In vitro effects of lidocaine on contractility of circular and longitudinal equine intestinal smooth muscle.

The purpose of the study was to compare the contractility-enhancing effects of lidocaine in equine jejunal circular (CSM) and longitudinal smooth muscle (LSM) in vitro. In previous studies, more pronounced effects of lidocaine were observed in ischaemia-reperfusion (IR) injured smooth muscle. Therefore in this study, effects were examined in both non-injured control tissues and tissues challenged by a defined, artificial IR injury. Isometric contractile performance of CSM and LSM, assessed by frequency (F), amplitude (A) and mean active force (MAF) of contractions, was defined as contractility. LSM featured lower basic contractility compared to CSM. Lidocaine provoked contractility-enhancing effects in both smooth muscle layers, but except for F at high lidocaine concentrations, contractility of LSM remained lower throughout the trial. Additionally, higher lidocaine concentrations were required to cause significant effects in LSM. No differences were observed in contractility of control and IR injured smooth muscle, but higher lidocaine concentrations were needed to provoke effects in IR injured smooth muscle. In contrast to CSM, contractility of LSM did not decrease at comparably high lidocaine concentrations. Differences in basic contractility of CSM and LSM might be explained by physiologically lower activity of LSM per se or by a thinner LSM layer with fewer smooth muscle cells taking part in contractions. The smaller thickness of the LSM layer may also contribute to persisting discrepancies in contractility following lidocaine application. Additionally, variations in lidocaine concentrations necessary for inducing significant effects could result from differences in the molecular structure of CSM and LSM cells.