Figure 3. Electrical properties of muscle bundle from the circular layer of guinea-pig distal colon.

A, the effects of injecting a series of hyperpolarizing and depolarizing currents on the membrane potential of a short bundle of muscle isolated from the distal colon. Note that the discharge of membrane noise was little affected throughout. The resting potential of this preparation was −44 mV; the time current and voltage calibration bars apply to recordings shown in A. B, spectral density curves illustrating that the discharge of membrane noise (•) was unaffected by adding 9-AC (○) to the physiological saline; the resting potential of the preparation remained unchanged at −40 mV throughout. C, the discharge of membrane noise (•) was reduced by adding MAPTA-AM (20 μm; ○) to the physiological saline for 10 min (Cc). Note that the reduction in membrane noise was associated with an increase in resting potential from −42 mV (Ca) to −51 mV (Cb). Atropine 1 μm and nifedipine (1 μm) were present in each experiment.