The significance of a fast exchanging superficial calcium fraction for the regulation of contractile force in heart muscle

Abstract

The force of contraction of isolated guinea-pig atria decreased with time during 5 h of incubation in Tyrode solution containing 0.9 m m-CaCl 2. The relative effect of ouabain strongly depended on the duration of incubation (“age”) of the tissue. Freshly dissected atria showed a 12% increase in contractile force, whereas aged atria (5 h of incubation) produced a 240% increase in tension in the presence of ouabain (1.5 × 10 −7 m). In freshly dissected atrial preparations, after an equilibration period of 30 mins, ouabain had no influence on Ca metabolism (total tissue Ca, rate of exchange, and size of exchangeable fraction). After aging of the tissue the rate of Ca exchange was significantly decreased, and ouabain considerably accelerated the Ca exchange rate. The presence of pentobarbital also slowed the Ca exchange rate. Ouabain was able completely to restore the rate of Ca exchange to control valves. Analysis of the Ca exchange curves yielded a fast and a slowly exchanging Ca compartment. The size of the fast exchanging fraction was significantly reduced by aging and by the treatment with pentobarbital. Under these conditions ouabain enhanced the size of this particular Ca compartment. It is suggested that the fast exchanging Ca compartment is superficially located and is responsible for rapid changes in contractile force. This Ca fraction may provide a Ca concentration of about 1.5 × 10 −5 m in the intracellular aqueous space, which is generally believed to be sufficient for the activation of contraction.