Increase In Amplitude Of Contractions Research Articles

The effects of blocking the Na-Ca exchange at intervals throughout the physiological contraction-relaxation cycle of single cardiac myocyte

The method of rapid superfusion of the single isolated ventricular myocytes of guinea-pig heart was used in order to inhibit the Na-Ca exchange throughout the physiological contraction-relaxation cycle. Superfusion of the cell at selected intervals during the contraction with the Na,Ca-free solution resulted in increase in its amplitude, increase in time to peak shortening and in delay of relaxation, albeit the cells relaxed before reperfusion of normal Tyrode solution. The largest increase in amplitude of contraction (to 134 ± 16%) was observed when the effective exchange of the cell's environment was attained ∼ 50 ms after the pulse stimulating contraction. The effects declined promptly when the delay was increased beyond 100 ms. In the cells treated with 10 m m caffeine superfusion with the Na,Ca-free solution after the delay of 50–100 ms resulted in decrease in extent of shortening. Increase in delay resulted in slight increase in extent of shortening with respect to control and strong inhibition of relaxation. The strongest effects were observed when the delay was ∼200 ms. Superfusion of the normal cells and of the cells treated with caffeine between contractions resulted in slight potentiation of the next beat. It is concluded that Na-Ca exchange provides an important mechanism of relaxation and outward Ca 2+ transport in the physiological contraction of the ventricular cardiomyocyte.

The involvement of potassium channels in the action of ciclazindol in rat portal vein.

1. In whole portal veins, ciclazindol (0.3-10 microM) increased the amplitude and duration, but decreased the frequency of spontaneous contractions. Glibenclamide (0.3-10 microM) produced a small increase in contraction amplitude and duration with a small reduction in contraction frequency. 2. In whole portal veins, ciclazindol (1-10 microM) antagonized the relaxant effects of BRL 38227 in a non-competitive manner. Under identical conditions, the effects of glibenclamide (0.3-10 microM) appeared to be competitive. 3. In whole portal veins loaded with 42K, ciclazindol itself (up to 3 microM) had no detectable effect on basal 42K exchange. However, the increase in 42K efflux produced by BRL 38227 (5 microM) was antagonized by ciclazindol (3 microM). Similar effects were produced by glibenclamide (up to 3 microM). 4. In freshly-isolated portal vein cells examined by the whole-cell voltage-clamp technique, ciclazindol (1-100 microM) inhibited the slowly-activating and inactivating transient outward current (ITO) which could be generated at potentials more positive than -30 mV. In addition ciclazindol (1-10 microM) inhibited the non-inactivating K-current (IKCO) induced by BRL 38227 (10 microM). 5. In freshly-isolated portal vein cells under current-clamp conditions, the hyperpolarization produced by BRL 38227 (10 microM) was reversed by ciclazindol (1-10 microM). 6. In porcine brain membrane fragments, glibenclamide (0.65 nM) displaced 50% of the binding of [3H]-glibenclamide whereas ciclazindol (up to 10 microM) had no effect. 7. It is concluded that ciclazindol is a K-channel blocker. Its action is not selective for the channel(s) which carry IKCO but also extends to those which carry ITO.Its inability to displace [3H]-glibenclamide from porcine brain fragments may indicate that antagonism of BRL 38227 by ciclazindol in smooth muscle is exerted at a site different from that of glibenclamide.

Expression and pharmacological characterization of a stimulatory subtype of adenosine receptor in fetal chick ventricular myocytes.

