Lack of correlation between the amplitudes of TRP channel-mediated responses to weak and strong stimuli in intracellular Ca imaging experiments

It is often observed in intracellular Ca²⁺ imaging experiments that the amplitudes of the Ca²⁺ signals elicited by newly characterized TRP agonists do not correlate with the amplitudes of the responses evoked subsequently by a specific potent agonist. We investigated this rather controversial phenomenon by first testing whether it is inherent to the comparison of the effects of weak and strong stimuli. Using five well-characterized TRP channel agonists in commonly used heterologous expression systems we found that the correlation between the amplitudes of the Ca²⁺ signals triggered by two sequentially applied stimuli is only high when both stimuli are strong. Using mathematical simulations of intracellular Ca²⁺ dynamics we illustrate that the innate heterogeneity in expression and functional properties of Ca²⁺ extrusion (e.g. plasma membrane Ca²⁺ ATPase) and influx (TRP channels) pathways across a cellular population is a sufficient condition for low correlation between the amplitude of Ca²⁺ signals elicited by weak and strong stimuli. Taken together, our data demonstrate that this phenomenon is an expected outcome of intracellular Ca²⁺ imaging experiments that cannot be taken as evidence for lack of specificity of low-efficacy stimuli, or as an indicator of the need of other cellular components for channel stimulation.