Different effects of simple anoxic lactic acidosis and simulated in vivo anoxic acidosis on turtle heart.

We compared responses of turtle heart at 20 degrees C to an anoxic lactic acidosis solution (LA) containing 35 mM lactic acid in an otherwise normal turtle Ringers equilibrated with 3% CO2/97% N2 at pH 7.0) to a solution simulating in vivo anoxic acidosis (VA), with elevated concentrations of lactate, Ca2+, Mg2+, and K+, and decreased Cl-, equilibrated with 10.8% CO2/89.2% N2 at pH 7.0. We examined mechanical properties on cardiac muscle strips and determined intracellular pH (pHi) and high energy phosphates on perfused hearts using 31P-NMR. Maximum active force (Fmax) and the maximum rate of force development (dF/dtmax) of muscle strips were significantly higher during VA than during LA superfusion. An elevation of Ca2+ alone (to 6 mM) in LA significantly increased both Fmax and dF/dtmax but the effects diminished toward the end of the exposure; however, hypercapnic anoxic lactic acidosis (addition of 20 mM HCO3- to LA, equilibrated with 10.8% CO2/89.2% N2, pH 7.0) did not significantly affect Fmax or dF/dtmax. During VA perfusion, pHi (6.73 +/- 0.01) was significantly higher than that during LA perfusion (pHi 6.69 +/- 0.013), but the difference is probably too small to have physiological significance. ATP, creatine phosphate, and inorganic phosphate were not significantly different in the two anoxic solutions. We conclude that the reduction of cardiac mechanical function in vivo is minimized by the integrated effects of changes of ionic concentrations, but the observed changes in Ca2+ and pHi cannot fully explain the effect.     

Inotropic effects of adrenaline on the anoxic or hypercapnic myocardium of rainbow trout and eel

Summary The effects of adrenaline on the development of force under anoxia and hypercapnic acidosis (13% CO₂ in 30 mM HCO ₃ ^– ) were examined in isolated, electrically stimulated cardiac ventricle strips of rainbow trout and eel.During anoxia or acidosis applied 15 min in advance, the adrenaline concentration of the bathing solution was increased in 5 steps from 0 to 10^–4 M with 5 min at each step. Before levelling off the contractile tension increased by 145±42% (mean±SE) in the anoxic, 80±14% in the acidotic and 152±41% in the control trout cardiac strips. Except for the acidotic strips the corresponding values tended to be lower for the eel strips being 46±9%, 57±17% and 57±9%, respectively. The inotropic affinity for adrenaline was lower in the trout than in the eel myocardium. For the trout myocardium it remained unchanged, while it decreased somewhat for the eel myocardium under anoxia or acidosis.Adding to the muscle bath 10^–5 M adrenaline resulted in an increase in force development by about 90% for the trout myocardium and 50% for the eel myocardium. 5 min later anoxia or hypercapnic acidosis was applied for 30 min followed by 30 min at control conditions. Relative to the force values recorded just before anoxia or acidosis was applied, the changes in contractile force during these periods were the same with and without adrenaline. Thus adrenaline appears to have a persistent positive inotropic effect in the fish myocardium during and after oxygen lack or acidosis.     

Oxygen consumption and force development in turtle and trout cardiac muscle during acidosis and high extracellular potassium

Relative to species such as rainbow trout, freshwater turtle shows a high tolerance to challenges involving acidosis and increases in extracellular K+. Therefore, the effects of acidosis or high K+ on twitch force and oxygen consumption were examined in ventricular ring preparations from these two species. The oxygen consumption associated with force development was estimated by net oxygen consumption (oxygen consumption during twitch force development minus that during rest). For turtle, elevation of CO2 from 2% (pH 7.7) to 12% (pH 6.9) in the gas equilibrating the muscle bath decreased twitch force by 20% without any effects on oxygen consumption. Decreasing pH from 7.7 to 6.9 with 22 mM lactic acid had similar effects. For trout, CO2-induced acidosis decreased twitch force by approximately 60%. Furthermore, force development became energetically less efficient as it fell disproportionately more than net oxygen consumption. This was not observed for lactic acidosis. For trout but not for turtle, acidosis resulted in an increase in oxygen consumption during rest. An increase in extracellular K+ from 2.5 mM to 10 mM depressed force and oxygen consumption proportionately for both species. Adrenaline (10 microM) increased twitch force for both species and oxygen consumption for trout; it attenuated the effects of high extracellular K+. Neither adrenaline nor high K+ influenced the ratio of force to net oxygen consumption. As opposed to high extracellular K+, acidosis appears to increase the energetic cost of contractility, particularly for the trout heart.     

