EFFECT OF FURA-2 ON ACTION-POTENTIAL STIMULATED CALCIUM-RELEASE IN CUT TWITCH FIBERS FROM FROG-MUSCLE

Cut fibers (striation spacing, 3.6-4.2 mum) were mounted in a double V aseline-gap chamber and studied at 14-15-degrees-C. One or both of the Ca indicators fura-2 and purpurate-3,3'diacetic acid (PDAA) were intr oduced into the optical recording site by diffusion from the end pools . Sarcoplasmic reticulum (SR) Ca release was elicited by action potent ial stimulation. With resting [fura-2] = 0 mM at the optical site, the [Ca] transient measured with PDAA was used to estimate SR Ca release (Baylor, S.M., W.K. Chandler, and M.W. Marshall. 1983. Journal of Phys iology. 344:625-666). With resting [fura-2] > 0 mM, the contribution f rom Ca complexation by fura-2 was added to the estimate. When resting [fura-2] was increased from 0 to 0.5-2 mM, both the amount of SR Ca re lease and the maximal rate of release were increased by approximately 20%. These results are qualitatively similar to those obtained in inta ct fibers (Baylor, S.M., and S. Hollingworth. 1988. Journal of Physiol ogy. 403:151-192; Hollingworth, S., A. B. Harkins, N. Kurebayashi, M. Konishi, and S. M. Baylor. 1992. Biophysical Journal. 63:224-234) and are consistent with a reduction of Ca inactivation of SR Ca release pr oduced by 0.5-2 mM fura-2. With resting [fura-2] greater-than-or-equal -to 2 mM, the PDAA [Ca] transient was reduced to nearly zero and SR Ca release could be estimated from DELTA[Cafura-2] alone. When resting [ fura-2] was increased from 2-4 to 5-6 mM, both the amount of SR Ca rel ease and the maximal rate of release were decreased by approximately h alf, consistent with a possible reduction of Ca-induced Ca release (Ja cquemond, V., L. Csernoch, M. G. Klein, and M. F. Schneider. 1991. Bio physical Journal. 60:867-873) or a possible pharmacological effect of fura-2.