An unknown endogenous inhibitor of Na/Ca exchange can enhance the cardiac muscle contractility.

The cardiac sarcolemma Na/Ca exchanger is a key system for controlling the intracellular calcium levels during the excitation-contraction coupling. Here, we test the hypothesis that the heart tissue contains a putative endogenous factor having a capacity to modulate the Na/Ca exchanger and muscle contractility. The concentrated cardiac extracts inhibit the Na(i)- or Ca(i)-dependent (45)Ca uptakes in isolated cardiac sarcolemma vesicles as well as the Na(o)-dependent Ca efflux, monitored by extravesicular Ca probe fluo-3. The inhibitory activity has been purified approximately 2000-fold by normal and reversed-phase HPLC procedures. The inhibitory activity is eluted from the Sephadex G-10 in the range of 350-550 Da, suggesting that the inhibitory factor is a low-molecular-weight substance. The mass spectra analysis shows a number of signals within m/z 380-560; however, it is not clear at this moment whether these recordings represent the mass of putative inhibitory factor or irrelevant impurities. The endogenous inhibitory factor of Na/Ca exchange does not resemble the properties (HPLC retention time, mass spectra, amino acid analysis, etc.) of autoinhibitory XIP peptide. The addition of inhibitory factor to muscle strip of guinea pig ventricles induces 2- to 5-fold enhancement of isometric contractions, thereby exhibiting a strong positive inotropic effect. This effect is a dose-dependent phenomenon, which can be reversed by washing the inhibitory factor from the organ bath. Assuming a molecular weight of 350-550 Da, the effective concentrations of putative inhibitor must be <10(-6) M. Therefore, the present findings demonstrate that the mammalian heart contains a low-molecular-weight factor that can inhibit Na/Ca exchange and enhance the cardiac contractility.

Effect of estradiol on rat ileum.

1. Sex hormones may influence gastrointestinal motility and thus may be responsible for symptoms that are common during pregnancy or hormone replacement therapy. The purpose of this study was to evaluate the effect of estradiol on the gut. 2. Segments of rat ileum (n=9) were suspended in an organ bath and exposed to increasing concentrations of carbachol, in the presence or absence of 17beta-estradiol. 17beta-estradiol markedly reduced the force developed by the ileum in response to carbachol. 3. These results suggest that estradiol reduces gastrointestinal motility.

Acute effects of 17 beta-estradiol on the rat heart.

OBJECTIVE: Our purpose was to study the acute effects of 17 beta-estradiol on mechanical and electrical activities of cardiac function and on coronary arteries in the rat heart. STUDY DESIGN: The effects of 17 beta-estradiol were studied on perfused working heart isolated from Charles River male rats. Heart rates, coronary flow, aortic flow, and left ventricular pressure were measured. To avoid coronary interaction, chronotropic and inotropic effects were also tested on isolated atria. Data were analyzed with the paired Student t test. RESULTS: 17 beta-Estradiol produced a dose-dependent negative chronotropic effect in right atria but did not affect the contractility of left atria. A decrease in heart rate was also observed in perfused hearts treated with 5 x 10(-6) mol/L 17 beta-estradiol. 17 beta-Estradiol (5 x 10(-6) mol/L) significantly increased coronary flow (p < 0.005) but had a negligible effect on cardiodynamic index values. A significant effect of 17 beta-estradiol on cardiac function was observed when coronary arteries were precontracted with acetylcholine. CONCLUSION: Both the experimental coronary vasodilatory effect and the negative chronotropic effect of 17 beta-estradiol support the clinical observations that suggest that this hormone may have an important role in prevention of cardiovascular diseases.

Positive inotropic effect in the heart produced by acetylcholine.

