A study of calcium transport in cells reveals the complexity of the intracellular medium

SILVINA PONCE DAWSON

Workshop; Transport Phenomena and Fluctuations in Small Complex Systems; 2023

Calcium (Ca2+) is a universal second messenger that participates in the regulation of an enormous variety of physiological processes. The way in which local elevations of the cytosolic Ca2+ concentration spread in space and time is key for the versatility of the signals. Having reliable estimates of the rate at which Ca2+ diffusion proceeds is thus basic to understand the dynamics of the signals and of the responses that can eventually be evoked. Ca2+ diffusion in the cytosol is hindered by binding/unbinding to/from proteins that act as buffers. The idea of a concentration-dependent effective diffusion is thus introduced to describe the net transport that results from the combination of the free (Einstein-like) diffusion of the ions and their interaction with these buffers. Effective diffusion coefficients reach free diffusion ones as the Ca2+ concentration increases, “overriding” the hindering effect of the interaction with buffers. Together with Cecilia Villarruel and Pablo Aguilar we performed a series of experiments in Xenopus laevis oocytes and in cells of Saccharomyces cerevisiae to estimate the Ca2+ diffusion coefficient in vivo. These experiments revealed that, in cells, Ca2+ diffusion is not only affected but the interaction with buffers but also by local changes in viscosity that yield widely variable free diffusion values. In this talk I will describe these results which led us to the conclusion that, in the cytosol of living cells, Ca2+ ions can freely diffuse at a higher rate than previously thought. I will also discuss briefly the implications of this conclusion for the propagation of the signals and the end responses that they can elicit.