Cat papillary muscles were depolarized by raising the external K concentration so that resting potential went from about −80 mV to −50 mV. This caused inactivation of the transmembrane Na system. The remaining Ca-mediated action potentials showed greater overshoot, reduced upstroke velocity and action potential duration, and higher threshold of excitation. A graded electrical response to stimulous strength was also present. These action potentials depended mainly on the extracellular Ca concentration [16]. Further, the specific Ca-antagonistic compounds verapamil and D 600 [10] or Ca-with-drawal completely abolished the ability of the K-depolarized fibres to respond to stimulation with propagated electrical activity. Conversely, extra Ca, epinephrine or isoproterenol overcame this inhibitory effect by increasing the Ca transmembrane influx. Also, within 1 min after returning treated muscles to normal K
0 Tyrode solution, the Na system was reactivated and apparently normal electrical activity possible. In this case, Ca-withdrawal and the Ca-antagonists lose their electrical inhibitory influence, while excitation-contraction uncoupling still persist.

Calcium-mediated action potentials in mammalian myocardium

The effects of Mn-2+-containing solutions on the mechanical and electrical responses of myocardial tissue were studied on guinea pig ventricular strips. At a concentration of 10 mM, Mn-2+ abolished the twitch responses and caused the development of a contracture in stimulated preparations but not in resting or in Ca-2+-depleted preparations. The duration of action potentials was shortened, and dV/dt-max was decreased. In Ca-2+-depleted (0.1 mM), Mg-2+-free medium, Mn-2+ also increased the amplitude of the overshoot; the increase was due to a slow phase of depolarization. Measurement of the Mn-2+ uptake showed an increased influx of these ions in stimulated preparations compared with that in quiescent preparations. These results suggest that Mn-2+ (1) generates a transmembrane current in guinea pig myocardial cells and (2) interferes with the excitation-contraction coupling process at two levels: the cell membrane and an intracellular site.

Depolarization-induced movement of Mn 2 cations across the cell membrane in the guinea pig myocardium.

Crucian carp (Carassius carassius L.) were acclimated for at least 4 weeks to 2°C or 22°C, and the consequences of thermal acclimation on force development, time-course of contraction and action potential duration of the ventricular myocardium were studied. In cold-acclimated fish contraction was activated at much lower external [Ca] than in warm-acclimated fish: [Ca] for half-maximal force was 0.9±0.15 and 3.1±0.92 mmol·l-1 (P<0.05) for cold- and warm-acclimated fish, respectively. Durations of contraction and relaxation were significantly longer in fish acclimated to 2°C than in fish acclimated to 22°C, especially at [Ca] below 2 mmol·l-1. In low-Ca solution ventricular action potential was prolonged both in cold- and warm-acclimated fish. In 0.5 mmol·l-1 Ca action potential duration at zero voltage level was longer in cold- than warm-acclimated fish. Although lengthening of action potential was evident in both acclimation groups, a marked prolongation of contraction duration by low-Ca solutions occurred only in cold-acclimated fish. This suggests that a plateau component of contraction is present in cold-acclimated fish but less well developed in warm-acclimated fish hearts. Contractions were strongly inhibited by sarcolemmal Ca-channel blocker, cadmium (100 and 300 μmol·l-1), in both warm- and cold-acclimated crucian carp hearts. However, the sarcoplasmic reticulum Ca release channel blocker, ryanodine (10 μmol·l-1), had no effect on the force of contraction in either acclimation group. These results suggest that the contraction of crucian carp heart is controlled by sarcolemmal mechanisms without contribution by sarcoplasmic reticulum Ca release. Since the Ca sensitivity of myofilaments was not altered by thermal acclimation, the results indicate that thermal acclimation alters Ca activation of contraction of the crucian carp heart at the level of sarcolemma.

Effect of extracellular calcium on the contractility of warm-and cold-acclimated crucian carp heart

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.

Prolonged potentials in gastrointestinal muscles induced by calcium chelation

The interrelationship of calcium-mediated action potentials and tension development in cat ventricular myocardium.

Guinea pig papillary muscle was exposed to Tyrode solution to which no Ca had been added (Ca-free) and to various concentrations of EDTA in either Tyrode or Ca-free Tyrode. The results were as follows:

1.

During 15 to 20 min exposure to Ca-free Tyrode the force of contraction of papillary muscles initially declined rapidly and then at varying rates and to varying degrees. Action potential duration and resting potential were reduced but maximum upstroke velocity (dV/dtmax) was increased. The effects were essentially reversed upon Ca replacement.




2.

Exposure to 0.8 mM EDTA in Ca-free Tyrode or 2.5 mM EDTA in Tyrode produced similar effects as exposure to Ca-free Tyrode although the rates of change were more rapid and contraction was always abolished.




3.

Exposure to 2.5 or 5.0 mM EDTA in Ca-free Tyrode or 5 mM EDTA in Tyrode caused a very rapid loss of measurable contraction, a decrease in resting potential, a marked increase in action potential duration and an increase ind V/dtmax. Replacement of Ca caused a beat to beat decrease in action potential duration, an increase in resting potential, a decrease ind V/dtmax and often an increase in resting tension. Contraction was initiated within 15 to 20 sec associated with marked after-contractions. As the after-contraction amplitude declined there was a decrease in resting tension. Action potential duration and resting potential recovered during 10 to 15 min after Ca replacement altough plateau duration usually did not return to control value.

H. E. Stierle

Papers

Effects of Ca-Free and EDTA-Containing Tyrode Solution on Transmembrane Electrical Activity and Contraction in Guinea Pig Papillary Muscle