Reversal of effects of acidosis on contraction of rat heart myocytes by CGP-48506

Abstract

In experiments reported here, we tested the ability of CGP-48506 to reverse the depressed cardiac contractility associated with hypercapnic acidosis in isolated rat cardiac myocytes. CGP-48506 is a cardiotonic agent that directly and specifically promotes the actin-cross-bridge reaction. Myocytes superfused at pH 6.8 demonstrated a significantly reduced extent of cell shortening, but an increase in the peak amplitude of the Ca 2+ transient. Moreover, cells in acidosis showed small, but significant, decreases in time to peak shortening to 50% relaxation. Superfusion of the cells with 3, 7, and 10 micro-molar CGP-48506 restored the inhibited contractility as a function of concentration with no significant effects on the Ca 2+ -transient. Moreover, 10 micro-molar CGP-48506 completely reversed the depressed myocyte contraction associated with an increase in time to peak shortening and time to 50% and 75% relaxation. Our results indicate that the depression of contractility associated with acidosis is due to a reduced myofilament response to Ca 2+ , which can be overcome by agents working downstream from troponin C through a direct effect on the actin-myosin interaction. KeywordsCa 2+ -sensitizer; Ischemia; Cardiotonic; Contractility; Myocyte Shortening; Ca-Transient INTRODUCTIONIn experiments reported here we have tested whether a cardiotonic agent with direct effects on the actin-myosin reaction is able to reverse the effects of acidosis on contraction of isolated cardiac myocytes. Our rationale for these experiments was based on: 1) evidence that the decrease in the response of the myofilaments to increased intracellular Ca 2+ during acidosis is the main mechanism for the decline of force production (1), and 2) the unique properties of CGP-48506 to directly and specifically affect the myofilament response to Ca 2+ with much less of an effect on cardiac relaxation than other agents working through this mechanism (2,3).Although acidosis inhibits the Ca 2+ current, the Ca 2+ pump activity of the sarcoplasmic reticulum (4), and the Na + -Ca 2+ exchange mechanism (5), direct measurements of intracellular Ca 2+ have shown that Ca 2+ delivery to the myofilaments actually becomes Send correspondence to: R. John Solaro PhD, Department of Physiology and Biophysics (M/C 901), College of Medicine, University of Illinois at Chicago, 835 South Wolcott Avenue, Chicago, IL 60612-7342, Tel: 312-996-7620, Fax: 312-996-1414 larger during acidosis (1). Apart from displacement of Ca 2+ from buffer sites including troponin C (6), it is known that acidic pH leads to an increase of intracellular Ca 2+ indirectly through changes in cytoplasmic Na + via Na + -H + mechanism (5). This increased Na + in turn induces an elevation of intracellular Ca 2+ via the Na + -Ca 2+ exchange mechanism. The increase in cytoplasmic Ca 2+ leads to increased Ca 2+ loading of SR and hence increases release from SR. Yet, despite this increase in cytoplasmic Ca 2+ , force falls, most likely due to an altered response of the myofilaments to Ca 2+ . Mecha...

Time to Achieve Target Mean Arterial Pressure during Resuscitation from Experimental Anaphylactic Shock in an Animal Model. A Comparison of Adrenaline Alone or in Combination with Different Volume Expanders

“…This was the case in our experiments in which the AD+HS group had the lowest pH values associated with hypernatraemia. The resulting hyperchloraemic acidosis can lead to a decrease in cardiac contractility [37][38][39][40][41] . HS has also been reported to decrease systemic vascular resistance [42][43][44] .…”

Prediction of Extravascular Burden of Carbon Monoxide (CO) in the Human Heart

“…63 An abrupt change in pH is adjusted rapidly (''within minutes'') 55 in cardiac myocytes by cardiac-specific isoforms of four transporters, carbonic anhydrase, the lactic acid transporter proteins MCT1 and MCT 4,46 and by gap junctions. The heart metabolizes lactate rapidly even at rest, and exercise at 40% of VO 2 max does not increase myocardial lactate production.…”

Muscle Fatigue from the Perspective of a Single Crossbridge

“…20,24,26,46,[58][59][60] Characterizing the mechanisms of fatigue at the cellular and molecular levels is important because many of the most promising therapeutic interventions act at this level to attenuate the debilitating effects of fatigue in clinical populations. 30, 51,57,74,87 In addition to directly inhibiting the crossbridge cycle, the metabolic by-products that are elevated during intense contractile activity are thought to indirectly affect contractile function by altering the ability of the muscle regulatory proteins (troponin [Tn] and tropomyosin [Tm]) to regulate actomyosin binding by making the thin filament less sensitive to Ca 2+ . 26,32,56,59 This mechanism is thought to play a particularly prominent role in fatigue at higher stimulation rates when the myoplasmic Ca 2+ concentration is rapidly compromised and muscular force drops precipitously.…”