Ventricular and atrial myocytes cultured from chick embryos 14 days in ovo were used as model systems to study cardiac adenosine receptors. In membranes of ventricular cultures, blocking of the A1-adenosine receptor pathway by the A1-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) or by pertussis toxin treatment of the myocyte resulted in a significant adenosine agonist-mediated stimulation of the adenylate cyclase activity. The maximal increases in adenylate cyclase activity caused by the equipotent or the A2-adenosine receptor-selective agonists (from 52.1 +/- 3% to 63 +/- 10% [mean +/- SEM]) were significantly greater than those caused by the A1-selective agonists (from 11 +/- 5% to 34.6 +/- 7%) (p less than 0.01, by t test, n = 4-8). However, in membranes of atrial myocytes, when A1-subtype had been blocked, the various adenosine agonists had no effect on the adenylate cyclase activity. Whether the stimulatory adenylate cyclase-coupled adenosine receptor is also capable of stimulating contractility in the intact ventricular myocyte was next investigated. In ventricular but not in atrial cells, the various adenosine agonists caused an increase in the contractile amplitude in the presence of DPCPX or in myocytes preexposed to pertussis toxin. The increase in contraction amplitude caused by each agonist was expressed as percent of maximum (maximum is the increase in contractility caused by 2.4 mM calcium). In the pertussis toxin-treated myocyte, the maximal increases caused by the equipotent or A2-agonists (NECA, MECA, CV-1808, and CGS21680, from 49.6 +/- 3% to 52.5 +/- 6%, n = 8-12) were significantly greater than those elicited by the A1-agonists (2-CADO, S-PIA, R-PIA, and DCCA, from 12 +/- 4% to 37 +/- 3%, n = 8) (p less than 0.05, by t test). These data demonstrated that a stimulatory adenosine receptor, likely the A2-adenosine receptor, was present on the ventricular but not the atrial myocytes and was linked directly to a stimulation of the cardiac contractility. The functional effects mediated by the A1-subtype became manifested in the presence of isoproterenol, as evidence by an inhibition of the isoproterenol-stimulated increases in adenylate cyclase activity and in cardiac contractility by adenosine agonists. Thus, both subtypes of adenosine receptors, each mediating opposing responses, were present on the ventricular myocytes, whereas only the A1-subtype was found in the atria. The presence of a stimulatory functional A2-adenosine receptor may help explain the absence of a direct negative inotropic response to adenosine in the ventricle.

Reduced beta-agonist sensitivity in single atrial cells from failing human hearts

Human myocytes were isolated from right atrial appendage, and contractile responses to inotropic agents were studied. Responses to inotropic agents of cells isolated from patients with mild heart disease [New York Heart Association (NYHA) classes I and II] were compared with those of myocytes from rabbit atria. Maximally effective concentrations of calcium, forskolin, and isoproterenol increased contraction amplitude to a similar extent (11.9, 11.5, and 11.3% change in cell length, respectively), but histamine produced a smaller effect (7.1%). The maximum responses of rabbit atrial cells to calcium (18.5%) and isoproterenol (15.0%) were significantly greater than human. In human cells, the velocity of contraction or relaxation was accelerated more by isoproterenol (P less than 0.05) or forskolin (P less than 0.01) than by high calcium. Only relaxation velocity was increased by isoproterenol in rabbit cells (P less than 0.05). Rabbit myocytes contracted and relaxed 10-30% faster than human (P less than 0.05). Cells from the atria of patients with New York Heart Association (NYHA) heart failure class III or IV were less responsive to isoproterenol than those from class I or II (P less than 0.01). Omitting data from patients who had been taking calcium-channel blockers or beta-adrenoceptor agonist or antagonist drugs did not affect the conclusions. Analysis of variance revealed a significantly greater between-patient than within-patient variation (P less than 0.001), indicating that cells from the same patient have a tendency to respond in a similar way. Responses to high calcium did not differ among NYHA classes. The effect of forskolin was not reduced in NYHA class III, although there was a decreased response in cells from two patients in NYHA class IV.(ABSTRACT TRUNCATED AT 250 WORDS)

Coregulation of calcium channels and beta-adrenergic receptors in cultured chick embryo ventricular cells.

