Flux detectors versus concentration detectors: two types of chemoreceptors

Dose-response curves relating the external stimulus concentration toreceptor occupancy differ in two types of chemoreceptor organs. In'concentration detectors' the receptor molecules at the receptor cellmembrane are directly exposed to the external stimulus concentration; theseorgans exhibit the well-known hyperbolic dose-response relationshipreflecting the association-dissociation of stimulus and receptor molecules.In contrast, 'flux detectors' accumulate the stimulus molecules in aperireceptor compartment. In flux detectors, deactivation of stimulusmolecules may be in balance with arrival, as a prerequisite for producing aconstant effective stimulus concentration at constant adsorptive flux ofstimulus molecules. In a simple model of a flux detector in which receptormolecules themselves catalyze the deactivation, the dose-responserelationship is linear. It reflects the rate of stimulus deactivation. Ifthe deactivation is catalyzed by a separate enzyme, the dose-responserelationship can be close to hyperbolic, or linear. In all cases, thereceptor molecules are maximally occupied if the adsorptive flux equals orexceeds the maximum rate of stimulus deactivation. The time course of thereceptor potential recorded from moths' pheromone receptors depends on theodor compound, which suggests that a peripheral process, possibly thestimulus deactivation, is the slowest, rate-limiting process of thetransduction cascade. Further evidence comes from experiments with stimulioversaturating the mechanism responsible for the decline of the receptorpotential.