Shortening fraction: its dependence on the Starling mechanism

The influence of increasing muscle length (ML) from L0 to 15 to 20% of L0 and calcium concentration (Ca2+) from 1.34 to 10 mmol . litre-1 on shortening fraction has been analysed in cat papillary muscles. Shortening fraction was calculated by dividing the amount of shortening by the muscle length at which that shortening occurred. When the muscle shortened at contrast total load, increasing muscle length from approximately or equal to L0 to approximately or equal to 15% above L0, increased the shortening fraction from 0.2 +/- 0.1% to 7.1 +/- 0.7% (P less than 0.01) and from 1.0 +/- 0.5% to 12.2 +/- 0.5% (P less than 0.01) at low and high (Ca2+) respectively. The highest shortening fraction values obtained (7 and 12%) correspond to calculated ejection fraction values of 20 and 32% respectively. At a given muscle length, increasing (Ca2+) significantly increased the shortening fraction (P less than 0.01). At constant afterload the shortening fraction increased from 3.5 +/- 1% to 9.1 +/- 1.9% when the muscle length changed from approximately or equal to 5% to approximately or equal to 20% above L0 and from 3,3 +/- 1.6% to 14.3 +/- 0.7% when the muscle was stretched from L0 to approximately or equal to 20% above L0 at low and high calcium respectively. Shortening fraction values of 9 and 14% correspond to calculated ejection fraction values of 25 and 37% respectively. The results indicate that the shortening fraction is altered not only by changes in cardiac contractility but also by the Starling mechanism.