To examine mechanisms whereby the abundance of functional Ca channels may be regulated in excitable tissue, Ca channel number was estimated by binding of the dihydropyridine (DHP) antagonist 3H (+)PN200-110 to monolayers of intact myocytes from chick embryo ventricle. Beta adrenergic receptor properties were studied in cultured myocytes using [3H]CGP12177, an antagonist ligand. Physiological correlates for alterations in DHP binding site number included 45Ca uptake and contractile response to (+)BAYk 8644, a specific L-type Ca channel activator. All binding and physiological determinations were performed in similar intact cell preparations under identical conditions. 4-h exposure to 1 microM isoproterenol reduced cell surface beta-adrenergic receptor number from 44 +/- 3 to 17 +/- 2 fmol/mg (P less than 0.05); DHP binding sites declined in number from 113 +/- 25 to 73 +/- 30 fmol/mg (P less than 0.03). When protein kinase A was activated by a non-receptor-dependent mechanism, DHP binding declined similarly to 68% of control. Exposure to diltiazem, a Ca channel antagonist, for 18-24 h had no effect on number of DHP binding sites. After 4-h isoproterenol exposure, 45Ca uptake stimulated by BAYk 8644 declined from 3.3 +/- 0.2 nmol/mg to 2.9 +/- 0.3 nmol/mg (P less than 0.01) and BAYk 8644-stimulated increase in amplitude of contraction declined from 168 +/- 7 to 134 +/- 11% (P = 0.02). Thus, elevation of [cAMP] in myocytes is associated with a time-dependent decline in Ca channel abundance as estimated by DHP binding and a decline in physiological responses that are in part dependent on abundance of Ca channels. Binding of a directly acting Ca channel antagonist for 18-24 h does not modulate the number of DHP binding sites.

Pharmacodynamics of BAY K 8644 Alone and in Combination with Dobutamine in the Isolated Rabbit Heart

The haemodynamic effects of the Ca-agonist BAY K 8644 alone and in combination with the selective beta-I-adrenoceptor agonist dobutamine were studied in the isolated rabbit heart. BAY K 8644 produced a concentration-dependent increase in contraction amplitude and shortening velocity, oxygen consumption and heart rate. In combination with a small fixed concentration of dobutamine (40 nM), Bay K 8644 produced similar alterations. When Bay K 8644 was infused at a fixed concentration (38 nM), dobutamine likewise produced similar increments in contraction amplitude, shortening velocity and heart rate, whereas oxygen consumption was considerably argumented. Both BAY K 8644 and dobutamine showed definite positive inotropic effects in the isolated rabbit heart. Combination of the two drugs did not yield a stronger positive inotropic effect than that seen on single drug administration, and oxygen consumption was even increased.

Effects of Low Density Lipoproteins and Mevinolin on Sympathetic Responsiveness in Cultured Chick Atrial Cells: Regulation of β-adrenergic receptors and αs

We have previously demonstrated that cultures of myocytes from embryonic chick atria grown in media supplemented with fetal calf serum from which lipoproteins have been removed demonstrate a nearly 10-fold increase in sensitivity of beating to the muscarinic cholinergic agonist carbamylcholine compared with cells grown with control medium. This increased response to carbamylcholine was associated with a 1.4-fold increase in total cell cholesterol, a 2-fold increase in the number of muscarinic receptors which bind agonist with high affinity, and a 2-fold increase in the levels of the alpha subunits of Go and Gi (Haigh, L. S., Leatherman, G. F., O'Hara, D. S., Smith, T. W., and Galper, J. B. (1988) J. Biol. Chem. 263, 15608-15618). In the studies reported here, we determined the responsiveness of cells grown in lipoprotein-depleted serum (LPDS) to beta-adrenergic stimulation. Isoproterenol stimulated a contractile response of 58% measured as an increase in amplitude of contraction with a half-maximal effect at 3 x 10(-7) M for cells grown in fetal calf serum, but had no significant effect on amplitude of contraction on cells grown in LPDS. In cells grown in media supplemented with fetal calf serum, isoproterenol (1 x 10(-3) M) stimulated adenylate cyclase activity 100% over basal with an EC50 of 7 x 10(-6) M compared with an increase of 32% in cells grown in media supplemented with LPDS. beta-Adrenergic receptor number as measured by the binding of 125I-pindolol decreased from 24 +/- 3 (+/- S.E., n = 6) fmol/mg protein in cells grown under control conditions to 12 +/- 2 (n = 6) fmol/mg protein in media supplemented with LPDS. The level of alpha s as measured both by ADP-ribosylation with cholera toxin in the presence of 32P-NAD and by immunoblotting with specific antibody to alpha s decreased by 3-fold in cells grown in media supplemented with LPDS compared with control. All of these effects of growth of cells in LPDS were reversed by incubating cells with LPDS plus 30 microM mevinolin, an inhibitor of endogenous cholesterol synthesis. These studies indicate that growth of cells in media supplemented with LPDS results in a coordinate decrease in the levels of beta-adrenergic receptors and alpha s. Taken together with our previous studies these data support the hypothesis that the receptors and guanine nucleotide-binding proteins which mediate sympathetic and parasympathetic responsiveness in the heart are reciprocally regulated.

