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Publisher Summary One requirement for efficient photon capture is a high optical density that evolution has achieved by packing photoreceptor cells with large amounts of rhodopsin (R). To translate efficient photon capture into high sensitivity, the signaling species downstream from rhodopsin must be abundant too and, indeed, there are high levels of transducin (T) and cGMP phosphodiesterase (PDE) in rod outer segments (ROS). This chapter presents experiments that constitute an example of what is possible with this technique of time-resolved microcalorimetry. The sensitivity of the instrument allows obtaining signals at physiological light levels of less than 100 R per rod. The enthalpy of cGMP hydrolysis is about -12 kcal/mol. With the proper instrumentation, this heat of reaction can be measured with enough time resolution and sensitivity to permit a fruitful kinetic exploration of the visual cascade at light levels resembling those in vivo . |
