Increase In Developed Tension Research Articles

Length dependence of staircase potentiation: interactions with caffeine and dantrolene sodium

In skeletal muscle, there is a length dependence of staircase potentiation for which the mechanism is unclear. In this study we tested the hypothesis that abolition of this length dependence by caffeine is effected by a mechanism independent of enhanced Ca2+ release. To test this hypothesis we have used caffeine, which abolishes length dependence of potentiation, and dantrolene sodium, which inhibits Ca2+ release. In situ isometric twitch contractions of rat gastrocnemius muscle before and after 20 s of repetitive stimulation at 5 Hz were analyzed at optimal length (Lo), Lo - 10%, and Lo + 10%. Potentiation was observed to be length dependent, with an increase in developed tension (DT) of 78 +/- 12, 51 +/- 5, and 34 +/- 9% (mean +/- SEM), at Lo - 10%, Lo, and Lo + 10%, respectively. Caffeine diminished the length dependence of activation and suppressed the length dependence of staircase potentiation, giving increases in DT of 65+/-13, 53 +/- 11, and 45 +/- 12% for Lo - 10%, Lo, and Lo + 10%, respectively. Dantrolene administered after caffeine did not reverse this effect. Dantrolene alone depressed the potentiation response, but did not affect the length dependence of staircase potentiation, with increases in DT of 58 +/- 17, 26 +/- 8, and 18 +/- 7%, respectively. This study confirms that there is a length dependence of staircase potentiation in mammalian skeletal muscle which is suppressed by caffeine. Since dantrolene did not alter this suppression of the length dependence of potentiation by caffeine, it is apparently not directly modulated by Ca2+ availability in the myoplasm.

Electropharmacological effects of UK-1745, a novel cardiotonic drug, in guinea-pig ventricular myocytes

Effects of (2 RS, 3 SR)-2-aminomethyl-2,3,7,8-tetrahydro-2,3,5,8,8-pentamethyl-6 H-furo-[2,3-e] indol-7-one hydrochloride (UK-1745), a novel cardiotonic drug with β-adrenoceptor blocking property and antiarrhythmic activity, on the action potentials of isolated papillary muscles and the membrane currents of single ventricular myocytes of guinea pigs were examined and compared with those of milrinone using conventional microelectrode and patch–clamp techniques. In papillary muscles, UK-1745 (3–100 μM) produced a mild positive inotropic response and depressed the maximum upstroke velocity of the action potential ( V̇ max) at 100 μM. Milrinone, a phosphodiesterase III inhibitor, markedly shortened the action potential duration with a significant increase in developed tension. In enzymatically-dissociated ventricular myocytes, UK-1745 failed to increase the L-type Ca 2+ current ( I Ca) at 10 and 30 μM and rather decreased I Ca at 100 μM. The high concentration of UK-1745 slightly inhibited the delayed rectifier K + current ( I K). Although UK-1745 antagonized the isoproterenol-induced increase in I Ca, it potentiated the histamine-induced increase in I Ca. On the other hand, milrinone per se significantly increased I Ca and markedly enhanced the isoproterenol-induced increase in I Ca. It can be concluded that UK-1745 is a unique cardiotonic drug possessing β-adrenoceptor blocking and weak phosphodiesterase-inhibitory actions in addition to direct inhibitory actions on the Na +, Ca 2+ and K + channels with its high concentrations.

Cardiac impairment and nitric oxide synthase activity in the chronic portal vein-stenosed rat.

