Prolonged potentials in gastrointestinal muscles induced by calcium chelation

Abstract

When stomach muscles of skate, toad, or frog or intestinal muscle of cat are treated with Ca-free physiological solutions containing 2-5 mM EGTA or EDTA, spontaneous spikes and slow waves disappear reversibly. With continued treatment, depolarization of 25-30 mV from resting potentials of -65 mV occurs and rhythmic prolonged potentials of several seconds duration appear. They show rapid depolarization to near zero and rapid repolarization and they may continue for several hours. The prolonged potentials disappear when Na is replaced by Li, Tris, or choline. They are insensitive to TTX. The EGTA-induced waves are abolished by Mn, Co, La, verapamil, and D 600. After 10-15 min in 5 mM EGTA, voltage-current and abolition of anomalous rectification. It is concluded that when bound Ca is removed by a chelator, nonspecific reduction in resistance occurs and Na ions may enter rhythmically through channels normally used by Ca.

Selectivity of calcium channels in rat uterine smooth muscle: interactions between sodium, calcium and barium ions.

“…This value is close to the expected Na equilibrium potential calculated from the Nernst equation (ENa = +38 mV assuming an internal Na concentration of 30 mm, at 30 TC). These results are in good agreement with previous observations on the ionic nature of action potentials recorded in Ca-free solutions containing chelating agents in gastro-intestinal (Biilbring & Tomita, 1970;Prosser et al 1977) and uterine smooth muscles (Mironneau et al 1982). There are several pieces of evidence that this new Na permeability was due to Ca channels that became permeable to Na when divalent cations were removed.…”

“…The action potentials are apparently mediated by voltage-dependent Ca channels since they vanish when external Na is removed or when Ca-channel blockers are added. Related findings were obtained on snail neurones (Kostyuk, Mironov & Shuba, 1983) and on muscle preparations (smooth muscle: Biilbring & Tomita, 1970; Prosser, Kreulen, Weigel & Yau, 1977; cardiac muscle: Garnier, Rougier, Gargouil & Coraboeuf, 1969;Miller, 1979; skeletal muscle: Potreau & Raymond, 1982; Almers, McCleskey & Palade, 1984). Uterine Ca channels are also permeable to divalent cations such as Ba and Sr (Jmari, Mironneau & Mironneau, 1986).…”

Ionic channels in smooth muscle studied with patch-clamp methods.

“…It seems that Ca ion controls ionic selectivity of Ca channel and that Na ions can pass the Ca channel under Ca-deficient condition. Similar results have been obtained with the sucrose-gap method in the muscle strips of guinea-pig taenia caeci [12], cat and guinea-pig small intestine [51], and rat myometrium [30].…”

Prolonged action potential of frog skeletal muscle membrane in Ca-free EGTA solution.

“…Similar action potentials produced in low or zero calcium with a chelator has been also observed in cardiac muscle fibers (CHANG and SCHMIDT, 1960;GOTO and ABE, 1964;GARNIER et al, 1969;TRITTHART et al, 1973;MILLER and MORCHEN, 1978) and in smooth muscle fibers (PROSSER et al, 1977).…”