Hiroshi Yamagishi

Myocardial Depolarizing Response to Glutamate in the Myogenic Heart of the Branchiopod Crustacean Triops longicaudatus

Abstract Fine structure of the heart and the effects on the heartbeat of some transmitter candidates in crustacean cardioregulatory system were examined in the myogenic heart of the branchiopod crustacean Triops longicaudatus. Electron microscopy revealed that, in each myocardial cell, myofibrils are confined in the part facing the epicardium and intercalated disks are present between the myofibrillar regions of adjacent myocardial cells. No neural elements were found in the heart, suggesting lack of extrinsic cardioregulatory nerves from the central nervous system. Gamma aminobutyric acid and acetylcholine produced no detect-able changes in the myogenic activity of the heart at concentrations up to 10−3 M, respectively. Glutamate induced a depolarizing membrane response in the cardiac muscle with a threshold concentration of approximately 1×10−5 M. The amplitude of the depolarizing response was concetration-dependent and saturated at approximately 1×10−4 M. The myogenic activity of the heart increased in frequency with glutamate of less than approximately 3×10−5 M. With higher dose of glutamate, action potential adaptation occurred in the cardiac muscle and the heart exhibited a systolic arrest.

Cardioacceleratory Neurons of the Isopod Crustacean, Ligia exotica: Visualization of Peripheral Projection onto the Heart Muscle

Abstract Innervation of the heart muscle by the cardioacceleratory neurons was morphologically and electrophysiologically examined in the isopod crustacean, Ligia exotica. Intracellular injection of neurobiotin into the first and second cardioacceleratory neurons (CA1 and CA2) revealed their peripheral axonal projections. Inside the heart, the CA1 and CA2 axons ran along the trunk of the cardiac ganglion. Finely arborized branches with many varicosities arose from the axon and projected over the heart muscle. Stimulation of either the CA1 or CA2 axon caused an overall depolarization in the muscle of a quiescent heart. The amplitude of the depolarization increased with increasing stimulus frequency. During stimulation, the membrane resistance of the heart muscle decreased. In a beating heart, the cardioacceleratory nerve stimulation caused multiple effects on the heart muscle activity and the heartbeat. The results suggest that the cardioacceleratory neurons of Ligia exotica regulate the amplitude of the heartbeat (inotropic effect) and the heart tonus (tonotropic effect) via the synaptic contacts on the heart muscle, while the heartbeat frequency (chronotropic effect) is regulated via the synapses on the cardiac ganglion neurons.

Aminergic Modulation of the Myogenic Heart in the Branchiopod Crustacean Triops longicaudatus

Abstract Although crustaceans typically have a neurogenic heart, the primitive crustacean Triops longicaudatus has a myogenic heart with the heartbeat arising from the endogenous rhythmic activity of the myocardium. In the present investigation, the effects of six biogenic amines, epinephrine, norepinephrine, dopamine, octopamine, serotonin and histamine, on the myogenic heart of T. longicaudatus were examined. Epinephrine, norepinephrine, dopamine and octopamine accelerated the heartbeat, increasing both the frequency and amplitude of the action potential of the myocardium in a concentration dependent manner. The ability of epinephrine and norepinephrine to produce the acceleratory effects was more potent than that of dopamine and octopamine; the threshold concentrations of epinephrine and norepinephrine were approximately 10−10 M and those of dopamine and octopamine approximately 10−7 M. Serotonin weakly inhibited the heartbeat, decreasing both the frequency and amplitude of the myocardial action potential in a concentration dependent manner with a threshold concentration of approximately 10−6 M. Histamine exhibited no effect on the heartbeat. The results provide the first evidence for direct effects of amines on the crustacean myocardium and suggest neurohormonal regulation of the myogenic heart in T. longicaudatus.

Dual Effects of Dopamine on the Adult Heart of the Isopod Crustacean Ligia exotica

Abstract In the adult heart of the isopod crustacean Ligia exotica, the cardiac ganglion acts as the primary pacemaker with the myocardium having a latent pacemaker property. We show several lines of evidence that dopamine modulates the heartbeat of adult L. exotica affecting both pacemaker sites in the heart. Dopamine caused positive chronotropic (frequency increase) and inotropic (amplitude increase) effects on the heartbeat in a concentration dependent manner. The time courses of these effects were considerably different and the inotropic effect appeared later and lasted longer than the chronotropic effect. Dopamine rapidly increased the frequency of the bursting activity in the cardiac ganglion neurons and each impulse burst of the cardiac ganglion was always followed by a heartbeat. Moreover, dopamine slowly increased the amplitude and duration of the action potential plateau (plateau potential) of the myocardium. When the myocardial pacemaker activity was induced by application of tetrodotoxin, which suppresses cardiac ganglion activity, dopamine slowly increased the amplitude and duration of the myocardial plateau potential while decreasing its frequency. These results suggest that dopamine modulates the heartbeat in adult L. exotica producing a dual effect on the two pacemaker sites in the heart, the cardiac ganglion and myocardium.

