Sarcomere length during contraction of isolated guinea-pig ventricular myocytes

An improved method was developed for measuring sarcomere length (SML) durin g twitch contractions of single cardiac ventricular myocytes, using a charg e-coupled photodiode array self-scanning at a rate of 1.5 ms/element. The a verage resting SML of 111 cells was 1.88+/-0.04 mu m (mean +/- SD). When co ntractions were triggered by action potentials under perforated-patch condi tions, the time course of SML shortening closely followed changes in cell l ength. A large variation was observed in contraction time course between my ocytes, some cells having a phasic component with a duration at 50% shorten ing (full-width at half-maximum; FWHM) of approximately 40 ms, while others shortened more slowly (FWHM of phasic component congruent to 100 ms). FWHM was highly correlated with relaxation half-time, but with neither action p otential duration nor resting SML. The kinetics of slowly contracting cells could not be converted to the rapid type by using conditioning trains or a pplying isoprenaline. The steady-state SML/pCa relation in ventricular myoc ytes was measured by applying solutions of various pCa immediately after lo calized punctures of the surface membrane using a focal laser beam. The Hil l coefficient, n(H), was congruent to 4-5 and K(1/2)congruent to 400-500 nM . but there was no evidence of two populations of cells with different Ca2 sensitivities.