Atrial myocardium is the predominant inotropic target of adrenomedullin in the human heart

Adrenomedullin (ADM) is an endogenous peptide with favorable hemodynamic effects in vivo. In this study, we characterized the direct functional effects of ADM in isolated preparations from human atria and ventricles. In electrically stimulated human nonfailing right atrial trabeculae, ADM (0.0001-1 micromol/l) increased force of contraction in a concentration-dependent manner, with a maximal increase by 35 +/- 8% (at 1 micromol/l; P < 0.05). The positive inotropic effect was accompanied by a disproportionate increase in calcium transients assessed by aequorin light emission [by 76 +/- 20%; force/light ratio (DeltaF/DeltaL) 0.58 +/- 0.15]. In contrast, elevation of extracellular calcium (from 2.5 to 3.2 mmol/l) proportionally increased force and aequorin light emission (DeltaF/DeltaL 1.0 +/- 0.1; P < 0.05 vs. ADM). Consistent with a cAMP-dependent mechanism, ADM (1 micromol/l) increased atrial cAMP levels by 90 +/- 12%, and its inotropic effects could be blocked by the protein kinase A (PKA) inhibitor H-89. ADM also exerted positive inotropic effects in failing atrial myocardium and in nonfailing and failing ventricular myocardium. The inotropic response was significantly weaker in ventricular vs. atrial myocardium and in failing vs. nonfailing myocardium. In conclusion, ADM exerts Ca(2+)-dependent positive inotropic effects in human atrial and less-pronounced effects in ventricular myocardium. The inotropic effects are related to increased cAMP levels and stimulation of PKA. In heart failure, the responsiveness to ADM is reduced in atria and ventricles.     

Acidosis counteracts the negative inotropic effect of K+ on ventricular muscle strips from the toad Bufo marinus

Strenuous activity is associated with acidosis, increased extracellular potassium concentration ([K+]o), and elevated levels of circulating catecholamines. Acidosis and elevated [K+]o are normally considered harmful to cardiac function, and a high sympathetic tone on the heart may lead to arrhythmia. During activity, however, the heart must be able to increase rate and strength of contraction. While the individual effects of [K+]o, acidosis, and adrenaline on contractile properties of cardiac muscle have been characterized for some ectothermic species, less information is available on their interactions. Here we examine the isolated and combined effects of [K+]o, acidosis, and adrenaline on ventricular muscle strips from the toad Bufo marinus. This study showed that increased [K+]o significantly reduced twitch force, while lactic acid significantly increased twitch force and more than counteracted the negative inotropic effects of elevated [K+]o. There was no inotropic effect of Na-lactate (neutralized lactic acid), which suggests that lactic acid stimulated twitch force through reduced pH and not by serving as a substrate. Adrenaline had a positive effect on twitch force in all preparations. Irrespective of treatment, twitch force decreased as stimulation rate increased. During high [K+]o, there was a severe reduction in maximal frequency of toad ventricular strips that was not alleviated by lactic acidosis and/or adrenaline, which suggests that high [K+]o influences twitch force and maximal rate by different mechanisms. In vivo levels of lactic acid, [K+]o, adrenaline, and heart rate previously observed during forced activity in Bufo did not significantly affect the contractile properties of heart muscle strips in vitro. Thus, although [K+]o significantly decreased twitch force, this detrimental effect was more than counteracted by the positive inotropic effect of lactic acid and adrenaline.     

Effects of prostaglandin E1 (PGE1) on the positive inotropic response of heart muscle to elevation of external Ca++-concentration, to increased driving frequency, and to paired stimulation.

