Positive inotropic but no relaxant effects of phenylephrine in cat papillary muscle.

The effect of phenylephrine (30 micron) in the presence of propranolol (1 micron) on electrically induced isometric twitches and on KCl-contractures was studied in papillary muscles from reserpine-pretreated cats at 25 degrees C. Under these conditions phenylephrine has previously been shown to act solely via alpha-adrenoceptors and not to increase c-AMP. Phenylephrine increased force of contraction, time to peak tension and relaxation time. Contracture tension remained unaffected. These data indicate that the stimulation of cardiac alpha-adrenoceptors, in contrast to beta-adrenergic stimulation, does not lead to so-called relaxant effects. This qualitative difference between the two responses is probably due to the different capacity of both stimuli to increase c-AMP.     

Positive inotropic effects of ouabain in isolated cat ventricular myocytes in sodium-free conditions

The inotropic and toxic effects of cardiac steroids are thought to result from Na(+)-K(+)-ATPase inhibition, with elevated intracellular Na(+)(Na)causing increased intracellular Ca(2+)(Ca) via Na-Ca exchange. We studied the effects of ouabain on cat ventricular myocytes in Na(+)-free conditions where the exchanger is inhibited. Cell shortening and Ca transients (with fluo 4-AM fluorescence) were measured under voltage clamp during exposure to Na(+)-free solutions [LiCl or N-methyl-D-glucamine (NMDG) replacement]. Ouabain enhanced contractility by 121 +/- 55% at 1 micromol/l (n = 11) and 476 +/- 159% at 3 micromol/l (n = 8) (means +/- SE). Ca transient amplitude was also increased. The inotropic effects of ouabain were retained even after pretreatment with saxitoxin (5 micromol/l) or changing the holding potential to -40 mV (to inactivate Na(+) current). Similar results were obtained with both Li(+) and NMDG replacement and in the absence of external K(+), indicating that ouabain produced positive inotropy in the absence of functional Na-Ca exchange and Na(+)-K(+)-ATPase activity. In contrast, ouabain had no inotropic response in rat ventricular myocytes (10-100 micromol/l). Finally, ouabain reversibly increased Ca(2+) overload toxicity by accelerating the rate of spontaneous aftercontractions (n = 13). These results suggest that the cellular effects of ouabain on the heart may include actions independent of Na(+)-K(+)-ATPase inhibition, Na-Ca exchange, and changes in Na.     

Positive and negative inotropic effects of elevated extracellular potassium level on mammalian ventricular muscle

The effect of moderate elevation in extracellular potassium concentration (up to 12 mM) on contraction of cat ventricular muscle was examined. Isometric force development was recorded from eight excised trabeculae and from six coronary-perfused in situ papillary muscle preparations. Contraction in the steady state was variably affected, sometimes decreasing monotonically, sometimes remaining unchanged, with increasing potassium level. In 11 of these 14 preparations, the steady state was preceded by a transient period in which the contraction was augmented. In addition, eight excised trabeculae were used in an experimental arrangement designed to distinguish between inotropic effects caused by potassium-induced alterations in the action potential and other, more direct, effects of this ion on contraction. The negative inotropic effect is attributable to a potassium-induced reduction in the amplitude and/or duration of the action potential plateau. The positive inotropic effect was found in experimental arrangements where effects of the potassium-rich medium on action potential time-course were effectively "buffered." The positive inotropic effect thus depends on the presence of the elevated potassium concentration and can occur independently of effects on the action potential time-course.     

Positive inotropic effects of Tityus cambridgei and T. serrulatus scorpion venoms on skeletal muscle

Toxins that block voltage-dependent K+ channels and those that modify Na+ channel gating exhibit positive inotropic effect on skeletal muscle. We compared the effect of the venom of Tityus cambridgei (Tc) and Tityus serrulatus (Ts) scorpions on mouse diaphragm force, in vitro. In indirect and direct (using D-tubocurarine 7.3 microM) stimulation, Tc, 10microg/mL, increased the contractile force, an effect prevented by tetrodotoxin (TTX) while Ts, 0.5 microg/mL, potentiated only indirectly stimulated diaphragm, thus indicating its activity is mainly mediated through acetylcholine release from nerve terminal. This effect is prevented by TTX and attenuated by the K+ channel opener cromakalim. In conclusion, our data show that while the positive inotropic effect of both venoms appears associated to the activity of Na+ and K+ channels, only Tc venom acts also directly on skeletal muscle. This finding call for further studies on Tc venom to identify the toxin responsible for its direct inotropic activity as it may have clinical applications.     