The effect of acetylcholine on cardiac muscle contractility and hemodynamics was investigated in human atrial strips and in isolated working rat heart. Activation of the muscarinic receptor in the heart muscle is generally known to result in negative chronotropic and inotropic effects. In our study, positive inotropic effects of acetylcholine (ACh) were observed in both human right atrial strips and in the working rat heart. Exposure of the human right atrial strips to ACh (10(-7)-10(-4) M) produced a dose dependent tri-phasic (positive-negative-positive) inotropic effect in approximately 40% of the strips. In muscle strips that exhibited only a negative inotropic effect, a positive response was observed following washout of ACh. Both positive and negative effects were antagonized by atropine. Exposure of the paced working rat heart to ACh (10(-7) - 10(-5) M) resulted in a dose dependent decrease in mean coronary flow followed by depression in cardiac function. When the heart was initially treated with the vasodilator adenosine (2 x 10(-6) M), exposure to ACh (10(-7) - 10(-5) M) had no effect on coronary flow and produced a dose dependent augmentation of all cardiodynamic indices: left ventricular pressure, isovolumic pressure, cardiac output, maximal aortic flow and stroke work. This positive response was antagonized by atropine. Exposure of the rat ventricular strips increased the formation of [3H]phosphoinositide breakdown products (e.g. inositol phosphates IP, IP2, IP3). These observations demonstrate that cholinergic muscarinic stimulation may produce positive inotropic effects in both human and rat cardiac muscle. Furthermore, our results suggest that IP3 may be a mediator in this process.

Myocardial response to alpha-agonist (phenylephrine) in relation to age.

We studied the inotropic effect of the alpha-agonist phenylephrine on human and rat myocardium in relation to age. Strips of human atrial myocardium, discarded during cardiac surgery, were obtained from pediatric (2 to 18 months) and adult (40 to 60 yr) patients, and their electrically driven contractions were studied. The inotropic response of pediatric myocardium to phenylephrine was five times larger than that of the adult myocardium (p less than .05). The beta-agonist, isoproterenol, was also administered to the same myocardial specimens, and the average pediatric myocardial response was three times larger than that of the adult myocardium (p less than .05). Strips of right ventricular myocardium were obtained from four age groups of rats: 14 days, 3 months, 6 months, and 18 months, and exposed to phenylephrine. We demonstrated two age-related phenomena: a) a biphasic response with an early short negative inotropic effect preceding a sustained positive inotropic effect was registered in most of the rats aged 3 months and older, but in none of the youngest age group. b) A high mean positive inotropic response (211% of basal response) was generated in the youngest age group, whereas the oldest age group of rats showed only a slight response (8% of basal response, p less than .05). We conclude that loss of myocardial responsiveness to alpha and beta-agonist with increasing age is a prominent feature. The pattern of the ventricular responsiveness to phenylephrine also changes during aging as it turns from monophasic to biphasic.

A novel cardiotoxic polypeptide from the venom of Atractaspis engaddensis (burrowing asp): cardiac effects in mice and isolated rat and human heart preparations.

A new cardiotoxic polypeptide isolated from the venom of the snake Atractaspis engaddensis has an LD50 of 15 micrograms/kg body weight in white mice. Intravenous administration in mice of lethal doses of the toxin causes, within seconds, marked changes in the ECG, consisting primarily of a transient slope elevation of the S-T segment, a temporary diminution of the S-wave and an increase in the amplitudes of the R- and T-waves. Concomitantly, and apparently unrelated to these changes, a severe A-V block develops and leads to complete cardiac arrest within a few min. Studies with rat and human isolated heart preparations showed that the toxin exerts a powerful coronary vasoconstriction (rats), and positive inotropic effects (rats and humans).

Nonopiate effect of naloxone on cardiac muscle contractility.