Electrically driven left guinea pig atria were exposed to positive inotropic stimuli which are thought to be related to the turnover of calcium ions. For increasing contractibility, the following procedures were used: a) varying the concentration of CaCl2 in the bath fluid; b) stimulation at frequencies from 1.0 to 3.0 Hz; c) paired stimulation. Positive inotropic responses to the increase of the rate of stimulation and to paired stimulation were not affected by 0.1 microgram/ml tetrodotoxin (TTX). This excludes the adrenergic contribution to the positive inotropic effects observed. Actions of the positive inotropic stimuli were studied both in the absence and in the presence of 0.1--1.0--10.0--1000.0 ng/ml of PGE1-PGE1 in the highest concentration used increased contractile force. The inotropic stimulus-response curves were not affected by PGE1 at any concentration. This finding suggests there is no interaction between Ca ions and PGE1 in the contractile mechanism of the guinea pig heart muscle.     

Force development at elevated [Mg2+]o and [K+]o in myocardium from the freshwater turtle (Trachemys scripta) and influence of factors associated with hibernation

The effects of high [Mg(2+)](o) on force development were examined for heart muscle of freshwater turtle. Plasma [Mg(2+)] during hibernation may increase drastically and like plasma [K(+)] approach values as high as 10 mM. Each experiment performed at either 20 or 5 degrees C involved four ventricular preparations of which one pair was exposed to 10, and one to 1 mMMg(2+). One preparation of each pair was furthermore exposed to 10 mM K(+), whereas the other was maintained at 2.5 mM K(+). During oxygenation, high relative to low [Mg(2+)](o) displayed a weak tendency to reduce twitch force; a tendency that was not reduced by elevations of [Ca(2+)](o). Severe hypoxia accentuated the negative effect of high [Mg(2+)](o). This effect disappeared, however, when hypoxia was combined with acidosis obtained by 24 mM lactic acid. In comparison to [Mg(2+)](o), high [K(+)](o) strongly depressed force development under both oxygenation and hypoxia, but no consistent interplay between the two ions was revealed. The negative inotropic effects of both high [Mg(2+)](o) and high [K(+)](o) were reduced or eliminated by 10 muM adrenaline. In conclusion the cardiac effects of elevations in [Mg(2+)](o) appear to be small during hibernation, in particular when considering the concomitant adrenergic stimulation and acidosis.     

Direct actions of prostaglandins E1, A1, and F2α on myocardial contraction

The inotropic and chronotropic actions of prostaglandin (PG) types PGE₁, PGA₁, and PGF_2α were studied in isolated guinea pig right and left atria, and papillary muscles; rabbit atria; and toad ventricular strips in order to more completely define the cardiac contractile properties of PG. All three prostaglandins, in muscle bath concentrations of 10μg/ml, exerted positive inotropic and chronotropic actions on guinea pig atrium. These contractile effects were persistent after removal of PG from the muscle bath and appeared to limit the relative response to a subsequent dose of PG. The inotropic action of PGE₁ was present over a wide range of bath calcium concentrations (1.1 to 4.4 mM/L). Beta adrenergic receptor blockade, histamine blockade, and pretreatment with reserpine failed to significantly affect the inotropic actions of PG. Norepinephrine and histamine produced more potent inotropic and chronotropic effects on guinea pig atria than did PG and these contractile effects did not exhibit persistence or tachyphylaxis. The prostaglandins did not significantly affect dose response curves for norepinephrine inotropic and chronotropic actions. The prostaglandins had no effect on the force or frequency of contraction in rabbit atria. PGE₁ exerted a positive inotropic effect on toad ventricular myocardium whereas PGA₁ had no effect and PGF_2α had a negative inotropic action.     

Lactic acid restores skeletal muscle force in an in vitro fatigue model: are voltage-gated chloride channels involved?

