Different electrical responses of outer and inner muscle of rabbit carotid artery to noradrenaline and nerves.

Electrical responses of outer and inner muscle of the rabbit carotid artery to electrical stimulation and noradrenaline were investigated. Mean values of the resting potential and space constant in the direction of the long axis of the cells were -47.3 mV and 1.61 mm for the inner muscle and -45.9 mV and 0.92 mm for the outer muscle. Both muscles showed strong outward-going rectification with no evoked action potential on continuous injection of depolarizing current. Such current spread not only in the direction of the long axis of the smooth muscle cells but also to a lesser degree transverse to this axis (space constant approximately 0.55 mm). There was little or no spread of current from the outer muscle layer to the inner muscle layer. At high frequencies of nerve stimulation (higher than 20 Hz), slow depolarizations representing very slow excitatory junction potentials (e.j.p.s) were recorded from the outer (innervated) muscle. However, e.j.p.s were not evoked from the inner (non-innervated) muscle at any rate of field stimulation. At low frequencies of stimulation (less than 5 Hz) no e.j.p. was observed in either the inner or outer muscle, although the muscle contracted. Noradrenaline (10(-6) M) depolarized inner but not outer muscle. High concentrations of noradrenaline (10(-5) M to 2 X 10(-4) M) caused large depolarization of the inner muscle, and also smaller depolarization of the outer muscle.