Myocardiocytes or cardiomyocytes are cardiac muscle cells that keep the heart beating and ensure that heartbeats are synchronized. Communication between cells is important for growth and development, the formation of new blood vessels from the restriction in blood supply to the heart and for coordination.
Myocardiocyte-myocardiocyte interactions occur in various ways. One such way is by cell-ECM adhesion and this aids in the stabilization of these cells. They allow the heart to function as a electromechanical syncytium. The sinoatrial node sends an impulse that spreads to and stimulates the cardiac muscles. This impulse results in a contraction of the myocardium. Because the stimulation of the myocardium is ordered, the heart contracts effectively allowing blood to be pumped throughout the entire body.
Muscle cells have a negative cell membrane potential at rest and volted gated ion channels. Stimulation induces the opening of these ion channels thus allowing cations to flow into each of these muscle cells. The ions entering are positively charged and therefore cause rapid and coordinated depolarization. This results in Ca2+ being released from the t-tubules. The high amounts of calcium causes a calcium- induced calcium release from the sarcoplasmic reticulum therefore, free Ca2+ causes the coordinated muscle contraction After a delay, potassium channels reopen the flow of K+ out of the cell and causes repolarization to the resting state. The gap junctions are also important in the conduction pathway because the wave of depolarization moves from one cell to the other through them. Gap junctions are pores in the membrane of the cells. They are one of the fastest ways in which cardiac muscle cells communicate as they facilitate cell-cell interaction, allowing the exchange of small signaling molecules such as the Ca2+. Gap junctions also allow these heart cells to contract simultaneously.
Interactions between heart muscle cells ensure that the electrical conduction system of the heart functions efficiently to produce coordinated and synchronized heartbeats. The heartbeats are important to supply the entire body with blood.
Erythrocyte – myocardiocyte communication!
Erythrocytes, red blood cells, are found within the blood which is a specialized form of connective tissue. Red blood cells have one main job and that is to to transfer oxygen around the body. They contain a pigment called haemoglobin,which binds to four molecules of oxygen, and form oxyhaemoglobin at high oxygen concentrations and at low oxygen concentrations oxyhaemoglobin dissociates to haemoglobin and oxygen. Areas at high oxygen concentrations are inside the alveoli in the lungs, while areas of low oxygen concentrations can be anywhere close to working cells such as those hardworking myocardiocytes!
Although myocardiocytes are surrounded by blood which the heart pumps, it would take too long for diffusion to be the mechanism by which they get oxygen and get rid of carbon dioxide, as well as nutrients and so on so a direct cell-cell interaction might be ineffective. Therefore, there is a network of blood vessels on the heart to support these hard working muscle cells and this is the means by which they interact.
The presence of CO2 helps the release of oxygen from haemoglobin , known as the Bohr effect and this is the mechanism used in RBCs transferring their oxygen to working myocardiocytes. When carbon dioxide diffuses into the blood plasma and then into the (erythrocytes) in the presence of the catalyst carbonic anhydrase, most CO2 reacts with water in the erythrocytes.
Carbonic acid, H2CO3, dissociates to form hydrogen ions and hydrogencarbonate ions. This is also a reversible reaction and undissociated carbonic acid, hydrogen ions and hydrogencarbonate ions exist in a dynamic equilibrium with one another.
Inside the erythrocytes negatively charged HCO3– ions diffuse from the cytoplasm to the plasma. This is balanced by diffusion of chloride ions, Cl–, in the opposite direction, maintaining the balance of negative and positive ions either side. This is called the ‘chloride shift’.
The dissociation of carbonic acid increases the acidity of the blood (decreases its pH). Hydrogen ions, H+, then react with oxyhaemoglobin to release bound oxygen and reduce the acidity of the blood. This buffering action allows large quantities of carbonic acid to be carried in the blood without major changes in blood pH.
Hb.4O2 + H+ HHb+ + 4O2
The importance of erythrocyte – myocardiocyte cell communication is to aid in the transport of blood throughout the body. This ensures that the mouse’s cells and heart have a good supply of oxygen. Blood is also composed of white blood cells which are important in creating antibodies and ‘attacking’ any foreign particles such as viruses. The movement of blood in and out of the heart also causes contraction of the heart muscle cells.
Alberts Bruce, Johnson Alexander, Lewis Julian, Raff Martin, Roberts Keith, and Walter Peter. Molecular Biology of the Cell. 4th ed. New York: Garland Science, 2002.