High interstitial K(+) concentration ([K(+)]) has been reported to impede normal propagation of electrical impulses along the muscle cell membrane (sarcolemma) and then also into the transverse tubule system; this is one considered underlying mechanism associated with the development of muscle fatigue. Interestingly, the extracellular buildup of lactic acid, once considered an additional cause for muscle fatigue, was recently shown to have force-restoring effects in such conditions. Specifically, it was proposed that elevated lactic acid (and intracellular acidosis) may lead to inhibition of voltage-gated chloride channels, thereby reestablishing better excitability of the muscle cell sarcolemma. In the present study, using an in vitro muscle contractile experimental setup to study functionally viable rectus abdominis muscle preparations obtained from normal swine, we examined the effects of 20 mM lactic acid and 512 μM 9-anthracenecarboxylic acid (9-AC; a voltage-gated chloride channel blocker) on the force recovery of K(+)-depressed (10 mM K(+)) twitch forces. We observed a similar muscle contractile restoration after both treatments. Interestingly, at elevated [K(+)], myotonia (i.e., hyperexcitability or afterdepolarizations), usually present in skeletal muscle with inherent or induced chloride channel dysfunctions, was not observed in the presence of either lactic acid or 9-AC. In part, these data confirm previous studies showing a force-restoring effect of lactic acid in high-[K(+)] conditions. In addition, we observed similar restorative effects of lactic acid and 9-AC, implicating a beneficial mechanism via voltage-gated chloride channel modulation.     

Positive inotropic effect of the inhibition of cyclic GMP-stimulated 3',5'-cyclic nucleotide phosphodiesterase (PDE2) on guinea pig left atria in eu- and hyperthyroidism

The significance of PDE2 on the atrial inotropy was studied in eu- and hyperthyroidism. The contractile force was measured and negative inotropic capacity of N6-cyclopentyladenosine (CPA) was determined on left atria isolated from 8-day thyroxine- or solvent-treated guinea pigs, in the presence or absence of EHNA (adenosine deaminase and PDE2 inhibitor) or NBTI (nucleoside transporter inhibitor). EHNA was administered to inhibit PDE2, while NBTI was used to model the accumulation of endogenous adenosine. The reduction of the contractile force caused by EHNA was smaller in the thyroxine-treated atria than in the solvent-treated samples. Contrary, NBTI induced a decrease in the contractile force without significant difference between the two groups. In addition, EHNA enhanced the efficiency of CPA in thyroxine-treated atria and did not affect it in solvent-treated samples, while the response to CPA was decreased by NBTI in all atria, especially in hyperthyroidism. On the basis of greater retention of the contractile force and sustained/enhanced responsiveness to CPA in the presence of EHNA we conclude that PDE2's inhibition has a significant positive inotropic effect in guinea pig atria and this effect is proven to be augmented in hyperthyroidism.     

Role of calcium channel in postvagal potentiation of contraction as evident from the effects of Mn2+, La3+, D-600, and deoxycholate on isolated guinea pig atria-vagus nerve preparation

The role of the calcium channel in the first large contraction (postvagal potentiation, PVP) of the atria at the end of the inhibitory phase of its response (IPR) to vagal stimulation has been investigated by studying the effects of agents acting on the calcium channel (e.g., Ca2+, Mn2+, La3+, and D-600) or sarcoplasmic reticulum (SR) (e.g., deoxycholate (DOC)). IPR was potentiated by high [Ca2+]o (3-16 mM) and also by the calcium channel blockers, Mn2+ (1 microM-0.5 mM), La3+ (0.1 microM-0.5 mM), D-600 (1.0-10 microM), and DOC (1 microM-0.5 mM). PVP was also potentiated by enhanced [Ca2+]o, but the PVP ratio, which employs a correction for the simultaneous changes in the force of spontaneous contraction was inhibited. This indicated greater potentiation of contractility during spontaneous activity by Ca2+ than during PVP. Mn2+, La3+, and D-600 and even DOC in the above concentrations inhibited PVP but increased the PVP ratio. High concentrations of DOC (greater than 1 mM), which disrupt SR, strongly inhibited PVP. It is concluded that the calcium channel plays a more prominent role in spontaneous contractions than in PVP in guinea pig atria. PVP is suggested to be generated by excessive triggered release of Ca2+ from SR leading to a marked increase in [Ca2+]i. The calcium channel and the calcium trapped in the glycocalyx also play significant roles in PVP.     

