Sarcoplasmic Ca2+ transients during the contractile cycle of single barnacle muscle fibres: measurements with arsenazo III-injected fibres

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Abstract
Summary Single muscle fibres fromBalanus nubilus were injected with the metallochromic Ca2+ indicator arsenazo III; multi-wavelength microspectrophotometry was used to monitor changes in the absorption spectrum of such fibres during isometric tension transients elicited by electrical stimulation. Differential absorbance changes, recorded in the 660–720 nm region of the spectrum, were characterized by rapid, nearly linear increases during constant current depolarizing pulses (25–200 ms); upon termination of electrical stimuli the increased differential absorbance described an exponential (k=0.6–2.0 s−1) decay toward baseline levels. The maximal increases in these absorbance signals preceded the peaks of the tension transients by 300–400 ms at 15–17° C. Calibration of these optical transients indicated that the initial velocity of the rising phase corresponded to a 0.03 µm ms−1 increase in sarcoplasmic free Ca2+. Appreciable tension was not generated until the apparent peak magnitude of the free Ca2+ transient exceeded 2 µm; a further three-fold increase (from 2 to 6 µm) in the maximal value of the free Ca2+ transient was accompanied by a 20-fold increase in the magnitude of the tension transient.