutyryl cyclic AMP, and alpha- and beta-adrenoceptor stimulators are all able to elicit inotropic effects. alpha- and beta-Adrenoceptor stimulation are known to change each myocardial contraction-relaxation cycle differently. In order to elucidate the myocardial function of cyclic AMP the effects of dibutyryl cyclic AMP on the contraction-relaxation cycle of isolated rat heart papillary muscle were examined and compared to the effects of alpha-and beta-adrenoceptor stimulation, respectively. Dibutyryl cyclic AMP (in the presence of propranolol) increased developed tension (Tmax) by 18%, rate of tension rise (T'max) by 46%, rate of tension fall (T'min) by 62% and onset-rate of relaxation (T"min) by 136%. These changes in the contraction-relaxation cycle were strikingly similar to those produced by isoprenaline (beta-adrenoceptor stimulation). The response to dibutyryl cyclic AMP, however, developed much more slowly than did the response to isoprenaline. The latter effect was associated with cyclic AMP elevation in a way indicating a trigger function for cyclic AMP. The alpha-adrenoceptor stimulation (by phenylephrine combined with propranolol), however, increased measures both for contraction and for relaxation by about the same degree, and the effects occurred without changes of cyclic AMP contents. Phenylephrine alone (combined alpha-and beta-adrenoceptor stimulation) elicited a substantial cyclic AMP elevation but gave mechanical effects only slightly different from the pure alpha-adrenergic response. Thus cyclic AMP effects did not seem to be fully expressed in this case. As a whole, the results indicate that the effects of both dibutyryl cyclic AMP and of isoprenaline are mediated by the cyclic AMP-system while alpha-adrenoceptor stimulation involves other mechanisms.

The role of calcium in supersensitivity to the inotropic effects of norepinephrine

Experiments using electrically stimulated rabbit left atria have demonstrated that supersensitivity to the inotropic effects of norepinephrine can be induced by either chronic reserpine pretreatment or hypothermia (lowering the temperature of the bathing medium). These two experimental conditions for inducing supersensitivity were not additive implying that they shared a common mechanism of action. Norepinephrine had no significant effect on the amplitude of a potentiated contraction of the rabbit atrium when the temperature was reduced from 37 to 30 degrees C or following pretreatment with reserpine (30 or 37 degrees C). Under these same conditions the ED50 of norepinephrine on the normal contraction was reduced. It is concluded that both reserpine pretreatment and hypothermia induce supersensitivity to the inotropic effects of norepinephrine by enhancing the cellular store of activator calcium while not affecting the ability of norepinephrine to release activator calcium.

Calcium fluxes and contractility in isolated guinea pig atrium: effect of A23187

The Ca2+ ionophore A23187 (10(-6) to 3 X 10(-5) M) increased the force of contraction is isolated guinea pig atria. In individual twitches, peak tension, maximum rate of tension development, time to peak tension, and total twitch duration were all increased by A23187. Tripelennamine, indomethacin, and atropine did not significantly alter the inotropic effect of A23187. Serotonin produced changes in individual twitches that differed qualitatively and quantitatively from those of A23187. Therefore, the inotropic action of A23187 is probably not mediated by release of endogenous histamine, prostaglandins, acetylcholine, or serotonin. 45Ca influx and efflux were increased by A23187. The enhanced 45Ca efflux exceeded that which would be predicted if the ionophore acted only to increase the passive Ca2+ permeability of the myocardial cell membrane. These results suggest that A23187 facilitates the entry of extracellular Ca2+ into the myocardial cell and the release of intracellular Ca2+ stores into the myoplasm. The resultant increase in intracellular Ca2+ activity could account for the positive inotropic action of A23187.

Effects of norepinephine, calcium, and rate of discharge on 42K movements in canine cardiac Purkinje fibers

We studied the effect of norepinephrine, calcium concentration, and rate of discharge in the presence of different [Ca2+]0 on radioactive potassium movements in cardiac Purkinje fibers. The following results were obtained: (1) norepinephrine increases potassium uptake in quiescent fibers and in fibers driven at constant rate, but more in the latter; (2) norepinephrine also increases potassium uptake in quiescent fibers depolarized at the plateau; (3) increasing [Ca2+]0 increases potassium uptake in fibers driven at constant rate; (4) increasing [Ca2+]0 may decrease K+ uptake in quiescent fibers; (5) increasing [Ca2+]0 decreases the rate of loss of tissue radioactivity in quiescent fibers and increases it in a driven fiber; (6) increasing the driving rate increases potassium uptake in low and high [Ca2+]0; (7) high [Ca2+]0 increases K+ uptake, especially at low rates; (8) norepinephrine is less effective in increasing K+ uptake in the presence of a high [Ca2+]0. We conclude that: (a) norepinephrine increases potassium uptake by different mechanisms; (b) calcium affects potassium movements when it is allowed to enter the cell, presumably by affecting potassium conductance; (c) the effect of an increased rate of discharge on K+ uptake may involve stimulation of active K+ uptake and may include a calcium-dependent component which is largest at high [Ca2+]0 and a slow rate of drive; (d) simultaneous application of two interventions results in a summation that is smallest when one of the mechanisms is already substantially activated.