Pharmacological characterization of an Endogenous Negative Inotrophic Factor (ENIF) from porcine heart

Recently we have been successful in isolating an endogenous negative inotropic factor (ENIF) from porcine left ventricular tissue. In this study, we have characterized its pharmacological properties. The results of the study demonstrated that ENIF produces a concentration-dependent negative inotropic response on both guinea pig left atria and right ventricular trabeculae. The maximal reduction in contractile force produced by 300 ul of ENIF (5 ml bath) on atria and trabeculae were 90.0 ± 0.8% and 77.5 ± 6%. Atria, however, was significantly more sensitive to ENIF than trabeculae. The ED 50 of ENIF for atria was found to be 38 ul as opposed to ED 50 of 100 ul of ENIF for trabeculae.Acetylcholine (ACh), a muscarinic receptor agonist, decreased the contractile force of guinea pig atria in a dose-dependent manner with a maximal decline in the contractile force of 90%. However, none of the concentration of ACh used affected the contractile function of the trabeculae. Atropine (1 uM) completely blocked the negative inotropic response on atria of all the doses of ACh used. The same dose of atropine, however, was unable to influence the negative inotropic effect of any of the doses of ENIF used on either the atria or trabeculae preparations in our study. The maximal decline in the contractile force of atria was e.g. 94 and 95% in the presence and absence of atropine respectively. These data demonstrate that the myocardial negative inotropic effect of ENIF is not mediated via the cholinegic receptor mechanism.     

Chronotropic and inotropic effects of PGE2 on isolated rat atria from normal and spontaneously hypertensive rats (SHR)

Prostaglandin E₂ (PGE₂) was tested for its ability to produce a positive chronotropic and inotropic response on isolated atria obtained from normal (NWR) and spontaneously hypertensive rats (SHR). There was no significant difference in the basal in vitro heart rate of atria from NWR and SHR. There was no significant difference in the increase in heart rate or contractile force produced by PGE₂ (0.568 μM and 5.68 μM) on atria obtained from NWR compared to SHR. It appears there is no difference in the in vitro cardiac reactivity to PGE₂ of atrial tissue from SHR compared to normotensive controls under the conditions of these experiments. This is in contrast to the previously reported decrease in contractile responses of atrial tissue of SHR to certain cardiac stimulants, and the diminished contractile response produced by PGE₂ on arterial tissue from SHR compared to NWR.     

The inotropic effect of endothelin-1 on rat atria involves hydrolysis of phosphatidylinositol

Endothelin-1 induces a positive inotropic response in isolated left atria of the rat with an IC50 value of 20 nM. The contractile effect of endothelin is larger than that of other inotropic hormones such as phenylephrine and epinephrine and smaller than that of Bay K8644. In the spontaneously active right atria, endothelin induces a positive inotropic effect with no chronotropic effect. Endothelin does not modify intracellular levels of cAMP under basal conditions or after stimulation with isoproterenol but stimulates the formation of inositol phosphates. Mobilization of inositol phospholipids is observed in the same range of concentrations as for the contractile action of endothelin. The contractile action of endothelin is not mediated by protein kinase C. It is antagonized by blockers of L-type Ca2+ channels, low external Ca2+ concentrations and drugs such as caffeine and ryanodine that interfere with Ca2+ release by the sarcoplasmic reticulum.     

Diabetic type of cardiomyopathy in food-restricted rats.

We have demonstrated that food restriction that is associated with weight loss can produce a type of cardiac dysfunction similar to that produced by diabetes. As in diabetic atria, the food-restricted atria had a 2-fold increase in contraction force, rate of force development, and rate of force decline compared with controls. Both food-restricted and diabetic atria could tolerate anoxia better than controls. The contractile function of the whole perfused heart from the food-restricted rat was reduced, as in the case of the diabetic heart. As the left ventricular volume was increased, the left ventricular developed pressure and the rate of rise and fall in pressure were significantly reduced in both food-restricted and diabetic hearts, compared with those of age- and weight-matched controls. The positive inotropic responses of atria and whole perfused heart to increasing concentrations of extracellular calcium were similarly altered in food-restricted and diabetic hearts. The possible molecular mechanisms of these findings and some of the differences observed between food-restricted and diabetic hearts are discussed.     

