Fluorescence signals from the Mg2+/Ca2+ indicator furaptra in frog skeletal muscle fibers.
The fluorescent Mg2+/Ca2+ indicator, furaptra, was injected into single frog skeletal muscle fibers, and the indicator's fluorescence signals were measured and analyzed with particular interest in the free Mg2+ concentration ([Mg2+]) in resting muscle. Based on the fluorescence excitation spectrum of furaptra, the calibrated myoplasmic [Mg2+] level averaged 0.54 mM, if the value of dissociation constant (KD) for Mg2+ obtained in vitro (5.5 mM) was used. However, if the indicator reacts with Mg2+ with a two-fold larger KD in myoplasm, as previously suggested for the furaptra-Ca2+ reaction (M. Konishi, S. Hollingworth, A.B. Harkins, S.M. Baylor. 1991. J. Gen. Physiol. 97:271-301), the calculated [Mg2+] would average 1.1 mM. Thus, the value 1.1 mM probably represents the best estimate from furaptra of [Mg2+] in resting muscle fibers. Extracellular perfusion of muscle fibers with high Mg2+ concentration solution or low Na+ concentration solution did not cause any detectable changes in the [Mg2+]-related furaptra fluorescence within 4 min. The results suggest that the myoplasmic [Mg2+] is highly regulated near the resting level of 1 mM, and that changes only occur with a very slow time course.
Konishi
M
M
Department of Physiology, Jikei University School of Medicine, Tokyo, Japan.
Suda
N
N
Kurihara
S
S
D000818:Animals; D001572:Benzofurans; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000378:metabolism*; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives*; D066298:In Vitro Techniques; D008274:Magnesium / Q000378:metabolism*; D009132:Muscles / Q000737:chemistry / Q000378:metabolism*; D010080:Oxazoles; D011896:Rana temporaria; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet
D001572:Benzofurans; D005456:Fluorescent Dyes; D010080:Oxazoles; C058776:2-(2-(5-carboxy)oxazole)-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic acid; D008274:Magnesium; D002118:Calcium; D016257:Fura-2