Action of pilocarpine on the normal frog heart and in pathology

Experiments on frogs were performed to examine the effect of the M-cholinomimetic pilocarpine on the heart. It was discovered that at concentrations of 10(-15)--10(-5) g/ml pilocarpine exerted only an adverse chronotropic effect on the perfused heart. When applied at a concentration of 10(-4) g/ml the drug produced a negative as well as a positive chronotropic effect. The latter occurred spasmodically (without progressive rise in the heart rate) in association with a slow heart rate. In some experiments such effects were preceded by a certain deceleration of the heart. In experiments with positive chronotropic effects, arrhythmias and sinoatrial dissociation were observed sometimes. Experiments with recording of the electrograms of the sinuses and lower parts showed that such effects were caused not by pacemaker acceleration but by the removal of the blockade of conduction, between the pacemaker and the atria. As far as the pacemaker is concerned, pilocarpine exerted only a negative chronotropic effect.     

Interaction of clonidine with chronotropic agents on isolated right atria

Clonidine was administered to isolated guinea pig right atria in order to characterize its chronotropic activity and its interaction with other chronotropic agents at the postjunctional level. Clonidine either had no significant effect (10(-7)--10(-4) M) or decreased (10(-3) M) atrial rate. Pretreatment of the atria with clonidine noncompetitively antagonized (10(-6)--10(-4) M) the positive chronotropic actions of isoproterenol, and competitively antagonized (10(-4) M) the negative chronotropic actions of pilocarpine. At doses of 10(-6) or 3 X 10(-6) M, clonidine also noncompetitively antagonized the positive chronotropic effects of 4-methylhistamine and glucagon. The results show that clonidine antagonizes both adrenergic and cholinergic influences on atrial rate at the postjunctional level and suggest that the antagonism of adrenergic influences does not involve a direct interaction with beta-adrenergic receptors.     

Interaction between forskolin and verapamil on spontaneous contractions of isolated guinea pig atria

Forskolin (0.16-1.3 mumol l-1 produced a concentration-dependent increase in both the isometric contraction and the atrial rate. The effects developed slowly, 5 to 8 minutes after the drug was added into the organ bath. Forskolin produced positive inotropic and chronotropic effects also in the presence of isoprenaline. The effects of forskolin were significantly inhibited in the presence of increased concentrations of verapamil in the organ bath. These results indicate that the action of forskolin is mediated by stimulation of the adenylate cyclase system, but in some steps of this action, calcium is also implicated.     

Insulin modulation of chronotropic response to Adrenaline in diabetic dogs.

In 8 unanesthetized dogs, 10-21 days post pancreatectomy, the cardiac chronotropic response to rapid infusion of a pharmacological dosage of Adrenaline was begun. During the subsequent month, the response was recorded electrocardiographically on 19 occasions. On 8 occasions, animals were treated with continuous intravenous administration of fluids and insulin up to the time of the test; on 11, insulin was omitted for 18 hours before Adrenaline injection. Insulin treated animals responded with the typical brief initial bradycardia, followed by some 2 minutes of ventricular tachycardia, and restoration of preinjection heart rate and electrocardiograph pattern within 5 minutes. On those occasions when insulin was omitted, the tachycardia was replaced by ventricular bradycardia. The altered chronotropic response of non-insulin treated dogs indicates impairment of their cardiac beta receptors.     

Effects of the lipolysis inhibiting agent beta-pyridylcarbinol (Ronicol) in acute myocardial ischaemia

The antilipolytic, nicotinic acid analogue beta-pyridylcarbinol (Ronicol) has previously been reported to decrease the free fatty acid (FFA) concentration of the arteria-blood, and to moderate the FFA-uptake and O2-consumption of the myocardium; on this basis, the drug may be expected to exert a cardioprotective action. The cardiac effects of Ronicol were therefore studied on a self-control, 'single-vessel' coronary artery ligature dog model. The left anterior descending coronary artery (LAD) was prepared in the in situ heart of anaesthetized, thoracotomized animals. Following the control ligation, a stabilization period and Ronicol infusion (1 mg/kg iv. during 10 minutes), the LAD was repeatedly ligated. The duration of the individual occlusions was 10 minutes. Ronicol significantly decreased the arterial FFA concentration and the epicardial ST segment elevation; its antilipolytic and anti-ischaemic effects were protracted and were still observed 120 minutes after pretreatment. The drug did not decrease the inhomogeneity of ventricular depolarization in the ischaemic myocardium and in the dose applied it had no influence on the heart rate, arterial blood pressure, left ventricular end-diastolic pressure and left ventricular contractility (LV dP/dtmax). In the canine myocardial infarction model employed it was observed that the duration of the anti-ischaemic effect of Ronicol (1 mg/kg iv.) is about 120 minutes. It has the advantage that it does not possess the unwanted cardiovascular side-effects displayed by nicotinic acid observed by us too in this model earlier (Cardiol. Hung. 13, 33-41, 1984).     