The effects of metabolic inhibition on intracellular calcium and pH in isolated rat ventricular cells.

1. Intracellular calcium concentration [( Ca2+]i) and pH (pHi) were measured in single, isolated rat ventricular myocytes using, respectively, the fluorescent indicators Fura-2 and BCECF (2',7'-bis(carboxyethyl-5(6)-carboxyfluorescein). Contraction was measured simultaneously. The intracellular calibration of BCECF is demonstrated. In a HEPES-buffered bathing solution of pH 7.4, pHi had a mean value of 7.16 +/- 0.05 (mean +/- S.E.M.). 2. Addition of NH4Cl (5-20 mM) produced an intracellular alkalosis that was associated with an increase of contraction amplitude. Removal of NH4Cl produced an acidosis and decrease of contraction. 3. The addition of 2 mM-cyanide (CN-) to inhibit oxidative phosphorylation had variable effects on contraction amplitude. Changes of contraction amplitude could largely be accounted for by changes in the systolic Ca2+ transient. 4. CN- addition increased lactic acid production. However, in the majority of experiments, this was not accompanied by an intracellular acidosis. 5. Anaerobic glycolysis was inhibited by either removal of glucose, addition of deoxyglucose, or addition of iodoacetate. Under these conditions the application of CN- decreased systolic [Ca2+]i and contraction amplitude. This was sometimes preceded by a transient increase of systolic [Ca2+]i and contraction amplitude. 6. When glycolysis was inhibited, the subsequent addition of CN- always increased diastolic [Ca2+]i and produced a contracture. The increase of [Ca2+]i occurred before the contracture. However, once the contracture had developed, decreasing [Ca2+]i (by removal of external Ca2+) did not cause relaxation. 7. With glycolysis inhibited, addition of CN- resulted in a large (0.51 +/- 0.05 pH unit) acidosis that was sometimes preceded by an alkalosis. This acidosis was unaffected by removal of external Ca2+ or external alkalinization. Calculations show that some of this acidosis may result from protons released by ATP hydrolysis. 8. If the acidosis produced by metabolic blockade was partly reversed by adding NH4Cl then a contracture immediately developed. This suggests that the acidosis delays the onset of the contracture. 9. We conclude that metabolic inhibition increases diastolic [Ca2+]i. The accompanying acidosis prevents contraction. Once the contracture has developed it is maintained by factors other than increased [Ca2+]i, possibly by a fall of [ATP].