Alterations in alpha 1-adrenoceptor stimulation of isolated atria from experimental diabetic rats

This purpose of this investigation was to determine the influence of experimental diabetes (3 months) on the responsiveness of rat isolated atria to alpha 1-adrenoceptor stimulation by phenylephrine. Diabetes was chemically induced with streptozotocin (65 mg/kg i.v.) in 42- to 43-day-old, nonfasted male Sprague-Dawley derived rats. Chronotropic (right atria) and inotropic (left atria) indices were recorded in response to alpha 1-adrenoceptor stimulation by phenylephrine. These experiments were performed in the presence of beta-adrenoceptor antagonism (timolol). Isolated right atria from diabetic rats demonstrated a greater increase in heart rate in response to phenylephrine than did corresponding control atria. Left atria were supersensitive (decrease in EC50 values) and hyperresponsive to alpha 1-adrenoceptor stimulation by phenylephrine when compared with stimulation of control left atria. Diabetic left atria in response to phenylephrine were observed to exchange more radioactive calcium (45Ca2+) than control left atria, whereas both diabetic and control left atria exchanged the same amount of 45Ca2+ during basal contractile conditions. Phenylephrine had no effect on 45Ca2+ efflux from either diabetic or control atria. These results indicate that 3 months of uncontrolled experimental diabetes in the rat produces an enhancement of alpha 1-adrenoceptor activation of isolated atria, and that there is an alteration in Ca2+ mobilization which may contribute to the enhanced receptor activation.     

The Effects of Zinc on Contractility,Membrane Potentials,and Cation Content of Rat Atria

Zinc depresses the contractile force of electrically driven rat atria logarithmically with time. The threshold concentration is about 5 x 10^-6 M zinc and the half-time for contractile depression at 10^-4 M is about 25 minutes. Zinc also depresses spontaneous activity of atria and alters the transmembrane potential parameters in a manner similar to quinidine. Unlike quinidine, zinc causes an elevation of the resting potential and an elevation of cellular potassium which varies with time in the same way as the resting potential. Exposure to 10^-4 M zinc for 60 minutes causes a statistically significant fall in atrial calcium content and an amount of radioactively labeled zinc is taken up which is quantitatively equal to the calcium lost. Zinc has no effect on rigor caused by iodoacetate but inhibits rigor caused by 1-fluoro-2,4 dinitrobenzene. It is postulated that zinc depression of contractile force is not due to metabolic inhibition, probably not due to quinidine-like action on the cell membrane, but may be due to an interference in the handling of calcium by the cell.     

Spontaneous Ca2+ release from the sarcoplasmic reticulum limits Ca2+- dependent twitch potentiation in individual cardiac myocytes. A mechanism for maximum inotropy in the myocardium

We hypothesized that the occurrence of spontaneous Ca2+ release from the sarcoplasmic reticulum (SR), in diastole, might be a mechanism for the saturation of twitch potentiation common to a variety of inotropic perturbations that increase the total cell Ca. We used a videomicroscopic technique in single cardiac myocytes to quantify the amplitude of electrically stimulated twitches and to monitor the occurrence of the mechanical manifestation of spontaneous SR Ca2+ release, i.e., the spontaneous contractile wave. In rat myocytes exposed to increasing bathing [Ca2+] (Cao) from 0.25 to 10 mM, the Cao at which the peak twitch amplitude occurred in a given cell was not unique but varied with the rate of stimulation or the presence of drugs: in cells stimulated at 0.2 Hz in the absence of drugs, the maximum twitch amplitude occurred in 2 mM Cao; a brief exposure to 50 nM ryanodine before stimulation at 0.2 Hz shifted the Cao of the maximum twitch amplitude to 7 mM. In cells stimulated at 1 Hz in the absence of drugs, the maximum twitch amplitude occurred in 4 mM Cao; 1 microM isoproterenol shifted the Cao of the maximum twitch amplitude to 3 mM. Regardless of the drug or the stimulation frequency, the Cao at which the twitch amplitude saturated varied linearly with the Cao at which spontaneous Ca2+ release first occurred, and this relationship conformed to a line of identity (r = 0.90, p = less than 0.001, n = 25). The average peak twitch amplitude did not differ among these groups of cells. In other experiments, (a) the extent of rest potentiation of the twitch amplitude in rat myocytes was also limited by the occurrence of spontaneous Ca2+ release, and (b) in both rat and rabbit myocytes continuously stimulated in a given Cao, the twitch amplitude after the addition of ouabain saturated when spontaneous contractile waves first appeared between stimulated twitches. A mathematical model that incorporates this interaction between action potential-mediated SR Ca2+ release and the occurrence of spontaneous Ca2+ release in individual cells predicted the shape of the Cao-twitch relationship observed in other studies in intact muscle. Thus, the occurrence of spontaneous SR Ca2+ release is a plausible mechanism for the saturation of the inotropic response to Ca2+ in the intact myocardium.     