Decreased cardiac contractility and beta-adrenergic responses have been observed in the chronic portal vein-stenosed (PVS) rat. Because nitric oxide (NO) may be increased in PVS and has been recognized as a negative inotropic agent, we investigated the induction of NO synthase (NOS2) and/or changes in constitutive NOS (NOS3) as factors in the cardiac dysfunction of the PVS rat. Ten to twelve days after portal vein stenosis or sham operation, cardiac function was evaluated in paced left ventricular papillary muscles (LVPM) and right ventricular strips (RV). To determine if NO modulation of contractile function was altered in PVS, we examined the increase in developed tension produced by the effect of Nomega-nitro-L-arginine (L-NNA) on the myocardial force-frequency relationship. Cardiac tissue NOS2 and NOS3 activities were assayed, Western blot analyses of NOS2 and NOS3 expression were performed, and circulating nitrate-nitrite (NOX) levels (an indicator of in vivo NOS activity) were assayed. Basal LVPM and RV contractile indexes were significantly reduced in PVS (30-50%), without a change in the relaxation rate. No between-group differences in the cardiac NOS2 or NOS3 enzymatic activities of PVS and sham-operated (SO) rats were observed. Western blots revealed no cardiac NOS2 expression in either SO or PVS rats. In contrast, NOS3 was expressed in both SO and PVS rats, but there was no quantitative difference in expression between the two groups. Changes in the cardiac force-frequency relationship (staircase effect) after L-NNA were consistent with NOS3 modulation of contractile function in both SO and PVS rats, but there was no between-group difference in the modulation. Circulating NOX concentrations did not differ between SO and PVS rats. In conclusion, protein expression data, enzymatic assays, end-product assays, and functional data indicate that between-group differences in NOS2 and NOS3 activity are not responsible for the cardiac impairment that has been observed in the chronic PVS rat.

T lymphocyte-dependent and -independent intestinal smooth muscle dysfunction in the T. spiralis-infected mouse.

We examined the profile of increased intestinal muscle contractility after primary infection with Trichinella spiralis in the mouse, correlating it with parasite expulsion. We also examined the extent to which the changes in muscle contraction were T lymphocyte dependent, by infecting athymic and SCID mice. Infection was accompanied by increased tension development by intestinal muscle. Two components of this response were identified, a rapid peak increase in tension generation observed on day 6 postinfection, and a smaller but sustained increase in tension evident thereafter in euthymic BALB/c mice. The peak muscle response was significantly delayed in infected athymic and SCID mice, along with a corresponding reduction in the magnitude of the sustained component. These changes were accompanied by reduced parasite expulsion in athymic and SCID mice, compared with euthymic mice. Reconstitution of T cell function in athymic mice restored both the acute and sustained profile of muscle contraction seen in euthymic mice, and this was accompanied by faster expulsion of the worms. These results identify T cell-dependent and -independent components of the muscle response to nematode infection in the mouse and suggest that the onset of the peak contractile response, as well as the magnitude of the sustained muscle response, contributes to parasite eviction from the gut.

Persistent enteric muscle dysfunction and expulsion of T. spiralis infections depend partially upon IL-5 and eosinophils

Interleukin (IL)-5 expression, eosinophilia and eosinophil activation have been shown to be increased early in the course of several gastrointestinal conditions including Crohn's disease, celiac disease and during enteric parasitic infections. However a functional role for eosinophils in either enteric host defense or in intestinal pathophysiology is unclear. IL-5 deficient (-/-) mice as well as their IL-5 +/+ controls were orally gavaged with 375 T. spiralis larvae for primary infections, and with a similar number for subsequent challenges. We measured isometric contraction of jejunal longitudinal muscle and counted adult worms as well as eosinophils within the infected small bowel, during both primary and challenge infections. In addition, as a measure of worm fecundity, we analyzed skeletal muscle larvae numbers. Following T. spiralis infection, intestinal eosinophil counts rose dramatically in IL-5 +/+ mice (0.8 to a maximum of 14.0 eosinophils/villus crypt unit (vcu)) during the primary infection and (1.0 to 9.5 eosinophils/vcu) in a subsequent infection. In contrast, no significant rise in eosinophil numbers was seen in IL-5 deficient mice. Increased tension development was observed in intestinal muscle from both IL-5 +/+ and IL-5 -/mice, over the course of infection and consisted of a rapid increase in tension by day 8 PI, followed by a smaller increase that was sustained over 21 days. While the magnitude of the rapid onset component was similar, that of the sustained component was significantly lower in IL-5 -/mice (1700 vs 1250 mg/mm2). Worm expulsion was also significantly reduced in the IL-5 deficient mice during the late stage of a primary infection (day 16 21 post-infection (PI)), with twice the number of worms recovered from IL-5 -/mice, compared to IL-5 +/+ mice. However there was no difference in the number of skeletal muscle larvae recovered. Subsequent T. spiralis infections showed an even greater dependence of intestinal worm expulsion upon the presence of IL-5 and eosinophils, with increased worm burdens evident in IL-5 4mice by as early as day 6 PI. These results indicate that IL-5 is essential for the generation of the intestinal eosinophilia that occurs during T. spiralis infections. In the absence of IL-5 and eosinophils, both parasite expulsion and the persistent increase in tension generation by muscle are reduced. Since differences between IL-5 +/+ and IL-5 -/mice emerged only after day 16 PI, these results indicate that eosinophils are not involved in the very early stages of expulsion, or in the initiation of altered muscle function. This study thus provides evidence that eosinophils play a role in enteric host defense and may be an important cell type in maintaining persistent motility disturbances following enteric infection and inflammation. Supported by MRC Canada.

