Increase In Contractile Force Research Articles

Lack of Importance in Humans of the Slow Component of the Response of Force to an Increase in Cardiac Muscle Length

1. An increase in length in isolated animal muscles causes an immediate increase in contractile force, which is followed by a slower further progressive increase: the slow component. However, the pressure-volume concept, used in characterizing left ventricular function, is dependent on a constant relationship between pressure and volume. 2. We therefore examined the possible occurrence of the slow inotropic component of the response to cardiac muscle stretch in man at cardiac catheterization. 3. Human subjects undergoing left heart catheterization for clinical indications were studied. The development of the slow component was studied by measurement of rate of rise in left ventricular developed pressure and the time course after an increase in end-diastolic volume. 4. No evidence for any slow component was elicited. 5. It is concluded that the slow component of force (or pressure) increase after an increase in cardiac muscle length (or volume) does not have a role in the human heart in situ.

Comparison of Different Iron Chelators as Protective Agents Against Acute Doxorubicin-induced Cardiotoxicity

Doxorubicin (Dox) is a widely used antineoplastic agent. Irreversible cardiomyopathy is a serious and dose-limiting side effect after chronic administration. The iron chelating bispiperazinedione ICRF-187 is currently the only drug which affords protection against Dox-induced cardiotoxicity. To compare the protective value of structurally unrelated iron chelators, isolated mice atria were exposed to Dox (30 μM) and either the hydroxamate desferrioxamine (DFO, 200 and 500 μM), EDTA (200 μM) or the hydroxypyridones CP44 (200 μM), CP51 (200 μM), and CP93 (200 μM) and ICRF-187 (200 and 500 μM). The nitroxide TEMPO (5 mM) lacks iron chelating properties but was used to prevent redox cycling or iron and scavenge superoxide. All iron chelators, except EDTA, CP93 and CP44, were modestly protective against a Dox-induced decrease in contractile force. As a single agent the hydroxypyridones decreased atrial contractile force. At a concentration of 200 μM, DFO was the most effective protector of the chelators tested. However, this effect disappeared when a concentration of 500 μM was used. This in contrast to ICRF-187 for which a concentration-dependent inhibition of Dox-induced decrease in contractile force was observed. TEMPO exerted a biphasic response consisting of a two-fold increase in contractile force, followed by a decrease in force and irregular contractions. In this model TEMPO lacked any perspective as a cardioprotectant. We conclude that at 200 μM, DFO was the most effective agent to afford protection against Dox-mediated atrial malfunction. However, at 500 μM, DFO was not effective whereas ICRF-187 afforded partial protection. Hydroxipyridones were found to be of limited value because of a negative inotropic effect on the isolated atria.

Signal transduction in vascular smooth muscle: diacylglycerol second messengers and PKC action

Agonist-stimulated phospholipid turnover can generate diacylglycerol (DAG), an intracellular second messenger that activates protein kinase C (PKC). DAG can be produced from the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by a phosphoinositide-specific phospholipase C and by the degradation of phosphatidylcholine (PC) by a phospholipase C or the concerted actions of phospholipase D and phosphatidate phosphohydrolase. In vascular smooth muscle, agonist-stimulated DAG accumulation is biphasic; PIP2 hydrolysis produces a transient increase in DAG, which is followed by a sustained phase of DAG accumulation from PC degradation. Metabolism of DAG attenuates PKC activation and thus results in signal termination. The metabolic fates for DAG include 1) ATP-dependent phosphorylation to form phosphatidic acid (DAG kinase), 2) hydrolysis to release fatty acids and glycerol (DAG and monoacylglycerol lipases), 3) synthesis of triacylglycerol (DAG acyltransferase), and 4) synthesis of PC (choline phosphotransferase). Hydrolysis through the lipase pathway is the predominant metabolic fate of DAG in vascular smooth muscle. Activation of PKC in vascular smooth muscle modulates agonist-stimulated phospholipid turnover, produces an increase in contractile force, and regulates cell growth and proliferation. Further research is required to investigate cross talk between signal transduction mechanisms involving lipid second messengers. In addition, spatial considerations such as nuclear PKC activation and the influence of diradylglycerol generation on the duration of PKC activation are important issues.