Differences between mouse and rat myocardial contractile responsiveness to calcium

Genetically altered mice have become an increasingly important tool for the study of mechanisms of cardiac function, and therefore it is vital to characterize the basic contractile properties of the mouse heart. As a first approach to this goal, we first optimized perfusion conditions and characterized the effect of incremental left ventricular balloon inflation on end-diastolic, systolic and developed pressures in the isovolumically-contracting mouse heart. Under constant loading conditions, we determined developed pressure in response to changing perfusate calcium (1.25, 2.5, 3.75 and 5.0 mM) and perfusate temperature (30 and 37 degrees C). We then compared the intrinsic inotropic responsiveness to changes in extracellular calcium of left ventricular myocardium from mouse to that from the rat. In the baseline state (1.25 mM extracellular calcium; [Ca2+]o), both isometric contraction duration and normalized active force at the peak of the active force-length relationship (Lmax) were less in mouse than in rat myocardium. Under isotonic conditions, temporal parameters of shortening and the relative shortening were less in mouse vs rat myocardium. Increasing [Ca2+]o from 1.25 to 2.5 mM markedly increased active isometric force and rate of force development (+dF/dt) in the mouse. However, rat myocardium responded to a lesser extent. Under isotonic conditions, peak shortening and the rate of shortening also increased to a greater extent in mouse relative to rat myocardium. Increasing the bath calcium concentration to 5.0 mM increased isometric force and +dF/dt further in the rat but not the mouse, suggesting that two species operate at different points on the force vs [Ca2+]o relationship. We conclude that mouse myocardium exhibits increased sensitivity to changes in [Ca2+]o within the physiologic range in comparison to rat. These differences do not appear to be due to differences in loading conditions. The data suggest that differences in inotropic responsiveness to calcium may reflect intrinsic differences in myocardial calcium sensitivity between species.     

A bradykinin potentiating peptide from Egyptian cobra venom strongly affects rat atrium contractile force and cellular calcium regulation

Peptide fractions were isolated from venoms of the Egyptian snake Naja haje haje (cobra BPP) and the scorpions Buthus occitanus (BPP(B)) and Leirus quenquestriatus (BPP(L)). The pharmacological effects of these peptides were bioassayed and showed bradykinin potentiating activities. Amino acid analysis revealed that 14 amino acids contribute to the structure of BPP(B) and 16 for BPP(L), while cobra BPP was composed of 15 amino acids. Treatment of rat atrial preparations with 50 microg/ml of cobra BPP caused a significant reduction (P<0.001) in myocardial force. Elevation of extracellular calcium concentration from 1.25 to 5 mM antagonized the effect of cobra BPP in a way that restored the atrial force development. Na(+)-channel blockers did not change the force development at 5 mM Ca(2+). Experiments with (45)Ca revealed that Ca(2+) uptake of cobra BPP treated atria was 0.52+/-0.07 microM/g wet mass and the force at the end of the uptake period was 55.0+/-2.0%. The corresponding values for non-treated preparations were 0.56+/-0.04 microM/g and 92.0%+/-3.0%, respectively. Our results revealed that cobra BPP did not exhibit any effect on Ca(2+) uptake by rat atrial preparations, but strongly affected cellular Ca(2+) regulation.