Evidence for cAMP-independent mechanisms mediating the effects of adrenomedullin, a new inotropic peptide.

Adrenomedullin (ADM), a new vasorelaxing and natriuretic peptide, may function as an endogenous regulator of cardiac function, because ADM and its binding sites have been found in the heart. We characterize herein the cardiac effects of ADM as well as the underlying signaling pathways in vitro. In isolated perfused, paced rat heart preparation, infusion of ADM at concentrations of 0.1 to 1 nmol/L for 30 minutes induced a dose-dependent, gradual increase in developed tension, whereas proadrenomedullin N-20 (PAMP; 10 to 100 nmol/L), a peptide derived from the same gene as ADM, had no effect. The ADM-induced positive inotropic effect was not altered by a calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP8-37, or H-89, a cAMP-dependent protein kinase inhibitor. ADM also failed to stimulate ventricular cAMP content of the perfused hearts. Ryanodine (3 nmol/L), a sarcoplasmic reticulum Ca2+ release channel opener, suppressed the overall ADM-induced positive inotropic effect. Pretreatment with thapsigargin (30 nmol/L), which inhibits sarcoplasmic reticulum Ca2+ ATPase and depletes intracellular Ca2+ stores, attenuated the early increase in developed tension produced by ADM. In addition, inhibition of protein kinase C by staurosporine (10 nmol/L) and blockade of L-type Ca2+ channels by diltiazem (1 micromol/L) significantly decreased the sustained phase of ADM-induced increase in developed tension. Superfusion of atrial myocytes with ADM (1 nmol/L) in isolated left atrial preparations resulted in a marked prolongation of action potential duration between 10 and -50 mV transmembrane voltage, consistent with an increase in L-type Ca2+ channel current during the plateau. Our results show that ADM enhances cardiac contractility via cAMP-independent mechanisms including Ca2+ release from intracellular ryanodine- and thapsigargin-sensitive Ca2+ stores, activation of protein kinase C, and Ca2+ influx through L-type Ca2+ channels.

Positive action of propionyl- l-carnitine on mechanical performance of papillary muscle from Syrian hamsters with hereditary dilated cardiomyopathy

Propionyl- l-carnitine has been shown to exert a beneficial effect on cardiac function in different experimental models of cardiomyopathy in the rat, most likely by improving cardiac metabolism and energy production. We have previously shown that, in a strain of hamsters with hereditary dilated cardiomyopathy (BIO TO.2), the mechanical activity of papillary muscle (length-tension, velocity of shortening, shortening, work and power relationship) is significantly depressed when compared to the same parameter in normal hamsters (BIO F1.B). The repeated oral treatment with propionyl- l-carnitine (60 mg/kg per os for 7 weeks) to BIO TO.2 hamsters had a significant positive inotropic effect, as indicated by an increase in developed tension up to the levels observed in papillary muscles from normal hamsters. This action is most likely associated with metabolic effects similar to those observed in rats.

Inotropic effects of alpha 1-adrenergic agonists in myocardium from rats with postinfarction heart failure.

