Enhanced chronotropic and inotropic responses of rat myocardium to cholinergic stimulus with aging.

The ability of the heart to respond to adrenergic stimulation diminishes with aging, and this may be one of the factors contributing to the age-associated decline in cardiac stress responsiveness. On the other hand, little is known about the impact of aging on the responsiveness of the heart to cholinergic stimulation. In this study, we determined the chronotropic and inotropic responses of the isolated, Langendorff-perfused hearts from adult (6-8 months) and aged (28-30 months) rats to cholinergic agonists so as to assess age-related alterations in postsynaptic cholinergic control of heart function. The results showed the following. (i) In isolated perfused spontaneously bearing rat hearts, the negative chronotropic response to acetylcholine (10(-9)-10(-5) M) was up to 4-fold greater in the aged compared with adult hearts; this age-related difference was less marked (2-fold) but not abolished in the presence of a maximally effective concentration (5 microM) of the cholinesterase inhibitor eserine. (ii) The cholinesterase-resistant agonist carbachol (10(-9)-2.5 x 10(-6) M) elicited a 2- to 3-fold greater negative chronotropic response in the aged compared with adult hearts. (iii) In isolated perfused, electrically paced (4 Hz) rat hearts, carbachol (10(-9)-10(-5) M) elicited a concentration-dependent negative inotropic response, which was 2-fold greater in the aged compared with adult heart at all carbachol concentrations. (iv) Acetylcholinesterase activities (micromoles per gram per hour) were 50-60% lower in the aged atria (83 +/- 21) and ventricles (24 +/- 6) than in adult atria (210 +/- 20) and ventricles (47 +/- 7).(ABSTRACT TRUNCATED AT 250 WORDS)     

The inotropic and chronotropic responses of the guinea pig and dog myocardium to isoprenaline and salbutamol.

The relative effects of isoprenaline and salbutamol on the inotropic and chronotropic responses of the denervated myocardium of the chloralose anesthetized dog and of the isolated guinea pig atrium, and the inotropic response of the isolated dog papillary muscle were studied. Both the in vivo dog heart and the in vitro guinea pig atrium displayed a similar relative response pattern to isoprenaline and salbutamol with regard to their inotropic and chronotropic responses. However, a comparison of the relative inotropic responses of the dog heart in vivo and in vitro showed that in vitro, salbutamol has a much lower affinity and efficacy for the adrenergic receptors than isoprenaline.     

Temperature and adrenoceptors in the frog heart

1. Cardiac adrenergic receptors in a frog, Rana tigrina, were examined in winter and summer months using isolated atria preparation maintained at 24 degrees, 14 degrees and 6 degrees C. Treatments included an examination of the atrial responses to selective alpha and beta adrenergic agonists (phenylephrine and isoproterenol respectively) and antagonists (phentolamine and propranolol). 2. Basal atrial beating rates differed between summer and winter months and increased with temperature. 3. Phenylephrine produced dose-dependent increases in the atrial beating rate and tension in the winter frogs only at 6 degrees C. These increases were blunted by phentolamine. 4. Isoproterenol produced positive chronotropic effects of 14 degrees and 24 degrees C but not at 6 degrees C in both summer and winter frogs; these effects were abolished by propranolol. Further, at 6 degrees C, the contractile response of the atrial tissue to isoproterenol was very sensitive. 5. Data suggests that the alpha adrenoceptor might be physiologically important to the frog in the low temperature environment of the cold season, during which period the cardiac beta adrenergic activity would be minimal or even absent.     

Use of echocardiography in synchronous evaluation of the inotropic and chronotropic activities of the heart

It is revealed as possible to use mathematical analysis of the cardiac rhythm according to the readings of echocardiography (ECG). It is advisable to use echocardiography in synchronous determination of the inotropic and chronotropic heart activity in donosologic diagnostics of the condition of the cardiovascular system and in estimation of an adaptive degree of the human organism to the environmental conditions.     

Simulated microgravity produces attenuated baroreflex-mediated pressor, chronotropic, and inotropic responses in mice

Whether myocardial contractile impairment contributes to orthostatic intolerance (OI) is controversial. Accordingly, we used transient bilateral carotid occlusion (TBCO) to compare the in vivo pressor, chronotropic, and inotropic responses (parts 1 and 2) to open-loop selective carotid baroreceptor unloading in anesthetized mice. In part 3, in vitro myocyte responses to isoproterenol in mice exposed to hindlimb unweighting (HLU) for approximately 2 wk were determined. Heart rate (HR) and mean arterial pressure (MAP) responses to TBCO were measured. In control mice, TBCO increased HR (15 +/- 2 beats/min, P < 0.05) and MAP (17 +/- 2 mmHg, P < 0.05). These responses were markedly potentiated in denervated control (DC) mice, in which the aortic depressor nerve and sympathetic trunk were sectioned before measurement. Baroreflex responses to TBCO were eliminated by blockade with hexamethonium bromide (10 microg/kg). In HLU (denervated) mice, HR and MAP responses were reduced approximately 70% compared with DC mice. In part 2, myocardial contractile responses to TBCO were measured with a left ventricular micromanometer-conductance catheter. TBCO in DC mice increased the slope of the end-systolic pressure-volume relation (end-systolic elastance) by 86 +/- 13%. This inotropic response was attenuated (14 +/- 10%, P < 0.005) after HLU. In part 3, contractile responses to isoproterenol were impaired in myocytes isolated from HLU mice. In conclusion, selective carotid baroreceptor unloading stimulates HR, blood pressure, and myocardial contractility, and HLU attenuates each response. These findings have important implications for the management of OI in astronauts, the elderly, and individuals subjected to prolonged bed rest.     