Stress induces alteration of esophageal pressures in healthy volunteers and non-cardiac chest pain patients

The present study was designed to explore the relationship between psychological stress and esophageal motility disorders. Nineteen non-cardiac chest pain patients (10 with the nutcracker esophagus and nine with normal baseline manometry) and 20 healthy control subjects were administered two acute stressors: intermittent bursts of white noise and difficult cognitive problems. The results indicated that the esophageal contraction amplitudes and levels of anxiety-related behaviors of non-cardiac chest pain patients and control subjects were significantly greater during the stressors than during baseline periods. All patients demonstrated significantly greater (P less than 0.01) increases in contraction amplitude and anxiety-related behavior during cognitive problems than during the noise stressor. The nutcracker esophagus patients showed a greater increase in contraction amplitude during the problems (23.50 +/- 9.42 mm Hg, X +/- SE) than control subjects (P less than 0.01), while the amplitude changes of chest pain patients with normal baseline manometry were not significantly greater than that of control subjects (9.00 +/- 1.91 mm Hg). The present results identified an increase in contraction amplitude as the primary esophageal response to stress. The possible interaction of esophageal contraction abnormalities, psychological stress, and the perception of chest pain is discussed.

Intravital microscopy: a new in vivo technique for visualizing and quantifying effects of regulatory peptides on choledochoduodenal junction motility

Using intravital microscopy, we studied the in vivo effects of regulatory peptides on choledochoduodenal junction motility in guinea pigs. During basal and hormone-stimulated periods, intravital microscopy documented rhythmic, asymmetrical, “milking” contractions of the sphincter ductus choledochi (SDC) which occurred independent of sphincter ampullae (SA) contractions or were followed by SA contractions. Cholecystokinin octapeptide (CCK-8) (≥ 0.01 μg/kg) increased the frequency of SDC contractions and at higher doses (≥ 0.1 μg/kg) increased the frequency of SA contractions. Pentagastrin (≥ 1.0 μg/kg) and secretin (10 μg/kg) decreased the contraction frequencies of both sphincters. Biliary manometry demonstrated similar effects of these peptides on the frequency of the SDC and SA contractions, but also showed that CCK-8 (0.1 μg/kg) increased the amplitude of SDC and SA contractions while pentagastrin (1 μg/kg) decreased the amplitude of only SDC contractions. Tetrodotoxin and atropine did not affect hormone-induced changes in frequency, but tetrodotoxin reduced the increase in amplitude of contraction caused by CCK-8. We concluded that intravital microscopy provides a sensitive, in vivo technique to visualize and quantify the complex motility of a small structure like the choledochoduodenal junction.

Actions of the novel gastrointestinal prokinetic agent cisapride on equine bowel motility.

The effect of cisapride was evaluated on the normal fasting bowel motility of four ponies with chronically implanted electromechanical transducers. Cisapride was infused over 60-min periods at 0.05 mg/kg (n = 4), 0.1 mg/kg (n = 5) and 0.25 mg/kg (n = 5). It produced marked and prolonged increases in electrical and mechanical activity at all sites examined. In the stomach there was increased total contraction activity with increased contraction amplitude and a slight reduction in rate. In the small intestine there was an increase in irregular (phase II) activity with an increase in number and amplitude of contractions and a decrease in the number of regular (phase III) activity fronts. There was a decrease in the number of phase III fronts that spread distally from the jejunum to the ileum. The phase II activity was coordinated temporally with prolonged activity in the stomach. Cisapride increased electrical and contractile activity in the left dorsal colon with increased contraction amplitude and an increase in electrical activity in the small colon. In the stomach and small intestine cisapride produced dose-dependent increases in activity but in the left dorsal and small colon the intermediate dose (0.1 mg/kg) produced the largest and most consistent responses. Side-effects observed were increased bowel sounds and frequency of defaecation, a slight increase in heart rate and transient signs of discomfort at the highest (0.25 mg/kg) dose rate.