In clinical and experimental heart failure, the inotropic response to beta-adrenergic receptor stimulation is depressed. Therefore, non-beta-adrenergic mechanisms may assume increasing importance for summoning inotropic reserve in the failing heart. To test the integrity of the inotropic pathway mediated by alpha 1-adrenergic receptor stimulation in a model of chronic ischemic heart failure, we administered phenylephrine to noninfarcted left ventricular papillary muscles isolated from sham-operated rats (n = 10) and rats with large (> 40% left ventricular circumference) anterior myocardial infarctions (n = 9). Isometric force was monitored, and intracellular Ca2+ (Cai2+) transients were recorded with the bioluminescent protein aequorin. Compared with muscles from sham-operated rats, contractility of muscles from rats with postinfarction heart failure was depressed at extracellular Ca2+ concentrations between 0.5 and 3.0 mM. Phenylephrine produced comparable dose-dependent increases in developed tension (126 +/- 4 vs. 125 +/- 7% of baseline) and peak rate of tension rise (125 +/- 4 vs. 140 +/- 9% of baseline) in muscles from sham and infarcted rats, respectively. There was no significant change in the time course of the isometric twitch or in the time course or amplitude of the Cai2+ transient after phenylephrine administration in muscles from either group. No evidence of Cai2+ overload, as defined by spontaneous Ca2+ release, was observed during phenylephrine administration in muscles from normal or failing hearts. The density of alpha 1-adrenoceptors measured with [3H]prazosin binding in crude membranes isolated from noninfarcted left ventricular tissue was not different in control and infarcted hearts (48 +/- 5 vs. 53 +/- 4 fmol/mg protein). These data indicate that the positive inotropic effect of alpha-agonists may be preserved in chronic ischemic heart failure. In both normal and failing myocardium, the inotropic effects of alpha 1-adrenergic stimulation occurred with little or no increase in Cai2+ availability and no apparent adverse effects on myocardial relaxation. Therefore, agents that act by similar mechanisms may have certain therapeutic advantages over traditional inotropic agents in patients with heart failure.

Endothelin-1 does not alter Ca2+ responsiveness in saponin-skinned ferret papillary muscles.

Endothelin is a potent vasoconstrictor and a positive inotropic agent in myocardium. Endothelin has been reported to increase myocardial contractility with little or no increase in intracellular Ca2+, thus apparently enhancing myofilament responsiveness to Ca2+. We investigated the effects of endothelin on tension development and Ca2+ responsiveness in both intact and saponin-skinned ferret right ventricular papillary muscles. Isolated ferret papillary muscles were stimulated for 2 h in the presence or absence of endothelin (100 nM). The muscles were then chemically skinned with saponin and exposed to relaxing and contracting solutions containing varying amounts of Ca2+, and the developed force of contraction was measured. The [Ca2+] required for half-maximal activation (pCa50) was determined by fitting force versus Ca2+ data to the Hill equation. In isometrically contracting muscles, endothelin (100 nM) caused a mean percent increase in developed tension of 34.7% +/- 11.3% (mean +/- S.E.). In muscles that were exposed to endothelin for 2 h and then skinned, neither the pCa50 nor the maximal Ca(2+)-activated force (Fmax) were significantly different from control skinned papillary muscles. After skinning, when endothelin (100 nM) was added to the Ca2+ buffers, both pCa50 and Fmax were significantly decreased. When papillary muscles were pretreated with phorbol 12-myristate 13-acetate (PMA) and then skinned, there was a significant increase in the pCa50. These results indicate that endothelin acts directly on the myofilaments to impair force development by directly decreasing the Ca2+ responsiveness of myofilaments.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular Ca 2+ loading and inotropic effects of doxorubicin in atrial muscle preparations isolated from rat or guinea-pig hearts