Negative chronotropic effects of thyroxine on the isolated bullfrog heart

An acute application of L-thyroxine (T4) to everted sinus-atrium preparations of bullfrog showed negative chronotropic effects consisting of two processes: an initial peak at 2-5 min and a delayed plateau at 40-60 min. The initial effect was blocked by Cd2+ and tetraethyl-ammonium (TEA). The delayed effect was suppressed by Cd2+ and NaCN.     

Naloxone's effect on the inotropic and chronotropic responses of isolated, electrically stimulated or spontaneously beating rat atria

Experiments were conducted to determine (i) how naloxone administration alone could modify the inotropic (in electrically stimulated (ES) rat atria) and both the inotropic and chronotropic responses (in spontaneously beating (SB) rat atria) isolated from normotensive and hypotensive (hemorrhaged) rats, and (ii) how naloxone administration would modify the inotropic and chronotropic responses of isolated rat atria previously administered an opiate agonist (morphine), a muscarinic agonist (carbachol), or an alpha- and beta-adrenergic agonist (noradrenaline). Naloxone (51-340 microM) added to ES atria caused a delayed but dose-related decrease in atrial tension (AT), whereas in SB atria, naloxone caused atrial heart rate (AHR) to fall and atrial tension (AT) to increase. Naloxone (68-340 microM), given to SB atria from acutely hypotensive rats, caused a similar increase in atrial tension as seen in the "normotensive" isolated (SB) atria and a similar decrease in atrial heart rate. Morphine sulphate (MS), 37-375 microM, administered to ES atria caused a delayed fall in AT; which was further decreased when naloxone (340 microM) was also added. In the SB atria, morphine caused a dose-related decrease in atrial heart rate whereas atrial tension increased. In SB preparations, atrial heart rate fell even further when naloxone was added to morphine compared with when morphine sulphate was given alone, whereas atrial tension was increased. Noradrenaline (3 or 12 microM) caused a positive, dose-related inotropic response in the ES atria, effects not influenced by the addition of naloxone. In the SB atria, naloxone caused no change in the dose-related increases in atrial tension and heart rate when combined with the lower dose of noradrenaline but decreased AT when combined with 12 microM noradrenaline, compared with when this dose of noradrenaline was given alone. Carbachol (683 nM-1.37 microM) caused a dose-related decrease in atrial tension in ES atria, which was reversed completely by the addition of naloxone. In SB atria, carbachol decreased both atrial tension and heart rate, and with the addition of naloxone (340 microM), a further slight drop in atrial heart rate occurred, but concurrently, a marked rise in atrial tension was observed. The results indicate that naloxone can act with receptors directly within atrial tissue to cause changes in atrial tension and heart rate. The comparable delayed responses of morphine and naloxone suggest their effects are mediated by nonopiate receptors which, in time, cause decreases in calcium influx into the atria.(ABSTRACT TRUNCATED AT 250 WORDS)     

The in vitro chronotropic and inotropic effects of vasoactive intestinal peptide (VIP) on the atria and ventricular papillary muscle from Cynomolgus monkey heart

The in vitro chronotropic and inotropic effects of vasoactive intestinal peptide (VIP) and of isoproterenol, two agents known to stimulate cardiac adenylate cyclase were compared on the heart from Cynomolgus monkey using the spontaneously beating right atrium, the electrically stimulated left atrium, and the electrically-stimulated ventricular papillary muscle. VIP increased concentration-dependently the rate of beating of the right atrium as well as the contractility of both atria but its efficiency was lower than that of D,L-isoproterenol. VIP also stimulated concentration-dependently, and this time as efficiently as D,L-isoproterenol, the contractility of papillary muscle. These VIP effects were unaltered by the neuronal blocker tetrodotoxin. In addition, the moderate inhibition exerted by the beta-adrenergic blocker D,L-propranolol on VIP effects argued against the implication of beta-adrenergic receptors in VIP effects. These results indicate that VIP exerts a direct stimulatory influence on the rate and contractility of Cynomolgus monkey heart.     

