Contractions induced by a calcium‐triggered release of calcium from the sarcoplasmic reticulum of single skinned cardiac cells.

Fragments of single cardiac cells were obtained by homogenization of ventricular tissue from adult rats. Remaining pieces of sacrolemma were removed by micro-dissection. Tension was recorded from the ends of the skinned (sarcolemma-free) cells with a photodiode force transducer. 2. In the presence of a strong buffering of the free Ca2+ with 4-0 mM total EGTA, a tonic tension was obtained that increased according to t sigmoid curve when the free (Ca2+ was increased from 10(-6-75)M to 10(-5-0)M. This curve was not modified by the destruction of the sarcoplasmic reticulum (SR) by the detergent Brij 58. Therefore, the tonic tension corresponded to the direct effect of the free Ca2+ present in the buffer on the myofilaments. 3. In the presence of a slight buffering of the free Ca2+ with 0-050 mM total EGTA, cyclic contractions were observed that were attributed to cyclic releases and re-sequestrations of Ca2+ by the SR. The absence of effect of azide and ruthenium red on the cyclic contractions obtained at a free Ca2+ lower than 10(-6-50)M demonstrated that the mitochondria played no role in the triggering of these contractions. 4. Cyclic contractions were induced by a slight variation of free Ca2+ in the buffer from 10(-7-65)M to 10(-7-40)M. Their amplitude at 10(-7-40)M free Ca2+ was equal to the tonic tension developed by a free Ca2+ 20 times higher applied to the myofilaments when the SR was destroyed by detergent or functionally inhibited by high total EGTA. It was concluded that these cyclic contractions corresponded to a Ca2+-triggered release of Ca2+ from the SR. 5. The cyclic contractions were induced by the filling of the SR with Ca2+ to a critical level at which it released a fraction of the Ca2+ it contained. Each contraction was followed by a re-sequestration of Ca2+, the kinetics of which conditioned the duration of the cycles. 6. The amplitude of the cyclic contractions increased when the free Ca2+ that triggered them was increased. This gradation was deemed incompatible with a simple regenerative process, which should produce an all-or-nothing response. Additional process, such as a modulation of the Ca2+ release by free Mg2+ and ADP may help to explain the gradation of the contractions. 7. It was concluded that a Ca2+-triggered release of Ca2+ from the SR of rat ventricular cells may amplify the Ca2+ flux crossing the sarcolemma during the plateau of the action potential, thereby permitting the activation of the myofilaments.