RELAXATION OF THE BOVINE RETRACTOR PENIS MUSCLE BY A SMALL K+ EXCESS AND THE ROLE OF K+ IN ITS NEUROGENIC RELAXATION

The mechanism of the relaxation of the bovine retractor penis muscle i nduced by 6.7 mM Kf as well as the role of K+ in the neurogenic relaxa tion of this muscle induced by nicotine, acetylcholine or electrical f ield stimulation, was studied. The relaxation induced by 6.7 mM K+ was , contrary to that induced by nicotine or electrical field stimulation , abolished by 10(-7) M ouabain. 15 min exposure to 10(-5) M N-G-nitro -L-arginine, 3.2 x 10(-6) M tetrodotoxin, 5.0 x 10(-4) M hexamethonium , 5.3 x 10(-4) M methylene blue or hypoxia, all known to inhibit the n eurogenic relaxation, did not affect the relaxation induced by 6.7 mM K+ which was also unaffected by 10(-5) M apamin, 3 X 10(-3) M 4-aminop yridine, 2.6 x 10(-2) M tetraethylammonium and 7.3 x 10(-4) M Ba2+. Ex posure to K+-free solution reversibly abolished the neurogenic relaxat ions. The relaxations caused by 5.0 X 10(-7) M cromakalim and 2.0 x 10 (-6) M pinacidil were totally blocked by 10(-5) M glibenclamide. Glibe nclamide and apamin did not affect the tone of the muscle or its neuro genic relaxations. 4-Aminopyridine 4.0 X 10(-5) to 3.0 X 10(-3) M and tetraethylammonium 10(-4) to 2.6 x 10(-2) M raised the tone and enhanc ed the relaxations elicited by electrical field stimulation. The resul ts indicate that the relaxation induced by 6.7 mM K+ is partly mediate d by activation of Na+-K+ ATPase and that its mechanism is thoroughly different from that of the neurogenic relaxations. K-f channels sensit ive to cromakalim and pinacidil exist in the smooth muscle cells of th e bovine retractor penis, but they or small conductance K+ channels ar e not involved in its neurogenic relaxation. The effects of the inhibi tory neurotransmitter(s) on the smooth muscle cells are not mediated b y K+ channels sensitive to 4-aminopyridine or tetraethylammonium, but such neuronal K+ channels may participate in the regulation of release of the inhibitory transmitter(s).