WB4101- and CEC-sensitive positive inotropic actions of phenylephrine in rat cardiac muscle.

This study was designed to determine the role of the alpha 1-adrenergic receptor (AR) subtypes in the positive inotropic action of alpha 1-adrenergic agonists in rat myocardium. Isolated left atrial and papillary muscle were suspended in oxygenated Krebs-Henseleit buffer (37 degrees C) containing 3 microM nadolol and paced at 3.3 Hz. Isometric tension was continuously monitored. Cumulative concentration-response curves for phenylephrine (3 x 10(-7) to 3 x 10(-4) M) were obtained in the presence and absence of WB4101 (4 and 10 nM) and with and without treatment with chloroethylclonidine (CEC; 10, 100, and 300 microM). WB4101 antagonized the effect of phenylephrine in both tissues, increasing half-maximal effective concentration (EC50) values in a concentration-dependent manner. CEC pretreatment also increased EC50 values in both tissues, and 300 microM CEC reduced the maximal positive inotropic effect of phenylephrine by approximately 48 and 38% in left atrial and papillary muscle, respectively. CEC alone elicited significant increases in contractile force that were not readily reversible. These data suggest that the positive inotropic effect of alpha 1-adrenergic agonists in rat atrial and ventricular myocardium results from stimulation of both WB4101- and CEC-sensitive alpha 1-ARs.

Effects of low sodium, ouabain and amiloride on chick myocardial contraction

1. 1. Effects of Ca 2+, low Na +, ouabain and amiloride on contractile force were examined in myocardial preparations from embryonic and hatched chicks. Measurement of contractile force was performed in an organ bath with whole hearts for the young embryo (5–6 days old) and with isolated strips from the right ventricles for the old embryos (16–18-day-old), hatched chicks (within 24 hr after hatching) and 1-week-old chicks. 2. 2. The maximum increase in contractile force above the basal value at each age was determined by increasing extracellular Ca 2+, which was smaller in young embryonic hearts than in older ones. 3. 3. Decreasing the Na + concentration to half of the normal value increased the contractile force in all ages examined: the increase in contractile force expressed as a percentage of the maximum increase at each age was significantly larger in young embryonic hearts than in older ones. 4. 4. Ouabain produced increases in contractile forces at all ages examined; the effective concentration was lower in young embryonic hearts than in older hearts. 5. 5. Amiloride prolonged the relaxation phase of contraction at all ages examined; the degree of prolongation was larger in young embryonic hearts than in older ones. 6. 6. The present results suggest that, in the chick myocardium, the relative role of Na +-Ca 2+ exchange in relaxation decreases during embryonic development, which may reflect the concurrent increase in sarcoplasmic reticulum function [Tanaka, Takagi and Shigenobu (1993) J. Dev. Physiol. 19, 235–240.

Effects of genistein on cardiac contractile force and atrial natriuretic peptide secretion in the isolated perfused rat heart

We examined the effects of a selective protein tyrosine kinase inhibitor, the isoflavonoid genistein, on haemodynamics and atrial natriuretic peptide (ANP) secretion in perfused rat heart preparations. The addition of genistein into the perfusion fluid at concentrations of 11, 22 and 37 μM for 30 min in the spontaneously beating rat hearts caused dose-dependent, sustained increases in contractile force, perfusion pressure and immunoreactive ANP secretion, while heart rate remained constant. The positive inotropic and vasoconstrictor effects of genistein were significantly (P < 0.001) greater in the paced than in spontaneously beating rat hearts. Infusion of the calcium-channel antagonist diltiazem (3 μM) inhibited the genistein-induced positive inotropic effect by 52% (P < 0.001), and KN-62 (1.5 μM), an inhibitor of Ca 2+/calmodulin-dependent protein kinase II, by 34% (P < 0.001). The genistein-induced increase in immunoreactive ANP secretion was completely blocked by diltiazem (P < 0.001) while KN-62 delayed (P < 0.02) the increase of immunoreactive ANP concentration in the perfusate. These results show that genistein, at concentrations known to inhibit the activities of protein tyrosine kinases, dose-dependently increased contractile force, coronary vascular tone and ANP secretion from isolated perfused rat hearts. These cardiac effects of genistein may be mediated by elevation of intracellular Ca 2+ concentration, as shown by the inhibition of inotropic and secretory effects by both L-type calcium channel antagonist and Ca 2+/calmodulin-dependent protein kinase inhibitor.

