Effect of acetylcholine on membrane currents in guinea-pig papillary muscle

1. The effect of acetylcholine (ACh) on the membrane current components in guinea-pig papillary muscle was studied by the single sucrose gap voltage clamp method. 2. The slow inward current (is) elicited by various depolarizations from the holding potential of -40 mV was evidently diminished by ACh greater than 5 x 10(-7) M. The amplitude of is, estimated as the difference between the peak of is and the current at 150 msec, was reduced to 56.8 +/- 12.9% (mean +/- S.D., n = 34) by 2 x 10(-8) M-ACh when estimated at the membrane potential where the amplitude was maximal. 3. is was suppressed almost completely by 2 mM-Co2+, is obtained by subtracting the current remaining in 2 mM-Co2+ was also diminished to the same extent by 2 x 10(-6) M-Ach with little change in its time course; the degree of decrease was not dependent on the membrane potential. 4. The time-dependent outward current during the depolarizing pulse was also diminished by 2 x 10(-6) M-Ach. Correspondingly, the positive tail current obtained when the membrane was repolarized to the holding potential of -40 mV after various 1 sec depolarizations was reduced in a proportionate manner to 68.6 +/- 7.3% (n = 10). 5. The time-independent inward rectifying outward current was not affected by 2 x 10(-6) M-Ach. 6. Atropine (10(-6) M) restored is and the time-dependent outward current which had been previously depressed by 2 x 10(-6) M-ACh. 7. ACh may decrease tension development by depressing is, thereby preserving the duration of the action potential with relatively little change, since, different from atrial muscle, ACh does not increase the outward current in mammalian ventricular muscle.

The effect of procaine on the passive electrical properties of guinea-pig ventricular muscle

Electrotonic potentials produced by rectangular hyperpolarizing current pulses were recorded intracellularly at various distances from guinea-pig papillary muscles and the results were analyzed according to the single-dimensional cable model. In normal Tyrode solution the space constant (lambda) was 0.62 +/- 0.12 mm (mean and S.D., nine preparations) and the time constant (tau) was 4.0 +/- 0.8 ms. Procaine (1 mg/ml) increased lambda to 1.20 +/- 0.29 mm and tau to 19.7 +/- 4.7 ms by increasing the membrane resistance, while the membrane capacity was only slightly increased and little change was observed in the internal longitudinal resistance. The dose-response relationship revealed that procaine inhibited the resting membrane conductance almost completely in a one-to-one manner with the dissociation constant of about 1 mM. It was suggested that potassium conductance predominant in the resting membrane conductance in guinea-pig papillary muscle was depressed by procaine and that electrical coupling is well maintained even when the membrane resistivity of apposed normal ordinary membrane at intercalated disks becomes high.

Manganese-dependent propagated action potentials and their depression by electrical stimulation in guinea-pig myocardium perfused by sodium-free media

1. Propagated action potentials were recorded in right ventricular papillary muscles from guinea-pig heart while exposed to Na-free, Ca-free and Mg-free solutions containing Mn. 2. When Na was totally replaced by 95 mM-Mn the overshoot was about 45 mV while the resting potential was about -90mV. 3. The overshoot of action potentials was increased by about 20-30 mV per tenfold increase of Mn concentration over the range of 2-50 mM. 4. Similar increases of overshoots with increasing of Mn concentration also occurred in the presence of 0-6 mM-Ca. Increasing of Ca from 5 to 20 mM had little influence on the overshoot but shortened the duration of the Mn-dependent action potential in the presence of 5 mM-MN. 5 Mn-dependent action potentials were not depressed by 3 X 10(5) M tetrodotoxin but by La. 6. These results suggest that Mn passes through the slow inward current channel to generate the action potential seen under the Na-free condition. 7. The overshoot and duration of the Mn-dependent action potential decreased with stimulation. At stimulus frequencies (Hz) of 0-5, 0-2, 0-1, 0-017 and 0-0033 the overshoot of action potential in 5 mM-Mn Tyrode decreased by 0-5-1 mV per an action potential. This depression of the action potential is explained by assuming intracellular accumulation of Mn.

Manganese action potentials in mammalian cardiac muscle

The membrane potential in guinea-pig's papillary muscles from right ventricle was recorded by glass microelectrodes and stimulation was effected by current pulses applied through a sucrose-gap. Action potentials with overshoot were recorded in the solution lacking Na+ and Ca++ but containing 2-95 mM Mn++. The overshoot was increased with the increase of [Mn++]o by about 30 mV/decade. Similar Mn++ dependent action potentials were also obtained in Na-free solution containing 0.6 mM Ca++. The results indicate that Mn inward current is sufficient to generate action potentials in cardiac muscle.

The mode of transverse spread of contraction initiated by local activation in single frog muscle fibers

Isolated single frog muscle fibers were locally activated by applying negative current pulses to a pipette whose tip was in contact with the fiber surface. In contrast to the graded inward spread of contraction initiated by a moderate depolarization, the contraction in response to a strong negative current was observed to spread transversely around the whole perimeter but not through the center of the fiber. This response was elicited only with pipettes of more than 6 micro diameter. The response was still present if the sodium of the Ringer solution was replaced by choline, or the chloride was replaced by nitrate or propionate. The duration of the response appeared to be independent of the duration of stimulating current in fresh fibers, while the contraction lasted as long as the current went on in deteriorated fibers. The contraction was first initiated at the area of fiber surface covered by the pipette, and spread around the perimeter of the fiber with a velocity of 0.8-6 cm/sec. Possible mechanisms of the response are discussed in connection with the properties of the transverse tubular system, the possibility of some self-propagating process along the walls of the tubules being suggested.