Understanding Cardiac Action Potential, Calcium Mobilization, And Inotropic Changes

a.Relate how the cardiac action potential arriving at the surface of the sarcolemma results in the mobilisation of intracellular calcium and subsequent cellular contraction.

b.Describe how the cardiac cell reduces the intracellular calcium concentration to bring about relaxation.

c.Give a detailed mechanism of action of a named drug or agonist that results in positive inotropic changes in the cardiac cell.Diagram

Cardiac Action Potential

Cardiac action potential is defined as the temporary phenomenon in which the difference between the internal and external membrane potential generates an electrical impulse (1). The variation in the electrical potential difference is evident in the different compartments of the heart. The excitory and contractile muscle systems detect these differences and constantly react to the potential maintaining the proper functionality of the heart intact (2). Autorhythmicity of the heart is maintained by the pacemaker potential, which is the ability of the heart’s excitory muscles, which detects spontaneous depolarization and maintains the flow of potential generation slowly, without any external influence (3).

Fig 1: action potential of cardiac muscles

Source: (4)

Phase 4 is known as the resting phase, which is constant at -90mV since the K⁺ ions keep leaking from the membranes through inward rectifier channels. The Na⁺ and Ca²⁺  remain closed during this time (4).

Phase 0 is the depolarization phase where, triggering of action potential makes the pacemaker cells raise the membrane potential from -90mV onwards. Na+ channels starts opening which allows the ions to enter the cellular compartment so that the membrane potential is now at -70mv creating an inward electronic pulse. Rapid Na+ influx causes membrane depolarization at 0mV temporarily called overshooting. During this time Ca2+ ions starts leaking inside the cell membrane slowly down the gradient making the membrane potential -40mV (4). (4)

Phase 1 is the early repolarization phase makes the membrane very positive and opens some of the K+ channels to facilitate movement of the ions outside of the cell making it 0mV (4).

Phase 2 is known as the plateau phase where the Ca2+ ions come inside through the long opening (L-type) receptors and K+ moves out of the cell and this process isss coupled by the excitation-contraction process (4).

Phase 3 is the repolarization Ca2+ channels close and the current electric potential returns to resting phase and the Na+ transmemebrane balance normalizes (4).

The excitation-contraction coupled phenomena are the cascade in which generation of electric pulse leads to contraction of heart sarcolemma by conversion of electrical energy to mechanical energy. The primary component for driving the phenomena is Ca2+ (5). The generation of action potential opens the very slowly after the phase 0 and greater influx of calcium is seen in the plateau phase. This causes calcium induction of calcium releases via Sarcoplasmic Reiculum (SR). The calcium binds to calmodulin complex protein, which in turn activates the myosin light chain kinase (MLCK), which phosphorylates the endings of myosin light chain by ATP hydrolysis. The phosphorylated ends form a cross-bridge with the actin filaments and troponin helps contracting the muscle fibers leading to whole muscle contraction (6). 

Excitation-Contraction Coupling

Fig 2: Excitation-Contraction Coupled reaction in heart muscles

Source: Benjamin Cummings, Addison Wesley Longman (7)

Cardiac muscles regulate the muscle contractility is different from vascular muscle contractility as it involves rapid alteration and generation of impulses which is controlled by contractile proteins called, actins and myosins and the regulatory proteins called troponin (6). The arrangements of these proteins are in such a way that all the components can freely function by keeping the tonicity of cardiac muscle contraction and reduction of diameter of the lumen. The release of calcium ions happen in the phase 3 of the cardiac muscle cycle, the repolarization phase where the L-type calcium channels starts to close down and the slow delayed rectifier (I_Ks ) K+ channels and the ions starts to move outward (8). The whole process creates a positive current with respect to the negative charge in the electric potentials of the membrane and makes the channels remain open (8). The calcium is driven out of the cell to balance the influx of K+ and Na+ balance. The calcium efflux phenomenon indicates the relaxation of the cardiac muscles. The calcium is removed from the cell compartment by active potential into the SR compartment after the completion of the action potential. This causes the removal of blocked troponin-myosin complex by inhibiting the active sites in the actin filaments. This causes relaxation of the cardiac muscles (8).

The intropic condition of the heart can be indced by physiological or pharmaceutical effects which alter the extendibility of the heart. The phenomena is called myocardial contractibility (9). Various factors affect cardiac contractibility, like the conduction among autonomic nervous which negatively regulate the atria. Certain condituion like extensive workouts, stress and hypertension induce production of epinephrines, which has andregenic effects on sympathetic nervous system (10). Heart rate elevation is also known to induce intropy. Physiological drugs like digoxins, beta-andregenic agonists (epinephrine, isoproterenol) and phosphodiesterase inhibitors like milrinone affect the entropy of the heart.  Phenylephrines are known to have positive effects of the cardiac intropy by activating the α-adrenoreceptors of the cardiac muscles. The mechanism is based on the activation of adenylyl cylclase and cyclic adenosine monophosphate (cAMP).  Increases of cardiac muscle contraction were evident by the action of phenylephrine depending on the concentration gradient (10). The positive intropic effects of phenylephrine were compared with the effect of phentolamine, which produced a concentration-based resposnse measured by the curve. α-adrenoreceptors induce the positive intropic effects of phelephrine while propanolol is present (11). This is due to increases of cAMP molecules.  The action potentials are dependent on the calcium ions which were observed to be increased productivity with time by the action of phenylephrine and a distinctive rate if elevation in the deporlarization (dV/dt_max) of the slower electric potentials. The intropic changes according to the phenylephrine action are reversible, but blocked by the effect of phentolamine. The increase of (dV/dt_max ) depolarization for slower potentials that the isoprenaline concentration. According to experiment conducted by 6 voltage gates clamp models were experimented on which showed that induction of the phenylphrine which increased the positive intropic effects  in heart slowed down the inward current peak in the given graphs as well as slowed down the inactivation of outward flow and were not changed with effect of phenylephrine (11).