Effects of inotropic agents on contraction and relaxation of papillary muscle.

The effects of epinephrine and caffeine on isometric rabbit papillary muscle preparations were observed after maximum peak tension was produced by adjustment of initial length and Ca⁺⁺ concentration. Without change in peak tension, epinephrine caused increased positive dP/dt (p < .005), increased negative dP/dt (p < .001) and decreased time to peak tension (TP) (p < .001), while caffeine resulted in decreased positive dP/dt (p < .02), decreased negative dP/dt (p < .005) and prolonged TP (p < .001). Caffeine added to muscles being perfused with epinephrine reversed the epinephrine effects. The data suggest that epinephrine increases the influx of Ca⁺⁺ into the cell and the uptake of Ca⁺⁺ by the sarcoplasmic reticulum, and that caffeine exerts opposite effects on Ca⁺⁺ exchange. However, an additional mechanism not dependent on changes in Ca⁺⁺ flux is suggested. Epinephrine and caffeine may directly effect the rates of actin-myosin interaction, the former agent increasing, the latter decreasing the rates of attachment and detachment of actin-myosin bridges.     

Positive inotropic effects of low dATP/ATP ratios on mechanics and kinetics of porcine cardiac muscle

Substitution of 2'-deoxy ATP (dATP) for ATP as substrate for actomyosin results in significant enhancement of in vitro parameters of cardiac contraction. To determine the minimal ratio of dATP/ATP (constant total NTP) that significantly enhances cardiac contractility and obtain greater understanding of how dATP substitution results in contractile enhancement, we varied dATP/ATP ratio in porcine cardiac muscle preparations. At maximum Ca(2+) (pCa 4.5), isometric force increased linearly with dATP/ATP ratio, but at submaximal Ca(2+) (pCa 5.5) this relationship was nonlinear, with the nonlinearity evident at 2-20% dATP; force increased significantly with only 10% of substrate as dATP. The rate of tension redevelopment (k(TR)) increased with dATP at all Ca(2+) levels. k(TR) increased linearly with dATP/ATP ratio at pCa 4.5 and 5.5. Unregulated actin-activated Mg-NTPase rates and actin sliding speed linearly increased with the dATP/ATP ratio (p < 0.01 at 10% dATP). Together these data suggest cardiac contractility is enhanced when only 10% of the contractile substrate is dATP. Our results imply that relatively small (but supraphysiological) levels of dATP increase the number of strongly attached, force-producing actomyosin cross-bridges, resulting in an increase in overall contractility through both thin filament activation and kinetic shortening of the actomyosin cross-bridge cycle.     

A study of inotropic mechanisms in the papillary muscle preparation

The length-tension diagram, the force-velocity relation, the characteristics of the series elasticity, and the duration of the active state have been studied on the papillary muscle preparation of the cat heart, and on other examples of cardiac muscle. Positive inotropic changes such as the staircase phenomenon and post-extrasystolic potentiation occur without lengthening, but frequently with shortening, of the duration of the active state. They are accompanied by an increased velocity of contraction, and may be caused either by an intensification of the active state or by an alteration of the force-velocity characteristics of the contractile component. The changes in the force-velocity relation point to an adaptation of the velocity-efficiency relation in dependence on the frequency of contraction.     

Internal viscoelastic loading in cat papillary muscle.

The passive mechanical properties of myocardium were defined by measuring force responses to rapid length ramps applied to unstimulated cat papillary muscles. The immediate force changes following these ramps recovered partially to their initial value, suggesting a series combination of viscous element and spring. Because the stretched muscle can bear force at rest, the viscous element must be in parallel with an additional spring. The instantaneous extension-force curves measured at different lengths were nonlinear, and could be made to superimpose by a simple horizontal shift. This finding suggests that the same spring was being measured at each length, and that this spring was in series with both the viscous element and its parallel spring (Voigt configuration), so that the parallel spring is held nearly rigid by the viscous element during rapid steps. The series spring in the passive muscle could account for most of the series elastic recoil in the active muscle, suggesting that the same spring is in series with both the contractile elements and the viscous element. It is postulated that the viscous element might be coupled to the contractile elements by a compliance, so that the load imposed on the contractile elements by the passive structures is viscoelastic rather than purely viscous. Such a viscoelastic load would give the muscle a length-independent, early diastolic restoring force. The possibility is discussed that the length-independent restoring force would allow some of the energy liberated during active shortening to be stored and released during relaxation.     

