Potassium-induced relaxation of the rat anococcygeus muscle

The nature of K-induced relaxations of the isolated rat anococcygeus muscle was studied in Tyrode's solution containing a variety of smooth muscle stimulants. When the tone of the preparation was raised by ACh (1 X 10(-6) M), the inhibitory responses to excess K (20-50 MM) were almost completely prevented by tetrodotoxin (1 X 10(-6) M) but not affected by ouabain (2 X 10(-4) M). When the preparation was contracted by barium (5 X 10(-3) M), tetraethylammonium (3 X 10(-2) M) or procaine (8 X 10(-3) M), relaxations by excess K (10-60 mM) were not affected by tetrodotoxin but completely inhibited by ouabain. It is concluded that two types of K-induced relaxations can occur depending on the smooth muscle stimulant used for elevating the tone of the preparation: the one is due to an activation of the electrogenic Na pump mediated by Na+, K+ ATPase of the smooth muscle membrane, the other is due to a stimulation of the inhibitory nerves by K.