Received for publication May 6, 2008.
Revised July 11, 2008.
Accepted for publication July 14, 2008.

Calcium sensitization in human esophageal muscle: Role for RhoA kinase in maintenance of lower esophageal sphincter tone

Abstract

A rise of the concentration of intracellular free calcium ([Ca²⁺]_i) is important for initiating contraction of smooth muscle, and Ca²⁺ sensitization involving RhoA kinase can sustain tension. We previously found that [Ca²⁺]_i was comparable in cells from the esophageal body (EB) and lower esophageal sphincter (LES) muscles, despite the fact that the LES maintains resting tone. We hypothesized that Ca²⁺ sensitization contributes to contraction in human esophageal muscle. Tension and [Ca²⁺]_i were measured simultaneously in intact human EB and LES muscle using the ratiometric Ca²⁺-sensitive dye fura-2. Spontaneous oscillations in EB muscle tension were associated with transient elevations of [Ca²⁺]_i. Carbachol caused a large increase in tension, compared to spontaneous oscillations, although the rise of [Ca²⁺]_i was similar, suggesting Ca²⁺ sensitization. The RhoA kinase blockers Y-27632 and HA-1077 reduced carbachol- and nerve-evoked contraction of the EB, accompanied by smaller reduction in the rise of [Ca²⁺]_i. Protein kinase C inhibitors reduced force to a lesser extent. RhoA kinase blockers caused concentration-dependent reduction of tension in spontaneously contracted LES muscle. Moreover, Rho A kinase blockers reduced intrinsic nerve-evoked and carbachol-evoked contraction. However, there was no effect on nerve- or nitric oxide-mediated relaxation of LES. Ca²⁺ sensitization mediated by the RhoA kinase pathway has an important role in contraction of human EB muscle and LES tonic contraction, a feature not previously recognized.

Key words: Ca2+, LES, esophagus, fura-2, human, sensitization