Wagner J, Li Y X, Pearson J, Keizer J
Institute of Theoretical Dynamics, Physiology and Behavior, University of California, Davis, California 95616, USA.
Biophys J. 1998 Oct;75(4):2088-97. doi: 10.1016/S0006-3495(98)77651-9.
In the preceding paper Fontanilla and Nuccitelli (Biophysical Journal 75:2079-2087 (1998)) present detailed measurements of the shape and speed of the fertilization Ca2+ wave in Xenopus laevis eggs. In order to help interpret their results, we develop here a computational technique based on the finite element method that allows us to carry out realistic simulations of the fertilization wave. Our simulations support the hypothesis that the physiological state of the mature egg is bistable, i.e., that its cytoplasm can accommodate two alternative physiological Ca2+ concentrations: a low concentration characteristic of the prefertilization state and a greatly elevated concentration characteristic of the state following the passage of the wave. We explore this hypothesis by assuming that the bistability is due to the release and re-uptake properties of the endoplasmic reticulum (ER) as determined by inositol trisphosphate (IP3) receptor/Ca2+ channels and sarcoendoplasmic reticulum calcium ATPase (SERCA) pumps. When combined with buffered diffusion of Ca2+ in the cytoplasm, our simulations show that inhomogeneities in the Ca2+ release properties near the plasma membrane are required to explain the temporal and spatial dependences of the shape and speed of these waves. Our results are consistent with an elevated IP3 concentration near the plasma membrane in the unfertilized egg that is augmented significantly near the site of fertilization. These gradients are essential in determining the concave shape of the Ca2+ fertilization wave front.
在前一篇论文中，丰塔尼利亚和努奇泰利（《生物物理杂志》75:2079 - 2087 (1998)）详细测量了非洲爪蟾卵受精钙离子波的形状和速度。为了帮助解释他们的结果，我们在此开发了一种基于有限元方法的计算技术，该技术使我们能够对受精波进行逼真的模拟。我们的模拟支持这样一种假设，即成熟卵的生理状态是双稳态的，也就是说，其细胞质能够容纳两种不同的生理钙离子浓度：受精前状态的低浓度以及波通过后状态的大幅升高的浓度。我们通过假设双稳态是由内质网（ER）的释放和再摄取特性所导致的来探究这一假设，而内质网的这些特性由肌醇三磷酸（IP3）受体/钙离子通道和肌质网钙ATP酶（SERCA）泵所决定。当与钙离子在细胞质中的缓冲扩散相结合时，我们的模拟表明，质膜附近钙离子释放特性的不均匀性是解释这些波的形状和速度的时间和空间依赖性所必需的。我们的结果与未受精卵质膜附近升高的IP3浓度一致，而在受精部位附近该浓度会显著增加。这些梯度对于确定钙离子受精波前的凹形至关重要。