Peptides from the N-terminal domain of chromogranin A (vasostatins) exert negative inotropic effects in the isolated frog heart

The negative inotropic effects of synthetic peptides derived from the N-terminus of chromogranin A (CgA) were studied in an avascular model of the vertebrate myocardium, the isolated working frog heart (Rana esculenta). The peptides were frog and bovine CgA(4-16) and CgA(47-66), and bovine CgA(1-40) with (CgA(1-40SS)) and without an intact disulfide bridge (CgA(1-40SH)). Under basal cardiac conditions, four of the peptides caused a concentration-dependent negative inotropism that was comparable to the negative inotropy reported for human recombinant vasostatin I (CgA(1-78)) and bovine CgA(7-57). By comparison of the structural characteristics of the bovine and frog sequences with their minimally effective concentrations ranging from 68 to 125 nM of peptide, the results were consistent with the natural structure (CgA(17-38SS)) being essential for the negative inotropism. In addition, the partial sequences of the frog and bovine vasostatin I were effective in counteracting the characteristic positive inotropism exerted by isoproterenol (1 nM) at minimally effective concentrations ranging from 45 to 272 nM. Taken together, these results extend the first evidence for a cardiosuppressive role of the N-terminal domain of chromogranin A known for its co-storage with catecholamines in the sympathoadrenal system of vertebrates.     

The inotropic action of acetylcholine on the heart ventricles during larval development in the frog Rana temporaria

The effects of acetylcholine and other cholinergic drugs on the isolated electrically driven larval frog ventricles have been studied. The negative inotropic response to acetylcholine appeared as early as stage 33 of the larval development (the stages were determined according to Dabagian and Sleptsova, 1975) and persisted through all the developmental stages including metamorphosis. The response is muscarinic in origin since it was reproduced with a muscarinic agonist methylfurmetide, blocked with atropine but was not modified with tubocurarine. At the stage 41 and following stages, the sensitivity to acetylcholine was decreased while to methylfurmetide was not. The decreased sensitivity to acetylcholine is most likely due to increase of activity of cholinesterases in the myocardial tissues.     

Physiological effects and biochemical properties of a serum protein that produces positive inotropic and chronotropic effects on isolated guinea pig atria

Isolated left and right guinea pig atria were used as a bioassay for the detection of an endogenous cardioactive substance in bovine serum. Serum, buffer exchanged to Krebs-Henseleit solution, produced positive inotropic and chronotropic effects on the isolated guinea pig atria. The cardiotonic effects were unaffected by the combined presence of propranolol and methysergide (both 10(-6)M) and were also dissimilar in time course from other known cardiotons such as catecholamines and cardiac glycosides. Following ultrafiltration (using XM100A Amicon membranes), activity was found solely in the retentate fractions and was therefore probably due to a large molecular weight (> 100 kDa) substance or a small molecule bound to a large protein. The cardioactive factor (CF) in the whole serum was heat labile, sensitive to acidification, exposure to potassium bromide and equilibration to physiological buffers of a low ionic strength. Isolation by conventional protein purification techniques was unsuccessful due to the labile nature of the active molecule(s) when exposed to non-physiological experimental conditions. Physical and biochemical properties of the CF which may help avoid inactivation are discussed for future experiments aimed at elucidating the nature and identity of the cardiotonic principle.     

In vitro quantification by image analysis of inotropic and chronotropic effects of drugs on cultures of cardiac myocytes

Quantitative image analysis is used to measure the inotropic and chronotropic effects of drugs on cultured heart cells maintained at 37°C on the stage of an inverted light microscope, and sequentially superfused with control and treatment media. The beating of the cardiac myocytes is evaluated by simultaneously selecting up to eight areas, including cell edges, from digitized video image. The sizes and positions of these areas are controlled by the operator. To analyze the motion of cell edges in each area, the computer measures the shift of the mass center of pixels' grey levels. Finally, a few parameters are calculated for the eight areas and displayed graphically. In order to assess treatment effects, appropriate statistical tests are performed on the data. Image analysis is an efficient screening test for evaluating the pharmacologic or toxic effects of a substance on isolated or cultured cardiac myocytes from various species.Abbreviations MEM Minimum Essential Medium     

The roles of alpha- and beta-adrenoceptors in the chronotropic responses to norepinephrine in carp heart (Cyprinus carpio)

1. The chronotropic effect of norepinephrine was studied in isolated spontaneously beating atrial preparations of carp (Cyprinus carpio) heart. 2. Norepinephrine, 0.1 microM, caused a positive chronotropic effect, while at 1 microM it caused either a positive or a negative chronotropic effect. The positive chronotropic effect, observed in 13 preparations, was potentiated by phentolamine and almost completely blocked by propranolol. 3. The negative chronotropic effect observed in the other 5 preparations was greater in the presence of propranolol, reduced by phentolamine and not affected by atropine. 4. These results indicate that alpha- and beta-adrenoceptors may coexist, mediating the negative and positive chronotropic effects, respectively, in isolated atrial preparations of carp heart.