The Effects of Chloride Flux on Drosophila Heart Rate

Abstract

Approaches are sought after to regulate ionotropic and chronotropic properties of the mammalian heart. Electrodes are commonly used for rapidly exciting cardiac tissue and resetting abnormal pacing. With the advent of optogenetics and the use of tissue-specific expression of light-activated channels, cardiac cells cannot only be excited but also inhibited with ion-selective conductance. As a proof of concept for the ability to slow down cardiac pacing, anion-conducting channelrhodopsins (GtACR1/2) and the anion pump halorhodopsin (eNpHR) were expressed in hearts of larval Drosophila and activated by light. Unlike body wall muscles in most animals, the equilibrium potential for Cl− is more positive as compared to the resting membrane potential in larval Drosophila. As a consequence, upon activating the two forms of GtACR1 and 2 with low light intensity the heart rate increased, likely due to depolarization and opening of voltage-gated Ca2+ channels. However, with very intense light activation the heart rate ceases, which may be due to Cl– shunting to the reversal potential for chloride. Activating eNpHR hyperpolarizes body wall and cardiac muscle in larval Drosophila and rapidly decreases heart rate. The decrease in heart rate is related to light intensity. Intense light activation of eNpHR stops the heart from beating, whereas lower intensities slowed the rate. Even with upregulation of the heart rate with serotonin, the pacing of the heart was slowed with light. Thus, regulation of the heart rate in Drosophila can be accomplished by activating anion-conducting channelrhodopsins using light. These approaches are demonstrated in a genetically amenable insect model.

Effect of Temperature on Heart Rate for Lucilia sericata (syn Phaenicia sericata) and Drosophila melanogaster with Altered Expression of the TrpA1 Receptors

“…But strong activation of the light-sensitive channel depolarizes the myocytes further and results in a similar effect with the full contraction ceasing and then causing the heart tube to quiver. This was shown in a video ( Supplementary Video S5 —of Stanley et al [ 44 ])…”

“…Exposure of serotonin increases the heart rate in Drosophila (Canton S strain) larvae at 10 °C, as well as at 21 °C [ 36 , 43 , 44 ]. In addition, the RNAi and background lines used in this study increased heart rate with exposure to 10 µM serotonin, but the over-expressors which ceased beating at 30 °C did not respond.…”

“…This also occurs when the anion pump halorhodopsin (eNpHR) is activated [ 44 ]. Thus, one might expect heart rate to slow down in larvae not expressing TrpA1 with increased temperature if the resting membrane potential becomes more hyperpolarized; however, the heartbeat increased in frequency in a synchronized manner.…”

Optogenetic Cardiac Pacing and Its Applications

“…Mice cardiomyocytes showed electric shock functions in electrical defibrillation and elderly humans and Drosophila may similarly significantly reduce heart rate via electrical pacing [36,39]. Rhythm disturbances were associated with an increase in age and light intensity was associated with NpHR stopping the heart rate in Drosophila [40,41].…”

<p>Optogenetic Pacing: Current Insights and Future Potential</p>

“…54 In Drosophila, NpHR stops the heart rate from beating in relation to light intensity. 55 During Drosophila metamorphosis glutamatergic neurons provide extensive innervation to the adult heart. Muscles of the first abdominal cardiac chamber showed pacemaker action potentials.…”