The glutamate-isolated mass receptor potential of the frog's retina. III. Calcium dependence.
The mass receptor potential of excised frog retinas was isolated by superfusion of the tissue with modified Ringer solutions containing sodium glutamate. The changes in the potential amplitude due to the variation of the calcium concentration were measured. Step by step reduction of the calcium concentration from 2 to 0.05 mM caused a mass receptor potential increase up to threefold; further calcium lowering diminished the amplitudes. Rods and cones, however, seemed to react in different ways to changed calcium concentrations. The responses to red light stimuli and the so-called rapid decay of the glutamate-isolated potentials, both generated mainly by cones, disappeared in calcium free solutions. Both the responses to green light stimuli and the so-called remaining part of mass receptor potentials, however, i.e. the potentials produced mainly by rods, were still larger in calcium free solutions compared to those in 2 mM calcium solutions. Changing the calcium concentration of the solutions altered both the Sodium and the potassium dependence of the mass receptor potential, and that differently for rods und cones. The results are used to support the hypothesis that calcium acts as internal transmitter in vertebrate photoreceptors. This view is confirmed by interpretation and critical comparison of data taken from the literature.
D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D004492:Edetic Acid / Q000494:pharmacology; D005071:Evoked Potentials / Q000187:drug effects; D005971:Glutamates; D018698:Glutamic Acid; D010775:Photic Stimulation; D011893:Rana esculenta; D012160:Retina / Q000187:drug effects / Q000502:physiology*
D005971:Glutamates; D018698:Glutamic Acid; D004492:Edetic Acid; D002118:Calcium