Effects of caffeine on intracellular calcium concentrations in frog skeletal muscle fibres.

Abstract

1. The mechanism of twitch potentiation by caffeine was studied in single muscle fibres dissected from m. tibialis anterior of Rana temporaria. Fibres were injected with a Ca2+-sensitive photo-protein, aequorin, and the resultant light signal and tension were simultaneously recorded. 2. In twitch responses triggered every 60 s, peaks of light and tension were maintained at a constant level. Low concentrations of caffeine (0.2-0.4 mM) potentiated twitch tension accompanied by a slight increase in light signal. 3. Although peak twitch tension was dose-dependently potentiated by caffeine, light peaks were suppressed at relatively higher concentrations of caffeine (0.6-1.5 mM) with prolonged decay of the light and tension signals. 4. Light intensity in the resting muscle (resting glow) was elevated by caffeine (0.2-1.5 mM) dose-dependently without detectable tension development. This effect of caffeine was suppressed by 0.5-1.0 mM-procaine or 10 mM-adenine, inhibitors of Ca2+-induced Ca2+ release. 5. In caffeine-treated preparations, peaks of light and tension were augmented by application of procaine (0.5-1.0 mM). Adenine (10 mM) affected the light signal in essentially the same way as procaine, but the effect varied from fibre to fibre. 6. From these results, the following hypothesis is proposed: low concentrations of caffeine directly induce Ca2+ release from sarcoplasmic reticulum (s.r.) in in the resting state, and facilitates Ca2+ release from s.r. induced by the action potential during twitch. At relatively higher concentrations of caffeine (0.6-1.5 mM), rise of resting [Ca2+]i (intracellular free Ca2+ concentration) before activation might be an important factor in twitch potentiation by caffeine. If the rise of resting [Ca2+]i induced by caffeine is inhibited by procaine and the content of Ca2+ in s.r. is well maintained, caffeine could facilitate Ca2+ release by depolarization and thus potentiate twitch tension.

The effects of caffeine on intracellular calcium, force and the rate of relaxation of mouse skeletal muscle.

“…Our results confirm earlier reports that caffeine increases tetanic force, increases [Ca2+]i during the tetanus and slows the rate of relaxation (Pagala, 1980;Konishi & Kurihara, 1987;Fryer & Neering, 1989 Wendt & Stephenson (1983) in skinned extensor digitorum longus fibres from rat. They showed that 5 mm caffeine produced a 0f06 log unit shift in Ca50 which compares with our observed 0-13 shift.…”

“…(2) Alternatively, caffeine might slow the release and/or uptake of Ca2+. There is already considerable evidence for this possibility as the time course of decline of [Ca2+]i has been shown to slow in the presence of caffeine (Konishi & Kurihara, 1987;Fryer & Neering, 1989; Simon, Klein & Schneider, 1989). …”

“…As a consequence of this action, moderate concentrations of caffeine increase twitch and tetanic force; at high concentrations it can produce a prolonged contracture in the absence of stimulation. However, caffeine also slows the time course in a twitch (Konishi & Kurihara, 1987;Fryer & Neering, 1989) and slows relaxation after a tetanus (Pagala, 1980) and these actions are less well explained. There are two main possibilities.…”

Effects of procaine and caffeine on calcium release from the sarcoplasmic reticulum in frog skeletal muscle.

“…Procaine in the millimolar range competitively inhibited caffeine contractures (Feinstein, 1963). The addition of 0-5-0-8 mM-procaine to fibres treated with 0-6 mM-caffeine inhibited the rise of resting glow induced by caffeine alone, but potentiated the peak of light and the peak of tension during a twitch (Konishi & Kurihara, 1987). The potentiated twitch response in these experiments could again be due to a prolongation of the action potential in the presence of procaine (Inoue & Frank, 1962 The present experiments involving brief (10-30 ms) depolarizations clearly indicate a specific inhibitory effect of procaine on the CICR induced by caffeine.…”

Resting cytoplasmic free Ca2+ concentration in frog skeletal muscle measured with fura-2 conjugated to high molecular weight dextran.

“…In amphibian skeletal muscle fibers, a cell type in which [Ca2+]i has been studied most extensively, the quantification of resting [Ca2+]i has been attempted with various methods, including Ca2+-sensitive microelectrodes (Tsien and Rink, 1980;Coray, Fly, Hess, McGuigan, and Weingart, 1980;Lopez, Alamo, Caputo, Dipolo, and Vergara, 1983;Weingart and Hess, 1984;Blatter and Blinks, 1991); aequorin (Blinks, Wier, and Snowdowne, 1980;Konishi and Kurihara, 1987;Blatter and Blinks, 1991); and tetracarboxylate indicators such as fura-2 (Klein, Simon, Szucs, and Schneider, 1988;Suda and Kurihara, 1991), fura red , and fluo-3 (Harldns, Kurebayashi, and Baylor, 1993). However, methodological difficulties encountered in the above studies make the quantification of [Ca~+]i somewhat uncertain, and consequently the estimated values for the resting [Ca~+]i vary widely.…”