Bradykinin induced a positive chronotropic effect via stimulation of T- and L-type calcium currents in heart cells

Using Fluo-3 calcium dye confocal microscopy and spontaneously contracting embryonic chick heart cells, bradykinin (10(-10) M) was found to induce positive chronotropic effects by increasing the frequency of the transient increase of cytosolic and nuclear free Ca2+. Pretreatment of the cells with either B1 or B2 receptor antagonists (R126 and R817, respectively) completely prevented bradykinin (BK) induced positive chronotropic effects on spontaneously contracting single heart cells. Using the whole-cell voltage clamp technique and ionic substitution to separate the different ionic current species, our results showed that BK (10(-6) M) had no effect on fast Na+ inward current and delayed outward potassium current. However, both L- and T-type Ca2+ currents were found to be increased by BK in a dose-dependent manner (10(-10)-10(-7) M). The effects of BK on T- and L-type Ca2+ currents were partially blocked by the B1 receptor antagonist [Leu8]des-Arg9-BK (R592) (10(-7) M) and completely reversed by the B2 receptor antagonist D-Arg[Hyp3,D-Phe7,Leu8]BK (R-588) (10(-7) M) or pretreatment with pertussis toxin (PTX). These results demonstrate that BK induced a positive chronotropic effect via stimulation of T- and L-type Ca2+ currents in heart cells mainly via stimulation of B2 receptor coupled to PTX-sensitive G-proteins. The increase of both types of Ca2+ current by BK in heart cells may explain the positive inotropic and chronotropic effects of this hormone.     

Role of adrenoreceptors in the effect of thyroliberin on lymphatic vessels

Formerly we showed that TRH had simulative effect on mesenteric bovine and rat lymphatic vessels (LV) in very low concentration--10(-12)-10(-18) M. In present paper, participation of LV alpha- and beta-receptors in realization of TRH activity on rat mesenteric lymphatic vessels was studied in situ. Propranolol increased the stimulative effect of TRH, isoproterenol exerted an opposite effect. Phentolamine, prazosin eliminated the simulative effect of TRH, yohimbine resulted in additional gain of effect, which seems to testify 1) presynaptic action of TRH or 2) increase of the output of norepinephrine, which is potentiated by alpha 2-adrenoceptor antagonists. Also the participation of adrenergic receptors in positive chronotropic effects of mesenteric rat LV was studied using the method of selective destruction of dopamine-containing neurons after 6-OHDA infusion. As it occurred, desympathization hindered development of stimulating action of TRH. Thus, the efficiency of TRH as a stimulator of LV is connected with activation of adrenergic mechanisms.     

The heart of Daphnia magna: effects of four cardioactive drugs

We used Daphnia magna bioassays to determine the LC(50) and the effects on the heart of the cardioactive drugs ouabain, verapamil, metaproterenol and metoprolol. Distinctions were made between the pharmacological and toxicological effects of these drugs and the adequacy of physicochemical characteristics of its habitat (reconstituted water). Video microscopy and digital image processing were used to study the pharmacological effects on the heart. D. magna exhibited the expected sensitivity to the reference toxicant sodium dodecyl sulfate with a LC(50) of 15.6+/-4.5 mg/l. All drugs were toxic with 48 h-LC(50) of 2.03 mg/l ouabain, 7.04 mg/l verapamil, 32.45 mg/l metaproterenol and 76.21 mg/l metoprolol. Ouabain was the most toxic and caused a positive concentration-dependent inotropic effect. Verapamil caused positive chronotropic and inotropic effects, while metaproterenol showed positive concentration-dependent chronotropic effects at high concentrations (10(-3) and 10(-4) M). Metoprolol induced a positive chronotropic effect at low concentrations (10(-8), 10(-7), 10(-6) M) and a negative chronotropic effect at high concentration (10(-4) M). Ouabain, metaproterenol and metoprolol in D. magna caused similar effects to those produced in mammals. In contrast, verapamil caused opposite effects. The results suggest the presence of Na(+), K(+)-ATPase receptors to verapamil and of non-specific adrenergic receptors in heart of D. magna.     

