Velocity and Extent of Electrically-Triggered and Spontaneous Contractions in Rat Ventricular Myocyte: Relation to Cell Size and Sarcomere Length.

Characteristics of velocity and extent of unloaded contraction were examined at a cellular and a sarcomere level in isolated rat ventricular myocytes both during an electrically-triggered twitch and a contraction wave which occurred spontaneously at an end of the myocyte and propagated toward the other end. Both the velocity and extent of shortening examined at the cellular level in the triggered twitch were independent of the length and width of the myocyte. Among the myocytes isolated separately from the right ventricle and the inner and outer layers of the left ventricle, no differences were observed in any of their length and width, the velocity and extent of shortening. The extent of shortening examined at the sarcomere level was independent of the sarcomere length (SL) before the contraction in both the triggered twitch and the spontaneous contraction in the range of the SL examined (1.6-2.1 microns). The shortening velocity decreased as a decrease of the SL in the triggered twitch at both the cellular and sarcomere levels, while independent of the SL in the spontaneous contraction as examined at the sarcomere level. On the assumption that the shortening velocity reflects the rate of increase of myoplasmic free Ca2+ concentration ([Ca2+]in) and therefore the rate of Ca2+ release from the sarcoplasmic reticulum (SR), the above result suggests that the rate of the SR Ca2+ release is dependent on the SL before the activation in the electrically-triggered, action potential-mediated twitch, while independent of the SL in the spontaneous contraction without development of any action potential.