Positive dromotropic effect of dibutyryl cyclic adenosine 3',5'-monophosphate on the atrioventricular node

The effect of atrioventricular (A-V) conduction of N6-2'-0-dibutyryl cyclic 3',5'-adenosine monophosphate (dibutyryl cyclic AMP) was investigated in comparison with those of norepinephrine, cyclic 3',5'-adenosine monophosphate (cyclic AMP), adenosine-5'-monophosphate (5'-AMP), and adenosine by the use of the isolated, blood-perfused A-V node preparation of the dog. Single injections of dibutyryl cyclic AMP (3-300 micronmol) and norepinephrine (0.1-1 nmol) into the posterior septal artery of the preparation (the upper part of the A-V node is mainly perfused through this artery) produced a dose-dependent decrease in the A-V conduction time. The time to the peak effect and the duration of the effect of dibutyryl cyclic AMP were much longer than with norepinephrine. The positive dromotropic effect of dibutyryl cyclic AMP was resistant to the beta-adrenoceptor blocking action of propranolol. Unlike dibutyryl cyclic AMP, cyclic AMP (above 30 nmol), 5'-AMP and adenosine (above 1 nmol) injected into the posterior septal artery prolonged the A-V conduction time in a dose-dependent manner. The results indicate that dibutyryl cyclic AMP has a mode of action on A-V nodal cells which differs distinctly from that of either norepinephrine or cyclic AMP, 5'-AMP, and adenosine.

Relaxation of heart muscle by catecholamines and by dibutyryl cyclic adenosine 3',5'-monophosphate. Similarity of beta-adrenoceptors mediating contractile and relaxant effects of catecholamines in kitten pipillary muscle

1. In isometrically contracting kitten papillary muscles, dibutyryl cyclic AMP (DBcAMP) enhanced peak tension, increased rates of contraction and relaxation, decreased tension of a phasic but not of a small tonic component of KCl-contractures, and caused aftercontractions. These effects resemble closely those of catecholamines. 2. The effects of DBcAMP on kitten papillary muscle were not influenced by (-)-bupranolol, a beta-adrenoceptor antagonist. 3. DBcAMP decreased KCl-contractures in strips of frog ventricle. 4. Phasic KCl-contractures in kitten papillary muscles were decreased by (-)- and (+)-isoprenaline. For similar effects, 100-fold higher concentrations of (+)-isoprenaline than of (-)-isoprenaline were required. 5. Increases in maximum rates of contraction and relaxation, increases in peak tension of isometric contractions and reduction of phasic KCl-contractures by catecholamines were antagonized competitively to a similar extent by (-)-bupranolol. Mean apparent equilibrium constants for the beta-adrenoceptor-(-)-bupranolol complex of 0.46-0.70 nM were estimated. These constants were quite similar irrespective of whether (-)-isoprenaline, (+)-isoprenaline or (-)-noradrenaline were used as agonists. 6. Increases in contractile strength, maximum rates of contraction and of relaxation of isometric contractions and decreases in KCl-contractures by (-)-isoprenaline were surmountably blocked by (+)-bupranolol. Mean apparent equilibrium constants for the receptor-(+)-bupranolol complex were 40-50 nM. 7. The equilibrium constants of (-)- and (+)-bupranolol for the receptors mediating positive inotropic and relaxant effects of catecholamines were not significantly different from constants for bupranolol-receptor complexes in cell-free membrane particles of kitten heart ventricle. It is suggested that the same beta-adrenoceptor triggers positive inotropic, relaxant and adenylyl cyclase-activating effects of catecholamines in kitten papillary muscle. 8. The partial agonist (-)-dichloroisoprenaline (DCI) (1 muM) reduced by 79% the phasic KCl-contractures of the kitten papillary muscles. DCI stimulates adenylyl cyclase activity of ventricle membranes to less than 1/4 of maximum stimulation by (-)-isoprenaline. If cyclic AMP produced by DCI is involved in the decrease of phasic KCl-contracture, small increase in cyclic AMP should be sufficient to induce this effect.