Localized magnetic resonance spectroscopy measurement of brain lactate during intravenous lactate infusion in healthy volunteers.

Proton magnetic resonance spectroscopy (1H MRS) localized to the left temporal-parietal region in 8 healthy volunteers detected a 2.1-fold +/- 0.7-fold increase (all values +/-SD) in brain lactate during intravenous infusion of 0.5 molar (M) sodium lactate (5 meq/kg over 20 minutes). Significant increases in brain lactate occurred within 5-10 minutes after starting lactate infusion, progressively rose during the infusion, then decreased towards baseline levels during 30 minutes post-infusion. Venous lactate concentration increased from 0.8 +/- 0.2 mM to 10.9 +/- 4.1 mM or 13.6-fold during the infusion. Flow phantom findings in vitro suggest attenuation of 1H MRS blood lactate signal from arteries and veins as a result of flow velocity effects. Correlations between paired blood and brain lactate measurements at each sampling time indicate a non-linear relationship between compartments during lactate infusion.     

Anti-arrhythmic effect of ryanodine in the guinea pig with glycoside poisoning

The effect of ryanodine on membrane potential oscillations in vitro (in isolated guinea-pig papillary muscle) and on ventricular arrhythmias in vivo (in glycoside intoxication) were studied. 3-5 minutes after ryanodine (0.5 microM) addition the membrane potential oscillations induced by ouabain (1 microM) were abolished. 4-5 minutes after intravenous ryanodine infusion (15 micrograms/kg) ventricular arrhythmias induced by ouabain intoxication (75-115 micrograms/kg) disappeared and 8-10 minutes later sinus rhythm was restored. It is suggested that antiarrhythmic effect of ryanodine is a result of the inhibition of diastolic membrane potential oscillations.     

Ligand-independent negative chronotropic responses of rat and mouse right atria to beta-adrenoceptor antagonists

Negative chronotropic effects of beta-adrenoceptor (betaAR) antagonists on right atria from reserpine-treated rats and mice were determined as a test of their inverse agonist activities. BetaAR antagonist ICI-118,551 and nonselective betaAR antagonists alprenolol, propranolol, and timolol produced negative chronotropic effects. In contrast, nonselective betaAR antagonists pindolol and nadolol as well as beta1AR-selective antagonists atenolol, acebutolol, and metoprolol did not cause a significant decrease in atrial rates. The neutral antagonist pindolol but not the inverse agonist alprenolol inhibited the negative chronotropic activities of ICI-118,551. Isoprenaline, salbutamol, and noradrenaline produced positive chronotropic effects; the chronotropic effects of isoprenaline and salbutamol but not of noradrenaline were antagonized by ICI-118,551. It is concluded that both beta1AR and beta2AR mediate positive chronotropic effects of catecholamines on rat and mouse atria but only beta2AR are constitutively active.     

Developmental changes in dopamine modulation of the heart in the isopod crustacean Ligia exotica: reversal of chronotropic effect

Developmental changes in dopamine modulation of the heart were examined in the isopod crustacean Ligia exotica. The Ligia cardiac pacemaker is transferred from the myocardium to the cardiac ganglion during juvenile development and the heartbeat changes from myogenic to neurogenic. In the myogenic heart of early juveniles, dopamine affected the myocardium and caused a decrease in the frequency and an increase in the duration of the myocardial action potential, resulting in negative chronotropic (decrease in beat frequency) and positive inotropic (increase in contractile force) effects on the heart. Contrastingly, in the heart of immature adults just after juvenile development, dopamine caused effects of adult type, positive chronotropic and positive inotropic effects on the heart affecting the cardiac ganglion and myocardium. During the middle and late juvenile stages, dopamine caused individually a negative or a positive chronotropic effect on the heart. These results suggest that the chronotropic effect of dopamine on the Ligia heart is reversed from negative to positive in association with the cardiac pacemaker transfer from the myocardium to the cardiac ganglion during juvenile development.     

