Effects of reduced external sodium concentration and multivalent cations on caffeine contractures in young ferret atrial trabeculae

The characteristics of caffeine (1.25–80 mM) transient contractures have been examined in small atrial trabeculae (diameters 50–250 μm) isolated from young (1 – 1.5 months) ferret hearts. In the control medium, the half-saturation constant and the maximum contracture strength (at infinite caffeine concentration) were 37.8 ± 10.2 mM and 0.9 ± 0.2 kN∙kg−1 (n = 11), respectively. The contractile response to caffeine was markedly enhanced following reduction of external sodium (70–0 mM). The perfusion of young ferret trabeculae with the sodium-free medium (up to 3 min) decreased the half-saturation constant by a factor of three (12.4 ± 1.6 mM, n = 8) with an increase in maximum contracture strength (1.09 ± 0.3 kN∙kg−1, n = 8). The effects of various divalent and trivalent cations have been tested on the 10 mM caffeine contracture in trabeculae perfused with Na-containing (140 mM) solution. The order of cation effectiveness is Gd3+ (half effect 0.04–0.07 mM) > Cd2+ (0.15–0.25 mM) > Ni2+ (2–2.5 mM) > Co2+ (7–7.5 mM) [Formula: see text] Mn2+. In conclusion, the present work has shown that in atrial trabeculae isolated from young ferret hearts, the strength of the caffeine contracture was markedly affected by the activity of the sarcolemmal Na–Ca exchange.Key words: caffeine, cardiac muscle, sarcoplasmic reticulum, sodium–calcium exchange, atrium, ferret.