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Differential calcium sensitivity of two types of quantum bumps in Limulus ventral nerve photoreceptors

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Ventral photoreceptors in Limulus polyphemus were continuously illuminated with dim light and produced two types of quantum bumps. Bumps of one type had a relatively symmetrical shape and fast rise and decay kinetics. In addition, bumps were detected with a small amplitude and slower kinetics. We termed these bump types “C2” and “C1 bumps”, respectively. The half width of a bump divided by its amplitude was found to be a reliable criterion to distinguish between the bump types. This shape quotient is smaller than 0.5 ms/pA for the C2 bumps and larger than 1 ms/pA for the C1 bumps. Lowering the extracellular calcium concentration from 10 mM to 0.25 mM caused an increase in the average amplitude of C2 bumps to 196%. After the injection of small amounts of 1.2 mM EGTA solution the amplitude of these bumps was reduced to about 30 and 50% in two cells studied. By contrast, in both cases C1 bump amplitudes were not affected significantly. We conclude that the two bump types are generated in one photoreceptor by the activation of different transduction pathways. These pathways are differentially sensitive to changes of the cytosolic calcium concentration.

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Dorlöchter, M., Yuan, W., Hennig, H.T. et al. Differential calcium sensitivity of two types of quantum bumps in Limulus ventral nerve photoreceptors. J Comp Physiol A 178, 863–869 (1996).

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Key words

  • Horse shoe crab
  • Phototransduction
  • EGTA
  • Cytosolic calcium
  • Current components