Calcium fluxes in internally dialyzed giant barnacle muscle fibers

Calcium-45 fluxes have been examined in isolated giant barnacle muscle fibers subjected to internal solute control by means of "internal dialysis". The 45Ca efflux was dependent upon the concentrations of both total and ionized internal Ca (Ca2+ buffered with EGTA). With a total Ca concentration of 2.0 mM and a 1:2 Ca/EGTA ratio (nominal [Ca2+]i =0.13 muM), the Ca efflux averaged 1.2 pmoles/cm2 sec. Under identical conditions, the mean Ca influx was only 0.36 pmoles/cm2 sec. The Ca efflux may not be attributed to leak of the CaEGTA complex, since a 2.5-fold increase in the EGTA concentration (nominal [Ca2+]i=0.032 muM) reduced the 45Ca efflux by one-third. Furthermore, when EDTA was used to buffer the internal Ca concentration (in the absence of internal Mg), the steady efflux of 14C-EDTA was only about 10% of the 45Ca efflux (in parallel experiments). The timecourse of the 45Ca fluxes also appeared anomalous in the 45Ca influx reached a steady level much more rapidly than 45Ca efflux in fibers of comparable diameters. If the muscle fibers are treated as right circular cylinders, these data imply that the apparent diffusion coefficient for inwardly-moving Ca is much larger than for outwardly-moving Ca. In contrast to Ca efflux, the outward diffusion of 22Na, 14C-EDTA and 3H2O appears to be limited primarily by the permeability of the dialysis tube wall. Some, but not all, of the anomalous behavior of the Ca fluxes can be reconciled if the deep, branched infoldings of the barnacle muscle surface membrane are taken into account.