Cardiac Nervous System in the Ostracod Crustacean Vargula hilgendorfii

Abstract We examined morphologically innervation of the heart of the ostracod crustacean Vargula hilgendorfii. The heart is single chambered and composed of a single layer of myocardial cells characterized by localization of myofibrils at the epicardial side. A nerve net in the heart was determined by vital staining with methylene blue. Electron microscopy revealed that a single neuron situated on the outer surface of the dorsal heart wall sends an axon into the heart wall. The axon of the dorsal neuron is branched widely and forms many neuromuscular junctions on the myocardial cells. A pair of extrinsic nerves, each of which contains several axons, enters the heart bilaterally and forms numerous nerve terminals on the dorsal neuron and myocardial cells, while no synaptic structures were found in the nerve terminals. The results suggest that the heart of V. hilgendorfii is neurogenic, with a single cardiac neuron having both pacemaker and motor functions.

Developmental changes in dopamine modulation of the heart in the isopod crustacean Ligia exotica: reversal of chronotropic effect.

Abstract Developmental changes in dopamine modulation of the heart were examined in the isopod crustacean Ligia exotica. The Ligia cardiac pacemaker is transferred from the myocardium to the cardiac ganglion during juvenile development and the heartbeat changes from myogenic to neurogenic. In the myogenic heart of early juveniles, dopamine affected the myocardium and caused a decrease in the frequency and an increase in the duration of the myocardial action potential, resulting in negative chronotropic (decrease in beat frequency) and positive inotropic (increase in contractile force) effects on the heart. Contrastingly, in the heart of immature adults just after juvenile development, dopamine caused effects of adult type, positive chronotropic and positive inotropic effects on the heart affecting the cardiac ganglion and myocardium. During the middle and late juvenile stages, dopamine caused individually a negative or a positive chronotropic effect on the heart. These results suggest that the chronotropic effect of dopamine on the Ligia heart is reversed from negative to positive in association with the cardiac pacemaker transfer from the myocardium to the cardiac ganglion during juvenile development.

Cardiac pacemaker mechanisms in the ostracod crustacean Vargula hilgendorfii

The heart of the ostracod crustacean Vargula hilgendorfii has a single intrinsic neuron that morphologically appears to innervate the myocardium. We, therefore, examined the heart activity electrophysiologically to determine whether the heartbeat is neurogenic. Each heartbeat is associated with a myocardial action potential composed of a spike potential followed by a plateau potential. The frequency of the action potential is not stable but changes successively over a wide range. The action potential is not preceded by a pacemaker potential and has an inflection in its rising phase. The myocardial cells couple electrically and fire almost simultaneously. The frequency of the action potential was unchanged by injection of depolarizing or hyperpolarizing current into the myocardium. However, slow oscillatory potentials appeared during the depolarization and its frequency was higher with increasing current intensity. Application of 1-microM tetrodotoxin (TTX) depolarized the myocardial membrane and completely prevented the action potential. During this depolarization, slow oscillatory potentials often appeared spontaneously. These results suggest that, although the myocardium has a property of conditional oscillator, the heartbeat is driven by the single cell cardiac ganglion that has both pacemaker and motor functions.

Acceleratory nervous regulation of juvenile myogenic hearts in the isopod crustacean Ligia exotica

We examined regulation of the myogenic heart by two identified cardioacceleratory neurons (CA1, CA2) in early juveniles of the isopod Ligia exotica. Repetitive stimulation of either the CA1 or CA2 axon increased the frequency and plateau amplitude of the action potential and decreased the maximum hyperpolarization of the cardiac muscle. These effects were larger with increasing stimulus frequency. The rate of increase in the frequency caused by CA1 stimulation was significantly larger than that by CA2. No impulse activity of the cardiac ganglion was induced by acceleratory nerve stimulation. The frequency of the muscle activity was decreased by injection of a hyperpolarizing current into the muscle during stimulation of the acceleratory nerve. In a quiescent heart, acceleratory nerve stimulation caused an overall depolarization in the muscle membrane and the amplitude of the depolarization induced by CA1 stimulation was significantly larger than that by CA2. These results suggest that CA1 and CA2 neurons regulate the myogenic heart affecting directly the cardiac muscle; the CA1 neuron produces more potent effects than does the CA2 neuron.

Isolation of Neurogenic and Myogenic Activities by Joro Spider Toxin in the Adult Heart of the Isopod Crustacean Ligia exotica

Abstract The effects of Joro spider toxin (JSTX), a specific glutamate antagonist, on the adult heart of the isopod crustacean Ligia exotica were examined. By application of JSTX, excitatory junctional potentials (EJPs) caused by the cardiac ganglion activity in the myocardium were gradually abolished. Subsequently, the cardiac ganglion and myocardium exhibited independent activities with their respective rhythms. In saline containing JSTX, no changes were observed in the muscle activity when the ganglionic activity was changed by current injection into the cardiac ganglion neuron. These results indicate that two pacemaker sites, the cardiac ganglion and cardiac muscle, are present in the adult heart of Ligia exotica and suggest glutamatergic neuromuscular transmission between them.

Cardioinhibitory Neurons in the Isopod Crustacean Ligia exotica

Abstract We identified electrophysiologically cardioinhibitory neurons in the central nervous system of the isopod crustacean Ligia exotica. A pair of the cardioinhibitory neurons are located at the ventral side in the anterior region of the 1st thoracic ganglion. Intracellular injection of neurobiotin into the cardioinhibitory neurons revealed that the each neuron sends the peripheral axon to the heart via a contralateral nerve root of its own ganglion. In the central nervous system, each of the neurons has arborization in the 1st thoracic and subesophageal ganglia and sends the longitudinal processes through the ipsilateral connective toward the circumesophageal connective and down to the 7th thoracic ganglion.