Extracellular ATP has a potent effect to enhance cystolic calcium and contractility in single ventricular myocytes

The effect of extracellular ATP on the contraction of single rat cardiac myocytes was investigated, together with the effect on the transient change in cytosolic Ca 2+ (Ca i) elicited by excitation and on the relationship between these two parameters. In unstimulated single myocytes, ATP caused a small increase in Ca i (measured as the ratio of fluorescence of indo-1 at 410 to that at 490 nm. In myocytes bathed in a medium containing 1.0 mM [Ca 2+] at 23°C and stimulated at 1 Hz, ATP (1 μM) resulted in a two-threefold increase in amplitude of contraction, as measured by video cinemicrographic techniques. The duration of the Ca i-transient was not altered but its amplitude was markedly enhanced, as was the amplitude of contraction. The relation between Ca i and contraction-amplitude was not altered by ATP, when measured over a range of extracellular [Ca 2+], suggesting that ATP does not affect the myofilament-Ca 2+ interaction. The primary site of action of ATP in increasing Ca i is at the sarcolemma since the addition to suspensions of myocytes of caffeine (10mM), which depletes the sarcoplasmic reticulum Ca 2+ load, does not prevent the subsequent increase of Ca i due to ATP. Further, lowering of the extracellular [Ca 2+] to less than 1 μM with EGTA abolishes the response of Ca i to ATP, though not the response to caffeine. Thus in rat cardiac myocytes ATP stimulates trans-sarcolemmal influx of Ca 2+: ADP, AMP and adenosine are ineffective. ATP markedly augments the amplitude of the Ca i transient elicited by electrical stimulation thus rendering it a potent inotropic agent.

Effects of ryanodine and caffeine on contractility, membrane voltage, and calcium exchange in cultured heart cells.

To investigate the mechanisms of action of ryanodine and caffeine, changes in mechanical and electrical activity caused by these agents were correlated with alterations in 45Ca fluxes and cell Ca contents in chick embryo ventricular cell monolayer cultures. Ryanodine (10(-10)-10(-5) M) irreversibly decreased contraction amplitude by 10-70% relative to control in a concentration-dependent manner with minimal effects on electrical activity. Ryanodine caused a slight decrease in rapid 45Ca uptake, but no change in total exchangeable calcium content or rapid 45Ca efflux. Caffeine (1-20 mM) caused a transient (less than 10 seconds) 5-12% increase in contraction amplitude followed by a sustained 9-76% decrease in contraction amplitude and a 10 mV decrease in diastolic membrane voltage. Caffeine caused a decrease in rapid 45Ca uptake, a decrease in total exchangeable calcium content, and an increase in rapid 45Ca efflux. These results suggest that caffeine produces a decrease in sarcoplasmic reticulum (SR) Ca2+ uptake, and/or an increase in SR Ca2+ release that eventually depletes the SR of Ca2+, presumably accounting for the negative inotropic effect. The ryanodine effects on contraction are more difficult to account for solely in terms of alterations of transsarcolemmal Ca2+ fluxes and Ca2+ contents. Our data indicate an important role for the SR in excitation-contraction coupling in cultured chick embryo ventricular cells and suggest that SR Ca2+ is part of the rapidly exchanging Ca2+ compartment noted in 45Ca flux studies.

Mechanism of the positive inotropic effect of milrinone in cultured embryonic chick ventricular cells

Milrinone, a potent positive inotropic and vasodilating agent, has shown promise in the clinical treatment of congestive heart failure, but significant controversy about its mechanism of action exists. To approach these mechanistic problems in a non-innervated, non-diffusion-limited system, the effects of milrinone on cultured embryonic chick ventricular cells were examined. At 37 degrees C in physiologic buffer, milrinone produced a rapid, concentration-dependent increase in amplitude of contraction that was 45% of the maximum increment in contraction produced by elevated extracellular calcium; the EC50 was 8 microM. This peak response was quantitatively similar to the contractile response produced by isobutyl methylxanthine, a potent phosphodiesterase inhibitor. Milrinone inhibited 70% of total phosphodiesterase activity of cultured ventricular cells with an EC50 of 11 microM. Exposure to 1 X 10(-4) M milrinone resulted in rapid increase in cyclic AMP content to levels greater than 100% above control within 4 min. The same concentration also produced a 43% increase in the rate of transsarcolemmal 45Ca uptake. The stimulation of 45Ca uptake rate was similar to the response produced by 1 microM isoproterenol and could be completely abolished by 10 microM verapamil. Thus, in cultured embryonic chick myocardial cells, the positive inotropic effect of milrinone is largely, if not entirely, attributable to phosphodiesterase inhibition, leading to intracellular cyclic AMP accumulation and stimulation of transsarcolemmal calcium influx via the slow calcium channel.

