Age-changes of the force-frequency-relationship and the duration of action potential of isolated papillary muscles of guinea pig.

The force-frequency-relationship and the duration of the action potential recorded intracellularly were compared on isolated isometrically arranged papillary muscles of young (3--4 months old) and senile (36--40 months old) guinea pigs. The muscles were in Krebs-Henseleit-solution at 37 degrees C electrically stimulated with frequencies between 30 and 420 imp./min. The following results were obtained: 1. The isolated papillary muscles of the two groups are almost similar with respect to length, weight and cross-sectional are. 2. In the range of stimulation rate from 30--420 imp./min the duration of action potential in the senile group was 120--135% of that in the young group. 3. The shape of slow repolarization of action potential was steeper in the senile group. 4. The time to peak tension and the relaxation time were longer in the senile group. 5. The maximally developed tension was the same in both groups, but in the senile group it was reached at a stimulation rate of 120 imp./min whereas in the young group, the optimal frequency was 290 imp./min. 6. The greatest value for the maximal rate of tension rise was 2.4 times smaller in the senile group. The results are discussed in relation to the age dependent intensity of membrane currents and to a possible altered activity of sarcoplasmic reticulum.