FLUX DETECTORS VERSUS CONCENTRATION DETECTORS - 2 TYPES OF CHEMORECEPTORS

Dose-response curves relating the external stimulus concentration to r eceptor occupancy differ in two types of chemoreceptor organs. In 'con centration detectors' the receptor molecules at the receptor cell memb rane are directly exposed to the external stimulus concentration; thes e organs exhibit the well-known hyperbolic dose-response relationship reflecting the association-dissociation of stimulus and receptor molec ules. In contrast, 'flux detectors' accumulate the stimulus molecules in a perireceptor compartment. in flux detectors, deactivation of stim ulus molecules may be in balance with arrival, as a prerequisite for p roducing a constant effective stimulus concentration at constant adsor ptive flux of stimulus molecules. in a simple model of a flux detector in which receptor molecules themselves catalyze the deactivation, the dose-response relationship is linear. It reflects the rate of stimulu s deactivation. If the deactivation is catalyzed by a separate enzyme, the dose-response relationship can be close to hyperbolic, or linear. In all cases, the receptor molecules are maximally occupied if the ad sorptive flux equals or exceeds the maximum rate of stimulus deactivat ion. The time course of the receptor potential recorded from moths' ph eromone receptors depends on the odor compound, which suggests that a peripheral process, possibly the stimulus deactivation, is the slowest , rate-limiting process of the transduction cascade. Further evidence comes from experiments with stimuli oversaturating the mechanism respo nsible for the decline of the receptor potential.