SPARK-TO-WAVE TRANSITION - SALTATORY TRANSMISSION OF CALCIUM WAVES INCARDIAC MYOCYTES

Using a modular approach, in which kinetic models of various mechanism s of calcium handling in cells are fine-tuned to in vivo and in vitro measurements before combining them into whole-cell models, three disti nct modes of transmission of calcium waves in mature and immature frog eggs have been defined. Two modes of transmission are found in immatu re eggs, where the inositol 1,4,5-trisphosphate receptor (IP3R) contro ls release of calcium from the endoplasmic reticulum (ER). The first m ode corresponds to an excitable physiological state of the cytoplasm a nd results in solitary waves that can appear as circular or spiral wav es in two dimensions with the wave speed proportional to the square ro ot of the diffusion constant of calcium. A second mode occurs when the state of the cytoplasm is oscillatory and because of the small size o f the buffered diffusion constant for calcium, the wave speed can appe ar to be weakly dependent on diffusion. In the mature frog egg, where the sperm-induced Ca2+ fertilization wave is a propagating front, the cytoplasm appears to be bistable and in this mode the wave speed is al so proportional to the square root of the diffusion constant. Here we investigate a fourth mode of propagation for cardiac myocytes, in whic h calcium release from the sarcoplasmic reticulum (SR) is dominated by clusters of ryanodine receptors spaced at regular intervals. In myocy tes a stochastically excitable myoplasm leads to the spontaneous produ ction of calcium 'sparks' that under certain conditions can merge into saltatory waves with a speed proportional to the diffusion constant. (C) 1998 Elsevier Science B.V. All rights reserved.