The role of calcium in the mechanical performance of cattle ruminal muscle

A viable preparation from the external muscle coat of cattle rumen for in vitro studies of contractility is described. The threshold Ca²⁺ concentration for contractile response in glycerol treated ruminal muscle was found to be pH dependent. At physiological pH it is situated between 10^-8 and 10^-7 M. Strips with intact membranes were studied in the depolarized state. In this preparation contraction to acetylcholine or histamine requires Ca²⁺ in the medium. However, contractility persists for several minutes in Ca²⁺-free solution at rest but disappears rapidly during stimulation. Recovery in a Ca²⁺-containing medium is much faster than the decline of responsiveness in a Ca²⁺-free medium. Tetracaine and D600 seem to inhibit contraction by blocking release of Ca²⁺ from and uptake of Ca²⁺ into hypothetical Ca stores inside the cell. The results are interpreted by assuming cellular Ca stores and two Ca pumps, one extruding Ca²⁺ into the medium and one accumulating Ca in the stores. Acetylcholine and histamine act by increasing Ca²⁺ permeability of both the membranes of the stores and the plasma membrane. The stimulator-induced and possibly the resting Ca²⁺ permeability in the depolarized state is reduced by tetracaine and D600 at both sites. The pumps are assumed not to be affected by stimulators and the mentioned drugs.