Effects of chronic streptozotocin-induced diabetes on the cardiac responses to milrinone

We have studied the effects of milrinone on various cardiac preparations obtained from 6-week streptozotocin diabetic rats. The basal rate of spontaneously beating right atrium from diabetics was significantly lower as compared with controls. Milrinone (5 X 10(-5) to 8 X 10(-4) M) produced a dose-dependent positive inotropic and positive chronotropic effect in left atrium and right atrium, respectively. The positive chronotropic response to milrinone was slightly increased in right atria from diabetic animals. In papillary muscle neither the maximum response nor the pD2 value of milrinone was altered significantly in diabetic animals. The pD2 values of milrinone in right atrium and left atrium were found to be significantly higher in diabetic preparations compared with controls. The data indicate that the responses to milrinone are either unchanged or enhanced in hearts from diabetic animals.     

A 31P-NMR study of some metabolic and functional effects of the inotropic agents epinephrine and ouabain,and the ionophore R02-2985 (X537A) in the isolated,perfused rat heart

1. Some metabolic effects of increased mechanical activity by the Langendorff-perfused rat heart have been characterized using ³¹P-NMR. Mechanical activity was increased by infusion of ouabain (0.9?7.0·10^?5 M), the ionophore R02-2985 (1·10^?5 M) or epinephrine (5·10^?8 M). 2. Similar metabolic changes accompanied infusion of each of the positive inotropic agents into hearts perfused with buffer containing 11 mM glucose as the substrate. In each case phosphocreatine concentrations decreased. During the period of epinephrine infusion the phosphocreatine began to recover its original concentration, although there were no significant changes in mechanical activity. 3. Comparisons of the metabolic changes accompanying the positive inotropic and chronotropic effects of epinephrine were made between hearts perfused with either glucose (11 mM), acetate (5 mM) or lactate (5 mM). A time-dependent decrease in phosphocreatine concentrations also accompanied infusion of epinephrine into hearts perfused with lactate as the sole exogenous substrate, but no statistically significant metabolite changes were observed after identical epinephrine infusions with acetate as the substrate. 4. Calculation of the concentration of free ADP assuming equilibrium in the creatine phosphokinase reaction allows estimation of the cytosolic phosphate potential ($\text{[}\text{ATP}\text{]}\text{[ADP][P}{}_{\mathrm{<mtext>i</mtext>}}\text{]}$), which appears to be dependent on a number of factors, including the nature of the exogenous substrate and the level of mechanical activity. 5. Thus, we conclude that there is no general correlation between the phosphate potential and the mitochondrial respiratory rate in the perfused rat heart.     

Capsaicin-sensitive structures as potential target sites for neurotensin and bradykinin in guinea pig atria

We tested the influence of capsaicin (CAP) desensitization on the positive chronotropic and inotropic effects of neurotensin (NT), bradykinin (BK), calcitonin gene-related peptide (CGRP) and noradrenaline (NA) in guinea pig isolated atria. The positive chronotropic and inotropic effects of NT and BK were completely inhibited, whereas those elicited by CGRP and NA were either slightly reduced (CGRP) or unaffected (NA), in CAP-desensitized compared to control atria. Cross-desensitization studies using CAP, NT and BK showed that the positive chronotropic and inotropic effects of CAP are slightly affected, whereas those evoked by BK are markedly reduced in NT-desensitized atria. On the other hand, the positive chronotropic and inotropic effects of CAP and NT were similar in BK-desensitized and control atria. The results were interpreted as an indication that NT, BK and CAP produce their excitatory effects in guinea pig atria by interacting with a common population of CAP-sensitive sensory nerve fibers (presumably substance P (SP)- and CGRP-containing nerve fibers). The absence of cross-desensitization between NT or BK and CAP, or between NT and BK, suggests that the activation and desensitization of atrial, CAP-sensitive sensory nerve fibers by the latter agents involve different receptors and/or mechanisms.     

The plasma vasoactive intestinal peptide (VIP) response to exercise is increased after prolonged strain, sleep and energy deficiency and extinguished by glucose infusion

The VIP response to a 30 minute bicycle exercise test with a workload of 50% of VO2 max was investigated in young men before and after 5 days of continuous physical activities, almost total sleep deprivation and limited amounts of food. It was shown that plasma VIP increased during physical exercise lasting for more than 20 minutes with a workload of more than 50% of VO2 max. A further increase took place in the early recovery period to a maximum level 5-10 minutes after the exercise. This response to exercise is even stronger after prolonged strain. Glucose infusion during the exercise almost abolished the increase of plasma VIP.     

