Calcium oscillations in blood platelets and their possible role in ‘interpreting’ extracellular information by cells

Intracellular Ca$^{2+}$ ions play an important role in the transmission and treatment of information that cells obtain from the ambient environment. Having received an external signal, a cell may increase the intracellular Ca$^{2+}$ concentration within fractions of a second by a factor of several hundred. This phenomenon triggers activation of various cellular systems that generate a response to the external stimulus. In many cells under the effect of external signal the concentration of Ca$^{2+}$ not only increases, but also starts oscillating. Both the frequency and amplitude of the oscillations are affected by the external signal strength. There are reasons to hypothesize that the conversion of the external signal into the oscillating intracellular signal has some important informational meaning. Methods to measure the dynamics of the intracellular Ca$^{2+}$ concentration and mechanisms that generate the oscillations are reviewed, and hypotheses on how the cell decodes Ca$^{2+}$ concentration oscillations are presented. Consideration is focused on the platelet, the cell that plays a key role in arresting hemorrhages. If a vessel is damaged, the platelet is rapidly activated. Identical platelets are divided in the process of arresting a hemorrhage into three populations with quite different missions. The platelet seems to somehow ‘interpret’ the set of external signals and uses the Ca$^{2+}$ concentration oscillations to ‘choose’ the population to which it will belong. Owing to the platelet's relative simplicity, one can expect that studies of that cell will shortly enable the decryption of the ‘code’ that drives Ca$^{2+}$ concentration oscillations.