Sir.—We read with great interest the article that appeared in the from research to relevance section of a recent issue of AJDC.1 In this article, Drs Cho and Pruitt very nicely review the applications of calcium channel-blocking drugs in clinical practice, with emphasis on their use in the pediatric age group.
We would like to point out, however, the erroneous explanation of the mechanism responsible for smooth-muscle contraction, particularly as it relates to the role of calcium and regulatory proteins. The authors stated that movement of extracellular calcium into the cell in response to an excitatory impulse stimulates the release of calcium from intracellular stores. This free cytoplasmic calcium binds to troponin, which removes its inhibitory effect on actin and myosin, thus allowing muscle contraction by the cyclic interaction of myosin with actin. Although this scheme of regulation generally holds true for both skeletal and cardiac muscles, it