Fig 3: Intropic effect, relaxation time and time to peak tension by the influence of pheenylephrine taking a mean of 12 experiments and isoprenaline (taking mean average of 11 experiments) shown in graph.

Source: 10

Analysis of the diagram Cumulative dose-inotropic response curves for isoprenaline and phenylephrine are shown in Fig 2. Show that positive inotropic effect was induced by isoprenaline in concentrations ranging from 10-9 to 10-7 M; the phenylephrine effect was obtained with concentrations ranging from 10-6 to 10-4 M. The endpoint of the concentration-effect curve for isoprenaline was considerably higher than that for phenylephrine: the maximum increase of contractile force (AFc) induced by the ,B-adrenoceptor agonist ( 10-7 M) was 337.1 ± 53.6 mg, while that induced by the ca-adrenoceptor agonist (10-4 M) was 220.0 ± 23.5 (mean values of 7 and 12 experiments respectively). Besides this quantitative difference in the inotropic effect, the two amines also had different effects upon the shape of the isometric contraction curves. Isoprenaline shortened the relaxation time (t2) whereas this parameter was not affected by phenylephrine. Time to peak tension (tl) was only slightly and not significantly affected by both substances (Figure 2). The increase in contractile force (AFc) induced by either isoprenaline or phenylephrine was linearly correlated to the increase in maximum velocity of force development (MVfd): the correlation coefficients (r) are 0.908 and 0.803, and the regression coefficients (b) are 0.020 and 0.023 respectively.

References:

1. O'Hara T, Virág L, Varró A, Rudy Y. Simulation of the undiseased human cardiac ventricular action potential: model formulation and experimental validation. PLoS computational biology. 2011 May 26;7(5):e1002061.
2. Hou JH, Kralj JM, Douglass AD, Engert F, Cohen AE. Simultaneous mapping of membrane voltage and calcium in zebrafish heart in vivo reveals chamber-specific developmental transitions in ionic currents. Frontiers in physiology. 2014 Sep 11;5:344.
3. Hurtado R, Bub G, Herzlinger D. A molecular signature of tissues with pacemaker activity in the heart and upper urinary tract involves coexpressed hyperpolarization-activated cation and T-type Ca2+ channels. The FASEB Journal. 2014 Feb 1;28(2):730-9.
4. Ikonnikov G, Yelle D. Physiology of cardiac conduction and contractility | McMaster Pathophysiology Review [Internet]. Pathophys.org. 2013 [cited 6 April 2018]. Available from: https://www.pathophys.org/physiology-of-cardiac-conduction-and-contractility/
5. .Eisner D, Caldwell J, Kistamás K, Trafford A. Calcium and Excitation-Contraction Coupling in the Heart. Circulation Research. 2017;121(2):181-195.
6. Lee YK, Ng KM, Lai WH, Chan YC, Lau YM, Lian Q, Tse HF, Siu CW. Calcium homeostasis in human induced pluripotent stem cell-derived cardiomyocytes. Stem Cell Reviews and Reports. 2011 Nov 1;7(4):976-86.
7. Mathews C, Ahern K, Van Holde K. Biochemistry. 3rd ed. San Francisco: Benjamin Cummings; 2012
8. Ledda F, Marchetti P, Mugelli A. Studies on the positive inotropic effect of phenylephrine: a comparison with isoprenaline. British journal of pharmacology. 1975 May 1;54(1):83-90.
9. Brückner R, Hackbarth I, Meinertz T, Schmelzle B, Scholz H. The positive inotropic effect of phenylephrine in the presence of propranolol. Increase in time to peak force and in relaxation time without increase in c-AMP. Naunyn-Schmiedeberg's archives of pharmacology. 1978 Jul 1;303(3):205-11.
10. Brückner R, Scholz H. Effects of α?adrenoceptor stimulation with phenylephrine in the presence of propranolol on force of contraction, slow inward current and cyclic AMP content in the bovine heart. British journal of 1984 May 1;82(1):223-32.
11. Fouad, F.M., Shimamatsu, K.A.Z.U.M.A.S.A., Hanna, M.M., Khairallah, P.A. and Tarazi, R.C., 1985. Impaired inotropic responses to alpha-adrenergic stimulation in experimental left ventricular hypertrophy. Circulation, 71(5), pp.1023-1028.