The inotropic effects of doxorubicin were examined under conditions that alter the Ca 2+ loading of myocardial cells. Atrial muscle preparations isolated from rat or guinea-pig hearts were used. Cellular Ca 2+ loading was altered by changing the temperature, extracellular Ca 2+ concentrations or the frequency of electrical stimulation. In guinea-pig heart muscle preparations stimulated at 2 Hz at 30°C in the presence of 1.2 mM CaCl 2, 30 μM doxorubicin caused biphasic (early and late phase) positive inotropic effects. A higher concentration (200 μM) of doxorubicin caused a transient positive inotropic effect followed by a gradual decrease in developed tension. An increase in CaCl 2 concentration from 1.2 to 2.4 mM decreased the positive inotropic effect of 30 μM doxorubicin and changed the inotropic effect of 200 μM doxorubicin from positive to negative after a transient increase in developed tension. At 0.5-Hz stimulation or 36°C incubation, 30 or 200 μM doxorubicin produced remarkable late phase positive inotropic effects. In rat heart muscle preparations, the patterns of the inotropic effects of doxorubicin were similar to those observed with guinea-pig hearts; however, the negative inotropic effect observed at 200 μM doxorubicin was greater. These results indicate that the early and the late phase positive inotropic effects of doxorubicin have different mechanisms. The pattern of inotropic effects of doxorubicin is affected by conditions that alter cellular Ca 2+ loading or the concentration of doxorubicin.

Comparison of positive inotropic effects of milrinone, dobutamine and ouabain.

This study compared the positive inotropic actions of milrinone in isolated rabbit myocardium with that of the conventional positive inotropic drug, dobutamine, and a cardiac glycoside, ouabain. Maximal increase in developed tension (g/mm2) was significantly (p < 0.05) greater with ouabain (from 2.0 to 4.4; 132 +/- 11%) than with dobutamine (from 3.9 to 6.0; 77.8 +/- 22.4%) or milrinone (from 2.4 to 3.6; 54.0 +/- 12.0%). Maximal augmentation of the rate of tension development (g/s/mm2), however, was similar with ouabain (from 14.3 to 39.7; 187 +/- 20%) and dobutamine (from 25.9 to 64.4; 174 +/- 35%), and both were significantly (p < 0.05) greater than with milrinone (from 18.2 to 30; 73.1 +/- 14.7%). In combination with dobutamine, however, the dose-response curve of milrinone was shifted to the left, and its ED10 was significantly (p < 0.001) reduced by 100-fold to 1.6 x 10(-7) M. Thus, milrinone is significantly less potent than dobutamine or ouabain in vitro and is without contractile effect at clinically relevant concentrations. However, data from the combined application of a catecholamine and milrinone in isolated myocardium suggest that milrinone may induce a direct positive inotropic effect at clinical concentrations in the presence of augmented sympathetic neurohumoral stimulation.

Endothelin-1 enhances cross-bridge function of ferret myocardium: role of second messengers

To investigate postreceptor pathways of endothelin and the site of action responsible for enhancing myocardial contractility, studies were performed on ferret papillary muscles loaded with the Ca2+ indicator aequorin. Endothelin-1 (ET) and the alpha 1-adrenoceptor agonist, phenylephrine (PE) produced similar dose-dependent increases in tension development and peak intracellular Ca2+ concentration ([Ca2+]i); moreover, pretreatment with PE eliminated effects of ET, suggesting similar postreceptor pathways. Because alpha 1-adrenoceptor activation is thought to cause the hydrolysis of phosphatidylinositol and generate D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and 1,2-diacylglycerol (DAG), the protein kinase C (PKC) activator 4 beta-phorbol 12-myristate 13-acetate (PMA) was used to determine whether activation of PKC was responsible for the myocardial actions of ET. In contrast to ET, PMA decreased tension development and peak [Ca2+]i, and pretreatment with PMA attenuated the myocardial action of ET; however, intracellular Ins(1,4,5)P3 levels were greatly increased by ET stimulation, suggesting that rather than DAG, Ins(1,4,5)P3 might be the second messenger for the actions of ET. To determine whether ET produced actions on the contractile elements, thereby enhancing myocardial contractility, 2,3-butanedione monoxime (BDM) was used to interfere with the interaction of myosin and actin. Pretreatment with 6 mM BDM did not alter the half-maximum effective concentration (EC50) of the [Ca2+]o-tension relation, but, in contrast, shifted the ET dose-response curve to the right, and increased the EC50 by approximately 1.0 log unit. In addition, ET partially reversed the downward shift of the peak [Ca2+]i-peak tension curve induced by BDM.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular pharmacology of the vasodilator-cardiotonic agent, 349U85.

