Light-dependent control of calcium in intact rods of the bullfrog Rana
Catesbeiana.
Younger, James P., Sean T. McCarthy and W. Geoffrey Owen.
Department of Molecular and Cell Biology, Neurobiology Division, University
of California, Berkeley, California, USA, 94720-3200.
APStracts 2:0270N, 1995.
SUMMARY AND CONCLUSIONS
1. Using Fura-2, we measured cytosolic free calcium concentrations in rod
outer segments of the bullfrog ( Rana Catesbeiana ) under a wide range of
steady-state, adapting light intensities. We also measured rod circulating
currents under the same adapting conditions. 2. Both the steady-state
cytosolic free calcium concentration and the steady-state circulating current
were halved by background lights that isomerized about 100 rhodopsin molecules
per rod per second. A just-measurable reduction in calcium was evoked by a
background one-tenth as bright. The steady-state calcium concentration and the
steady state circulating current were proportional to each other over the full
range of intensities tested, the brightest of which suppresses the circulating
current by 75% and reduces flash sensitivity by about two orders of magnitude
relative to the rods' sensitivity in darkness. 3. Additional experiments, in
which the Na + :Ca 2+ ,K+ exchanger in the rod outer segment was inactivated
while maintaining intracellular calcium at physiological levels, demonstrated
that steady-state calcium concentrations in the outer segment are set only by
the influx of calcium through the light-sensitive channels and its efflux via
the Na + : Ca 2+ ,K + exchanger. We found no measurable light-induced release
of calcium from internal stores and no evidence of any other calcium flux
between the cytosol and intracellular compartments. 4. Taken together, these
findings lead us to conclude that over the normal operating range of the rod,
the selectivity of the cGMP-gated channels for calcium does not change as a
result of background illumination. 5. Our data also suggest that a major role
of calcium-dependent cGMP synthesis by guanylyl cyclase is to stabilize both
the circulating current and the cytosolic free calcium concentration in
darkness. This would minimize the dark noise of the rod and thereby increase
the reliability with which dim stimuli can be detected in the dark-adapted
state.
Received 11 April 1995; accepted in final form 23 August 1995.
APS Manuscript Number J239-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.