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Veterans Affairs Medical Center, Durham, North Carolina 27705
We have estimated the rate of diffusion of
calcium ions in the transverse tubules of isolated cardiocytes by
recording changes in peak calcium current
(ICa) caused by
rapid changes of the extracellular calcium concentration
([Ca]o) at various
intervals just preceding activation of
ICa. Isolated
ventricular cells of guinea pig heart and atrial cells from rabbit
heart were voltage-clamped (whole cell patch), superfused at a high
flow rate, and stimulated continuously with depolarizing pulses (0.5 Hz, 200- or 20-ms pulses from a holding potential of 
45 or
75 mV to 0 mV). In ventricular cells, the change in peak
ICa following a
sudden change of [Ca]o
increased rapidly as the delay between the solution change and
depolarization was increased, up to a delay of ~75 ms [time
constant (
) 
20 ms, 30-40% of total current change), and
then increased more slowly ( 200 ms, 60-70% of total
current change); 400-500 ms were needed to achieve 90% of the
total current increase. In atrial cells, a clear separation into two
phases was not possible and 90% of the current change occurred within
85 ms. The slow phase of current change, which was unique to the
ventricular cells, presumably reflects the slow equilibration of ions
between the bulk perfusate and the lumina of the transverse tubules. If
the lengths of the transverse tubules were equal to the cell thickness, the slow rate of change of current would be consistent with an apparent
diffusion coefficient for calcium ions of 0.95 × 10
6
cm2/s, considerably smaller than
the value in bulk solution (7.9 × 106
cm2/s). Most likely, this
discrepancy is due to a high degree of tortuosity in the transverse
tubular system in guinea pig ventricular cells or possibly to ion
binding sites within the tubular membranes and glycocalyx.
guinea pig ventricular cells; calcium; excitation-contraction coupling
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