Effects of cisapride on the motility of the digestive tract in dogs and guinea pigs

Effects of cisapride on the motility of the digestive tract in vivo in dogs and the guinea-pig intestine in vitro were studied. Cisapride (0.05-2.0 mg/kg, i.v.) produced an increase in amplitude of spontaneous contractions and basal tone in the stomach, duodenum, jejunum and proximal and distal colon in dogs. In some animals, however, it induced an inhibition with decrease in amplitude and tone. It also induced an increase in amplitude of contractions in the gallbladder and the sphincter of Oddi in dogs. The tone of the gallbladder was elevated by the same dose of cisapride, but the tone of the sphincter of Oddi was decreased. The drug produced a reverse response in some animals. These excitatory responses to cisapride were abolished by atropine (0.2 mg/kg, i.v.). Motility of the guinea-pig isolated ileum and colon was enhanced with an increase in their amplitude of contractions and basal tone at low concentrations of cisapride (10(-9)-10(-6)M) but it was inhibited at higher concentrations (10(-5)-10(-4)M). Atropine abolished the excitatory response of the ileum to cisapride in all cases. It abolished the excitation of the colon in some preparations but reduced only in some degree in the other. The inhibitory effect of cisapride on isolated preparations was unaffected by tetrodotoxin. From these results, it is concluded that cisapride enhances motility of the gastrointestinal tract and biliary tract by acting on myenteric cholinergic neurons and inhibits it by acting on the smooth muscle itself.

Modulation of gallbladder motility by intrinsic cholinergic neurons.

The mechanical contraction and release of acetylcholine (ACh) were investigated by use of an isolated guinea pig gallbladder preparation subjected to transmural field stimulation. An increase in contraction amplitude was found to be related to the frequency (5-60 Hz) of applied stimulus. Treatment with tetrodotoxin and atropine prevented these electrically induced contractions. A simultaneous increase in the release of ACh was noted with electrical stimulation or depolarization by veratrine hydrochloride, and this increase was susceptible to blockade by tetrodotoxin. Neither contractions nor ACh release could be detected with stimulation at 0.1 Hz. When field stimulation was applied to contracted gallbladder induced by a supramaximal concentration of cholecystokinin octapeptide (CCK-8), a further increase in muscle tone was observed. The magnitude of this increase was frequency dependent. Although the onset of response to field transmural stimulation was relatively more rapid, the maximal response attained was only 70% of that elicited by CCK-8. When tissues were exposed to 2.2 X 10(-6) M CCK-8, the muscle contracted maximally but there was no increase in the efflux of [3H]ACh. These data provide a pharmacological characterization of the intrinsic gallbladder cholinergic neurons and demonstrate a possible role they may play in the control of gallbladder motility.

Effects of verapamil on contraction and relaxation of cultured chick embryo ventricular cells during calcium overload

The calcium channel blocking agents verapamil and nifedipine have been reported to lessen abnormalities of left ventricular relaxation in patients with severe left ventricular hypertrophy or coronary artery disease. Whether these effects in human beings are related in part to a direct effect on myocardial calcium metabolism is difficult to determine because of complicating drug influences on ventricular loading via systemic arterial vasodilation, on myocardial blood supply via coronary artery vasodilation and on reflex changes in sympathetic tone. For this reason, the effects of verapamil were investigated in a cellular model of impaired relaxation using spontaneously contracting tissue cultured monolayers of chick embryo ventricular cells exposed to high external calcium ([Ca]o). Under control conditions ([Ca]o, 0.9 mM), verapamil (2 X 10(-8)M) induced a 57 +/- 8% decrease in amplitude of cell contraction monitored with a phase contrast microscope video motion detector system. Elevation of [Ca]o from 0.9 to 8.0 mM resulted in a decrease in amplitude and velocity of contraction and a decrease in velocity of relaxation associated with an upward shift in diastolic cell wall position, suggesting a failure of normal myofilament dissociation. These abnormalities were completely reversible on reperfusion with [Ca]o, 0.9 mM. On re-exposure of the cells to [Ca]o, 8.0 mM, in the presence of verapamil, there was an increase in amplitude of contraction (0.56 +/- 0.11 to 1.03 +/- 0.09 micron, p less than 0.01) and velocity of relaxation (4.97 +/- 0.89 to 9.94 +/- 0.87 micron/s, p less than 0.01) compared with exposure to [Ca]o, 8.0 mM, alone, and an attenuation of the upward shift in diastolic cell wall position.(ABSTRACT TRUNCATED AT 250 WORDS)