Ontogeny of positive inotropic responses to sympathomimetic agents and of myocardial adrenoceptors in rats

Positive inotropic efficacies (maximal increase in contractile force) and potencies of the alpha-adrenoceptor agonist methoxamine and beta-adrenoceptor agonist isoprenaline were determined on electrically driven (1 Hz) ventricular strips from rats aged 0.5, 1, 2, 3, 6, and 10 (adult) weeks. The inotropic response to methoxamine significantly decreased after 2 weeks of age. The inotropic potency of isoprenaline was slightly but significantly lower at all ages than at 0.5 weeks of age. Up to 2 weeks of age, the maximal inotropic effect of methoxamine was comparable with that of isoprenaline, thereafter it was but markedly less. Phenylephrine behaved like methoxamine, and noradrenaline like isoprenaline. The effect of methoxamine was antagonized by prazosin but not by propranolol; the reverse was true for isoprenaline. Injections at birth of triiodothyronine and dexamethasone exerted minimal effects on the inotropic responses to methoxamine and isoprenaline. Chemical sympathectomy with 6-hydroxydopamine caused supersensitivity to the inotropic effects of isoprenaline but produced subsensitivity to responses to methoxamine at 1 week; effects of methoxamine at 3 and 6 weeks of age were not altered by sympathectomy. No significant differences in alpha 1- or beta 1-adrenoceptor densities or affinities in ventricular membranes from 7-day-old and adult rats were found. It is concluded that the positive inotropic responses to sympathomimetic amines decline with age, the decline is most marked in the case of alpha 1-adrenoreceptor- mediated effects, and these changes do not appear to be due to a decrease in the number or affinity of alpha 1- and beta 1-adrenoceptors.

Pharmacological actions of SDZ 218-135, a novel positive inotropic agent.

The effects of the new inotropic agent, SDZ 218-135 [(+)-(S)-4-[3-(4-diphenyl-methyl-1-piperazinyl)-2-hydroxy-propyl]- 6-(2-hydroxyethyl)-5-methyl-1,2,4-triazolo-[1,5-a]pyrimidin-7(4H)-one], were investigated using in vitro and in vivo techniques. In isolated rat atria, SDZ 218-135 elicited a dose-dependent increase in contractile force (+50% at 10 microM), which was paralleled by an increase in functional refractory period. In anesthetized rats SDZ 218-135 enhanced left ventricular (+)dP/dtmax by 100% at 10 mg/kg without influencing heart rate, arterial blood pressure, and cardiac output. In contrast to its predecessor, DPI 201-106, cardiac relaxation remained essentially unimpaired. The positive inotropic action was also maintained in a rabbit model of depressed heart function after myocardial infarction, where SDZ 218-135 increased peak acceleration of blood in the aorta. The prolongation of the effective refractory period in rat atria suggested possible antiarrhythmic effects. Indeed, SDZ 218-135 showed a dose-dependent marked reduction in reperfusion arrhythmias after coronary artery occlusion in rats. This effect was most likely due to a Class III action, since SDZ 218-135 significantly increased action potential duration (+10% at 10 microM/l) of the isolated guinea pig papillary muscle. In conclusion, SDZ 218-135 is a novel positive inotropic agent with an interesting profile of action. It does not impair cardiac relaxation and shows antiarrhythmic effects in a model of reperfusion-induced arrhythmias. The in vivo and in vitro data are consistent with a mechanism of action via sodium channel agonism.