349U85 is a chemically novel, nonglycoside, noncatecholamine cardiotonic-vasodilator agent with a unique cardiovascular profile in vitro and in vivo. 349U85 and milrinone, 10(-6) to 3 x 10(-5) M each, produce concentration-dependent increases in tension development of 33-60% and 37-60%, respectively, with corresponding 5-18% and 17-55% increases in contractile rate, respectively, in guinea pig spontaneously beating isolated paired atria. In anesthetized dogs, 349U85 at 0.03-1.0 mg/kg i.v. produces dose-dependent increases in left ventricular contractility (dP/dt) of 12-159%, decreases in total peripheral resistance of 11-38%, and increases in heart rate of 3-26%. Milrinone, Cl-914, and enoximone produce comparable increases in dP/dt and decreases in peripheral resistance yet increase the heart rate a maximum of 71, 49, and 41%, respectively. Intra-arterial injection of 349U85 into the vascularly isolated hindlimb of anesthetized dogs produces dose-dependent direct vasodilation. The inotropic effect of 349U85, following a single intravenous dose, is sustained in excess of 4 h while comparable initial inotropic effects of milrinone and enoximone are sustained less than 1 and 2.5 h, respectively. 349U85 effectively reverses acute cardiac depression in anesthetized dogs with a duration exceeding that of milrinone. In conscious dogs, 349U85, at 0.1-1.0 mg/kg p.o., produces a dose-dependent positive inotropic effect (15-73%) with no significant effect on heart rate. Following a single oral dose of 349U85, the inotropic effect is sustained in excess of 6 h. Results of these studies indicate that 349U85 is a potent, long-lasting positive inotropic and vasodilator agent with minimal heart rate effect in vitro and in vivo and is different from a number of reference inodilators.

An analysis of the mechanism of the inotropic action of some milrinone analogues in guinea‐pig isolated atria

1. It has been reported previously that the milrinone analogues, ethyl 5-cyano-1,6-dihydro-2-methyl-6-oxo-3 pyridine carboxylate (I) and ethyl 5-cyano-1,6-dihydro-2-ethyl-6-oxo-3 pyridine carboxylate (II) exert a positive inotropic effect (EC50 = 15.6 +/- 0.2 microM and 40.3 +/- 0.1 microM) both on spontaneously beating and on electrically driven atria from reserpine-treated guinea-pigs. In the present study the mechanism of the inotropic action of these two agents was investigated. 2. In electrically driven left atrium from reserpine-treated guinea-pigs the EC50 values for inotropic activity for compounds (I) and (II) corresponded to that of milrinone (EC50 = 25 +/- 0.1 microM) but compound (I) induced a greater maximum effect. This corresponded to a percentage increase in developed tension over control of 63 +/- 0.3 whereas the maximum inotropic effect of milrinone was 48 +/- 0.3 and that of compound (II) was 47 +/- 0.2. 3. The inotropic activity of compounds (I) and (II) (10-100 microM) was resistant to propranolol (0.1 microM), thus excluding the involvement of beta-adrenoceptors. 4. Since the inotropism induced by compounds (I) and (II) was not reduced by carbachol (1 nM-0.5 microM), an action involving changes in adenosine 3':5'-cyclic monophosphate (cyclic AMP) can be excluded. 5. The inotropic action of compounds (I) and (II) was blocked selectively by 8-phenyltheophyline (10 microM) or adenosine deaminase (2 u ml-1). 6. Both (I) and (II) inhibited, in an apparently competitive manner, the negative inotropic effect induced by N6-(L-phenylisopropyl) adenosine (L-PIA), a stable adenosine agonist. The pA2 values for (I) and (II) were 4.79 and 4.36, respectively.7. In rat brain compounds (I) and (II) inhibited the specific binding of N6-cyclohexyl[3H]-adenosine- ([3H]-CHA) with an IC50 of 0.18 + 0.01 mM and 0.25 + 0.02 mm, respectively, which were similar to their IC50 values for blocking the PIA-induced negative inotropic effect and which are also in the range of concentrations that are effective in inducing positive inotropism in guinea-pig atria.8. The results from the present study suggest that antagonism of endogenous purines causes positive inotropism without affecting intracellular cyclic AMP levels.

