Presystolic calcium-loading of the sarcoplasmic reticulum influences time to peak force of contraction. X-ray microanalysis on rapidly frozen guinea-pig ventricular muscle preparations.

Guinea-pig ventricular small papillary muscles and trabeculae were rapidly frozen presystolically after prolonged rest following positive inotropic interventions which strongly influenced peak of force and time to peak force. The possible sources of activator calcium for the different types of contraction were investigated. After rest in the presence of noradrenaline (10(-5)mol/l) the first post-rest contraction showed a retarded activation and a "late" peak of force. Muscle strips frozen after a rest period of 5 min in a bath solution containing noradrenaline were cryosectioned and analyzed with X-ray microanalysis for elemental distribution: although at this time an applied stimulus would induce a potentiated contraction, intracellular membrane systems such as sarcoplasmic reticulum and mitochondria failed to reveal any accumulation of calcium. After rest in a low sodium Tyrode the first post-rest contraction showed an "early" peak of force. Muscles frozen after rest in a low sodium solution revealed intracellular Ca accumulation on the sarcoplasmic reticulum, in the network at the level of the Z-lines. The results support the hypothesis that 1. the sarcoplasmic reticulum (SR) accumulates calcium presystolically when "early" contractions follow stimulation; 2. the network of sarcoplasmic reticulum at the level of the Z-lines is a crucial source of activator calcium; 3. the activator calcium for late contractions is probably of extracellular origin.