Mechanical activity and ionic currents in frog atrial trabeculae.

Abstract

1. The relationship between ionic currents and contraction has been investigated in sinoauricular trabeculae of the frog. A double “sucrose gap” technique combined with an optical system permits the measurement of the contraction of the small area where current was recorded. Changes in light intensity were similar to isotonic mechanical response reported for cardiac muscle. 2. In Ringer's, above the mechanical threshold, depolarizations longer than the action potential induced contractile responses associated with inward, outward and tail currents. Variation of the depolarizing step duration revealed the presence of two components in the contraction. 3. A fast contraction with a threshold between + 35 and + 45 mV was evoked preferentially by short depolarizations (below 100 msec). This contraction was not markedly influenced by the fast inward current (as shown by the effect of TTX), on the other hand the slow inward current and contraction are both abolished by Mnsolutions. When [Na]0 is reduced to 10% of the normal, the slow inward current and the contraction are altered by variation of the depolarization amplitude or of the [Ca]0 in a similar way, results which favour a link between the slow inward current and the fast contraction. 4. A slow contraction, with a threshold in the region of + 70 mV, was observed in solutions containing manganese with depolarizations above 100 msec duration. It is suggested that this contraction may be induced by release of calcium from intracellular stores.