To test the hypothesis that naloxone exerts a direct positive inotropic effect on the cardiac muscle, we employed two in vitro models. In one set of experiments we demonstrated that injection of 1 mg naloxone into an isolated perfused rat heart produced a significant increase in the amplitude of contraction. In another set of experiments we exposed an isolated and spontaneously contracting rat right atrium in a tissue bath to naloxone, and demonstrated that the amplitude of contraction increased significantly within a few minutes of naloxone administration. We showed that this effect of naloxone was not related to opiate receptors, since a similar effect was obtained with d-naloxone (the stereoisomer that is inactive as an opiate antagonist) and it was not affected by pretreatment with morphine. We also demonstrated that addition of alpha- and beta-adrenergic antagonists phentolamine and propranolol, in doses that effectively block alpha- and beta-adrenergic agonists, did not have any effect on naloxone's inotropic action. We validated our results in two electrically driven strips of human atrial myocardium in the tissue bath. A positive inotropic response to naloxone, measured as an increase of 80 and 50% in the amplitude of contraction, was noted. We postulate that naloxone's previously described cardiovascular pressor effect in states of shock may not only be related to reversal of the effects of endorphins but also to its direct inotropic action.

Positive inotropic response to alpha-adrenergic stimulation in an electrically driven rat left atrium: the role of extracellular calcium.

We studied the positive inotropic response induced by alpha-adrenergic receptor stimulation in an electrically driven rat left atrium. alpha-Adrenergic stimulation resulted in a prolonged positive inotropic response that reached its maximum within 5-7 min. The kinetics of the onset of the positive inotropic response were different for pure alpha-adrenergic, pure beta-adrenergic, and mixed adrenergic stimulation. The positive inotropic responses to alpha- and beta-adrenergic agonists were not additive. The relative inotropic response to alpha-adrenergic stimulation decreased when external calcium concentration was increased and disappeared when external calcium concentration was raised to 7.0 mM. The divalent cation ionophore A23187 (1 microM) produced a threefold increase of the contractility of the atrial preparation at 1.0 mM extracellular calcium, and no further alpha-adrenergic response was observed in its presence. Calcium channel antagonists verapamil and nifedipine markedly inhibited the response to alpha-adrenergic stimulation, with little effect on the beta-adrenergic stimulation, at a calcium concentration of 0.5 mM. The inhibitory effect of calcium channel antagonists could be fully reversed by increasing the extracellular calcium concentration. Our data suggest that the alpha-adrenergic contractile response in the rat atrium involves the mobilization of extracellular calcium through verapamil-sensitive calcium channels in a mechanism different from that for the beta-adrenergic response.

Phenytoin pharmacokinetics in catamenial epilepsy.

It is not known why the frequency of seizures sometimes increases in the perimenstrual period (catamenial epilepsy). We have examined the possibility that changes in anticonvulsant pharmacokinetics may be responsible. Seventeen women with seizures who were taking phenytoin (DPH) and whose seizures were more frequent perimenstrually were examined twice each, once on the first or second menstrual day and again after 2 weeks. Mean serum DPH levels were lower during the menses. In seven women with seizures unrelated to menses, the fall of DPH levels was much smaller. In catamenial epilepsy, the fall was due to increased DPH clearance. At the time of ovulation, hepatic DPH metabolism may be slowed by competition from steroid hormones.

Life-threatening theophylline intoxication in a hypothyroid patient.

In a hypothyroid patient who suffered also from chronic obstructive lung disease and mild congestive heart failure, treatment with 1 g/day theophylline administered orally, was followed by a life-threatening theophylline intoxication manifested by repeated epileptic fits and ventricular fibrillation, successfully reverted to sinus rhythm. The plasma theophylline was 34.7 micrograms/ml when the life-threatening arrhythmia occurred. Pharmacokinetic studies conducted during the hypothyroid state revealed a markedly prolonged theophylline plasma half-life of 29.5 h. 2 months later, after reestablishment of an euthyroid state, theophylline plasma half-life was shortened to normal, i.e. 5.7 h and the theophylline plasma level was 13.5 micrograms/ml, while the daily intake was 1 g. We conclude that hypothyroidism may predispose to theophylline intoxication, probably because of the decreased activity of the hepatic microsomal drug-metabolizing enzymes, responsible for the degradation of theophylline.