Depolarization-Stimulated Contractility of Gastrointestinal Smooth Muscle in Calcium-Free Solution: A Review

Abstract

The membrane of most gastrointestinal smooth muscles shows slow waves, slow rhythmic changes in membrane potential. Slow waves serve to bring the membrane potential of smooth muscle cells to a threshold level that elicits a second electrical event known as the spike or action potential. The inward current of the spike, in most gastrointestinal smooth muscle preparations, is carried, at least in part, by calcium. Indeed, considering the narrow diameter of smooth muscle cells, some have hypothesized that the influx of calcium during the spike is sufficient for activation of the contractile machinery. Findings consistent with this include marked reduction in contractility during exposure of muscle segments to blockers of L-type calcium channels or following reductions in external calcium levels. However, it has also been observed that following exposure of muscle segments to external bathing solutions containing no added calcium plus 5 mM EGTA to remove any remaining extracellular calcium, contractions can be triggered following membrane depolarization. It is noteworthy that in isolated smooth muscle cells or in small muscle segments, during incubation in calcium-free solution, depolarization does not induce contractions. The present paper discusses the evidence in support of depolarization-mediated contractions occurring in gastrointestinal smooth muscle segments during incubation in solutions devoid of calcium.

Characterization of Cecal Smooth Muscle Contraction in Laying Hens

“…Without calcium, cross-bridge cycling cannot take place, resulting in declining and an end to muscle contraction. Studies in mammals and bufos confirmed the absence of spontaneous smooth muscle contractions during incubation in calcium-free solution [29].…”

From depolarization-dependent contractions in gastrointestinal smooth muscle to aortic pulse-synchronized contractions

“…Prolonged potentials may persist for hours in calcium-free solutions and have a voltage excursion similar to the spike3,8 (Figure 1). In several gastrointestinal and visceral smooth muscle preparations, prolonged potentials trigger rhythmic contractile activity7–10 (Figure 2).…”

“…Recordings of electrical and mechanical activity in vitro from gastrointestinal smooth muscle segments show rhythmic mechanical activity that is triggered by corresponding rhythmic changes in membrane potential. 1 – 3 The underlying membrane potential changes are usually composed of two events, starting with a slow rhythmic change (often denoted as slow waves), which serves to depolarize the membrane potential to a sufficient level to elicit a second event, which is the spike or action potential ( Figure 1 ). Although in some gastrointestinal smooth muscles slow waves themselves may trigger contractions, 4 the usual physiologic trigger for contractions is considered the spike.…”

“…After the incubation of gastrointestinal smooth muscle segments in solutions devoid of calcium and containing the chelating agent ethylene glycol tetraacetic acid (EGTA), normal slow waves and spikes were eliminated, and an alternative rhythmic activity (prolonged potentials) develops 6 – 8 ( Figure 1 ). Prolonged potentials may persist for hours in calcium-free solutions and have a voltage excursion similar to the spike 3 , 8 ( Figure 1 ). In several gastrointestinal and visceral smooth muscle preparations, prolonged potentials trigger rhythmic contractile activity 7 – 10 ( Figure 2 ).…”

“…These and other explanations for the failure of some to note contractions in gastrointestinal smooth muscle during incubation in calcium-free solution have recently been described. 3 …”