Proctolin and an Endogenous Proctolin-Like Peptide Enhance the Contractility of the Limulus Heart

ABSTRACT Synthetic proctolin increases the force but not the rate of heart contractions of Limulus in a time- and dose-dependent manner. The threshold of this effect is 3 X 10−10M and the EDS0 is approximately 10−10M. At concentrations up to 10−7 M, proctolin has no effect on the rhythmic electrical activity of the isolated cardiac ganglion, and it does not change the simple and compound postsynaptic potentials recorded at the cardiac neuromuscular junction. Proctolin acts directly on the cardiac muscle fibres. Electrically stimulated myocardia show a proctolin-induced increase in contraction amplitude with the same concentration dependence as the intact heart. A compound with an apparent molecular weight of 700–800 occurs in the Limulus nervous system, particularly in the cardiac ganglion. This compound resembles proctolin in being heat-stable, resistant to trypsin and chymotrypsin cleavage, and losing activity in a time-dependent manner in response to treatment with leucine aminopeptidase or pronase. This peptide induces spontaneous contractions and a contracture of the cockroach hindgut in a manner similar to proctolin. Moreover, the Limulus inotropic peptide, like proctolin, increases the force of contraction of the Limulus heart without affecting beat frequency. It is concluded that an endogenous, proctolin-like peptide is an inotropic modulator of the Limulus heart, acting directly on the muscle fibres and not affecting cardiac ganglion activity.

Spontaneous activity of foregut and hindgut visceral muscle of the locust, Locusta migratoria—II. The effect of biogenic amines

1. At 10 −8 M, 5-HT increased both the frequency and amplitude of contractions of isolated locust foregut. At 10 −4 M the foregut general tonus was increased. 2. Both spontaneously active and quiescent hindguts were less sensitive to 5-HT, showing only an increase in amplitude of contraction at 10 −6−10 −5 M. 3. The Hill plot suggested that although the 5-HT receptor populations in these two gut divisions differed in affinities, they were essentially homogeneous. 4. Octopamine (10 −5−10 −4 M) increased foregut contraction frequency but diminished amplitude. 5. Octopamine action on the hindgut was varied. At 10 −8 M it slightly increased tonus, while at 10 −5 M it increased contraction amplitude without affecting frequency. 6. At 10 −4 M octopamine suppressed activity of spontaneously active preparations and lowered the tonus of quiescent preparations. 7. Tyramine induced dose-dependent inhibition of foregut responses to 5-HT. The hindgut was exceptionally sensitive to tyramine, at only 10 −8 M it suppressed 5-HT responses. 8. Octopamine inhibited fore- and hindgut responses to 5-HT, but was less effective than tyramine. 9. Locust fore- and hindgut have remarkably different pharmacological properties reflecting differences in innervation and in extrajunctional monoamine receptor affinities.

Esophageal peristalsis before, during, and after food intake in healthy people.

Esophageal peristalsis in the middle and distal esophagus was analyzed in nine healthy people before, during, and after food intake. A significant increase in amplitude of contractions occurred during and after eating. Furthermore, a significant increase in duration and propagation time of contractions occurred in the distal part of the esophagus during food intake.