Interactions of a Novel Catecholamine, GP-2-128, with Adrenoceptors

GP-2-128 is a novel catecholamine designed for transdermal iontophoretic delivery in patients with limited mobility to prevent deconditioning and muscular wasting. We characterized the interactions of this agent with alpha- and beta-adrenoceptors in vitro. In electrically stimulated rat left atria, GP-2-128 produced a concentration-dependent increase in contractile force. pD2 values for GP-2-128, isoproterenol (ISO), and dobutamine (DOB) were 10.6 +/- 0.12, 8.55 +/- 0.02, and 7.0 +/- 0.20, respectively. Metoprolol caused a shift in the concentration-effect curves for the three agonists. In spontaneously beating rat right atria, pD2 values of GP-2-128, ISO, and DOB are 10.4 +/- 0.24, 8.82 +/- 0.18, and 6.92 +/- 0.18, respectively. The affinity constant (KA) of GP-2-128, ISO, and DOB for cardiac beta 1-adrenoceptors was determined by competition binding assays to be 8.09, 6.04 and 4.49, respectively. In guinea pig trachea precontracted with histamine, GP-2-128 and ISO produced a concentration-dependent relaxation. pD2 values were 10.0 +/- 0.1 and 8.2 +/- 0.1, respectively. DOB was more potent than GP-2-128 in contracting isolated rat aortic rings (alpha 1 effect) and in displacing [3H]rauwolscine (alpha 2 effect). We also studied the interactions of GP-2-128 and ISO with the atypical beta-adrenoceptors (beta 3) in guinea pig ilea and rat and hamster adipocytes. Both agents inhibited twitches produced by transmural nerve stimulation in the presence of 10(-5) M nadolol. The EC30 for GP-2-128 and ISO at this atypical receptor site were 4.25 x 10(-10) and 5.05 x 10(-8) M, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac performance in semi-isolated heart of the crab Carcinus maenas

A semi-isolated, in situ heart preparation of the shore crab, Carcinus maenas, supported by its alary ligaments, pumps vigorously for hours at a mean heart rate of 49.7 beats/min and cardiac output of 30 ml.kg-1.min-1. These hearts show no adaptive responses to changes in pericardial sinus pressure, outflow resistance, or afterload. Direct perfusion-induced stretch of the heart wall causes increases in contractile force but minimal changes in heart rate. Stroke work and power are lower than comparable values for animals with myogenic hearts and closed circulatory systems. The values for heart rate and cardiac output are lower than in vivo values and may in part reflect the technique used as well as intrinsic performance of the heart without neural and neurohormonal inputs. Morphometrically the heart represents 0.2% of whole body weight, and the mean stroke volume of 0.35-0.45 ml/kg represents an ejection fraction of 27-34% of ventricular volume (1.4 ml/kg).

Declining inhibition in ipsi- and contralateral lumbar motoneurons during contractions of an ankle extensor muscle in the cat

1. Motoneurons of pretibial ankle flexor and knee flexor and extensor muscles were recorded intracellularly in chloralose- or pentobarbitone-anesthetized cats during sustained submaximal contractions of either ipsi- or contralateral gastrocnemius medialis muscle (GM). 2. In a majority of ipsilateral motoneurons, a sustained GM contraction elicited inhibitory potentials that quickly subsided before the end of the contraction. An abrupt increase in contractile force could elicit a new series of inhibitory potentials, which declined again in spite of a maintained force level. 3. Contraction-induced effects were only exceptionally detected in contralateral triceps surae and plantaris motoneurons. In a small number of pretibial flexor and knee flexor and extensor motoneurons, declining inhibitions were observed during sustained contractions of the contralateral GM muscle. 4. At the onset of GM contractions, a variety of motoneurons uniformly receive inhibitory inputs that are quickly filtered out. Although the functional significance of this widespread initial inhibition remains to be elucidated, its rapid decline seems useful to allow subsequent recruitment of motor units as may be required for coordination of posture and movement. 5. Tendon organs are activated during muscle contraction, but it is not certain whether Ib inputs from GM can account for all the effects observed. Contribution of other afferents was considered and tested using a different experimental approach. The companion paper reports observations suggesting that effects elicited by group II afferents may cooperate in the contraction-induced inhibition of motoneurons.

Voltage-dependent potentiation of the activity of cardiac L-type calcium channel alpha 1 subunits due to phosphorylation by cAMP-dependent protein kinase.

