Time course of the initial [Ca2+]i response to extracellular ATP in smooth muscle depends on [Ca2+]e and ATP concentration

Spatial-temporal dynamics of collective chemosensing

Although the process of chemosensing by individual cells is intrisically stochastic, multicellular organisms exhibit highly regulated responses to external stimulations. Two key elements to understand the deterministic features of chemosensing are intercellular communications and the role of pacemaker cells. To characterize the collective behavior induced by these two factors, we study the spatial-temporal calcium dynamics of fibroblast cells in response to ATP stimulation. We find that closely packed cell colonies exhibit faster, more synchronized, and highly correlated responses compared to isolated cells. In addition, we demonstrate for chemosensing the existence of pacemaker cells and how the presence of gap junctions impact the first step of the collective response. By further comparing these results with the calcium dynamics of cells embedded in thin hydrogel films, where intercellular communication is only possible via diffusing molecules, we conclude that gap junctions are required for synchronized and highly correlated responses among cells in high density colonies. In addition, in high density cell colonies, both communication channels lead to calcium oscillations following the stimulation by external ATP. While the calcium oscillations associated with cells directly exposed to external flows were transient, the oscillations of hydrogel trapped cells can persist with a fundamental frequency and higher harmonics. Our observations and measurements highlight the crucial role of intercellular signaling for generating regulated spatial and temporal dynamics in cell colonies and tissues.

ATP enhances spontaneous calcium activity in cultured suburothelial myofibroblasts of the human bladder

Background

Suburothelial myofibroblasts (sMF) are located underneath the urothelium in close proximity to afferent nerves. They express purinergic receptors and show calcium transients in response to ATP. Therefore they are supposed to be involved in afferent signaling of the bladder fullness. Since ATP concentration is likely to be very low during the initial filling phase, we hypothesized that sMF Ca²⁺ activity is affected even at very low ATP concentrations. We investigated ATP induced modulation of spontaneous activity, intracellular calcium response and purinergic signaling in cultured sMF.

Methodology/Principal Findings

Myofibroblast cultures, established from cystectomies, were challenged by exogenous ATP in presence or absence of purinergic antagonist. Fura-2 calcium imaging was used to monitor ATP (10⁻¹⁶ to 10⁻⁴ mol/l) induced alterations of calcium activity. Purinergic receptors (P2X1, P2X2, P2X3) were analysed by confocal immunofluorescence. We found spontaneous calcium activity in 55.18%±1.65 of the sMF (N = 48 experiments). ATP significantly increased calcium activity even at 10⁻¹⁶ mol/l. The calcium transients were partially attenuated by subtype selective antagonist (TNP-ATP, 1 µM; A-317491, 1 µM), and were mimicked by the P2X1, P2X3 selective agonist α,β-methylene ATP. The expression of purinergic receptor subtypes in sMF was confirmed by immunofluorescence.

Conclusions/Significance

Our experiments demonstrate for the first time that ATP can modulate spontaneous activity and induce intracellular Ca²⁺ response in cultured sMF at very low concentrations, most likely involving P2X receptors. These findings support the notion that sMF are able to register bladder fullness very sensitively, which predestines them for the modulation of the afferent bladder signaling in normal and pathological conditions.

Modeling of basolateral ATP release induced by hypotonic treatment in A6 cells

ATP is released from the basolateral membrane of A6 epithelia in response to hypotonic treatment. This study addresses the problem of ATP diffusion through the permeable supports used to culture the cells. A theoretical analysis of a recently introduced experimental protocol is presented and a model of ATP diffusion through the compartments of the measuring system is proposed. The model provides the ATP profiles near the cell layer and in the measurement chamber. Comparison of results from computer simulations and experimental data showed that the permeable support introduces a marked delay for ATP diffusion, supporting the correlation of apparently time-separated events: the mobilization of Ca(2+) from internal stores and release of ATP from the cell. The model is consistent with experimental data obtained with the luciferin-luciferase pulse protocol and provides an indirect proof of related processes like the closure and opening of the lateral interspace that occur after imposing the hyposmotic shock. The influence of the pore structure of the permeable support in modulating the measured release rates revealed by computer simulation is experimentally validated for two types of Anopore filters.