The regulation of the calcium conductance of cardiac muscle by adrenaline

1. The effect of adrenaline on the Ca-dependent slow inward current, Is, of mammalian cardiac muscle has been investigated by the voltage-clamp method. The mechanism of the increase in the conductance, gs, was analysed on the basis of a kinetic scheme (Hodgkin & Huxley, 1952) applicable to this system. 2. The rate constants alphad and betad, of activation of gs were not influenced by adrenaline, although the limiting conductance, gs, was greatly increased. 3. Reduction of [Ca]o from 1-8 to 0-2 mM decreased the amplitude of inward tail currents when gs was fully activated; however, the relative decrease of the current amplitude was the same with and without adrenaline. The reversal potential, ER, of Is was not changed by the drug. This indicates that the catecholamine has no influence on the selectivity of these conductance channels. 4. An increase in the number of functional conductance channels by adrenaline is discussed as a possible mechanism for the increase in Gs.

Restoration by histamine of the calcium-dependent electrical and mechanical response in the guinea-pig papillary muscle partially depolarized by potassium

Electrophysiological and mechanical effects of histamine were observed in guinea-pig papillary muscle which had been depolarized and rendered inexcitable by elevation of potassium concentration in Tyrode solution to 27 mM. 1. Histamine (3 X 10(-7) to 3 X 10(-5) M) restored the action potential and tension development. The amplitude of the action potential was increased by 31.6 mV/10-fold increase in extracellular Ca2+ concentration. Nifedipine (10(-6) M) abolished the electrical and mechanical responses which had been restored by histamine (10(-5) M) but TTX (10(-5) M) did not affect them. Reduction of the extracellular Na+ concentration to one half decreased the amplitude and the maximum rate of rise of the action potential restored by histamine (10(-5) M) while the peak tension was increased and an after-contraction occurred. 2. The maximum rate of rise and the amplitude of the action potential restored by histamine (10(-5) M) decreased with increase in stimulus frequency from 0.1-1.6 Hz. The peak tension decreased and then increased. The shape of the developed tension was also changed. In the presence of caffeine (1 mM), the only effect of an increase in stimulus frequency was a decrease in peak tension but the change in the shape of developed tension did not occur. 3. The electrical and mechanical responses restored by histamine (3 X 10(-6) or 10(-5) M) were depressed by metiamide (3 X 10(-6) M) but not by diphenhydramine (10(-5) M) or bufetolol (10(-6) M). 4. The electrical response restored by histamine (10(-6) or 10(-5) M) was enhanced by papaverine (10(-5) M) and depressed by N-methylimidazole (10 mM). It is concluded that histamine may enhance the slow inward Ca2+ current mediated by histamine H2-receptors and the adenylate cyclase system in ventricular muscle and that the positive inotropic action of histamine may be attributed to these mechanisms.

Possible role of cyclic AMP in the relaxation process of mammalian heart: effects of dibutyryl cyclic AMP and theophylline on potassium contractures in cat papillary muscles

The effect of dibutyryl cyclic AMP (DB-c-AMP; 3 X 10(-4)-3 X 10(-3) M) on electrically induced twitch and high potassium (142.4 mM KCl)-induced contracture tension was studied in papillary muscles from normal and reserpinized cats ([Ca]0 1.8 mM; 25 degrees C; pH 7.4). In both groups of preparations, the increase in twitch tension evoked by DB-c-AMP was accompanied by an abbreviation of the time to peak force and of relaxation time. In the same preparations, the high potassium contracture was markedly depressed by DB-c-AMP in a concentration-dependent manner. Similar results were obtained with the N6-monobutyryl derivative of cyclic AMP. The relaxing effects of the cyclic nucleotides on KCl contractures did not appear to be due to possible non-cyclic breakdown products: adenosine, 5'-AMP and sodium butyrate did not attenuate contracture tension at concentrations up to 3 X 10(-3) M. The same applies to ATP and non-cyclic N6-2'-0-3'-0-tributyryl-adenosine-monophosphate. Theophylline (10(-2) M) was found to prolong the relaxation time of the twitch and to enhance the high KCl contracture. It is concluded that cyclic AMP may be capable of modulating the relaxation process of mammalian heart and that not only the positive inotropic but also the relaxant effects of catecholamines on myocardium described before may be mediated by the cyclic AMP system. The relaxant effects of cyclic AMP derivatives on intact myocardial preparations are attributed to a stimulation by cyclic AMP of the calcium transport of the sarcoplasmic reticulum (SR) and are interpreted to be a corollary to the effects of cyclic AMP previously obtained on isolated SR preparations.

On the mechanism of histamine action in cardiac muscle

The effect of histamine on electrical and mechanical properties of guinea-pig ventricular muscle fibers was investigated. It was found that the drug decreased in rate of tension development, the rate of fall of the twitch and decreased the time to peak tension. The height of the plateau of the action potential was increased by histamine and this seems to explain, at least, part of the drug action on heart contractility. The uptake and efflux of 45Ca were both increased by the drug. The positive inotropic action of histamine is largely dependent on the external calcium concentration and temperature and is abolished in low Na solution. It was found that histamine has a marked relaxing action which is probably related to increased Ca sequestration by the sarcotubular system. The drug also produced a decrease in post-extrasystolic potentiation.