The relaxant effects of atrial natriuretic factor on vascular smooth muscle

Extracts prepared from rat atria, which cause natriuresis and diuresis when injected into bioassay rats, relax aortic smooth muscle preparations. A family of atrial peptides has been isolated, purified and synthesized which elicit similar biological responses as the atrial extracts. The in vitro vasodilator profile of synthetic atrial natriuretic factor (sANF) exhibits many similarities to sodium nitroprusside including inhibition of agonist-induced but not high-K+-induced contractions, relaxation independent of the vascular endothelium and elevation of cyclic GMP in aortic smooth muscle coincident with relaxation. Aortic rings remain relaxed in the presence of sANF but can be recontracted following a sufficient washout period. sANF causes a significant activation of the particulate (but not soluble) form of guanylate cyclase which is seemingly consistent with the presence of high affinity receptors for sANF in plasma membranes prepared from aortic tissue. Both species and regional vascular differences exist for the vasodilator activity of the synthetic atrial peptides.     

cAMP and in vitro inotropic actions of secretin and VIP in rat papillary muscle.

Secretin and VIP stimulate cardiac adenylyl cyclase activity and exert a positive inotropic action in several mammalian species. This study examined positive inotropic activity and cAMP levels in rat papillary muscle. Isoproterenol and secretin increased contractions by 150+/-31% and 129+/-27%, respectively. VIP increased contraction by 30+/-21% only at 10 microM. Isoproterenol significantly increased cAMP levels by 82%, whereas increases by secretin (58%) and VIP (56%) were not significant. These results are consistent with reports that secretin and VIP stimulate cardiac adenylyl cyclase in the rat, but suggest that cAMP tissue levels cannot totally explain the positive inotropic responses to secretin and VIP.     

Positive inotropic and negative lusitropic effects of endothelin receptor agonism in vivo

The endothelin (ET) system is involved in the regulation of myocardial function in health as well as in several diseases, such as congestive heart failure, myocardial infarction, and septic myocardial depression. Conflicting results have been reported regarding the acute contractile properties of ET-1. We therefore investigated the effects of intracoronary infusions of ET-1 and of the selective ET(B) receptor-selective agonist sarafotoxin 6c with increasing doses in anesthetized pigs. Myocardial effects were measured through analysis of the left ventricular pressure-volume relationship. ET-1 elicited increases in the myocardial contractile status (end-systolic elastance value of 0.94 +/- 0.11 to 1.48 +/- 0.23 and preload recruitable stroke work value of 68.7 +/- 4.7 to 83.4 +/- 7.2) that appear to be mediated through ET(A) receptors, whereas impairment in left ventricular isovolumic relaxation (tau = 41.5 +/- 1.4 to 58.1 +/- 5.0 and t(1/2) = 23.0 +/- 0.7 to 30.9 +/- 2.6, where tau is the time constant for pressure decay and t(1/2) is the half-time for pressure decay) was ET(B) receptor dependent. In addition, intravenous administration of ET-1 impaired ventricular relaxation but had no effect on contractility. Intracoronary sarafotoxin 6c administration caused impairments in left ventricular relaxation (tau from 43.3 +/- 1.8 to 54.4 +/- 3.4) as well as coronary vasoconstriction. In conclusion, ET-1 elicits positive inotropic and negative lusitropic myocardial effects in a pig model, possibly resulting from ET(A) and ET(B) receptor activation, respectively.     

Positive inotropic effect of ryanodine on rabbit ventricular muscle: dependence on the intracellular calcium load

Two types of electrical and mechanical responses to 1 mumol/l ryanodine, depending on the intracellular calcium load, were observed in rabbit papillary muscles. In a normal calcium solution, ryanodine induced a transient decline followed by a stable increase in the developed force (by 20 +/- 5% of the pretreatment level; n = 30) and prolonged the action potential (AP). The positive ryanodine response showed an increased time-to-peak force and was completely suppressed by 2 mumol/l nifedipine, partially blocked by 50 mumol/l tetracaine (Ca2+ release blocker), but greatly potentiated by 20 mmol/l CsCl or (-) Bay R 5414 which prolonged the AP. The prolonged time-to-peak force of the positive ryanodine response was shortened by procedures raising the content of Ca2+ in the sarcoplasmic reticulum (SR). It is suggested that the initial decline in the force amplitude results from Ca2+ leakage from the SR which is further compensated for by an elevation of both the transmembrane Ca2+ entry and intracellular Ca2+ release. In calcium overloaded myocardium, 1 mumol/l ryanodine caused irreversible contracture and dramatic AP shortening, explained by a massive Ca2+ release from the overloaded SR into the cytoplasm. It is concluded that the calcium content in the SR is the main modulator of the electrical and mechanical effects of ryanodine in ventricular myocardium.