Electrical properties of the cardiac muscle cell membrane and its role in the excitation-contraction coupling.

During extracellular stimulation, the electrical properties of the cardiac muscle cell membrane were investigated by means of intracellular microelectrodes. The current-voltage characteristic was S-shaped, and showed non-linearity over given voltage ranges in hyperpolarizing and depolarizing directions as well. The space constant of the membrane was 1.7 +/- 0.1 mm, which decreased after treatment with Ca-free or hypertonic solutions. The mean membrane time constant was 70 msec. and the input resistance was about 8 Mohm. The specific membrane resistance (Rm) and capacity (Cm) were 20 000 ohm.cm2 and 5--6 muF/cm2, respectively. After glycerol treatment the Rm decreased to 10 000 ohm.cm2 and the Cm to 4--4.5 muF/cm2. On the basis of our results it seems reasonable to suggest that the functional relationships between muscle cells might be realized through low resistance junctions, and the effectiveness of these junctions appears to be weakened both by Ca-free solution and glycerol treatment. The specific membrane capacity was lowered by 20 per cent, after treatment with a hypertonic solution, however 60 per cent of the contractility remained unchanged. Thus, we conclude; 1. a part of the Cm originates from the infoldings of the surface membrane, and 2. the snail heart muscle cells are insensitive to glycerol treatment. Since the excitation-contraction (E-C) was not affected by the glycerol treatment it can be assumed that in snail heart the coupling of the E-C is a "direct coupling" similarly to other heart, smooth and slow muscles.