The effect of quinidine on calcium accumulation by isolated sarcoplasmic reticulum of skeletal and cardiac muscle.

Quinidine potentiates twitch tension and (at higher concentrations) causes contracture of skeletal muscle whereas the same drug reduces tension development of cardiac muscle. To gain insight into the possible differences in the excitation-contraction coupling mechanism of the two types of muscle the effect of quinidine on calcium accumulation by isolated sarcoplasmic reticulum from skeletal and cardiac muscle was investigated. In a medium containing ATP, Mg(++), oxalate, and (45)Ca, pharmacologically active concentrations of the drug inhibited calcium accumulation by both skeletal and cardiac sarcoplasmic reticulum.

The site of calcium binding in relation to the activation of myofibrillar contraction.

Skeletal muscle myofibrils, in the presence of 2 mM MgCl(2) at pH 7.0, were found to have two classes of calcium-binding sites with apparent affinity constants of 2.1 x 10(6)M(-1) (class 1) and approximately 3 x 10(4)M(-1) (class 2), respectively.

Contractility and ultrastructure in glycerol--extracted muscle fibers. II. Ultrastructure in resting and shortened fibers.

Glycerol-extracted rabbit psoas muscle fibers were examined by electron microscopy both before and after ATP-induced isotonic shortening. Ultrastructural changes were correlated with the initial sarcomere length and the degree of shortening. The ultrastructural appearance of the resting fiber at rest length was identical with that described by H.

Contractility and ultrastructure in glycerol--extracted muscle fibers. I. The relationship of contractility to sarcomere length.

This study was undertaken to determine whether glycerol-extracted rabbit psoas muscle fibers can develop tension and shorten after being stretched to such a length that the primary and secondary filaments no longer overlap. A method was devised to measure the initial sarcomere length and the ATP-induced isotonic shortening in prestretched isolated fibers subjected to a small preload (0.02 to 0.