Relaxation and hyperpolarization of the smooth muscle of the rat tail artery following electrical stimulation.

1. The smooth muscle of the rat tail artery was made to constrict with noradrenaline or 5-hydroxytryptamine. Electrical stimulation of the contracted artery caused a transient hyperpolarization and relaxation. 2. The hyperpolarization and relaxation could be recorded from denervated arteries, showing that these responses were independent of the perivascular nerves. 3. Removal of the vascular endothelium caused only a small reduction in the relaxation. 4. Raising the external K+ concentration to 20 mM abolished or greatly reduced the relaxation, and caused some reduction of the hyperpolarization. 5. 1 mM-tetraethylammonium chloride abolished both the hyperpolarization and the relaxation. 6. From the membrane potential-contraction relationship for this artery it appeared that the hyperpolarization could account for some but not all of the relaxation.