Differential effects of contractile potentiators on action potential-induced Ca2+ transients of frog and mouse skeletal muscle fibres.

Muscle fibres, isolated from frog tibialis anterior and mouse flexor digitorum brevis (FDB) were loaded with the fast dye MagFluo-4 to study the effects of potentiators caffeine, nitrate, Zn2+ and perchlorate on Ca2+ transients elicited by single action potentials. Overall, the potentiators doubled the transients amplitude and prolonged by about 1.5-fold their decay time. In contrast, as shown here for the first time, nitrate and Zn2+, but not caffeine, activated a late, secondary component of the transient rising phase of frog but not mouse, fibres. The rise time was increased from 1.9 ms in normal solution (NR) to 3.3 ms (nitrate) and 4.4 ms (Zn2+). In NR, a single exponential, fitted the rising phase of calcium transients of frog (τ1 = 0.47 ms) and mouse (τ1 = 0.28 ms). In nitrate and Zn2+ only frog transients showed a secondary exponential component, τ2 = 0.72 ms (nitrate) and 0.94 ms, (Zn2+). We suggest that nitrate and Zn2+ activate a late slower component of the ΔF/F signals of frog but not of mouse fibres, possibly promoting Ca2+ induced Ca2+ release at level of the RyR3, that in frog muscle fibres are localized in the para-junctional region of the triads and are absent in mouse FDB muscle fibres.