Barium currents mediated by the alpha 1 subunit of the cardiac L-type Ca channel expressed in Chinese hamster ovary (CHO) cells were increased up to 10-fold during dialysis of the cell with the catalytic subunit of cAMP-dependent protein kinase. After partial activation by exogenous kinase, the activity of the alpha 1 subunit was also reversibly potentiated up to 3.5-fold by prepulses to voltages in the range of 0 to +150 mV. Potentiation at +48 mV developed with a biphasic time course with time constants of 131 ms and 8 s. Reversal at -60 mV was biphasic with half-times of 12 ms and 100 ms and was blocked in the presence of the phosphatase inhibitor okadaic acid. Both the increase in calcium-channel activity during dialysis with kinase and the voltage-dependent potentiation were accompanied by shifts in the voltage dependence of activation to more negative membrane potentials. The increases in Ba current due to protein phosphorylation and to the dihydropyridine Ca channel agonist Bay K8644 were approximately additive. The results show that the alpha 1 subunit of the cardiac L-type Ca channel is sufficient for substantial modulation of Ca-channel activity by cAMP-dependent protein kinase and for potentiation by state-dependent protein phosphorylation. Voltage-dependent potentiation of the activity of the alpha 1 subunit may contribute to the increase in contractile force in response to increased rate of stimulation, the positive staircase effect in heart muscle.

Mechanisms for hypothermia-induced increase in contractile force studied by mechanical restitution and post-rest contractions in guinea-pig papillary muscle.

Lowering myocardial temperature increases contractile force, presumably by increasing intracellular calcium content. To study the mechanisms behind this, we compared the effects of some known inotropic interventions with hypothermia on mechanical restitution and post-rest contractile force in isolated guinea-pig papillary muscles. In four groups (n = 6 per group), the effects of: (1) reducing the ability for Na/Ca exchange to extrude Ca2+ (a) by increasing [Na+]i with ouabain or (b) by increasing [Ca2+]o; and (2) activation of calcium channels with Bay-K 8644, were compared with lowering temperature from 37 to 27 degrees C. Normally (at 37 degrees C and 2 mM CaCl2), mechanical restitution could be described by a rapid recovery phase with a time constant between 180 and 220 ms, followed by a slowly decaying phase with a time constant between 5000 and 8000 ms and post-rest contractions (1-10 min rest) were markedly depressed compared to steady-state contractions. Steady-state developed force was markedly increased at 27 degrees C, after 1 microM ouabain, 6 mM CaCl2 or 0.1 microM Bay-K 8644. At 27 degrees C the rapid recovery phase of restitution was delayed while the slowly decaying phase was not affected. Ouabain and increased [Ca2+]o caused elevation of the slowly decaying phase of restitution and markedly attenuated the post-rest depression of developed force, which may be attributed to a reduced diastolic extrusion of Ca2+ via the Na/Ca exchanger. Hypothermia and Bay-K 8644 on the other hand, augmented this post-rest depression. Hence, this study suggests that increased Ca2+ influx due to delayed inactivation of calcium channels may account for the increased developed force during hypothermia rather than reduced diastolic extrusion of Ca2+ via the Na/Ca exchanger.

Postischemic dysfunction of the heart induced by small numbers of neutrophils via formation of hypochlorous acid.

The role of polymorphonuclear neutrophils (PMN) in the injury of the heart following ischemia and reperfusion is still controversial. The aim of this study was to investigate whether small numbers of PMN may cause myocardial dysfunction in an isolated system, how the resulting loss of function can be characterized and whether the formation of hypochlorous acid (HOCl) can be responsible for the PMN-mediated effect. Isolated working guinea pig hearts were subjected to a 90% reduction of coronary flow for 30 min, with or without intracoronary infusion of homologous PMN (approximately 1-2 x 10(5) cells/min, i.e. about 5-10% of normal blood count). This ischemia was followed by a 15 min reflow period in a non-working ("Langendorff") mode before work was resumed. In hearts perfused only with buffer, post-hypoxic heart function recovered to 75-80% of the initial value. Inclusion of unstimulated PMN did not further attenuate cardiac function. However, cardiac output was decreased to 42% of the initial value, provided thrombin (0.3 U/ml) and H2O2 (10(-5) M) were also present, and the retained PMN (about 10% of those infused) were additionally stimulated during reflow by application of FMLP (10(-6) M for 1 min). In these instances, coronary flow at any time of the experiment and release of lactate or purines during ischemia and reflow did not differ significantly between hearts perfused with or without PMN. There was no substantial release of myoglobin in controls and in PMN-treated hearts. Inotropic stimulation of the hearts with noradrenaline or exogenous Ca2+ caused a sustained increase in contractile force. However, the response was significantly reduced in PMN-perfused hearts in comparison to control hearts. The myocardial contents of high-energy phosphates with and without inotropic stimulation proved to be identical irrespective of whether experiments had been performed in the absence or presence of PMN. A similar loss of myocardial function as mediated by PMN could be produced by infusing chemically generated hypochlorous acid (HOCl, 5 x 10(-7) M for 10 min). Strikingly, that portion of the infused HOCl which actually reacted with cardiac tissue was comparable to the amount shown to be generated by stimulating 10(6) PMN retained in the coronary system (about 7 nmoles). Supplementing the perfusate with the scavengers L-methionine (10(-4) M) or uric acid (5 x 10(-4) M) prevented the attenuation of heart function provoked by PMN. The results indicate that small numbers of PMN, sufficiently activated, can depress cardiac function after 30 min of ischemia.(ABSTRACT TRUNCATED AT 400 WORDS)

