['Proximal colon from dystrophic mice develops spontaneous tone increment, but the mechanisms involved in its development have not been investigated. This study examined whether alterations in the properties of cell membrane calcium channels and/or sarcoplasmic reticular (SR) Ca2+-adenosine triphosphatase (ATPase) contribute to tone development.', 'Effects of calcium-free solution, nifedipine, pinaverium (calcium channel blockers), and cyclopiazonic acid (CPA; SR Ca2+-ATPase inhibitor) on the contractile activity of colon from mdx and control mice were determined.', 'Calcium-free solution abolished spontaneous contractions in both preparations, but decreased the tone only in mdx mice. Nifedipine or pinaverium abolished phasic contractions, acting with different sensitivities on the 2 preparations. They also decreased the tone in colons of mdx mice, and Ca2+-free solution did not cause any further loss of tone. CPA, after an early contractile effect, abolished spontaneous contractions in control animals. It did not suppress the contractile activity in mdx mice. CPA inhibited the repletion of intracellular calcium stores in both tissues to the same degree.'] Is increased calcium influx responsible for the sustained mechanical tone in colon from dystrophic ( mdx ) mice?