Dynamic, Spatially Non-Uniform Calcium Regulation in Frog Rods Exposed to
Sean T. McCarthy, James P. Younger, and W. Geoffrey Owen.
Department of Molecular and Cell Biology, Neurobiology Division, University
of California, Berkeley, California.
APStracts 3:0098N, 1996.
SUMMARY AND CONCLUSIONS
1 . In intact rods of the bullfrog, Rana Catesbeiana , that were loaded with
Fura-2 by incubation, we made high-resolution measurements of Na:Ca,K exchange
currents and measured cytosolic free calcium concentrations during exposure to
steps of illumination. The calcium dynamics we observed are indicative of
unmanipulated rods because Fura-2 had little effect on calcium buffering
within the outer segment. 2 .. In the dark, the total concentration of calcium
within the outer segment, determined by integrating the exchange current, was
near 50 [mu] M. The free calcium concentration in darkness was 200 - 400 nM
indicating that 99% or more of the internal calcium was bound to buffer
molecules or equivalently sequestered. 3. During saturating illumination, the
concentration of free calcium near the membrane (assayed by the exchange
current) fell more rapidly than the space-averaged free calcium concentration
(measured with Fura-2), but both had time courses that were best described by
a sum of three exponential terms. The time constants were the same for each
assay, but the weighting factors were different. 4. The relationship between
the exchange current and space-averaged calcium concentration is consistent
with significant concentration gradients within the outer segment resulting
from high buffering power, diffusional restrictions and the fact that all net
gain and loss of calcium occurs at the membrane. The data further indicate
that effective buffering, and hence calcium mobility, is not uniform within
the outer segment. 5. Calcium kinetics were independent of the calcium
concentration indicating that the dominant buffers effectively have a low
affinity for calcium, (K D >> ). 6 . The dynamics of calcium changes and of
exchange currents evoked by saturating and non-saturating illumination are
completely predictable from changes in the circulating current. Calcium and
current are related by a linear transformation indicating that calcium fluxes
within the outer segment are passive and that buffers equilibrate rapidly. 7.
Although calcium concentrations change slowly with respect to changes in the
circulating current, both measured and calculated calcium dynamics are well
correlated with changes in light adaptation. Responses to test flashes
depended weakly on the detailed time course of the adapting stimulus, but
strongly on the free cytosolic calcium concentration at the time the test
flash was delivered.
Received 21 February 1995; accepted in final form 6 May 1996.
APS Manuscript Number J132-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 June 96