5-Hydroxytryptamine increases contractile force in porcine right atrium but not in left ventricle.

Positive chronotropic as well as inotropic effects of 5-hydroxytryptamine (5-HT) have been observed in pig atrial tissue, but no data are available about the direct effects of 5-HT on ventricular tissue. In the present study we investigated inotropic effects of 5-HT on atrial and ventricular trabeculae obtained from hearts of 3 months old pigs. The baseline isometric contractile force was significantly higher in ventricular (4.14 +/- 1.25 mN) than in atrial tissue (0.47 +/- 0.11 mN). A noradrenaline concentration-response curve (0.01 to 10 mumol/l) was used to check contractile responsiveness of the tissue and all responses were expressed as percentage of the response to 10 mumol/l noradrenaline. Noradrenaline caused a concentration-dependent increase in contractile force in both atrial and ventricular trabeculae. In contrast, though 5-HT (0.01 to 100 mumol/l) did increase force of contraction in atrial tissue (maximum: 72 +/- 20% of the response to noradrenaline 10 mumol/l), the contractility of ventricular trabeculae was not significantly affected (maximum: 12 +/- 6%). The present data show that, in contrast to atrial tissue, contractile force of ventricular tissue could not be significantly affected by 5-HT. To our knowledge, this is the first study to show that an agent which increased force of contraction in the atrium, did not have a corresponding effect on the ventricle. These findings may have important implications for a better understanding of the physiology and pharmacology of cardiac contractility.

Mechanisms for hypothermia-induced increase in contractile force studied by mechanical restitution and post-rest contractions in guinea pig papillary muscle

Mechanisms for hypothermia-induced increase in contractile force studied by mechanical restitution and post-rest contractions in guinea pig papillary muscle

Action of alpha 2A-adrenergic receptors in circular smooth muscle of canine proximal colon.

Addition of the alpha 2-adrenergic receptor agonist clonidine (1 microM) to tetrodotoxin-treated strips of canine colonic circular smooth muscle resulted in a significant increase in contractile force that was blocked by addition of the alpha 2-antagonist yohimbine (0.1 microM). The alpha 2-receptor antagonist radioligand [3H]rauwolsine bound rapidly and reversibly to a single class of saturable sites (Bmax, 38.4 +/- 6.2 fmol/mg protein) in colonic circular smooth muscle membranes with an affinity (KD = 5.1 +/- 0.9 nM) characteristic of alpha 2-adrenergic receptors in smooth muscle. Studies in cells freshly isolated from circular muscle of canine proximal colon verified the smooth muscle origin of these receptors. Rank order of potency of alpha 2-adrenergic receptor antagonists in competition for [3H]rauwolsine binding was yohimbine greater than oxymetazoline much greater than prazosin. Affinity of alpha 2-receptors for yohimbine was indistinguishable from that of its optical isomer, rauwolsine, in both competition studies (KI = 3.4 nM) and in saturation-binding experiments employing [3H]yohimbine directly (KD = 4.2 nM). The alpha-receptor agonist epinephrine, in competition studies employing [3H]rauwolsine, revealed high-affinity binding sites that were converted to low-affinity binding sites for agonist by addition of 100 microM GTP gamma S. Addition of the alpha 2 more-selective agonist clonidine (100 microM) resulted in inhibition of adenylate cyclase activity that was abolished by pretreatment of tissue strips with pertussis toxin suggesting coupling of the alpha 2-receptor in colon to adenylate cyclase via the GTP-binding protein Gi. Our data demonstrate a physiological role for adenylate cyclase-coupled receptors of the alpha 2A-subtype in canine colon circular smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Contraction of single vascular smooth muscle cells by phenylephrine at constant [Ca2+]i.

