Magnesium dependent contraction of glycerinated smooth muscle

Abstract

The effect of Mg²⁺ on the contraction of glycerinated smooth and skeletal muscles was examined. In the absence of Ca²⁺, glycerinated smooth muscle contracted slowly when exposed to 5 mM adenosine triphosphate (ATP) and Mg²⁺. The magnitude of this contraction depended on the concentration of Mg²⁺ in a range of 1–20 mM. When 20 mM Mg²⁺ was used with ATP, glycerinated smooth muscle contracted almost maximally in a Ca²⁺ free environment, and it did not contract further on subsequent addition of 0.1 mM of free Ca²⁺ (EGTA-CaEGTA buffer). In contrast, in skeletal muscle a Ca²⁺ dependent contraction could always be elicited in the presence of 1–20 mM Mg²⁺ and 5 mM ATP. Tension development in smooth muscle induced by Mg²⁺ and ATP was not due to a contamination by Ca²⁺ or the removal of a Ca²⁺-sensitive protein. In the present experimental conditions, glycerinated smooth muscle developed a Mg²⁺-dependent contraction in a Ca²⁺ free medium, but glycerinated skeletal muscle did not.