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The sensory dark current of vertebrate retinal rods is believed to be controlled by light activation ofa chain of coupled biochemical cycles that finally regulate the cationic conductance of the plasma membrane by hydrolytically reducing the level of cGMP in rod outer segment cytoplasm. The scheme has been tested by measuring heat production by live frog retinas when stimulated with sequences of light flashes of progressively increasing energy. Using pyroelectric poly(vinylidene 1,1-difluoride) detectors that simultaneously measure transretinal voltage and retinal temperature change, four heat effects assignable to known biochemical cycles in rods have been found. As the dark current shuts down after a flash causing 180-1800 rhodopsin photoisomerizations per rod, a heat burst, qj, raises the retinal temperature 1-2 IAK. ql is closely regulated in size and slightly precedes dark current shutdown. Isobutylmethylxanthine slows and enlarges qj, de-laying the dark-current response. Increasing cytoplasmic Ca2+ stops the dark current without affecting qj. Although rod heat production is consistent with splitting of 1-3 IAM of free cytoplasmic cGMP during transduction, the kinetics of the two processes do not match the predictions of current cGMP control models.A vertebrate retinal rod responds to photons absorbed in its outer segment (OS) disk membranes by briefly reducing the dark current flowing through the overlying plasma membrane, hyperpolarizing the cell (1, 2). Since OSs contain a light-activated cGMP phosphodiesterase and the cationic conductance of OS membranes increases rapidly and reversibly on exposure to concentrations of cGMP >5 taM (3)(4)(5), transduction based on control of the dark-current conductance by free cytoplasmic cGMP has been proposed (6, 7). In this work we test this idea by comparing the heat produced by frog retinas during transduction with that generated when cGMP is split by phosphodiesterase. Although cGMP yields 11 kcal/mol of heat on hydrolysis to 5'-GMP (8) Using an infrared viewer, the disks were placed on the detector with receptor layer down ( Fig. 1) and excess fluid was blotted away.Detector. Fig. 1 Physical Parameters. Flashes (6 .us) with a spectral width 70 nm centered at 500, 550, or 640 nm from a xenon flash tube were attenuated by a circular graded wedge and fed by a light pipe whose end was projected to a 6-mm circle at the retina. A computerized stepping motor turned the wedge so that flashes of various intensities could be quickly delivered. The light input at the retina was measured with a silicon photovoltaic diode (International Light, Newburyport, MA). Exposures of the OS were calculated for a rod density in the retinal mosaic of 1.76 x 106 cm-2 (unpublished measurements on live frog retinas). Cone and green rod pigments were neglected, and rod porphyropsin (11) was lumped with rhodopsin. Taking the quantum efficiency of rhodopsin photoisomerization (PI) to be 0.6 (12) and its molar absorbance at 550 nm to be 2.0 x 104 M-1cm-1 (13), its isomerization cross-s...

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