EMD 53998 sensitizes the contractile proteins to calcium in intact ferret ventricular muscle.

EMD 53998 (a thiadiazinone) is an inotropic drug that produces a pronounced increase in the Ca2+ sensitivity of the contractile proteins in skinned cardiac fibers. The present study was undertaken to determine whether this effect on Ca2+ sensitivity could explain the increase in tension observed in intact ventricular muscle. The experiments were performed on isolated ferret papillary muscles that had been microinjected with aequorin to measure the intracellular Ca2+ concentration. Force and intracellular Ca2+ concentration were monitored before, during, and after application of EMD 53998. EMD 53998 (5 microM) increased developed tension by 230%; aequorin light transients increased by only 85%, and this increase was reduced and became insignificant in the presence of agents that prevent catecholamine release. When a similar increase in developed tension was produced by elevation of extracellular calcium, the aequorin light transients increased by 240%. Thus, EMD 53998 produces a substantial Ca2+ sensitization in intact ventricular muscle, and this can explain most of its inotropic effect. In addition, EMD 53998 caused a small prolongation of the time course of contraction and a small reduction of the time course of the aequorin light transient. A computer model is described that shows that both these effects can be explained by the effect of EMD 53998 on Ca2+ sensitivity. At much higher concentrations, EMD 53998 also caused an increase in resting tension. EMD 53998 is the first agent for which much of the inotropic effect in intact cardiac muscle can be accounted for by increased Ca2+ sensitivity of the contractile proteins. Inotropic agents with this mechanism of action cause increased force production with much less increase in the intracellular Ca2+ transients than conventional agents and, therefore, increase the energy efficiency of the myocardium and are less likely to cause Ca(2+)-activated arrhythmias.

Inhibitory role of endothelium‐derived relaxing factor in rat and human pulmonary arteries

1. The inhibitory role of endothelium-derived relaxing factor was studied in both rat and human pulmonary arteries in vitro by inhibiting its synthesis with the L-arginine analogue NG-monomethyl-L-arginine (L-NMMA). 2. In rat pulmonary arteries, L-NMMA pretreatment (10-300 microM) dose-dependently inhibited acetylcholine-induced relaxation (which is endothelium-dependent). NG-monomethyl-D-arginine (D-NMMA, 100 microM) was without effect. L-Arginine, but not D-arginine, dose-dependently reversed this inhibition. L-NMMA had no effect on relaxation induced by sodium nitroprusside. 3. In human small pulmonary arteries L-NMMA (100 microM) pretreatment similarly inhibited the acetylcholine-induced relaxation but had no effect on the sodium nitroprusside-induced relaxation. 4. In both rat and human pulmonary arteries, L-NMMA, but not D-NMMA, always caused contraction of preconstricted tissues whereas it had no effect on baseline tone. In the rat this contraction was completely prevented by prior treatment with L-arginine. 5. L-NMMA (100 microM) pretreatment mimicked the effect of endothelium removal on phenylephrine-induced vasoconstriction, both resulting in an increase in tension development at each concentration of phenylephrine. This enhancement was greatest at low concentrations of phenylephrine but was still present even at the highest concentrations. Pretreatment with L-NMMA (100 microM) also significantly increased the responses to single doses of phenylephrine. 6. These results suggest that endothelium-derived relaxing factor from endothelial cells both mediates the relaxation response to acetylcholine and also acts as a physiological brake against vasoconstriction in pulmonary vessels.

Metabolic consequences of positive inotropy in rat and guinea-pig hearts

The metabolic and physiological responses of hearts from male rats and guinea-pigs to isoprenaline and SK&F 94120 (a phosphodiesterase III inhibitor), have been studied. Doses which gave similar chronotropic stimulation gave different inotropic responses. In both species, isoprenaline generated a greater increase in developed tension than SK&F 94120. With both drugs the inotropic response in the rat was less than that in the guinea-pig. 31P-NMR investigation of high-energy phosphate levels showed reduction in PCr concentration and an accompanying acidosis in the isoprenalineperfused rat heart only. In both species, lactate production was stimulated by SK&F 94120 but not by isoprenaline. These results are discussed with reference to G-protein activation of ion channels and differences in Ca 2+ handling by the two species.