Membrane currents and tension in cat ventricular muscle treated with cardiac glycosides

The effect of cardiac glycosides on membrane currents and tension in cat ventricular muscle was studied using the single sucrose gap voltage clamp method. Complete tension-voltage and current-voltage relations were obtained in five preparations before and during treatment with dihydro-ouabain (DHO, 1.7 X 10(-5)M). After 1-2 minutes of DHO, the developed tension was 15% greater than control, but there was no change in either the slow inward (calcium) current (Ica) or the level of the outward current flowing at the end of a 300-msec depolarization (Iout). After 6-8 minutes of DHO, there was a 60% increase in developed tension, a noticeable increase in resting tension, a 20% decrease in Ica, and a smaller increase in Iout. It seems possible that the reduction of Ica was due to a reduced driving force. In preparations treated with ouabain (5 X 10(-7)M, 3-5 minutes), developed tension was 45-150% greater than control with no change in Ica or Iout between -45 and + 15 mv. We conclude that the inotropic action of these cardiac glycosides is not mediated by an increase in Ica.

Localization of beta adrenergic receptors, and effects of noradrenaline and cyclic nucleotides on action potentials, ionic currents and tension in mammalian cardiac muscle

1. Isoprenaline and noradrenaline were applied iontophoretically to cardiac Purkinje fibres. Intracellular application of the drugs had no effect, while extracellular application of the same amounts of charge caused acceleration of pace-maker activity and a shift of the plateau level of the action potential. These results indicate that beta-adrenergic receptors are located at the outside of the cardiac cell membrane.2. A systematic comparison of the effects of cyclic AMP derivatives and noradrenaline on action potentials and isometric tension of ventricular myocardial preparations showed that the nucleotides and the catecholamine increase the plateau height and the duration of the action potential and also increase tension. However, there are quantitative differences in the action of these drugs.3. Cyclic AMP derivatives and noradrenaline increase the slow inward current, I(Ca), in ventricular myocardial preparations. Voltage clamp analysis of I(Ca) showed that the kinetic parameters of this membrane current are not affected by these drugs. However, the membrane conductance to Ca ions is greatly increased by noradrenaline and to a smaller extent by dibutyryl cyclic AMP.4. Concentration-response relations of the membrane effects of noradrenaline on plateau height of the action potential and on I(Ca) could be fitted by the same theoretical log concentration-response curve. The Hill plot of this concentration-response curve had a slope of 2. The half maximal response occurred at 5 x 10(-7)M.5. The results are compared with other membrane effects of catecholamines and cyclic nucleotides in cardiac muscle. The effects on I(Ca) are related to the positive inotropic effect of the drugs.

Relationship between theophylline uptake and inotropic effect in the guinea-pig heart

1 The time course of the positive inotropic effect of theophylline was compared with the time course of the uptake and release of [(3)H]-theophylline in guinea-pig isolated, electrically driven hearts perfused by the Langendorff method.2 Formation of theophylline metabolites could not be detected under the experimental conditions used.3 Theophylline entered myocardial tissue very rapidly in two different phases. The first process (half-time 21 s) amounted to 93% and the second (half-time 5 min 50 s) to 7% of the total uptake. The development of the positive inotropic effect of theophylline was about four times faster than even the rapid component of the uptake of the drug into the myocardium.4 The amount of theophylline accumulated in myocardial tissue (after 10 min perfusion) incerased proportionally with theophylline concentrations in the perfusion media and no signs of saturation were detected. The tissue-medium ratio did not exceed 1. The water content of the myocardial tissue amounted to about 80% at all theophylline concentrations examined.5 The uptake of theophylline (3 mg/ml) was diminished by 9.2% after pretreatment of the hearts with caffeine (1 mg/ml). Theophylline uptake was also decreased by 13.5% when caffeine-pretreated hearts were perfused with a solution containing theophylline (300 mug/ml) plus caffeine (1 mg/ml).6 Theophylline release from the hearts was also very rapid. The efflux curve was composed of three components (half-times: 24 s; 1 min 24 s; 6 min 18 seconds). The intercepts of the linear portions of the efflux curve occurred at 61%, 38% and 1%, respectively. Contractile force and theophylline content in myocardial tissue declined in a similar manner.7 It is concluded that theophylline enters myocardial tissue very rapidly by passive diffusion. Theophylline distributes itself in the heart as freely as in the perfusion medium. A very small amount may be bound within the cell in a relatively specific way.8 It seems possible that the positive inotropic effect of theophylline is partly due to an action of the drug on intracellular calcium binding or storage sites. However, the principal action of theophylline is assumed to be on the sarcolemma where it increases calcium influx from the extracellular space. This conclusion is based on the fact that the time courses of the increase in contractile force and of theophylline uptake into the cell were dissimilar.