The mechanism of alpha-adrenergic agonist-mediated force generation was investigated in single hyperpermeable vascular smooth muscle cells. By use of a previously described method, force was recorded from individual ferret aortic cells made hyperpermeable by exposure to saponin. When the cells were clamped at pCa 7, addition of protein kinase M (PKM), the constitutively active form of protein kinase C (PKC), caused a sustained increase in force, which was reversible upon addition of the PKC pseudosubstrate inhibitor peptide (PSSI) corresponding to residues 19-31 of PKC. Phenylephrine at pCa 7 caused a dose-dependent increase in contractile force of the hyperpermeable cells, which was reversible on addition of phentolamine. The phenylephrine contraction could also be inhibited by the same concentration of PSSI that was effective toward the PKM-induced contraction. The response of the cells to a constant [phenylephrine] in different Ca buffers showed a lack of Ca dependence between pCa 8.6 and 7.0. The addition of PSSI to unstimulated cells caused a small but significant drop in basal tone. Taken together, these results suggest that a fraction of the basal tone, as well as the phenylephrine contraction that occurs in the skinned cells at constant intracellular free Ca2+ concentration, is the result of activation of a Ca-independent isozyme of PKC.

Effects of toxin Ts-γ, purified from Tityus serrulatus scorpion venom, on the isolated rat atria

By using a pair of silver/silver-chloride electrodes it was possible to record, simultaneously, the atrial electrogram and the atrial contractile force of rat atria, in an organ bath, containing Krebs-Ringer solution (30°C, pH 7.4, bubbled with 95% O 2 and 5% CO 2). Addition of toxin Ts-γ, purified from Tityus serrulatus scorpion venom, into the bath (1 μg/ml), evoked complex effects characterized by an initial reduction of both rate and contractile force, followed by increase of force and reduction of rate and finally by reduction of both rate and force. The increase of contractile force was prevented by metoprolol and is, therefore, adrenergic in nature. The reduction of rate was concomitant with changes in the atrial electrogram in which a positive P wave was replaced by a diphasic P wave, while the positive Ta wave was depressed. Experiments with tetrodotoxin, atropine and physostigmine indicate that these effects are due to the release of acetylcholine from vagal endings.

Effects of the new cardiotonic agent loprinone hydrochloride (E-1020) on left ventricular diameter in normal and experimental heart failure dogs and its action potential characteristics in isolated guinea pig cardiac muscles and sinus nodes

In anesthetized dogs, loprinone hydrochloride (LP, 10 approximately 100 micrograms/kg, i.v.) dose-relatedly increased cardiac contractility (LV dP/dt max), enhanced cardiac index and decreased systemic vascular resistance with relatively small reduction in mean aortic pressure (MAP) and a mild increase in heart rate (HR). LP also decreased left ventricular internal diameter (ID), increased fractional shortening (LVFS) and caused a leftward shift and an increase in the slope of the LV end-systolic pressure-ID relation. In the heart failure models induced by propranolol, LP rapidly reversed the cardiac depression and the enlargement in cardiac size. The extent of changes in MAP and HR by LP was smaller than those observed with several other cardiotonic agents. In guinea pig cardiac muscles, LP (10(-6)-10(-4) M) produced a concentration-related increase in contractile force and the maximum rate of rise of Ca(2+)-action potential. In the sinus nodes, LP induced a concentration-related decrease in action potential duration and increase in the slope of slow diastolic depolarization, resulting in an increase in beating rate. These influences of LP on sinus nodes were smaller than those of milrinone. These results indicate that LP increases LVFS, reduces cardiac size and improves the heart failure due to its cardiotonic and vasodilating properties. Moreover, electrophysiological studies suggest that at least a part of the lower chronotropic effects of LP may be due to the mildness of its direct stimulating effects on sinus nodes.