Characterization of MS-857, a New and Potent Cardiotonic Agent, in Isolated Mammalian Cardiac Muscles

The positive inotropic activity of MS-857, a novel cardiotonic agent, was assessed in isolated atrial and ventricular muscle of several mammalian species. MS-857 (0.1-300 microM) increased the developed tension of the dog, rabbit, monkey, cat, and guinea pig ventricular muscles in a concentration-dependent manner. However, MS-857 failed to cause a positive inotropic effect in isolated rat ventricular muscle. In isolated guinea pig atria, MS-857 caused both positive inotropic and chronotropic effects. MS-857 was more selective for inotropy than for chronotropy as compared with either milrinone or 3-isobutyl-1-methylxanthine (IBMX). The positive inotropic and chronotropic effects of MS-857 do not appear to be mediated by direct stimulation of alpha- or beta-adrenoceptors, histamine H1- or H2-receptors, or by release of endogenous catecholamines. The inotropic response to isoproterenol or histamine was potentiated by a relatively low concentration of MS-857. MS-857 increased both developed tension and intracellular cyclic AMP level in isolated dog right ventricular muscle. The time course of the elevation in cyclic AMP level was paralleled by that of increase in developed tension. Carbachol inhibited the increase in developed tension and cyclic AMP level caused by MS-857. These findings support the idea that cyclic AMP may be involved in the positive inotropic effect of MS-857.

Profile of Cardiac and Coronary Vasodilator Effects of MS-857, a Novel Cardiotonic Agent, Assessed in Isolated, Blood-Perfused Heart Preparations of the Dog

MS-857 is a novel orally active nonglycoside and nonsympathomimetic cardiotonic agent. Cardiovascular properties of MS-857 were assessed in isolated, blood-perfused papillary muscle, sinoatrial (SA) node, and atrioventricular (AV) node preparation of the dogs. MS-857 (0.3-100 nmol) was injected intraarterially (i.a.). In paced papillary muscles, the drug produced a dose-dependent increase in developed tension. In spontaneously beating papillary muscles, MS-857 was ineffective on the ventricular automaticity. The drug increased sinus rate in SA node preparations and shortened AV conduction time by accelerating AV nodal conduction in AV node preparations. In all preparations, MS-857 increased coronary blood flow. The drug produced no arrhythmia in all doses tested. At doses which MS-857 produced a 50% increase in developed tension of papillary muscle, the drug also produces coronary vasodilatation with minimal chronotropic and dromotropic effects. In having such a cardiovascular profile, MS-857 resembles both milrinone and MCI-154 among few cardiotonic agents.

Comparative Positive Inotropic Effects of TA-064 (Denopamine), a New Cardiotonic Agent, and Isoproterenol and Ouabain on Guinea Pig Ventricular Muscles

The effects of TA-064 (TA) on "fast" and "slow" response action potentials and contractile tensions were studied in isolated guinea pig papillary muscles, and the findings were compared with those of isoproterenol (ISP) and ouabain. The results are as follows: (a) TA produced a dose-dependent (10(-7)-10(-5) M) increase in developed tension, with no significant changes in the resting membrane potential (RMP), action potential duration (APD), and maximum rate of rise of action potential (Vmax). (b) The concentration required to increase the developed tension by threefold ("equivalent concentration") were 10(-6) M with both TA and ouabain, while that of ISP was 5 x 10(-8) M. (c) TA increased the maximum rate of rise, dP/dtmax, and the maximum rate of fall, dR/dtmax, of the developed tension to the same extent, unlike those of ISP and ouabain. (d) Positive inotropic effects of TA (10(-7)-10(-5) M) were not completely abolished by atenolol (3.8 x 10(-5) M), a specific beta 1 blocker, whereas those of ISP were completely abolished. (e) The increasing effects of "equivalent concentration" of TA (10(-6) M) on the Vmax of the slow response were less than those of ISP (5 x 10(-8) M). These results suggest that the positive inotropic effects of TA are mainly due to stimulation of beta 1 adrenoceptors, but that the mode of action of the drug differs in several respects from that of ISP or ouabain.