[Mechanisms of excitation-contraction coupling and calcium liberation in striated muscles of vertebrates]

The effects of Agauria salicifolia leaf extract on calcium current and excitation-contraction coupling of isolated frog muscle cells

The effects of the ethanolic extract of Agauria salicifolia (AS) previously tested on sodium currents of normal and TTX-treated voltage-clamped skeletal and cardiac muscle cells was investigated on mechanisms involved in the excitation-contraction coupling of the same biological preparations. AS (10(-6) g/l) prolonged the action potential duration whereas the associated contraction was depressed. This is not due to the blocking action of AS on the tubular sodium current since similar results were obtained in the presence of high tetrodotoxin (TTX) concentrations. This is not due to some blocking action of AS on the L-type calcium current since this type of calcium current was enhanced by the extract. In contrast, the cardiac T-type calcium current was decreased by 10(-6) g/l AS. AS exerted a dose-dependent (tested for concentrations ranging from 5 x 10(-10) to 5 x 10(-5) g/l) inhibition of the two components of the contractile response elicited by durable depolarizations, with a prominent effect on the tonic phase. This effect was partially relieved by increasing the external divalent cation (Ca2+ or Cd2+) concentration. At the same concentration it shifted the inactivation/potential relationship for tension by 20 mV towards negative potentials. It is concluded that the inhibitory action of AS on excitation-contraction coupling is partly or completely due to the enhancement of the voltage- and Ca(2+)-dependent inactivation processes of the voltage-sensor.

Action of heptaminol hydrochloride on contractile properties in frog isolated twitch muscle fibre

1. Heptaminol stopped or delayed the progressive decline in tension which characterizes the phenomenon of fatigue in frog isolated twitch muscle fibre. 2. Heptaminol had no action on the sodium, potassium and calcium voltage-dependent ionic conductances. 3. The hypothesis of an action via an internal alkalinization was tested by comparison with the action of NH4Cl. Both substances increased the tension. 4. The action of heptaminol was suppressed in sodium-free (TRIS) solution or in the presence of amiloride while the action of NH4Cl was always observed. 5. These results could be explained by a stimulation of the Na/H antiport by heptaminol.