Interactions between peripheral-type benzodiazepine receptor ligands and an activator of voltage-operated calcium channels.

The effects of the peripheral-type benzodiapine receptor (PBR) ligands Ro 5-4864 and PK 11195 were studied in the spontaneously beating guinea pig atrium and in a model for myocardial ischemia in the rat. In the former, Bay K 8644 produced positive chronotropic and inotropic responses; intracarotid administration of this agonist (5 or 10 micrograms kg-1) to anesthetized rats elicited a transient increase in mean arterial blood pressure accompanied by alterations in the ECG pattern. Ro 5-4864 and PK 11195 (10 microM) completely blocked the positive chronotropic effect of Bay K 8644 in the atrium, PK 11209, a structural analog of PK 11195 with a low affinity for PBR, was inactive, and the central benzodiazepine receptor ligand clonazepam had a marginal effect. Ro 5-4864 potentiated whereas PK 11195 inhibited the myocardial ischemia produced by Bay K 8644 in the rat. Furthermore, PK 11195 blocked the combined response to Bay K 8644 and Ro 5-4864. Addition of Ro 5-4864 (10 microM) to the organ bath potentiated the inotropic effect of Bay K 8644 in the atria; PK 11195 at the same concentration inhibited this effect. Clonazepam and PK 11209 were both inactive in this regard. Nifedipine, a potent calcium channel antagonist, completely blocked the inotropic and chronotropic responses to Bay K 8644. PK 11195 and Ro 5-4864 did not affect this action. These findings strongly suggest that there is a functional association between PBR and voltage-operated calcium channels in the guinea pig atrium and rat cardiovascular system.     

Studies on bradycardia mechanism in the moderately hypothermic dog.

Chronotropic action of isoprenaline on the heart was studied in anesthetized dogs, in euthermic and moderate hypothermic conditions, before and after intravenous administration of atropine and oxprenolol or a cervical bilateral vagotomy. In moderate hypothermia we observed: i) larger duration of the positive chronotropic response to isoprenaline with a delayed and slightly lesser intensity in its maximum; ii) relating to euthermic conditions, delayed but superimposed potentiation of the chronotropic isoprenaline response in atropinized or vagotomized dogs; iii) a small negative chronotropic response to isoprenaline 15 min after oxprenolol, that diminished after atropine; iiii) oxprenolol induced a marked bradycardia nearly twice as intense as in euthermic dogs, almost completely blocked subsequently by atropine. It is concluded that progressive bradycardia in the moderately hypothermic dog is due, among other factors, to a cholinergic action but not to a lesser ability of beta-adrenergic cardiac effectors to chronotropic responses.     

Delta sleep-inducing peptide as a modulator of mediators acting on the heart

Experiments on 51 isolated rabbit hearts have documented, that delta sleep-inducing peptide (6 X 10(-6) M/l) has a modulating effect on the mediators influencing the heart. This peptide enhances negative chronotropic effect of acetylcholine (1 X 10(-6) M/l) and decreases positive chronotropic effect of noradrenaline (1 X 10(-6) M/l). Such effect may be one of mechanisms of changes in the extracardiac regulation on the heart influenced by this peptide.     

Chronotropism and blood flow patterns following teratogenic doses of catecholamines in 5-day-old chick embryos.

In vivo heart rates of 5-day-old chick embryos were recorded from electrodes placed in close proximity to the heart. L-epinephrine (4X10(-10) mole), 1-norepinephrine (1X10(-9)mole) and 1-isoproterenol (1.6X10(-10)mole) in 5 microliter of isotonic saline transiently accerlerated the mean heart rate by almost 9 percent. L-phenylephrine (2X10(-9)mole/5microliter) and the experimental procedure produced no appreciable effect. The positive chronotropic effect of the catecholamines was found to be highly significant (P less than 0.0005) as computed by Student's t test. However, no direct relationship could be established between the chronotropic response and the aortic arch anomalies produced. A prolonged reduction of blood flow in the primitive heart tube and the sixth aortic arch after administration of epinephrine and isoproterenol is apparently related to the induction of hypoplastic right pulmonary artery with absent or hypoplastic right ductus arteriosus.